RSS/Realism overhaul - how do you build your rockets?

[dino1984]

How do you build your rockets? I mean methodology. I try to build a lunar impactor (probe to smash into lunar surface), so I made a top part - 3,5 km/s of dV with antena some science and RCS. Now I try to add whole rest of the rocket to lift it into LEO, so additional 10km/s of dV. I've already got 310 tons with 5000 kN of thrust... and it is still not enough (I need to add more avionics to be able actually to steer it and when I add avionics... its getting heavier and losing its dV).

I have a feeling that I am extremely overbuilding this due to lack of knowledge. Can you tell me some rocket science behind rocket building (rocket equation - I know)?

[Streetwind]

A general rule of the thumb is to make all stages have a similar fuel-mass fraction. That is important because a.) the more fuel you add to one stage, the less additional dV you will get out of that fuel you just added (i.e. diminishing returns), and b.) having an upper stage being heavier than average can choke the dV of all stages below it.

In a simplified world, where all engine Isp's are identical, that would mean that each stage has roughly the same dV. Something like three stages of 3300 m/s each in order to get to low Earth orbit. In practical application, Isp will vary, especially on upper stages that have the ability to use high efficiency fuels and vacuum optimized bells. So your dV per stage will differ. If the fuel-mass fraction remains the same, then your upper stages will have more dV than your launch stage. The trick is to find the fuel-mass fraction where the disparate dV numbers of the stages summed up still give you enough dV to reach orbit without overbuilding. Example: three stages with 2800, 3300 and 3800 m/s respectively, all with the same fuel-mass fractions and just different Isp.

If your vessel still ends up struggling, bolt some SRBs to the first stage for a quick boost off the pad. Even if they don't add much dV themselves, they will also reduce gravity losses by shortening your fully vertical flight time. And since there's no stage below the launch stage that could be choked by the extra weight, overloading your launch stage is free!

 

Edited March 29, 2016 by Streetwind

[dino1984]

How do You understand fuel-mass fraction? Fuel/oxidizer? 

[CSE]

The fuel mass fraction is the percentage of the with-full-tanks mass of the rocket that will be used up by flying it. Fuel in this sense is Liquid Fuel, Oxidizer, or any other substance that your engines will have pushed out of the back of your rocket by the time you get to your destination.

There's a good tutorial on methodically designing a rocket. The methodical approach is particularly important in real-scale solar systems because the target range between "good enough for the mission" and "the limit of current tech" is much narrower than in stock KSP.

I also think that you should aim to need less avionics, as a percentage of rocket weight, in a real-scale launch. The job of the avionics is (roughly) to rotate your rocket from vertical to horizontal, regardless of the size of planet you launch from, and in RSS the launches take longer so you have much more time to do that rotating.

[Streetwind]

4 hours ago, dino1984 said:

How do You understand fuel-mass fraction? Fuel/oxidizer? 

Well, I must admit I made an oopsie - the number I meant is actually simply referred to as "mass ratio". No "fuel" in there, apologies. But, as for what it actually is...

It's easiest understood when looking at a singular fuel tank, like the Rockomax X200-16. This tank masses 9 metric tons in total. That mass is called the "wet" mass, because it is measured when the tank is full of liquid.

This total mass is split into 3.6 tons LiquidFuel, 4.4 tons Oxidizer, and 1 ton empty structure. The last part, the empty structure, is the so-called "dry" mass, because it contains no fuel at all.

The mass ratio, then, is a very simple thing: it's the ratio of the fully-fueled "wet" mass (3.6 + 4.4 + 1 tons) to the "dry" mass that remains after all the fuel is gone (1 ton). For the Rockomax X200-16 (and indeed almost all tanks in stock KSP), this ratio is simply "9:1", or 9.

The same number can be calculated for any given rocket stage. You take its total ("wet") mass, subtract all things that are fuel, and you receive the empty ("dry") mass. Then you divide the wet mass by the dry mass and you get the mass ratio.

 

Understanding this is kind of important, because it's one of two halves of the all-important Rocket Equation. The other half is specific impulse. Or rather, effective exhaust velocity (a number that Isp directly translates into via simple multiplication with gravity).

dV = Isp * 9.807 * ln(wet/dry)  <---- here is the mass ratio!

