A study regarding the feasibility of airships using real-world physics within the scope of KSP.

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A study regarding the feasibility of airships using real-world physics within the scope of KSP.

Disclaimer: First and foremost, this is not a rant.  Don't portray it as one as you'll be promptly ignored.  Secondly, this is not a criticism of any work by any modder.  God knows there are fewer and fewer of us left and I'm not one to chase any of them off.  This is merely my opinions and nothing more and is in no way intended to be insulting to anyone.

Years ago, I fell in love with a mod.  And not long after it's birth, it was buried.  This dissertation, wall of text, manifesto, ramblings of a mad man, call it what you wish, is me thinking out loud to the KSP public about the creation of a very similar mod. 

This thread will serve 2 purposes.

  1. A place for me to place these ideas and a future reference for those who will want to understand my reasonings.
  2. A place for you to discuss and, more likely, correct my mistakes.

A brief history of the KSP Airship.

Hooligan Labs Airships - This was the first airship mod to grace KSP.  Coming to the scene in 2013, it was the first mod that attempted to broach the lighter than air concept (as far as I know).  The models used to the parts were very futuristic in concept.  Some appear completely hollow yet somehow generate lift.  While this in itself isn't a bad thing, it did require a bit of imagination to convince yourself that they were airships.  After passing through several hands, it's still pretty much as it was when released having spent the last couple of years in maintenance mode.  You can find it here.

pros: It got fans of airships off the ground and allowed players to experience flight in the slow lane... or maybe the static lane would be a better choice of words.  It worked on any planet with an atmosphere.  You could stick them on top of a rocket and they'd normally survive reentry. They also featured envelopes that expanded procedurally which meant that you could leave them uninflated until arrival at a launch destination.
cons: Unrealistic flight model.  There were virtually no real-world physics calculations involved with Hooligans.  While atmospheric density and g-force were included in the calculations, the buoyancy model was such that they were more like helicopters.  You could maintain precise altitudes by just setting a slider and then rocket your balloon around mere meters off the ground.

Procedural Airships - This mod entered the scene less than a year after Hooligan's.  Based on the Procedural Parts mod, Procedural airships allowed the player to not only size and scale the airship, it also allowed different meshes to be used for parts like end caps.  This meant, instead of adding yet another part to the collection, the user clicked a button to cycle through those available.  The first nail in it's coffin was a major update to the Unity UI system.  I once spoke to @RadarManFromTheMoon about reviving it and he basically said the entire UI was trashed and didn't have the time to rework it. The final nail came when Procedural Parts became orphaned for a period of time.  Without this dependency working, ProcAirships wouldn't work even with a new UI.  Though it hasn't worked since 0.90, you can find it here.

pros: Actually looked like real-world airships.  Being procedural, they could be scaled and resized in the editor to suit the players needs.  It used many real-world airship features like ballast dumping and balloonets.  It also had the ability to vent lifting gasses.  It's flight model took into account not only stock atmospheric modelling but that of FAR and NEAR.  Most importantly, it used the ideal gas law as the basis for it's lifting calculations and took the molar mass of lifting gasses into account.  Once you learned how to fly them, it 'felt' like you were piloting a zeppelin.
cons:  Dependent upon another mod for all of it's parts.  Instead of using familiar lifting gasses RadarMan created his own, Liftium and Combustium, which roughly translated into Helium and Hydrogen. I list that as a con because KSP is a game replete with resources and the more familiar ones could have been used without detriment to the mod.  Difficult to fly.  This was the first mod I ran into that pretty much required you to follow it's built-in tutorial.  There's a review of the mod by @Kottabos where he detonated an airship while sitting on the runway from accidentally overpressurizing it.  Blowing them up was wayyy too easy.  Limited parts. There were only 3 parts, 2 envelopes and 1 ballast tank.  But, they were procedural so it felt like more.

Aerostats - This mod came out in early 2016 as a WIP and, unfortunately, never progressed beyond that state.  @wasml, the author of that mod, had the basis for an airship that followed real-world physics.  Using Hydrogen and Helium for the lifting gasses he also applied the ideal gas law in order to achieve lift.  Because this mod never moved into release status, I'm not going to list any pros or cons.  Suffice it to say that what he had and, from the code I've seen, what he was planning on, would easily rival the two previous mods.  One thing I will note is that wasml intended on having his envelopes leak (which is very realistic), and in the WIP I found, leak they did.  They stated leaking the instant I launched.  If I still had any lifting gas left and managed to get airborne, I spent more time clicking on the envelopes and then clicking on his 'repair' buttons than I did flying the airships. I did get this mod working but I believe it required that I recompile the plugin.  You can find it here.

Heisenberg Airships - Also coming out in 2016, @Angel-125 has done a fine job modelling airships and of all the airship mods, his has the most extensive part collection and allows for very historic looking airships.  Because this mod has Hooligan's asa dependency, I won't go into any pros or cons for it either.  However, I will say that because Hooligan's physics are the underlying code for lift,  it allows for some rather unrealistic, albeit very unique, designs.   You can find it here.


I've been informed that I kinda forgot one though I'm not sure it qualifies:

KerBalloons - Originally by JoePatrick1, KerBalloons are very simple devices designed primarily to gather science.  While I have seen some cases of people strapping engines onto them and using them for sky-diving platforms, I don't really consider them to be airships.  An entertaining mod and useful for gathering cheap science on a variety of planets, they don't use any real-world physics, including the fact that their vertical ascent is hard-coded.  Though they do simulate balloons to a reasonable degree and they are capable of carrying Kerbals aloft, the only methods for controlling vertical ascent is to completely deflate them or wait for them to expand to the point they pop.  Just recently JoePatrick1 gave me his blessings to take over this mod.  And now that you have an idea of what's on my brain I assume you can see my interest in it.  If these experiments go well, KerBalloons may be using real-world physics in the future. You can find it here.

*end edit*

So, if you haven't guessed, my preference toward airship mods is slanted toward those which are based on reality.  Unfortunately, it's been a couple of years since such a mod existed or was even being planned.  Now before you start shaking a finger at me and pointing toward some of my more unrealistic mods and resources (compressed water), yes, I do have my quirky side and I fully realize that the only way to simulate real life is with... well, real life.  But for the most part, I do try to base my mods on reality (ok, 1869 is an exception where I just do what I want.).  When it comes to flying an airship tough, I want it to feel like I'm wearing a tophat and try to relive some of their victorian history.

The basic physics of airships

So, before I go jumping off any bridges, an understanding of how airships work is in order.

Airships, and naval ships for that matter, create buoyancy by displacing something heavier.  A ship on the ocean floats because it creates a hole in the water.  It does this by shoving water aside and replacing it with something much lighter, air (for the most part).  Airships employ the same principle.  They are able to float because they create holes in the atmosphere.  Their envelopes allow them to shove air aside and replace it with something much lighter, lifting gas.  When this hole is created, lift is the result.   In the world of Kerbals, air and water are intangible objects.  You can't create holes in them.  But, the physics around them are accurate enough that you can simulate a hole.  For the purposes of an airship, this simulation can be accomplished using the Ideal Gas Law.  The ideal gas law basically states that if you know a few things about a gas, you can discover others.  For example, if you have a gas inside a balloon and if you know what elements the gas is comprised of and what the pressure and volume is, you can calculate the temperature.  As strange as that sounds, the balloons you played with as a kid can act as a thermometer.   Fortunately in KSP, a lot of those pieces of information are already programmed into the game like air pressure, temperature and substance densities.  Volume can be calculated by performing some basic geometry on the parts in the game.  So, all of the numbers necessary to plug into the ideal gas law either already exist in-game, or they can be calculated with a bit of math.

Once you start looking into the ideal gas law, you find out you can come up with some other calculations to help simulate the hole.  For example.  If you know the density of the air outside your craft (which KSP provides) and you can calculate the density of your lifting gas, finding out how much your airship can lift is easy.  It's (atmospheric density - gas density) * gas volume.  You're replacing a volume of air with the same volume of a lifting gas.  So, the amount you can lift is equal to the weight of the air you shoved out of the way.  If you take that number and multiply it by the local gravity (which KSP provides), that formula will tell you how many newtons of force that volume of gas can exert.  If you know KSP, you know all thrust in the game is based on newtons.

Lifting Gasses

Lifting gasses are what we replace the air in the atmosphere with when we create those holes I mentioned.  Traditionally, we're familiar with hydrogen, helium and hot air.  The requirement for a lifting gas is that it have a density less than the surrounding atmosphere.  The ultimate lifting gas isn't a gas at all.  It's a vacuum. Francesco Lana de Terzi (known as the father of aeronautics) realized as long ago as 1670 that a vacuum would be the perfect lifting medium because, unlike any lifting gas, it has no mass.  Every bit of air shoved out of the way by a vacuum would be converted into lift.  The problem there is, the mass of the container you need to hold a vacuum would weight more than the air you moved out of the way.  Perhaps in the future when material science creates something strong and light enough, a vacuum airship will be feasible.  Today, it's not.

