[1.8+] Real Fuels

[regex]

You can easily see the formulas that are used in the spreadsheet itself. Plus, engines can be manually adjusted, and some might be, to account for different plumbing. If you are using a certain engine config the spreadsheet that generates those configs is often provided by the config author.

[fallout2077]

Okay, then. Thank you.

[regex]

As promised, here are the adjusted densities for CRP integration. The changes probably won't affect any engine configs with the exception of those that use MMH, which will need to be redone before we do the integration. HTP could also be a concern but I think that was only used on very early engines. Please let me know if there are any concerns or questions with this.

ClF5 4.5 -> 1.9

Diborane 1.22 -> 0.421

Ethane 1.28 -> 0.544

Ethylene 1.18 -> 0.568

HTP 1.44 -> 1.431

LqdMethane 0.42262 -> 0.448

MMH 0.866 -> 0.88

MMH was adjusted to 0.88 because that is the density at STP insofar as I can tell. Only two sources give that number, everything else gives either 0.86 (25C) or 0.874 (20C). This was done for consistency.

LqdMethane was adjusted per Northstar's post here, which makes good sense.

Originally I had decided to advocate the density of liquid ammonia at -50C but instead decided to go with the density at boiling point for consistency with pretty much every other fuel. The difference isn't very pronounced. I'll be suggesting the "density at boiling point for cryogenic fuels, density at STP for everything else" convention in the CRP thread for consistency reasons.

[fallout2077]

One more question, Regex, and I promise I won't bother you anymore. Where are said formulas? The only indication of any formulas that I can find are in the Engines sheet is what appears to be code referring to the EngineOut sheet, which displays more code-like behavior. An example of the EngineOut entry referred to is: ="@PART["&D28$Engines.C25&"] //\:Final //"&$Engines.A25&" { "&IF(AND($Engines.D25<>0,$EngineConsts.$A$18)," @title = "&$Engines.D25&" ","")&IF(NOT(ISBLANK($Engines.E25))," @description = "&$Engines.E25&" ","")&" @mass = "&($Engines.F25+$Engines.G25)&" @maxTemp = "&B25&" "&$Engines.V25&" "&D25&F25&G25&M25&" !MODULE[ModuleEngineIgnitor] {} %category = Propulsion "&N25&T25&" }"

Is this the correct display, or is LibreOffice screwing up the formula? Once again, thank you for taking your time to help me out.

[regex]

Is this the correct display, or is LibreOffice screwing up the formula? Once again, thank you for taking your time to help me out.

It looks right. The calculations are in the individual cells themselves. On the Engines page, check out cell I4:

=ROUND(VLOOKUP(G4,CHOOSE(VLOOKUP(H4,EngineTypes,2,FALSE()),TypeO,TypeU,TypeUplus,TypeSplus,TypeL,TypeLplus,TypeA,TypeS,TypeN),2)*K4,0)*IF(AA4>0,1.07,1)

This, along with J4, is the calculation that determines the the base isps for the engine. It is further modified by the fuel used from EngineConsts page on the table comprising columns J ~ O.

As far as the mass of the engine is concerned, I'm not sure that there ever really was an automatic calculation, the mass was simply set and the TWR checked for being within a good bounding range, then the isp was adjusted based on the TWR. The Tech Level only really comes into play in the game itself, when the engine is changed from its base stats.

[NathanKell]

fallout2077: actually, there's two things at play here. First is the goal TWR at any given tech level. That you can get from the spreadsheet (or RealSettings.cfg, which is the export). Second is what happens when you upgrade an engine. Here is the source. Here's the tl;dr though: if there is no massMultiplier assigned (and there pretty much always isn't), then the mass will be multiplied by either (a) oldTWR / newTWR [for constant thrust engines like SRBs] or ( oldIspVac / newIspVac *and* thrust will be increased by newTWR / oldTWR * oldIspVac / newIspVac. The reason for the complexity in case ( is so that when tech level increases, there is *both* an increase in thrust *and* a decrease in mass.

Kitspace: You have come across a rather fundamental issue. RealFuels has to try to replicate the dry masses and volume utilization of *all* real launch vehicles. That means replicating very early, low-tech high-dry-mass vehicles like the R-7, but also modern lightweight things like Falcon or old even-lighter-weight things like Titan I or II. Consider that Titan I's first stage massed 76.2t wet and only 4t dry, including almost a ton of engine and also the interstage parts. That's 2 tons of tank mass for 72.2 tons of propellant (ratio 2/74.2 = 0.027)--pretty incredible!