Because of this, the mass ratio is each rocket stage's single most important number. If your stage has a mass ratio of 4:1, and your engines deliver 300s worth of specific impulse, then that stage will deliver exactly the same dV as all other stages with a mass ratio of 4:1 and engines with 300s Isp. It doesn't matter how many parts any given stage contains, how much it weighs in total, whether it's a lower or an upper stage, which size the main stack is, how much stuff is sticking out the side, how much it cost, which fuel type it is running on, or anything else... they will perform exactly the same.

On top of that, the mass ratio of your fuel tanks defines the upper limit of the dV an engine can achieve. In stock KSP, no engine can give you more dV than its own Isp, multiplied by the product of the gravity constant and the natural logarithm of 9 (which ends up being roughly Isp * 21.55). Not only that, but because rocket engines and payloads have mass too, you won't even get anywhere near that value in practical application. Much rather than approaching 20, you'll find that most rocket stages give you no more than 15 times the engine's Isp worth of dV. Beyond that, the diminishing returns just get too strong to bother adding more fuel. In Realism Overhaul, these numbers will be different, but the math works just the same.

Therefore, always be aware of your mass ratios

 

Edited March 29, 2016 by Streetwind

[NathanKell]

It depends on whether your upper stage can relight its engines (or if not if you're doing direct ascent with no parking orbit) and how much you're throwing at the moon.

For reference, early moon probes were launched by:

R-7 with a Blok E (RD-0105) upper stage, probe mass around 300kg

Thor-Able (Thor, Able liquid stage, Altair kick stage) - probe mass around 20kg.

Juno II (Jupiter lower stage, Baby Sergeant cluster upper stages) - probe mass 4kg

 

Later probes were launched by Molniya (R-7 with an RD-0110 second stage and S1.5400 upper stage) or Atlas-Agena.

 

How heavy is your impactor?

[NathanKell]

Oh, and also what tech do you have available?

 

Now, as to the general question, here's how I go about it.

1. Decide on a mission. Will this LV take payload to low Earth orbit? Will it take payload to beyond LEO? Will it be designed for both roles? Will it be designed to take payload to LEO but have a higher-than-required TWR to allow the addition of an extra stage for BLEO work?

2. Decide if you are making a 2 stage or 3 stage LV (or even more), and/or if it will use stage 0 (i.e. runs during core burn) boosters.

3. Consider: do you have restartable engine(s) for your upper stages? If not, and you are not doing a direct injection ascent, your final stage(s) will need to precisely handle the injection burn after being placed into orbit by the rest of the LV. If you have restartable engines, or you are doing a direct injection launch, then you can use part of the final stage(s) to perform parking orbit insertion and the rest for BLEO injection.

 

With that decided, I generally create stages that take advantage of the full rated burn time of the engine I use. Due to Isps and possible mass ratios varying between stages, trying to keep delta V equivalent between stages doesn't work that well. Also you can (and should) have much longer-burning upper stages anyway, since TWR isn't very important after your first 2km/sec of speed.

[OhioBob]

8 hours ago, Streetwind said:

Well, I must admit I made an oopsie - the number I meant is actually simply referred to as "mass ratio". No "fuel" in there, apologies.

Just to confuse the issue, there is something called "propellant mass fraction".  It is the fraction of the total mass that is propellant.

The term "fuel" is often use (as a layman's term) when it would be more proper to use the term "propellant".  For example, KSP calls it tanks "fuel tanks" when they should really be called "propellant tanks".  Fuel is only part of the contents, with oxidizer being the other.  The fuel and oxidizer are reactants that are mixed together and burned.  The resulting combustion products form a reaction mass that is expelled from the engine to propel the rocket.  Since both the original fuel mass and the original oxidizer mass are expelled as reaction mass, both fuel and oxidizer are propellants.
 

Quote

The mass ratio, then, is a very simple thing: it's the ratio of the fully-fueled "wet" mass (3.6 + 4.4 + 1 tons) to the "dry" mass that remains after all the fuel is gone (1 ton). For the Rockomax X200-16 (and indeed almost all tanks in stock KSP), this ratio is simply "9:1", or 9.