While we're familiar with most of the common lifting gasses, there are literally dozens of possibilities.  And with the KSP system of resources all of those are possible.  It'll even be possible to simulate coal gas by converting in-game ore into a gas.  So none of those are out of the question.  But if you read closely, at the bottom of that list, you'll see something KSP doesn't do well, mixing of resources. 

Up till now, lifting gasses in KSP airship mods have been one gas at one density.  In the real world, gasses can be easily mixed.  And the math behind mixing of gasses is easy.  You add the pressure created by one gas to the pressure created by the other gas and you have the total.  Here's the problem.  When you mix two gasses, like hydrogen and helium, the density changes.  All KSP resources have a fixed density. If you mixed two gasses together and came up with a new density, you'd have a new resource.  That also means that even a small fractional change in the mixture of gasses, would also change the density and thus, yet another new resource.

So, as you can see, mixing of gasses in KSP would be a bit impossible .  You'd essentially have infinite resources clogging your computer memory.  Unless, that is, if you didn't consider it a resource at all.  It would be easy enough to track how much of each gas (say hydrogen and helium) go into a bottle.  By remembering how much goes in, you could calculate the densities and thus, it's lifting potential.  So mixing gasses is possible and realistic results could be achieved. The down-side to that is, you wouldn't be able to use those gasses as a KSP resource.  Meaning, moving the gasses between various parts wouldn't happen.

The question is then, which would be more realistic?  Allow the mixing of lifting gasses or have a single gas  but treat it like a resource?

There is another option, and this is something no other airship mod has attempted (to my knowledge).  Liquified gasses.  Hydrogen, like many gasses, can be compressed into a liquid form.  Suppose an airship mod allowed those liquified gasses to be moved around, loaded onto trucks or rockets and even bolted onto the side of the airship.  When the airship needed a lifting gas, it would simply open the valves and instantly create the gas.  You'd have a very compressed form of the lifting gas, a resource that several mods already include as collectable/minable and you could take a supply with you.  Once converted from liquid to a gas inside the airship, it would be a 'use it or lose it' though.  And, lifting gasses would not be a resource you could fill in the editor.  It would be something you'd have to create after you launch. 



Weighing in at roughly 0.68 mt at launch, this is a rather small test airship.  Using hydrogen it's total lift was just over 0.91mt. 
In this image 700 L of liquid hydrogen was used to fill over 800 m3 of volume (I blew a lot out the forward vent because I over-inflated the nose, was tail heavy and lost a lot of gas balancing.)

Other lifting methods

So, if you took the time to look over the available lifting gasses on Wikipedia, you probably noticed there are quite a few.  Most, if not all of those can be turned into a lifting gas in KSP.  So, the physics needed to get airships off the KSP runway exists.  There is a slight problem though: being able to make the ship realistic yet, light enough for the gasses to lift it.  While my testing is showing that even a small airship of a metric ton or so with enough space to carry a couple of Kerbals around can get off the ground, the envelopes are filled almost to capacity, leaving very little room for cargo.   Fortunately,  lifting gasses aren't the only way to get an airship off the ground.



The USS Macon (above), given to the US as part of Germany's WW1 reparations (corrected, that was the Los Angeles) had a rather unique engine arrangement.   By placing the engines inside the airship skin and mounting them at a 90 degree angle to the direction of travel, it allowed the props to be rotated.  But, if you look at the second and third image, which is supposedly from the USS Akron, the Macon's sister ship, you'll notice that the mounting brackets prevented the props from rotating directly upward, meaning they were most likely used exclusively for descent.  Whether they were connected to a transmission that allowed the prop rotation to be reversed, I wasn't able to discover.  Either way, this was a huge improvement in engine design compare to earlier airships like the USS Shenandoah.  Prior to rotating props, airship engines were so large that the designers actually called them cars and they were large enough that the crew worked inside the compartment.  Today, modern airships employ the rotating engine technique.


Most everyone knows that airship, the Goodyear blimp.  Here's a good shot of two of it's engines rotated for an assisted take-off.

But hey, we all play KSP and we all know the ultimate solution to getting off the ground is always moar boosters. Well, apparently that was thought of too when it comes to airships.  The first attempt at moar boosters was with the Piasecki PA-97 helistat - a conjunction of helicopter and aerostat.  If EVER there was a Kerbal airship, this would have been it.  What do you get when you take a 50 year old blimp, weld up a bunch of pipe and then, bolt on 4 junked Sikorsky H-34 helicopters with their tail rotors chopped off?  And then, you put a pilot in ONE of those helicopters with a bunch of cables running to the others so he can control them.... well, you have a Kerbal day in the making.

USN_Piasecki_Helistat_1985.jpg 300px-Sikorsky_S-58_landing_(cropped).jp

Now keep this in mind.  This thing was built in the 1980's.  Yes... that's right.  The Concorde had been flying for over 10 years and this thing, a $40 million investment in 1980's dollars, was supposed to be a technological leap forward?  Well, as I said, it was a Kerbal day in the making.  All it took was a gust of wind across the tarmac a vibration ensued and all 4 helicopter rotors became unbalanced, ripped through the blimp and then promptly broke off the tube frame.  Unfortunately, the pilot lost his life in that accident.  Fortunately, that's when the project came to a screeching halt.

But.  That was not to be the last word on the marriage of a helicopter and an airship.


Boeing has teamed up with a Canadian company (who holds the patents) and are in the early stages of creating the SkyHook (HLV) heavy lift vehicle.  While this is just one artists rendition of it, if you do some digging you'll find the images presented by Boeing are considerably different.  I can't post them here, they want MONEY for their images.  While this is still in the early stages, they're claiming it can lift 40 tons and carry it some 200 miles.

And then, there's the hybrid airships.  Some include wings, like aircraft and some, like the hybrid being built by Lockheed-Martin, is a lifting body,

So now you've seen some of the alternate lifting methods that have been associated with airships.  Here's the good news.  None of these designs are outside of the scope of KSP physics.  All the lift and thrust elements are there.  Now, which ones are going to be practical to design?  Rotating engines? No problem.  Helicopter rotors?  No problem.  Wings? Already there. Lifting bodies? Already in the game but... that's a stopping point.  While it would be possible I don't foresee at least me doing it.  And the big reason would be, it would require a specific shape.  Old airship and blimp designs may have some lifting properties to their shape but it would be so negligible in KSP that I'm not going to spend that much time trying to factor it in.  Creating an airship specifically with lifting body properties would lock players into a pretty rigid design.  And that's not what KSP is about.

But helistats? Yea... I want one of those.  Even if it is a Kerbal design in the making.



At roughly 1.2 metric tons, this airship was the first test flight of a 2 Kerbal gondola.  Still learning the balancing properties, I wasted a lot of liquid hydrogen getting it adjusted.
If you look at the cell inflation bar, it's at about 90% so there's still room to get a bit more lift out of it.


But really?  A comparison of fruits.

So far in this rant, dissertation, manifesto, I've focused on the physics of getting airships off the ground.  And, as you've seen from the screenshots, those numbers can be applied to KSP and made to work.  And I've even delved into the fanciful of helistats and from the screenshots in some of my replies, even they appear to work.  But seriously. How close do these airships come to what really files?

Thus far, the airships I've shown have been rather tiny by historical comparisons and pretty small even by modern standards.  Even the helistat I posted below was breaching the limits of it's capabilities just lifting a 9mt weight.  Up until now, I've been testing the lower limits of what was possible.   I wanted to make certain that at least on Kerbin that a simple 2-Kerbal airship was doable and easy enough to fly.  From what I've learned, if an airship is light enough you can get away with not much more than 1,000 m3 of gas.  When you compare those small airships I've been flying around to say, the Hindenburg, they're microscopic.  The Hindenburg's envelopes held right around 200,000 m3.  And by most accounts, it was even slightly over-pressurized when it lifted off.  So, maybe my numbers are way off.  Maybe I have some calculation that's not quite right that allows these tiny ships to fly when they're 1/2% the size of a real airship.  Maybe I'm nuts, wrong or both.

Today, I decided to test that theory against a known airship that we're all probably familiar with, the GoodYear blimp.  For this comparison, I'm using the stats available directly from GoodYear.  For the sake of you Euros, I'm going to be converting the relevant numbers into metric.

Stat                       WingFoot KSP LTA   
Overall Length 75.1 m 78.2m
Maximum Envelope Width 14.1 m 18
Envelope Volume 8425.03 m3 8231.3 m3
Maximum Weight 8.97 mt 6.32 mt
Maximum Speed 32.6 m/s 46.2 m/s
Gondola Seating Up to 14 humans1 2 Kerbals2
Total Usable Lift 1.83 mt 2.93 mt
Number and Type of Engines 3 Vectored 2 vectored
Engine Power 200 hp each 5 kN each
Endurance 24-40 hours3 7 hours w/200 LF4








1Seating configuration for the new Goodyear blimp is subject to adjustment as assignments and lift conditions vary.
2What lift conditions. Every day is pretty much perfect for airship flying

3Based on cruise power, maximum fuel load and optimum atmospheric conditions. Maximum endurance is in conjunction with optional extended range kit.
4 Approximate flight time at maximum speed. Maximum flight duration undetermined due to leaks not being incorporated yet.