Oh, and cryogenic is the type of highly-insulate tank that debuted in the late 1970s for hydrolox; for kerolox, use Default. If you want something closer to R-7's dry mass fraction, try using Structural. Or do like we usually have to do with replicas: add some LeadBallast.

As for too much fuel--decrease the Utilization slider. That's what it's there for. Volume utilization will depend on stage design, tank design, and other factors; rather than try to calculate that, preventing replicas from being built, we just let you set it as needed.

[Kitspace]

Nathan:

Structural?

What do you mean in that case?

Is it a type of parts capable of holding fuel?

I thought it was a separate part not containing any tanks and so it was meant to be lighter than tanks?

Why does the utilization slider affect the dry mass of a part with no tanks yet present?

I think it would be more convenient to affect the dry mass of a tank directly through a setting not via a resource if it is possible at all. The ballast takes the fuel space and vice versa and it depends on the utilization slider meaning that you can not set the dry mass and then fill the tank and set the full mass.

What I do not yet understand from all that is how is it even technically possible to build something sane out of tanks as light as you described above. Both here and in real life. In your example you take a first stage of a launcher that still has something comparably heavy on top of it to push. That means that the thrust to weight ratio will increase like three or four times from launch to burnout. But what if it happens on a top stage? What do they do with it in real life? I guess I am doing something wrong but in game for me it is a common occasion to have an increase in acceleration of more than twenty times while the fuel is consumed. No matter how small the fuel tank size is there is just an insane difference in mass with and without fuel. In this case it is obviously impossible to choose engines.

Basically that leads to situations where there is a tough limitation of the minimal size of the payload. In some cases that minimum limit defined by the near burnout acceleration is higher than the maximum limit defined by the overall lifting capability making such a rocket impossible at all.

And one more time regarding the symmetry bug.

While there is yet no fix for it ready is it possible to get around it somehow and have working procedural side tanks?

And thank you for your detailed answers!

Edited March 14, 2015 by Kitspace

[NathanKell]

Kitspace: Structural is one of the tank types, like Default or Cryogenic. Keep switching type until you get it.

The utilization slider simply affects how much volume RF thinks the part holds.

Regarding TWR increase, can you actually show some pictures (with MJ's stats window open on 'all stats') of your craft?

I don't encounter the issue often and I'm not aware of a workaround. I will try to get the next version out soon, but no promises (RL has been very busy).

[fallout2077]

Thank you, NathanKell, for the very useful information. I never realized just how complex the Real Fuels mod really is.

[Kitspace]

Nathan:

So what the structural tank type simulates then?

The utilization slider slightly affects the dry mass of the tank for some sort of reason if I remember correctly.

I will probably be able to get the pictures by tomorrow.

Thank you.

[NathanKell]

The structural tank type simulates, more or less, a heavier Default tank. The utilization slider affects the total volume of the part; since dry mass is a function of volume, it will affect dry mass as well. You're most welcome!

[Kitspace]

Strangely enough I seem to be unable to recreate the problem. I do not remember correctly what those rockets were made of. Maybe I somehow forgot to put a launch guidance pod on them and that made them too light. Or did the update increase the mass of the command pods? The Titan guidance package however seems even lighter than most pods. Is its current mass considered correct? If those rockets are indeed that light in real life then what actually limits their burnout acceleration?

However regarding my own rockets the amount of payload very strongly affects the burnout acceleration. That means that the payload mass fraction of a dry empty stage is still very high. Is it normal or should the stage be even heavier?

What can be considered a normal classic payload mass fractions range of a dry final stage? What about a whole dry rocket stack?

[jonanlsh]

As a player who has just installed Real Fuels, I have a few questions that I am seeking clarification for. Please forgive my ignorance if these questions have been asked before.

Firstly, what are the different tank types for and what differences do they have?

Secondly, must certain tank types be used with certain types of engines?

Thirdly, is it possible that the FAQ on the front page could include a short section dedicated to explaining the different tank types?

While I might have missed out the information, I do believe that a small overview of what the different tank types mean for the user on the FAQ would benefit new users of RF.

[Raptor831]

As a player who has just installed Real Fuels, I have a few questions that I am seeking clarification for. Please forgive my ignorance if these questions have been asked before.

Firstly, what are the different tank types for and what differences do they have?

Secondly, must certain tank types be used with certain types of engines?

Thirdly, is it possible that the FAQ on the front page could include a short section dedicated to explaining the different tank types?