In this example, the total mass is 9 tons, of which 8 tons is propellant.  Therefore, the propellant mass fraction is, 8/9 = 0.889.  Propellant mass fraction is just another way of looking at mass ratio, but it is mass ratio that is important in solving the rocket equation.

[Blaarkies]

22 hours ago, Streetwind said:

A general rule of the thumb is to make all stages have a similar fuel-mass fraction. That is important because a.) the more fuel you add to one stage, the less additional dV you will get out of that fuel you just added (i.e. diminishing returns), and b.) having an upper stage being heavier than average can choke the dV of all stages below it.

In a simplified world, where all engine Isp's are identical, that would mean that each stage has roughly the same dV. Something like three stages of 3300 m/s each in order to get to low Earth orbit. In practical application, Isp will vary, especially on upper stages that have the ability to use high efficiency fuels and vacuum optimized bells. So your dV per stage will differ. If the fuel-mass fraction remains the same, then your upper stages will have more dV than your launch stage. The trick is to find the fuel-mass fraction where the disparate dV numbers of the stages summed up still give you enough dV to reach orbit without overbuilding. Example: three stages with 2800, 3300 and 3800 m/s respectively, all with the same fuel-mass fractions and just different Isp.

If your vessel still ends up struggling, bolt some SRBs to the first stage for a quick boost off the pad. Even if they don't add much dV themselves, they will also reduce gravity losses by shortening your fully vertical flight time. And since there's no stage below the launch stage that could be choked by the extra weight, overloading your launch stage is free!

 

Stock/Vanilla ksp:
I realized it helps to build stages so that, the bottom stage is about twice as heavy as the top stage(2 stage rocket).
So a 1ton payload gets pushed by a stage with an additional mass of 2ton...that gets pushed by a bigger stage with additional mass of 2x3ton(the 1ton top and its own stage is considered the payload of this bottom stage)...and so forth.

What bothers me is that, this 2/3 stage ratio is pulled out of thin air(i cannot get a definite point on a dv/mass plot, Ln is really smooth, even at 1st and 2nd derivatives). Do you know of any way to calculate this?

[Streetwind]

1 hour ago, Blaarkies said:

Stock/Vanilla ksp:
I realized it helps to build stages so that, the bottom stage is about twice as heavy as the top stage(2 stage rocket).
So a 1ton payload gets pushed by a stage with an additional mass of 2ton...that gets pushed by a bigger stage with additional mass of 2x3ton(the 1ton top and its own stage is considered the payload of this bottom stage)...and so forth.

What bothers me is that, this 2/3 stage ratio is pulled out of thin air(i cannot get a definite point on a dv/mass plot, Ln is really smooth, even at 1st and 2nd derivatives). Do you know of any way to calculate this?

It's a simple and direct consequence of the 9:1 fuel tank mass ratio.

Let's say you have a 1 ton payload, and all your engines have a KSP-typical Isp of 300s and a KSP-typical base TWR of 20.

You add a 0.05 ton decoupler. You then add an engine to give you roughly 1.0 TWR. To do that with your target mass of 3 tons, you need about 30 kN. And since the engine's base TWR is 20, that means such an engine weighs 0.15 tons. Finally, you add 1.8 tons tons worth of fuel tanks (because your payload + engine + decoupler weighs 1.2 tons), only 8/9th of which is actual fuel.

Your stage now masses 3 tons, out of which 1.6 tons are fuel. Your mass ratio is 3:(3-1.6) = 3:1.4 = roughly 2.143

You plug that in the rocket equation, and you get about 2240 m/s dV.

For stock KSP, that's a pretty reasonable number for dV contained in a single stage. After all, the average launch vehicle has two stages, and the average player flying an average launch vehicle expends about ~3500 m/s to orbit. If anything, this stage is slightly over budget, but having a margin is always good. If you build another stage below this one, and make it again twice the weight of the "payload" above it (which now includes the upper stage), that stage too will have 2240 m/s, assuming equal ratios of of mass devoted to engines and structural elements like decouplers. Though you would in fact devote more mass to engines, because you want to actually be able to lift off the pad, and to do so comfortably, you want a TWR of maybe around 1.5 instead of 1.0. So your final dV will be a little less - though still above 4000 m/s in total for the whole rocket. All in all, this is a pretty decent launch vehicle for an orbital mission that requires a bit of maneuvering (like a rescue contract), and it is that way because the fuel tanks and engines available in stock KSP have the stats they have.