So, I attempted to build an airship pretty close to the WingFoot stats in order to compare apples to apples.  While the shape is a bit more pointed due to the parts I created, most of the stats are pretty close.  The length and width I had to increase to get closer to the size of the WingFoot envelope volume.  I'm having to use some pretty basic geometry to calculate the envelope volumes and it's not 100% accurate but it's within a few percent. There's also a fair difference in the weight of the two.  Then again, mine doesn't have a 12,384 pixel lighted sign with the accompanying hardware.


LTA medium sized rigid airship parked outside the SPH.  This airship is an attempt to size/stat compare an LTA airship to the GoodYear WingFoot blimps.

Taking the WingFoot's billboard into account, the usable lift difference would probably be a lot closer were it removed.  While the WingFoot has what appears to be a considerably longer endurance and a much larger seating capacity, one thing to keep in mind is that WingFoot is pretty modular. They're able to adapt it to the mission.  If you noticed the notes at the bottom, the WingFoot has an 'extended range kit'  they can install.  Because this was the first flight of my replication, I over judged the amount of gas needed and ended up packing around an extra 1,300+ liters of liquid hydrogen.

Oh, and for us "Yanks" I did the reverse math on the fuel usage.  That's about 13.7 mpg.  You tell me if that sounds decent enough or should it be less (it's definitely not going to be more).

Another thing you'll see in a screenshot below is that the envelopes on mine are only filled to about 80% capacity.  For this particular airship, I didn't place any ballast on it.  The WingFoot, last I read, used 75 lb. sand bags.  Were I going to keep this one around, I'd definitely add in a ton or two of ballast so that the envelopes could be filled closer to capacity where the balloonets would become effective and aid in ascent/descent.  In this case, I ended up using the engine vectoring to assist in landings and takeoffs (which worked beautifully).

So, the purpose of this test was to compare apples to apples and I'm going to have to say that I'm satisfied.  This little 'blimp' replica, even with just the 3 ailerons/rudders like WingFoot flew beautifully.  I did bounce a bit on touchdown, but once I got stopped, I reversed and rotated the engines to push downward so the brakes on the 2 wheels would hold better and that worked.  I'm much more confident that the the numbers I'm using are close enough for KSP and reality.  If you want to get picky, yea, you'll find things are off a bit.  Of course, if you are that picky, you could always call Zeppelin NT and buy your own blimp and all the numbers would always be perfect.

Next post, I'm going to explain a bit more about how the balloonets and ballast system is going to work.


On the way to the Island Runway


About to land on the Island Runway. This is the only shot I got on this short trip that shows the UI and fuel gauges.  Note, the vectored engines are rotated and reversed so that they push down and backward.  It worked much better than the wheel brakes.


Balancing on 1 wheel long enough for Val to hop out and take a selfie.


Getting them back down again

So far, my ramblings here have been focused on how to get airships off the ground and performing tests to verify that the numbers look good enough for KSP (government) work.  And so far, I'm pretty satisfied with the results.  One other test I conducted that I won't go into in extreme detail is an attempt to replicate the lifting capacity of one of, if not the, largest airships ever, the Hindenburg.  That test is still ongoing however and so that I don't spoil all the fun of making your own I won't say a lot.  However, I will say that the Hindenburg was able to lift somewhere around 230 metric tons.  I was able to match the basic length, width and envelope volumes and the lifting capacity was very, very similar.  And, there was enough lift left over that I had to put on a BUNCH of ballast.  Which brings me to the topic of this section, bringing them back down again.

At the turn of the 20th century, when airships were starting to get lots of attention, inventors, scientists and engineers realized that getting an airship off the ground was easy enough.  You filled it with a gas that had a really low density compared to air and, up it went.  And getting them back down again was pretty easy too, you poked a hole in the envelope, let the gas out and down it would come.  There were two problems with this method. Hydrogen, the best possible lifting gas, is highly explosive.  It was easy enough to produce but flying around strapped to a ticking time bomb wasn't exactly the ideal solution.  Helium, which is very close to hydrogen in it's lifting abilities was the next best solution.  But, it was, and still is, impossible to manufacture.  It's only found near deposits of radioactive material.  While Helium is plentiful in the universe, not so on earth. The only way we could possibly manufacture it is in a fusion reactor, which we still haven't perfected.  To make matters worse, at that time, only one source was known to exist with any viable quantity and that was in the United States.  And that meant, if the Americans didn't like you, your helium got cut off.

Hydrogen valuable enough and helium was hard enough to get that the designers behind balloons and airships started coming up with ideas on how to conserve lifting gasses.  Even those engineers in the US where helium was easy to get realized that there was a finite quantity and they needed to conserve as much as possible.  The theory that came out of all of this was that once they put the lifting gas into the envelopes, do everything possible to keep it there.

In order to try to replicate the value placed on this resource in KSP, you won't be able to fill gas envelopes inside the VAB/SPH.  You'll have to do it the old fashioned way and fill your envelopes using a compressed resource like liquid hydrogen outside the editor.  Once you fill these envelopes, recovery of the lifting gas won't be possible. You'll have to pack along some extra or find a way to have some delivered to you.  This also means that keeping what gas you have in the envelopes will be something you'll have to consider. Blowing it out the release valves won't be a long term solution.

Engineers building early airships realized that blowing gasses out the release valve  wasn't a long term solution either. The Hindenburg held 7,062,000  cubic feet of lifting gas. During a typical Atlantic crossing, they'd vent some 1,500,000 cubic feet of gas.  The reason this was necessary was to compensate for fuel loss.  At normal speeds, the Hindenburg would consume some 130 kilos of diesel fuel per hour.  At it's best, an Atlantic crossing from Germany to the US, it took almost 53 hours of non-stop flying. (For us 'Yanks' the Hindenburg got less than 2 m.p.g.). Some method had to be developed to help compensate for that loss of weight.

Venting - While the least desirable method of descending, it's still a very valid one. You just have to realize that each time you do vent, you lose precious lifting gas.  Like modern and historical airships, this mod will have valves that you can open and close. Unlike other mods, these vents will be actual parts that you have to place and they will have to be on an envelope to work.  The way they'll work is pretty simple, with both a manual and an automatic mode.  In automatic mode, you set the pressure at which you want them to open (somewhere between 1 and 5000 Pascals).  Should the internal pressure of the envelope exceed that amount, they start venting your gas.  You an also turn them on manually to vent as needed.  The bad news.  These vents have a fixed rate at which gas can escape through them.  The good news, you can set up action groups to turn them on and off and you have have multiple vents on an envelope.

Ballast - While ballast alone doesn't prevent the loss of lifting gasses, it be came, and still is, necessary for the proper operation of airships.  Later, it became important in conserving the lifting gas.  Rigid airships have a structured frame with one or more balloons inside them.  In order to keep that balloon from flopping around and risking puncture, it had to be inflated to a minimum volume. Even today's airships are non-rigid or semi-rigid and have to have a minimum amount of lifting gas to retain their shape.  Depending on the payload, ballast had to be added or removed in order to help achieve a neutral buoyancy, where it neither ascended or descended.  Even today, the GoodYear blimps are designed to have an expected payload.  When that payload isn't exactly met, sand bags are added.  To give you an idea of how critical ballast has become, in 2011, a GoodYear blimp pilot lost his life as a result of ballast loss.



In a condensed version, the pilot of the blimp and 3 passengers began to smell fuel during a flight.  Making an emergency landing, a fire aboard the airship broke out and the pilot instructed the 3 passengers to disembark.  The ground crew hadn't arrived to secure the blimp and as a result, it shot back into the air due to the ballast loss.  Ultimately, an investigation by German authorities attributed much of the disaster to pilot error. He was also credited with being a hero for saving the life of his passengers.  While tragic, the point I'm trying to make here is that historically, and even with today's airships, ballast plays such a critical role in maintaining a perfect balance in these airships that even the loss of a few hundred pounds is enough to radically alter their aerodynamics.

Balloonets - The idea behind balloonets dates back to the late 1700's.  Their purpose was to decrease lift by adding to the envelope a much heavier gas, air.  Whenever the airship needed to descend, a balloonet was inflated inside of it, creating added weight and the airship descended.  Thus far, I've tested this in KSP and, surprisingly it works.  In a video I posted further down the thread, you can see one of the test airships lift, fly around and descend without a significant loss of lifting gas by inflating and deflating balloonets inside each cell.  Balloonets also had another important function.  By inflating or deflating them and depending on their location in the airship frame, they could be used to trim the pitch of the airship.