While I might have missed out the information, I do believe that a small overview of what the different tank types mean for the user on the FAQ would benefit new users of RF.

Mostly, the different tank types have different mass ratios and can hold different resources. Default holds your basic fuels and oxidizers. Cryo tanks have less boiloff for cryogenic fuels (LH2, LOX, etc) but are heavier due to insulation. ServiceModule can hold pretty much any resource and have them pressurized (important if you use an ignition/ullage mod). Balloon tanks are a bit lighter than regular tanks. Fuselage is like an airplane tank, so it's similar to Default but with planes in mind.

No, certain tanks to no have to be used with certain engines. As I mentioned, if you use an ignition/ullage mod (like EngineIgnitor, but I don't think that's ready for 0.90) then your tank selection can change how you can use the engine, but they all still work.

Mostly, unless you're replicating real-world rockets, you'll end up using mostly Default, Cryo, and ServiceModule (And possibly Fuselage if you work with spaceplanes). Any time you have large amounts of cryogenic fuels you need to store for longer than a launch, use Cryo tanks. If you need to store RCS fuels or life support resources or electric charge, use a ServiceModule. Otherwise, Default will do you just fine.

[Kitspace]

I like to operate often with smallish very light payloads and the very high burnout acceleration issue persists for me.

I need to slap some batteries like three times the mass of the payload on the top stage to keep it controlable which seems just not right for a thing like a rocket.

All the guides and tutorials just tell us to put a payload with a fairing and then a tank. Nobody seems to add absolutely anything but the payload on top of the final stage tank.

Also laucnhing different payloads on a typical rocket is even harder. We have been talking some time ago that a general rocket should be capable of flying normally in range from zero payload to full payload with the final burnout acceleration within reasonable limits. How should a rocket look like to be able to? Are all real rockets capable of flying with a range payloads down to zero payload mass? With most my light rockets even going to half the payload affects the acceleration really significantly which I do not like. It does not seem right and it contradicts with the data from the real world and prevents the rocket from carrying the complete full range of payloads it should be able to carry.

All that goes back to the question on just how to build a sane launcher out of tanks that light and how do they do it in real life?

What is a normal payload mass fraction and normal fuel mass fraction for a final rocket stage?

And another thing of the same sort. Russian modular launchers series all based on the R-7.

They all differ by the presence and size of the third stage while the first two remain the same. Also the payload capability is higher with versions which already have bigger and heavyer third stages.

How does that kind of system work actually?

Some pictures of the usual situation with a small light payload and the smallest engines availible around.

[NathanKell]

Kitspace: what Titan Guidance Package? If you mean the Titan ICBM guidance package, I'm not sure there's a correct RO config for it, and in reality it's inside the second stage anyway. It's certainly not payload.

I mentioned residuals as one thing that limits burnout TWR. Rockets are diverse enough there is no 'classic' payload mass fraction for the final stage; on a solid kick stage I'd expect a burnout TWR of 40:1 or so (Explorer 1) whereas a 3G max is obtainable on something like Soyuz (with its many stages). For uncrewed stuff using all-liquid, I'd expect no more than 6Gs at burnout for any stage, higher and it's probably unoptimized.

jonanish: I thought I had a description in the FAQ, but it seems I don't. Raptor (as usual ) has it mostly covered, the exception being the Structural is the aircraft-equalavent of Default; Fuselage is the aircraft equivalent of ServiceModule. Also, ServiceModule (and its twin, Fuselage) have Cryo-level insulation.

Kitspace: Upper stages are generally much more than just tank+engine, in particular guidance, three-axis control, ullage motors, batteries, payload fairing base, etc. drive up the dry mass. KSP, however, does not force you to add any of that, so if it's literally just tank and engine, with a tiny tiny PLF base on top (as your pic shows), then your burnout TWR will be quite high (though 9Gs is still quite controllable).

Instead try making a constant-diameter upper stage with a nice big payload fairing base on top, and in addition add a nice heavy probe core for guidance, some RCS, some hydrazine, some solid ullage motors...it adds up.

The R-7's payload capacity increases because it started with a high TWR so there was plenty of room to add more/larger upper stages, and because the RD-107 and 108 engines were also upgraded over time for even more thrust.

I'm...really not sure why you're confused by "Also the payload capability is higher with versions which already have bigger and heavyer third stages." -- if you don't go under the minimum liftoff TWR, adding more, larger upper stages will increase your payload, because you're taking advantage of what staging does

[Kitspace]

Nathan:

Yes I mean that guidance package thing which I think was created out of something by the Realism overhaul actually and what bothers me that it is still very light compared to the counterparts.