If the tanks had a mass ratio of just 3:1, you would have to add a whole lot more of them to achieve 2240 m/s of dV in one stage: adding 4 tons of tankage and a 50 kN engine will give you a stage massing 5.25 tons, 2.666... tons of which are fuel, for a mass ratio of 2.032 and a stage dV of 2086 m/s. Maybe you should have added 5 tons of fuel and a 60 kN engine instead! Suddenly your "rule of the thumb" no longer states "each stage below should have two times the mass of the payload above" but rather says "each stage below should have five times the mass of the payload above". For no other reason than the fuel tanks having different stats.

Mass ratios: important!

 

Edited March 30, 2016 by Streetwind

[Blaarkies]

1 hour ago, Streetwind said:

...

If the tanks had a mass ratio of just 3:1, you would have to add a whole lot more of them to achieve 2240 m/s of dV in one stage: adding 4 tons of tankage and a 50 kN engine will give you a stage massing 5.25 tons, 2.666... tons of which are fuel, for a mass ratio of 2.032 and a stage dV of 2086 m/s. Maybe you should have added 5 tons of fuel and a 60 kN engine instead! Suddenly your "rule of the thumb" no longer states "each stage below should have two times the mass of the payload above" but rather says "each stage below should have five times the mass of the payload above". For no other reason than the fuel tanks having different stats.

Mass ratios: important!

 

That makes a lot more sense, thanks!

See I just eyeballed the dv/mass plot's derivative, and made a judgement call about at the point of where "the dv per 1ton" gets too low for my like. Isp is just linear multiplier added to whatever the Ln(wet/dry) outputs. To be clear, I am working on the assumption that you can just keep adding stages...because in stock its not that much of a problem. So the ideal solution is probably something nearing infinite stages, but that is unpractical.

This all started when I was building an Eve ascent lander that doesn't use asperrigus staging. This was a pure stack rocket, about 50ton to get a probe+command-seat into orbit(inside a fairing ofcourse :P). 5 Stages got the job done...very nearly.

[dino1984]

So - if I understood good - every stage should keep "dry to wet mass ratio" on the same level, including all previous stages, right?

So 1st stage dry mass will be mass of tank, engine and total mass 2nd and 3rd stage. Right?

And 1st stage wet mass will be the same but with 1st stage full of fuel.

 

Looking at natural logarithm graph, optimal wet/dry ratio would be around 6, as later additions are getting really small. How do you think?

Edited March 30, 2016 by dino1984

[NathanKell]

@OhioBob yes, I've tried to never say fuel in the new tuts, only propellant, but a lot of parts are still named "Fuel Tank" so it's confusing. >.>

 

@dino1984 No, that is a rule for stock KSP. In RO it pays to ignore that rule completely, as I mentioned. Here is a perhaps better method for you.

1. Add  a probe core and a leadballast tank

2. Add a fairing base under that.

3. Add a proc tank under that.

4. Add your best upper stage engine under that, and extend the tank until you get to that engine's rated burn time.

5. Size the leadballast payload until you get 3400m/s out of the stage.

6. Add an interstage below that.

7. Add a new tank and your best second stage (if doing traditional staging) or sustainer (if doing booster+core) engine. Size the tank to get that engine's rated burn time.

8. Add a new stage below that (or side boosters) with your best lower stage engine, or more likely a cluster of them/multiple boosters. Size that stage and add engines, or copies of the booster, until you get 12,800m/s vacuum delta V and a SLT (or, in KER, atmospheric TWR) of >= 1.2.

Done!

[dino1984]

Well.. I followed a rule of keeping 6:1 mass ratio, and I saved like 10 tons - 298 intead of 308. HURAY!