By TraceyR [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or FAL], from Wikimedia Commons


In KSP I've had to take a slightly different approach to balloonets.  In the image above you can easily see that anytime the inner balloon is inflated, the volume of the outer balloon doesn't change but it's size does.  Because the airships I'm first trying to replicate are of a rigid design, I've had to allow for some creativity in how balloonets work.  It'll be easier to think of them as two opposing pistons inside a cylinder.  Each balloonet can be inflated up to 25% of the capacity of the cylinder. One of the problems with replicating a balloonet has to do with the density of air.  In the image above, no calculation has to take place. You blow up the inside balloon and it works.  In KSP, temperature and air pressure all have an effect on how the ballonet works and need to be factored in.  In the first iteration of this mod, it's going to be much simplified.  The air added to the balloonet won't have any weight.  Instead, this is how it will work.  Up until the point that the lifting gas envelope is 75% filled with lifting gas, the balloonet will have no appreciable effect.  Once beyond that 75%, the balloonet effectively reduces the maximum volume that the gas cell can hold.  This causes an increase in the pressure of the lifting gas and, as a result, it's density increases.  This also has the effect that since the total volume of lifting gas is reduced, the 'hole' it creates in the atmosphere gets smaller and thus, the airship isn't able to lift as much mass.



How balloonets will work in this mod.

The mass of air in the balloonet isn't insignificant.  When you start working with airships the size of the Hindenburg, 50,000 cubic meters of air has a significant weight.  And eventually, I will be adding this mass but not at first.  The reason being is it will take some careful mathematics.  For example. Assume you're on Kerbin and you fill your balloonet to maximum.  Let's set the air density there = 1.   When you're ready to lift off, you deflate the balloonet and you're on your way.  Now let's assume you're doing some high altitude flying and you're at say 20km.  Up there, the density is only 0.25.  That means that the mass of the air for the same 25% volume is only 1/4 of what it was at the surface.  To make it even more interesting, assume that you deflated the balloonet at the surface and very slowly, as you ascended, you started inflating it.  In order to calculate the mass of the air in the balloonet, you'd have to constantly sample the air. Take that air and it's density and then add that volume and density to what's already in the balloonet and come up with a new density and thus, a new mass.  Now this can all be done, I'm just not going to do it in the first iteration of the mod.

Blau Gas - Blau Gas was a rather unique idea.  It was a gas distilled from naphtha that had two very desirable properties. First, it was flammable. Second, it's density was very close to that of air.  This allowed airship designers to add a fuel that the engines could run off of and as it was consumed, replace it with air.  In essence, this created a 0 sum game whereby weight wasn't lost or gained.  I do plan on adding this into the mod but it too will take some work.  In KSP there's basically two choices that I'm aware of. 1. Make an engine that runs specifically off of a type of fuel, which would require more parts, or 2. Write a plugin that would allow specific engines to 'switch' fuel types at the user's choosing.  For now, I'm not sure which I'm going to pursue but I do want this fuel type in the game.  The burnable gas may come into existence as a by-product of creating a lifting gas from KSP ore.  *edit* I just did a bit of research and this will be a lot easier than I initially thought.  RAPIER engines already have a dual mode.  The only thing that would be needed to switch and engine between LF and Blau Gas would be a .cfg file.  Problem solved.

Ballast Reclamation - One of the other solutions to loss of mass was to create your own ballast on-the-fly.  Interestingly enough, about half of what comes out of the exhaust pipe of an engine is carbon monoxide.  The other half, is water.  Engineers during and after WWI set their attention to collecting this water and using it to replace mass loss.  One engineer, Robert F. Kohr, wrote a paper describing how he and a team of engineers in the 1920's worked on a method to extract this water from the exhaust of airship engines.  Ultimately, their method consisted of creating condensers and mounting them on the sides of airships.  They conducted numerous experiments and the math gets pretty heavy.  But the basic conclusions they drew was that between 70% and 90% of the mass lost to fuel consumption could be collected from engine exhaust as water.

If you're interested in this work, you can read about it here: https://nvlpubs.nist.gov/nistpubs/nbstechnologic/nbstechnologicpaperT293.pdf

Kohr and his team experimented with LOTS of different fuel types and atmospheric conditions. For the sake of my sanity, I'm not nearly going to be delving into the mathematics that he did.  But, I will be adding a part into the mod that will reclaim water from the engines in a MUCH simplified manner.  The amount of that water reclamation will have a number of factors involved including air pressure, air temperature & engine thrust (or throttle position).   And, if you took a look at the pictures in that .pdf, there will be condensers that you can place on the airship to collect this water which you can turn on and off.  Another thing that I will factor in is which planet you happen to be on.  While I haven't seen anything that indicates the oxygen content of any particular planet's atmosphere, breathable air will obviously provide better results.

Engine Vectoring - 

In the pictures above there is a shot of the USS Macon or Akron's engines.  As far as I can determine, it was one of the first, if not the first, to use vectored thrust.  By taking the forward propulsion engines and rotating them up or down, engineers essentially added or subtracted lift.  This mod will also employ vectored thrust.  Thus far, and as far as I can plan, all of the engines designed for forward propulsion will be able to rotate 180 degrees, front to back.  You'll also be able to adjust that rotation speed to your liking and stop the rotation at any point.  You'll be able to assign action groups to this rotation and stopping so that, if you set all of the speeds the same, you can have the engines point any direction you like and stay there.  And, with all of them having reverse thrust that can be placed in an action group, that gives a full 360 degrees of thrust rotation.  In the replica of the GoodYear blimp above I was able to exclusively use vectored thrust from just 2 small engines to ascend and descend as well as move forward and in reverse without changing the volume of the lifting gas.  I will be creating both an LF and an electric version of each engine used so that you can still use them on any planet with an atmosphere.


Time to get serious.

If you've managed to read this far in this rambling, you've probably noticed that thus far, I've been playing around with some pretty small airships.  Nothing I've attempted yet has even come close to those monsters that once used to blot out the sun.  Well, in this installment, I'm going to attempt just that.  It's time to get serious and see just how the physics and my math come together when the numbers get really big.

The Kindenburg

The largest airship ever built was the Hindenburg.  It was 245m x 41.2 m and held some 200,000 m3 of hydrogen.  It's total lift?  Well over 200 metric tons.  In order to see if those type of numbers could be achieved using KSP physics, the first task was to build a monster of roughly the same size.  It took me several attempt to come up with a working design even with the knowledge of knowing how all the physics should work.  What I ended up with was this:

  Hindenburg Kindenburg
Length: 245m 236m
Width: 41.2m 40m at the envelope edge
53.6m with the engines
Envelope Volume: 200,000 m3 207,000 m3

219 mt fully loaded

116.9mt empty
149.5 mt as tested
Estimated lift 232 mt 229 mt

You've probably noted a rather large difference in the weights of each of these.  The engineers on the Hindenburg had years to calculate how much it could carry and they had it down so precise that the Hindenburg even had an aluminum piano on board.  For my test, I purposely didn't try to get close to that weight since I wasn't positive the numbers would all work out correctly.  What I discovered was, I didn't have nearly enough weight.  As it was I put on about 35 tons of water ballast, entirely too much fuel for the short jaunt to the island runway, I ended up blowing out gobs of gas because I underestimated the inflation of the cells.  I had all of the balloonets fully inflated for the entire trip just to try to keep the beast from launching into the stratosphere and still had had to use the vectored thrust to keep it's altitude low enough to have a prayer of landing.  After this test flight, I had the gut feeling that I could have easily went into the 200+ mt range with ballast and still made the flight.  Here's some screenshots of the test:


Coming in for a landing at KSC I had to patiently wait for the envelopes to deflate.

fKUzGx1.pngA view from inside one of the buildings on the Island Runway.  Here I have  a number of the GUI's open to show some detail.  One thing to keep in mind is this is all part of the development. Much of this detail won't be shown to the end users.
Ymmxkc8.pngThose tiny white dots on the top of the envelopes are relief valves.  You'll be able to attach them to envelopes and they will allow the lifting gas to escape.  You'll be able to turn them on manually by right-clicking or with an action group.  You'll also be able to set a pressure at which they automatically open.


Heading back to KSC.


This shot of the landing on the Island Runway should give you an idea of just how large the Hindenburg was.  While I was able to even park the smaller airships inside those hangars, the Kindenburg wouldn't quite fit.

c29e7Iw.pngLanding at KSC.  Being still over-inflated I chose the option of vectoring the engines downward in order to land.


Another shot of the flight back. In this shot you can see the UI has had a minor change.  The pressure bar now changes to yellow and red to help indicate when you're reaching over inflation.  After my friend inadvertently blew up his airship he politely informed me that he had no indication it was in any danger. Now, he does.  Another ting to notice is the two white spots.  Those are end caps on the water ballast tank.  You'll be able to start dumping your water ballast via the main UI or you can right click and manually turn on each one.


This was my attempt to partially emulate the rear propeller arrangement on the newer Zeppelin NT's.  While it did work, it required several action keys as main forward engines also had to be shut down.