Just I am still trying to understand how seriously should a change in payload mass affect the acceleration and what is considered normal? What do you mean when you say unoptimized? How to optimize in this case?

How heavy is all that stuff expected to be? Those nine gees are with the payload. If I even try to cut the payload in half it will be somewhere around fifteen. And that is exactly what looks unrealistic for me.

The Titan launcher family is a good example indeed. This time let us take the Titan II.

Its second stage full mass is around twenty nine metric tons and its dry mass is around just two tons. The increase in acceleration is fifteen times if I am correct. Its rated payload is three and a half tons. However it was launching things like the Clementine probe massing two hundred and twenty kilograms being a mere tiny fraction of a Gemini.

However obviously this is actually the same rocket in both cases and it was capable of flying safely in both cases. The burnout gees are already high even with the full payload.

What is the actual trick here?

Or a Soyuz rocket capable of carrying a Soyuz or a satellite about ten times smaller in size.

Or a Sputnik rocket actually capable of carrying five tons but also a hundred kilograms Sputnik.

I believe the original liftoff acceleration is slightly below two gees for a two stage R-7. It massed around two hundred and seventy tons. The three stage versions can mass well over three hundred tons. The liftoff thrust of all versions is four hundred tons. That does not look like a lot of room to stack things on top of the existing rocket. Surprisingly enough they still have quite a quick liftoff.

Thank you.

[Kitspace]

As far as I remember only one of Soyuz rockets versions uses the slightly different engines with improved fuel combustion conditions and consequently higher thrust but that increase is really slight. And anyway the later versions went back to the classic ones.

I just built an launched the Gemini using the replica parts.

The masses are a bit off even for replica parts unless I am missing something but that is not that important.

What is important is that with that replica Mechjeb gives the burnout of twenty one gees without payload. Putting three hundred kilograms payload gives nineteen. Does not look very realistic even without fuel leftovers. I think that acceleration would just smash a fragile satellite payload and a nearly empty tank structure itself. Do you agree with that?

[StoryMusgrave]

@Kitspace: My suggestion is to stop worrying about these things because in real life, they don't matter. Satellites are built to withstand quite a bit of just pure acceleration. No different than your cellphone being dropped from a 23 story window and hitting the pavement below. It can survive the acceleration alone. It's the impact damage, be it the atmosphere or another object (like the ground) that can wreck havoc on a satellite (or your phone). Now a human rated craft is different, a Gemini launch really was at ~7G on SECO, but regardless, what you are seeing is correct, it is real, accept it. It's obviously been done before, so start launching. Also think that such a small payload will increase the dV budget for a launcher. The difference between a Titan w/ Gemini (that barely could make orbit) and a 333kg payload is roughly ~3000m/s. Which means that a lot of performance of the launcher is wasted to achieve the same orbit. The 2nd stage will just shut down that much sooner.

Realistically, such a small payload these days could be launched as a secondary payload or picking a smaller vehicle. Not quite as extreme, but you don't use a Delta IV Heavy to launch just a cubesat.

[Starwaster]

...but you don't use a Delta IV Heavy to launch just a cubesat.

But it's fun!

[NathanKell]

Kitspace: As I said above, Titan's actual guidance was inside the upper stage. I'm not sure what RO is basing that "ICBM guidance unit" on--probably the computer that controlled MIRV deployment? Dunno. If it bothers you that much I encourage you to research it and find out what it should mass.

As for burnout: yes, uncrewed spacecraft can suffer accelerations of upwards of 40Gs. Look up just how many Gs the final stage of Juno I was, for the Explorers. As Story Musgrave said, that's how it works. Most launchers most of the time launch the paylaods they're designed for, and their burnout TWRs are generally <10 (if your TWR goes from 0.5 to 14 with your rated payload, you need to turn that stage into two stages) but if you insist on putting a 5kg payload on something that's designed for 5 tons, it's going to be crazy. Yes, burnout Gs for Clementine atop the Titan 23G were about 15, but it's not a human being, it's fine. Heck, human beings survived a 20+G suborbital reentry from a during-ascent abort (one of the Soyuz flights).

Sputnik 8K71PS1 was capable of 500kg to orbit, massed 267t, and had a liftoff TWR of 1.48. Since 1.15 is the minimum practical liftoff TWR, there was lots of room for upper stages. The modern Soyuz LV (that's what can lift 5 tons) can mass somewhat over 300t, yes, but also has 12% more powerful first stage engines (for ~440 tonnes force). Its liftoff TWR is still around 1.4. I'm sorry to say this, but you really, really need to do more and better research.