Why 12,800 m/s ? 10km/s to orbit+3,4km/s to TLI is more like 13,4km/s?

 

It is very interesting, because I saw video on youtube where moon impactor had initial dV of something slightly above 11 km/s and it done it! What is a math behind it?

It starts at 7:15

 

[OhioBob]

48 minutes ago, NathanKell said:

@OhioBob yes, I've tried to never say fuel in the new tuts, only propellant, but a lot of parts are still named "Fuel Tank" so it's confusing. >.>

The game actually burned me once because of its inconsistent and confusing use of the word "fuel".  I had accepted a contract to place in orbit a station that included 4 tons of "fuel".  I figured this requirement would be fulfilled by including a "fuel tank" holding 4 tons of contents.  But no, the contract wanted 4 tons of actual fuel, i.e. not oxidizer.  This type of mixing of terms I find to be really frustrating.  We shouldn't have a contract ask for "fuel" but then have the contents of a "fuel tank" not fulfill the contract.  I really think the game should change all references to "fuel tank" to "propellant tank."

[OhioBob]

Quote

It is very interesting, because I saw video on youtube where moon impactor had initial dV of something slightly above 11 km/s and it done it! What is a math behind it?

Are you sure that was dV and not just velocity?  Earth escape velocity is about 11 km/s, so it makes sense that someone would accelerate their lunar impactor to 11 km/s.  However, adding in for gravity and drag losses, it should take a dV of at least 12.5 km/s or so to make it from the ground to the moon.

[Guest]

On 3/29/2016 at 10:00 PM, dino1984 said:

How do you build your rockets? I mean methodology.

1. Determine final payload mass.

2. Do I need a transfer stage?  Will one of my pre-existing ones fit the bill?  Can my upper stage pull double duty as transfer?  If not, build one.  There may even be more than one transfer stage depending on the mission.

3. Payload + any transfer stage(s) is payload to orbit mass.  Do I have a rocket that can handle that mass to LEO?  Do I have a rocket that can handle the mass and inject the payload into a transfer if I have no transfer stage?  If not, step 4.

4. How many stages will it take to get my payload to orbit?  For me, this is generally determined by the diameter of the payload fairing since that will determine the upper stage diameter which will influence the lower stage diameter.  Since I prefer taller rockets to wider rockets, this also influences the engines that will be used.  Since I fly all missions by hand I find an upper stage TWR less than @0.75 uncomfortable if it has a very long burn time which means I'll stack additional engines as needed (upping TWR and reducing final burn time, and delta-V slightly).  I look for a space delta-V from my launcher of around 10.5km/s, given that I've chosen proper lifting and upper stage engines suited for their individual tasks.  Most of my rockets end up being 2+1 (2 stages + boosters) or 3 (no boosters) stage affairs.

5. Double check everything.  Did I remember RCS on my payload?  On my transfer stage?  Is there adequate power?  Life support?  Do I need to provide roll control for the upper stage?  Lower stage?  etc...

 

Step 4 is the part where everyone will diverge into their own opinions on how to design things.  Yes, if you're going for nothing but efficiency there is really only one or two ways to build a rocket, but we are free to explore concepts and accommodate the way we enjoy playing in KSP.

Edited March 30, 2016 by regex

[LeuZ]

@dino1984

You need about 13 km/s delta-V to get to the moon (9.8 For LEO & 3.2 for Moon transfer burn)

I think Scott Manley didn't have just 11 km/s delta-V because sometimes Kerbal Engineer Redux shows not the right delta-V, but Mechjeb does. 

[Nich]

That is atmospheric dv in the screen shot KER will increase the DV for each stage as you gain altitude. 

[NathanKell]

Don't ever try to use the atmospheric delta V columns. They really aren't helpful. You need to know your liftoff TWR (the SLT column in MechJeb) for the first stage, but everything else should be vac.

[Matuchkin]

I try to get approximately 4500 dv on the second stage. The first stage has to have a twr of less than 1.10 but more than 1.05. In general, I try to keep my total Dv above 10,500. Unfortunately, I'm having trouble managing that, with the best engine mods being gone forever. Also, does anyone know how to download SSTU so that the engine models will actually be visible?