My own initial impressions:  I'm glad I wasn't an airship captain.  The smaller airships I've been flying around up to this point have been mostly fun.  They haven't taken a lot of thought or planning and I've been able to balance out the envelopes just by eyeballing it.  This monstrosity was a beast to fly.  Not that it was uncontrollable, it actually flew quite well. The maneuvering of it was where it took some effort and that shouldn't have come as a surprise. When you have 150 tons which is essentially dangling on a thread, the laws of physics SERIOUSLY come into play.  When it starts to turn it wants to keep turning until you make it stop.  Once going forward, it keeps going forward. While there are drag cubes on these parts and while it will slow down eventually, 150 tons moving at 60m's is like a freight train.  You don't stop on a dime.   One of the things I HAD to do was to set the port and starboard engines to have their reverse thrust set to separate action keys.  Once this thing came to a stop, if I needed to turn, reaction wheels were NOT going to be much good in an atmosphere.  This, was a challenge to fly.  And the more I think about it, the more I realize just how much planning and forethought it must have taken to fly a real one.

This was also a test of a new part.  In the previous section I gave some details as to how real airship engineers attempted to reclaim water from exhaust gasses in order to compensate for the loss of weight due to fuel consumption.  Trying to emulate their methods I have created a condenser.  Here's 3 of them on the port side of the Kindenburg.


The way they'll work is something like this.  First, each one will have a random efficiency (which you won't see or even know exists) assigned to it.  As long as it's part of the vessel, that won't change.  As your engines generate thrust, the condensers will capture that information and begin to generate water.  A number of other factors will also be considered. There will be a temperature range at which they're the most effective.  As the temperature gets colder or hotter, you can expect reduced efficiency... to the point that they freeze up or can't condense because the water is boiling off. And I'll also admit that the engine to evaporator ratio to achieve maximum efficiency is NOT 1:1.  You'll have to experiment to find out which setup works best for your situation.

In the first few sections of this rant, I described basically how airships worked.  In the last few I've dug into some details and described how this mod will work.  For just a moment, I'm going to go off-rails and give you an idea of how this mod can work.

Because the envelopes are going to be resizable... and because they'll be independently controlled by the plugin, you won't be restricted to what does exist.  While the basic shape of the envelope is fixed.  It's arrangement and the number you choose to use, won't be:



The whole point of this mod is going to be the emulation of real-world physics as it applies to airships.  The engineering creativity you decide to come up with in using that physics, will be entirely up to you.


The beans are burning (Well, it was a good idea at the time)

So in this part of my experimentation I decided to work on using the Kerbin atmosphere for a lifting gas.  The theory is pretty simple.  Hot air balloons work because when air gets hot, the molecules move around a lot faster.  In an enclosed environment like a balloon, this causes one of two things to happen: either the volume increases or the pressure increases.  In either case the density of the air lowers.  When the density inside the balloon is less than the density outside the balloon, it creates lift.  So, the theory is pretty simple.  What I discovered is the implementation wasn't.

The first problem to solve was a somewhat realistic way to make the air hot.  The second problem to solve was how to make the air cold again.  We all understand the basics of thermodynamics thanks to soup.  If you put a pan of soup on the stove and turn on the burner, it gets hot.  If you turn the burner off, it gets cold.  But that's just the basics.  Lots of other factors apparently go into determining what the actual temperature of that soup is, like how much soup are we talking about? What's the temperature of the burner it's sitting on? What's the temperature of the air in the kitchen? How much surface area of the soup contacts the air? And then, there's a magic number called the heat transfer coefficient which is different for everything you encounter.  Pea soup has a different coefficient than chicken soup which means, they would heat and cool at different rates.  And this is just scratching the surface. Then there's things like the 'heat equation' which states: "The heat equation is a parabolic partial differential equation that describes the distribution of heat (or variation in temperature) in a given region over time."  Ok, I'm trying to simulate... simulate mind you... airships.  I am not working on a master's thesis.


"All of physics is either impossible or trivial.
It is impossible until you understand it, and then it becomes trivial."

the 'father' of nuclear physics



In this case, the trivial became obvious thanks to Sir Issac Newton.  In 1701, Newton published an article that describe the relationship between the temperature of an object and time.  While he didn't express any formula in that article, he did lay the foundation for what's become "Newton's law of cooling."  His law basically says that the heat loss of an object is proportional to it's temperature and the surrounding temperature.  What I discovered is that the equation used by this law could be turned around and used to determine the temperature of an object over time:


In this equation

T is the temperature of the object's surface and interior. (SI unit: K)
Tenv is the temperature of the environment. (SI unit: K)
t is the time.
r is that mysterious heat transfer coefficient.

KSP gives 2 of those variables. The atmospheric temperature and the time it takes for the code to run an update.  The temperature of the object (air in this case) I could change and that I could just plug some numbers in and see what happens.  To make a long, boring physics story short, what I discovered is, for the purposes of hot air, this formula works great.  The hotter the air is, the faster it cools toward the atmospheric temperature.  The closer it is to atmospheric temperature, the less cooling occurs.  This formula could also be used in reverse. If, instead of the environmental temperature I substituted the temperature of say, a big gas flame, then I could also simulate heating (just so happens Newton's law is also called a law of heating, yay!).

There are few things this formula didn't take into account, like the volume of air and the surface area of the envelope where the gas was.  But, these numbers could be plugged in later.  This was enough to allow me to test hot air in a somewhat realistic manner.  Heating would happen in a gradual slope.  As more heat was applied the temperature would slowly rise toward the burner temperature. The closer it got to the temperature of the burner, the faster it would rise.  In reverse, the cooling process would work in as rapid cooling process when there was a big difference between the gas temperature and the atmospheric temperature.  The closer the temperature got to air temperature, the less it would cool.  Hopefully, somewhere in the middle, the heating and cooling would meet and find a balance.

Ok, so here was the goal. According to Wikipedia a hot air balloon should have these basic characteristics:

The lift generated by 100,000 ft³ (2831.7 m³) of dry air heated to various temperatures may be calculated as follows:

air temperature air density air mass lift generated
68 °F, 20 °C 1.2041 kg/m³ 7517 lb, 3409.7 kg 0 lb, 0 kg
210 °F, 99 °C 0.9486 kg/m³ 5922 lb, 2686.2 kg 1595 lb, 723.5 kg
250 °F, 120 °C 0.8978 kg/m³ 5606 lb, 2542.4 kg 1912 lb, 867.3 kg

So, with my new formula plugged in and a couple of new parts to try out, I started testing:


In this screenshot you may need to zoom in to see the numbers.  But, what I did was first, inflate an envelope to roughly 2,800 m3 with Kerbin atmosphere at the atmospheric temperature.  I then turned on my 'burners' and heated up the air.  Once I reached around 427 degrees Kelvin (307k ambient + the 120C from the table above) I looked at the UI.  It was saying that it should lift around 1.2 metric tons.  This was about 50% more than what Wikipedia suggested.  Ok, so the hot air was generating more lift than expected.  At least it wasn't less than expected. I could live with this minor discrepancy.  Now, let's see what it takes to get this 5 ton airship off the ground.


Here's the things to note from this screenshot:

First is the cool burners I've created and placed on the bottom of the airship.  Next is the odd shaped item just in front of the burner. That's a 'blower' that can be turned on and off to pump outside air into the envelope. What you'll want to take close note of is the numbers in the UI panel.  It's showing that the highest temperature of the air inside the airship is over 900 degrees Kelvin! And in this test I even cheated and didn't use any type of fuel to run the burners.

Hot air balloons are extremely light.  The gas volume of the balloon compared to the payload is a rather large ratio.  When it comes to airships, that ratio drops significantly.  Airships are inherently more massive than a balloon.  Plus other problems arise. In all of my testing, each envelope was it's own unique part.  For lighter than air gasses, this has worked great.  For hot air, it's a nightmare to control.  Imagine 3 balloons bolted together with each having it's own independent temperature and volume of gas.  This means that air density in each one could be vastly different from the others. In the screenshot above, the rear envelope was at 900+ degrees while the forward one was at around 750 degrees.  This creates vastly different amounts of lift between the front and the rear.  Combine that with the fact that at these temperatures the cooling is (and should be) pretty rapid, I was having to fire up the burners every 30 seconds or so to stay aloft.  And when they cooled, they cooled rapidly when meant, it had a tendency to start dropping like a rock.  Next add in the fact that because the envelopes are at different temperatures, adding more heat creates even more varied temperatures.  And finally, there was that minor issue of temperatures that could melt aluminum.

While I still haven't explored all the possibilities (like having a uniform temperature across all envelopes) the biggest problem I see is that in order to generate enough lift to get something as massive as an airship off the ground, that air needs to be hot, really, really hot.  The only other viable solution I can see is to vastly decrease the mass of the airship itself.  Either way, if hot air is going to be an option for a lifting gas then there's going to be unrealistic temperatures or unrealistic mass.

Since my plan was to make an airship mod based on real-world physics, perhaps the best answer is neither.  Then again, there may be another option.  Using hot air to boost the lift of helistats without having to carry around a lifting gas.  Hummm....