As for Gemini, it sounds like something is getting built wrong, or there's an error in the configs; burnout Gs are indeed ~7 at SECO as Story Musgrave says. There was a mass-related error in the Gemini files that just got fixed, so that might be it.

[Kitspace]

Nathan:

If the guidance system is supposed to be inside the actual part as it is in game then it is too light and needs to be four hundred kilograms heavier.

Yes I know about that case with people surviving those crazy gees but that was certainly way over the safe limit for a human.

I believe you mean that famous case of uncontrolled ballistic entry after a third stage ignition failure?

I believe that pretty much every existing manned spacecraft out there is the full rated payload for its respective launcher. That does make perfect sense.

Are you saying that the actual burnout acceleration of an unmanned stage is basically uncontrolled and unlimited? So it can and does go that crazy on a classic big and complex liquid fuel stage? I think the early pioneer designs especially with solid motor stages are a different story?

And regarding the Russian launchers yes it was my mistake.

Two stage version lifted five tons to suborbital only. The first one capable of five tons to orbit was the Voshod as I believe with the new big third stage.

The more powerful first stage engines?

Are you talking about the RD-108A? Seems to be my mistake again. I thought about the RD-118.

Where do you get the acceleration data? Are you calculating it yourself?

Sorry if I ask questions that look stupid.

Than you.

[StoryMusgrave]

@Kitspace: The guidance package for the Titan II in FASA is part of the actual ICBM, and is part of the Mk 6 re-entry vehicle, which had a mass of 3.69t. With the warhead itself being 2.8t. So that other ~.9t or so is the entire base of the re-entry vehicle, payload shroud, etc. I think the mass is a bit off, at 3.76t, but pretty close. I'm not sure of real numbers, but it is what it is, payload of the Titan II isn't that far off.

Yes the 'didn't exist' Soyuz 18 that the 3rd stage did ignite but due to failure of 2nd/3rd stage separation caused some tumbling which caused the computer to start the abort.

Uncontrolled and unlimited, no of course not. The goal for launchers is to get something to a specified orbit, if things need to shut down early, then that happens, even on solids there is a thing called thrust termination, which basically kills most/all forward thrust by venting out the sides. Not all solids have it, but it's an option on some. You again also have to realize that people are going to use a launcher appropriate for the payload. No need for Atlas 552 when a Delta II 7390 will do.

[stratochief66]

Unmanned payloads, like manned payloads, can handle very high acceleration for very short periods of time. During the last few seconds of any stage the Jerk is at its maximum (http://en.wikipedia.org/wiki/Jerk_%28physics%29)

the acceleration goes through a wide range of values, and a great deal of velocity is added. This is indeed a stressful period for the payload, manned or unmanned. R&D would determine when these stresses exceeded the capacity of the launcher and/or payload. For instance, NASA determined that it was better to shut down the centre engine of the first and second stages of the Saturn 5 before burnout to reduce the range of accelerations the craft would be placed under. A payload can be built in anticipation of these great accelerations and forces, the greatest threat appears to be to the rocket engine itself in many cases, literally vibrating itself apart. (http://en.wikipedia.org/wiki/Pogo_oscillation)

[Faster]

Is there a patch for this with Engine Ignitor? For example, to fix the LV-909 which shows in the VAB as supporting only Aerozine 50 + NTO or LH2 + LOX but then appears on the launch pad wanting generic hypergolic. Granted, it does burn the Aerozine 50 and maybe the current approach is the smoothest KSP itself makes feasible; just wondering if there's any patches already floating around that clean up these discrepancies so, for example, the automatic fuel selection options for fuel tanks work smoothly and the Delta-V readouts are correct?

[update 1] Apparently getting the 909 to burn Aerozine 50 as hypergolic was a fluke, I can't seem to repeat that--so as it stands RF and Ignitor seem incompatible without a patch (though the chatter in this thread would seem to indicate otherwise.)

[update 2] Now I can't get a 909 to not burn Aerozine 50; after all the convolutions of trying to figure the interplay of RF and EI I feel like I've been sniffing Aerozine. So, I'm back to the original hypothesis: the oddity of showing "Hypergolic" and then only Aerozine is just the best that can be done with the limitations of KSP. (Ignore the comment about incorrect delta-v above, that came after the Aerozine sniffing began.)

Edited March 27, 2015 by Faster