*edit Updating the Hummm...*

Performed a quick test with a helistat.  The basic results were, it's viable. The theory behind helistats is is that the airship lifts itself including the rotors.  The rotors are then used to lift the intended cargo.  While hot air won't achieve that goal, it can help.  In a quick test of the screenshot below, at roughly 520 degrees Kelvin (still considerably warmer than a standard hot air balloon but not an insane temperature) the hot air was able to lift about 3.5 metric tons out of the total 8 ton airship weight.  While that still puts a 4.5 ton demand on the rotors it does have one large advantage over lifting gasses... I didn't need any.




-- to be continued --

Feel free to follow this thread.  I'll create a new post when I update this OP.  And while this is technically a WIP, it's no where near ready for testing.  I will start a WIP thread when it is ready but for now, I'd hope to start a discussion about my ideas and your expectations.

Edited by Fengist

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On 8/27/2018 at 6:16 PM, Fengist said:

And I just realized, this probably should have been in the addons forum.  I'm sure some generous mod will move it eventually. 

[Redacted as the thread has been moved]

Edited by Gargamel

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(To mention an additional option).
Like here: https://en.wikipedia.org/wiki/Thermal_airship

An airship containing two independent volumes of gases:

  • lightweight gas (basically He) providing bouyoancy equal to the constant part of the airship weight (construction + cargo).
  • hot air (or another heatable gas) providing additional bouyoancy for vertical maneuvering and balancing variable part of the airship weight

So, originally the airship with both volumes filled with corresponding gases has a slightly negative bouyoancy.
Then a heater starts warming the hot-air balloon, it expands, and the airship takes off.
While flying, the heater keeps holding neutral bouyoancy of the airship by warming/not warming the hot-air balloon. Vertical maneuvering is being performed in the same way.
On arrival it stops warming, the air balloon slowly cools down, the airship lands.

On the heater malfunction, the heating obviously stops, the air balloon slowly cools down, the airship safely lands where it was.
(This option requires proper mass balancing.)


(Best use with nukes which can both heat and power the propellers with no fuel spending. Also they can electrolize water to produce hydrogen from the local water).




On 8/28/2018 at 1:16 AM, Fengist said:

Allow the mixing of lifting gasses or have a single gas  but treat it like a resource?

Unlikely you will be separating the gas mixture after the flight.
And as gases (and especially hydrogen) are leakable, they will be leaking, with different rates. So, the mixture composition will be probably permanently changing. 
So, the LiftingGas resource would be constantly changing its physical properties, say, from 90% H + 10% He to 90% He + 10% H. Including density, molar mass, and heat capacilty.
And its lifting force would vary.

(Say, even fuel tanks made of metals are loosing light gases quickly, and here is just an envelope).

On another hand, you would be probably pumping the most leakable component to keep the LiftingGas composition constant.
But this in turn again requires to keep somewhere amounts of the LiftingGas components, and you anyway will face this problem of their separated counting when out of liquid hydrogen.

So, maybe it would be better to keep somewhere in memory numbers of the currently being used lifting volume components but with no possibility to use them by, say, engines, or move back into the tanks. Only vent them out after the flight.

Also, 700 l of LH2 * 0.070 kg/l = 49 kg of hydrogen, so unlikely you would leave it inside the balloon between the flights, you would anyway loose them for leaking and need to check&repair the envelope.


If you don't fill with gases a single large bubble, but separate it into a bunch of small ballonets covered with a main lightweight envelope, any hole vents out just a small portion of total lifting volume, while other ballonets stay intact.
In this case you also don't need to mix gases. You can fill various ballonets from various tanks. (Unlikely you can have LH+LHe mixture, probably you would have two separated cryotanks).
So in this case you have separated amounts of non-mixed gases (say,  34 ballonets filled with H, 25 filled with He at the moment).

Unlikely you can have more than 2 lifting gases at once. Say "good"+"safe" (H and He) or "safe"+"cheap" (He and H), but why fill the tanks with N, H, and He at once.

So, maybe you would limit the components amount with two resources ("A" and "B") in separated tanks, use separated ballonets to avoid mixing and to make refilling easier.
Say, in GUI you don't select "H" or "He", but "A-gas" and "B-gas" and have amount of ballonets: "currently filled with A", "currently filled with B", "empty/vented".

As a game simplification you can take that ballonets are leaking one by one, so you always have say:
77.6 ... 77.5... 77.4... ballonets currently filled with component A
38.2 ... 38.1..  38.0.. ballonets currently filled with component B
24... 24... 25... empty ballonets

If you want, say, less He, more H at the moment, you do:
"Vent out 2 ballonets currently filled with compoment A".
(so, you get 2 more empty ballonets).
"Fill 2 ballonets with compoment B".
(so, you get 2 more ballonets filled with desired fluid).

On ground you originally have tanks for "A" and "B" components, probably select KSP resources in menu (as you anyway should keep somewhere that LiquidHydrogen gives Hydrogen, while Uranium is unlikely an option at all, so the list of appropriate resources will be fixed and limited).
Then you fill your tanks with corresponding resources. 
And never refer to them as to "H" or "He" in Airship GUI, only as "coponent A" and "component B", because say on Titan or Venus there could be Nitrogen or other resources.

When your flight is finished, you can pump out the remains of the components from the tanks back to a refueling rover.
As your tanks are just simple KSP/CRP tanks for KSP/CRP resources, you just pump LiquidHydrogen and LiquidHelium. ("A" and "B" they are only in your balloon GUI),

Then you vent out the components from your ballonets, getting them all empty.


Also if implement a thermal airship, you can additionally have component C.
But as this component C is to be taken from outside (local "air"), you don't need to calculate its in ballonets or have some tank for it, you just have a constant amount of component C in the Heatable Ballonet "currently warmed up to 81°C", so its amount is counted much easier.


An ""Emergency venting out" action probably could be also useful.
The code would calculate amount of ballonets currently filled with the lightest component of A and B to make bouyoancy neutral (taking into account inertia, so not at the current altitude, but at some greater altitude where the undesired lifting will presumably stop) and vents them out in standard way.

Using discrete ballonets also automatically allows to have partially damaged ballons.
Say, 2 ballonets are damaged and cannot be used until a repair.
Also they allow to combine in calculation several balloons with different ballonet amounts.

(finished editing)

Edited by kerbiloid

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6 hours ago, kerbiloid said:

(To mention an additional option).

Thanks for the long reply @kerbiloid.  I'll try to get a response and the next installment of my ramblings out tomorrow.  Don't have much time today.

But, I did get one thing done.  Here's a quick video to browse.  Sorry for the 480p!  I know it really sucks.  I didn't realize it was such low quality till after I uploaded it.  But, I still think you'll get the gist of the testing going on and keeping the details secret might just be a nice hype generator... eh?  


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Update to the OP - alternate lifting methods

On 9/1/2018 at 9:38 AM, kerbiloid said:

(To mention an additional option).
Like here: https://en.wikipedia.org/wiki/Thermal_airship

I have actually taken a look at that design and, here's the good news. The method I'm currently using is what I call mole counting.  While pressure, volume and temperature will change, one thing that will remain constant (assuming no leaks) is the number of molecules of a specific gas.  And, the airships in the screenshots are already using that method.  Basically, I have a list with a bunch of info, including the number of mol's of each gas.  From those mol's, every other part of the ideal gas law can be calculated.  This list also means, I can track the temperatures of each individual gas.  While heated & non heated gasses in the same envelope is going to be on the list, it's not on the top.  But the infrastructure in the code to have a hot air/helium airship already exists.

But... I don't do nukes.  To me, reactors in the game are like RTG's.  They're just too cheaty for me.  I'll come up with another solution but it'll likely use some kind of fuel.

As for emergency venting, already there as well.  While other mods have vented gasses, this one will be a bit different. If you look at the first screenshot, on the top of the envelopes, you'll see 3 black dots. One forward, mid and aft.  These are vents.  You can also just see one in the second screenshot on the top.  The way they'll work is via manual and automatic settings (which you can turn off).  You give them a pre-set pressure and should the envelope exceed that pressure, they'll begin venting.  The bad news.  The rate of that venting will be fixed.  The good news, for just a few more kilos... or pounds... you can add more vents.  In the video I posted you can see the automatic vents fire off a few times when I expanded the balloonets a bit too much during landing.

As for damaged gas cells, for now I don't plan on that and here's a quick reason why. A good friend and former modder, InfiniteDice has been tossing in idea (and laughing at me) through this mind bending coding process.  Once I had the physics down to the point that the airships actually flew, his response was, "now, make them fun."  While they will pop if you over-inflate them, just like any good KSP part should do, having random damage appear isn't in the foreseeable future.

Thank you for the long and well thought out reply! It's greatly appreciated.

Edited by Fengist

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Making some dangerous assumptions here based solely on your screen shots – you've likely though of some or all of these. That said I'll toss out some thoughts in hopes of triggering some ideas on your side.


Have a balloonet in both end caps to adjust AOA.

Pro – adjustable AOA so you can point up or down and the engines can drive you up and down faster that just the lift (or lack of).

Con – more complex UI/ UI uses more screen space.

Could have one fill slider and one balance slider – balance would just move air from one balloonet to the other without changing lift volume/balloonet volume ratio.

Problem – you know people are going to build crazy contraptions that won't have a clear front/back (think an end cap at front/back/left/right/top/bottom) and the poor balance slider code is going to have a melt down.


Rigid airships with multiple internal cells (Wikipedia says the LZ 129 Hindenburg had 16). No balloonets unless I misunderstand.

Con – potential nightmare UI unless control could be condensed – maybe a full/empty slider and a gradient slider?


Open cell (think hot air balloon) – pressure is always ambient and temperature alone drives lift.



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Get 'er done. I love your thinking, and you've obviously got a lot of research done, so whatever you pull out of this imagining is definitely going to get my download.

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Wow, this is amazing. I saw your post on making GUI's in the Plugin Dev forum, and was wondering what was in the making. Followed!

If I may offer: You might be able to do something with a "heating element" for creating hot gasses, something that would use electric charge like a water heater, maybe at a lesser rate. I think that might be a viable alternative to RTG's or reactors, because one would have to create energy storage and production, as well as have enough air heaters to create enough hot air for the balloon.

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2 hours ago, wasml said:

Making some dangerous assumptions here based solely on your screen shots – you've likely though of some or all of these. That said I'll toss out some thoughts in hopes of triggering some ideas on your side.


Have a balloonet in both end caps to adjust AOA.

Pro – adjustable AOA so you can point up or down and the engines can drive you up and down faster that just the lift (or lack of).

Con – more complex UI/ UI uses more screen space.

Could have one fill slider and one balance slider – balance would just move air from one balloonet to the other without changing lift volume/balloonet volume ratio.

Problem – you know people are going to build crazy contraptions that won't have a clear front/back (think an end cap at front/back/left/right/top/bottom) and the poor balance slider code is going to have a melt down.


Rigid airships with multiple internal cells (Wikipedia says the LZ 129 Hindenburg had 16). No balloonets unless I misunderstand.

Con – potential nightmare UI unless control could be condensed – maybe a full/empty slider and a gradient slider?


Open cell (think hot air balloon) – pressure is always ambient and temperature alone drives lift.



Right now there are only 3 envelope parts.  I'll explain why in an update.  But, balloonets are optional from within the VAB or SPH.  If you go traditional and have one fore and one aft the main UI has buttons that will inflate or deflate both equally.  Right clicking on the part will give you buttons to inflate or deflate just that balloonet.  If I remember, I'll also set up actions for that so you can inflate or deflate with a keystroke.  But, with all the other things that this mod is already doing, you may not have enough keys to go around.

For now, these are considered rigid.  Once you get into non-and semi rigid, you get into a nightmare when it comes to resizing the envelopes outside the hangar.  Multiple cells, while entirely doable present another problem which you quickly identified, controlling them all.  There will be a 'solution' available if you wish to go that route with this mod tho.  You could theoretically have a lot more than the Hindenburg.  I've yet to test this tho, still trying to get them off the ground without exploding... (foreshadowing).

As for open cells, well... dunno if you heard but I was recently allowed to take over development of KerBalloons.  Dunno why I'd have an interest in that mod.


2 hours ago, JH4C said:

Get 'er done. I love your thinking, and you've obviously got a lot of research done, so whatever you pull out of this imagining is definitely going to get my download.

Thank you sir.  Yea, I did a bit of research.  And face-desking,  I'm a couple months and several thousand lines of code into this.  

1 hour ago, Benjamin Kerman said:

Wow, this is amazing. I saw your post on making GUI's in the Plugin Dev forum, and was wondering what was in the making. Followed!

If I may offer: You might be able to do something with a "heating element" for creating hot gasses, something that would use electric charge like a water heater, maybe at a lesser rate. I think that might be a viable alternative to RTG's or reactors, because one would have to create energy storage and production, as well as have enough air heaters to create enough hot air for the balloon.

Haven't quite decided on how to handle that.  At the moment, my testing is with just hydrogen and helium.  And for those, heating is a case of diminishing returns.  Hot helium or hot hydrogen don't offer that much lift over ambient.  Where heating gasses really works is when they're dense like air.   I'm going to have to fiddle with numbers (weight vs lifting advantage) to see how that'll all play out.  But that's down the road.  Still trying to get parts designed that look half-way decent but are light enough that these things get off the ground.


Oh, and I kinda got helistats working today along with rotating engines.

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The USS Macon (above), given to the US as part of Germany's WW1 reparations, had a rather unique engine arrangement.

A minor detail, but the USS Macon was not one of the rigid airships given to the USA by Germany. The Macon was constructed in Ohio at the Goodyear-Zeppelin manufacturing plant. You may have been thinking of the USS Los Angeles, which was indeed a rigid airship constructed in Friedrichshafen, Germany, and given to the USA.

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6 minutes ago, Supercheese said:

A minor detail, but the USS Macon was not one of the rigid airships given to the USA by Germany. The Macon was constructed in Ohio at the Goodyear-Zeppelin manufacturing plant. You may have been thinking of the USS Los Angeles, which was indeed a rigid airship constructed in Friedrichshafen, Germany, and given to the USA.

I stand corrected.  Yep, I was thinking of the L.A.  The Macon/Akron came a good bit later and were noted for being (loosely termed) aircraft carriers. I'm not sure I'd have had the umm. intestinal fortitude to try to land an aircraft by flying close enough that a hook on top of the wing would snag a 'trapeze' bar. Don't think I want my passenger airliners doing circus tricks.

So, the fun for the day.  Testing helistats.

Here we have the trusty little airship I've been flying around the past week or so with a few additions.

First, the engines I have been using, now rotate. There's 4 small electrics used primarily for forward propulsion that now rotate 180 degrees and have reverse thrust. So, if you like action keys, it has a full 360 degree thrust rotation.  Don't expect much from these little guys.  Though they will shove the airship around at 20+ m/s, they only have 1.2kN of thrust.

Next, the rotors. Liquid fueled and reasonably efficient. At 50kN they're not in the neighborhood of a Chinook but, they perform adequately on this little ship.  If you recall, my past flights the airship has been around 1-1.2 metric tons. With the addition of the helo rotors and 400 units of LF, that jumped to 5.5 tons.  Something the airship alone (at this size) couldn't lift.  (For a comparison, the Kuey rotors from KAX are rated at 100kN)

But, combined, 4 rotors plus the airship, well... here's some screenshots of it flying around with a 9.44 mt cargo underneath (KAS winches ftw).  This is pretty much at it's weight limit and though the takeoff was NOT pretty (snatching 9+ tons off the ground and then trying to control it), once I got the winches reeled in a bit, it leveled out nicely.





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You could simulate mixing gasses while considering them as resources by having multiple resources be able to go into the tanks like the liquidfuel/oxidizer tanks.  The player could set how much of the max gas capacity would be hydrogen or helium or something and have that be switchable on the go.

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1 hour ago, 0something0 said:

You could simulate mixing gasses while considering them as resources by having multiple resources be able to go into the tanks like the liquidfuel/oxidizer tanks.  The player could set how much of the max gas capacity would be hydrogen or helium or something and have that be switchable on the go.

I suppose that would be possible.  I came up with the idea of the list and didn't put a lot more thought into it because it worked. Thinking about it, here's the problem I see.  Potentially there will be a half dozen or more lifting gasses available.  Trying to set up a single part so that the user could potentially rearrange the volumes for specific gasses would be a bit of a nightmare from the code side and rather tedious from the user side.  Limiting them to only say 2 gasses would place some undue limits on them.

Let's say, for example, you manage to get an airship to Duna. You packed along helium and hydrogen.  But, your airship has been there for a while and most of those gasses have leaked out or you've chucked them out the release valves. To get more, you'd have to have it delivered.  But, if you happened to have a mining op with the 'special' ore-to-ore gas converter, you could make a lifting gas. If the cells were pre-set in the hangar to only accept hydrogen and helium, well, plan on a supply run.   Resetting them in the field, while doable, I think would add considerably more code to what's already several thousand lines.

It's a good idea but I still think what I came up with will work best for now.  It allows the flexibility to use what you have on hand and not have to pre-define or, as you suggest, redefine what gasses you're going to use.  For that matter, you could mix all 3 the way it is now.  Not only that, it's a bit more realistic.  And, since the gasses can't be recovered anyway (or rather I've found no examples of them being recovered), they'll either leak out or get shoved out anyway.

Plus, while not exactly practical, the only limit to the number of gas cells you can have is your hangar size.  Managing volumes for a dozen or more would parts would be... rather boring.

Thanks for the idea though. Keep em coming.

Edited by Fengist

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13 hours ago, Mecripp said:

I'm glad you found this.  I remember watching this same video years ago thinking how cool it was.  So, let's explore the possibility.

First, the modelling.  Modelling the exact thing that's in the video isn't going to happen.  Adding in the physics of cloth and the unfolding and inflating would probably have a very bad effect on the physics engine.  But, that's not to say it can't be done. KerBalloons resizes it's balloons.  It cheats though.  The model was created full size and then massively scaled down in order to simulate expansion.  And that, is possible with airships as well.  So, can a model be created that at least simulates what happens in the video?  Sorta, kinda, maybe.

Now, let's examine the physics.  Venus has an atmosphere that's extremely heavy in CO2.  Because CO2 is a very dense gas, the plan with HAVOC was to use good old Earth air as a lifting gas.  Duna has a very, very thin atmosphere so air isn't going to work.  While Eve has a much, much denser atmosphere, it's gravity is nothing like Venus.  Venus is very close to Earth gravity.  Eve's gravity is almost double that of Kerbin.  Unfortunately, there is no planet like Venus in the Kerbol system.  So let's look again at Duna.

The simple formula to determine how much lift you can get from a lifting gas is:


d1 = Atmospheric density in Kg/m3
d2 = Lifting gas density in Kg/m3
v = volume in m3
L = lift in Kg

For this calculation, gravity is unimportant thanks to Archimedes' principle.

So, here's some numbers for Duna:

Atmospheric Density = 0.0963
Hydrogen Density on Duna = 0.00448  (pressure and temperature affect this number so it's very different than on Kerbin)

Now that looks actually pretty good.  You want the lifting gas to be much less dense than the atmosphere.  If we subtract one from the other, we get  0.09162 Kg.  That means, that 1m3 of hydrogen on Duna can lift around .09 Kilos of mass.  Ok, so far so good.  Converting that to metric tons (which KSP uses exclusively) we get 0.00009162 Mt.

So, some quick math tells us that if we want to lift 1 metric ton of mass on Duna, we'd need 10,914.65 cubic meters of lifting gas.  Ok, that's a bit.  If you look at the screenshots of the little airships I've been flying around, they're only around 1,000 m3.  So, we'd need something 11 times that size.  Well, that's still doable.  But there's another problem, the weight of the Hydrogen itself.

Depending on where you look, hydrogen gas weighs in at 0.083 kg per m3.  Convert to metric tons and we have 0.000083.  Now, multiply that by 11,000 m3 of hydrogen and we get 0.913 mT.

This means that 11,000m3 of hydrogen gas could only lift 87 kilos on Duna.  (1mt - 0.913mt)

So if it takes 11,000m3 of gas to lift 87 kilos, then to lift 1mt we'd need 126,436 cubic meters of hydrogen.... just to lift 1 metric ton?  126 of the little airships I've been flying around???

That's.... well that's this big:


Wait? Are those numbers correct?

Of course not.  Other factors get involved, like the fact that Duna's air pressure is so low that you can fill 11,000 m3 with a very small amount of hydrogen so it's weight is going to be a LOT less.   And then, because Duna's gravity is much lower than that on Kerbin the amount of lift (in kilo newtons) generated by the lifting gas is going to be much higher than it is on Kerbin.  So, less gas and more lift would also figure into these equations.

What I've done here is to intentionally give a somewhat misleading example of the kinds of calculations that need to be done in order to figure out the real physics behind airships.  I don't want to give you all of the numbers you need in order to determine if an airship of a given volume and given weight will get off the ground on any particular planet.  That would defeat half the fun of figuring it out yourselves.  Is it possible to fly an airship on Duna? Yes.  With the first release of this proposed mod? Maybe, maybe not. The current airships are designed to be rigid airships and will intentionally have a weight that supports that design.  It may take a non-rigid blimp with some extremely lightweight materials.  You'll have to experiment and find out.

So, does this mean your suggestion can be done?

Yes, and no.  The physics and the modelling are possible.  Where this runs into issues is with KSP, which I mentioned in another recent reply to KerBalloons. Once you leave the ground, a vessel is considered flying. You can't leave that vessel while it is and have it remain where it is.  If you try, you're pitched into a wormhole and warped backward in time to your last game save.  There is a technique called air parking whereby a vessel is taken 'off rails' and is fooled into thinking it's landed. The problem there is, when it is taken off rails, physics no longer applies to it and it becomes sorta frozen.

That means, that while the airships may be doable, having them perform as in HAVOC won't be.  You'd have to either land them or cheat and air park them.  And as for the air colonies that were proposed in phase 5 of HAVOC.  Again, you'd have to cheat and force them to be landed.

Then again, thinking about it, there may... I say may... be a way around all that cheaty stuff but that will take a lot of digging and testing. It may be something for the future.

Thanks for the idea and it'll stay stuck in me 'ead for now.



Edited by Fengist

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And sure you have seen this one http://curious-droid.com/302/zeppelins-mars-havoc-venus-nasas-new-planetary-airships/

And they don't have to land if I remember right you just have to have a long cable that we used the winch and anchor but still that's close to landing and some one did make a great mod of a airship part will try and found it


Edit- Does anyone still have the mod ( Discreet aircraft parts ) 

looks like it was nuked by the forms https://forum.kerbalspaceprogram.com/threads/123273-Discreet-aircraft-parts

Edited by Mecripp

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1 hour ago, Mecripp said:

And sure you have seen this one http://curious-droid.com/302/zeppelins-mars-havoc-venus-nasas-new-planetary-airships/

And they don't have to land if I remember right you just have to have a long cable that we used the winch and anchor but still that's close to landing and some one did make a great mod of a airship part will try and found it


Edit- Does anyone still have the mod ( Discreet aircraft parts ) 

looks like it was nuked by the forms https://forum.kerbalspaceprogram.com/threads/123273-Discreet-aircraft-parts

I've seen several examples of how real airship/balloon probes would function. Most of those are of the probe variety which I'm going to eventually attempt with KerBalloons.

As for anchoring methods. I did check one idea and realized why it won't work.

To be considered landed, a part has to actually touch the ground.  KSP has a limited distance at which parts can be from each other, which is why no one has yet been successful at building a space elevator.  This is also bad news for anchoring or tethering anything that floats.  In order to do it the proper way, you have to be within that visible range.  If you are, it is possible to fake it and essentially attach a part on to the ground, connect it to the vessel and, you're landed.  However, controlling the physics of a tethered vessel could be a bit tricky, but not undoable.

And I was able to find a copy of Discreet parts on Curse.  If it's the same mod, with a blimp, then it uses Hooligan for it's base code.  Hooligan does have a method called 'anchoring'.  Two problems exist with it. 1. The current maintainer of Hooligan's has made the code all rights reserved and it's been that way for a few years.  I doubt he has any intent on changing that. (which is one of the big reasons I'm writing my own code from scratch).  2. Hooligan's anchoring is just another name for air parking.  As a matter of fact, the code looks pretty damned similar.  And looking at one of Discreet's .cfg files, it used Hooligan's anchoring.

So, in theory, it's possible to tether at distances farther than KAS does and likely consider the vessel landed.  That's something I'd have to experiment with well down the road but tethering a colony 20km into the atmosphere, don't hold your breath.

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Update to the OP - a comparison of fruits.

In the latest of my ramblings I attempt to build a GoodYear blimp and compare stats and capabilities.


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Update to the OP  - Getting them back down again

In the next installment, a babbling of how to get airships back on the ground.

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Update to the OP - Time to get serious

In this installment I take the Lighter Than Air mod and attempt to recreate the Hindenburg (crashes and burns excluded).  Also, some details on how the exhaust gas condenser will work and how you will be limited by physics, but not by your imagination.

Further research - Which of these gasses would make a potential lifting gas and how will changes in temperature affect their lifting ability.


Edited by Fengist

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Update to the OP - The beans are burning

In this installment I experiment with creating hot air (which I too am full of) and stuffing it inside an airship.

Further research - How much more could you lift if you heated helium or hydrogen?

Edited by Fengist

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On 9/1/2018 at 9:38 AM, kerbiloid said:

(To mention an additional option).
Like here: https://en.wikipedia.org/wiki/Thermal_airship

An airship containing two independent volumes of gases:

  • lightweight gas (basically He) providing bouyoancy equal to the constant part of the airship weight (construction + cargo).
  • hot air (or another heatable gas) providing additional bouyoancy for vertical maneuvering and balancing variable part of the airship weight

Took a while to get here but the addition to the OP is what you suggested.

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Very cool.  I really would love to play with realistic airships.  

If you click on my avatar, it is a simple representation of a lifting balloon(s) I would like to build in real life. 


Steam is an excellent lifting gas.  Steam has an advantage over helium in that a steam balloon can be operated at ambient pressure and small leaks will not ruin the lifting performance.  The difficult thing with steam is to keep the temperature high enough to prevent condensation.  I propose to do this using the greenhouse effect.  The surfaces of the balloons are variously transparent, reflective, or highly absorbant to focus solar energy on the inner balloonet.  The inner steam balloonet is insulated by surrounding hot air.  



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