More liquid fuel (only) tanks

[Behemot]

Hello world

 

I often use atomic engines in KSP but it seems the offer of liquid fuel (only) tanks is still kinda limited in KSP. Dragging totaly useless oxidizer all the way who knows where is not really a good idea when you have no use for it, on the other hand with only a couple of existing tanks ships tend to get unnecessarilly large and bulky.

 

So what would especially help would be Mk3 to 2.5m Adapter liquid-fuel only variant. Or possibly even converting Rockomaxx tanks to liquid-fuel only variants as well, together with Kerbodyne ADTP-2-3.

 

Edited January 23, 2019 by Behemot
space

[klgraham1013]

Fuel variants for tanks is a very good idea.  I'd, again, like to think modders for making this happen until Squad maybe gets to it.

[Geonovast]

1 hour ago, Behemot said:

Dragging totaly useless oxidizer all the way who knows where is not really a good idea when you have no use for it

Not to put anything against your suggestion, I agree with you completely, but you can empty the oxidizer out of the tank in the Editor if you don't need it.  Granted, it sucks hauling a half-empty tank, but it is better than hauling a tank half full of something you don't need.

 

1 hour ago, Behemot said:

Or possibly even converting Rockomaxx tanks to liquid-fuel only variants as well, together with Kerbodyne ADTP-2-3

Seems like too much work when it should just be configurable across the board.

 

That said, there are of course mods to get you by until the game does what you want in stock.  Recent thread on it:

 

[septemberWaves]

I have two, somewhat conflicting, opinions on this. The first is that using large LFO tanks for nuclear engines and simply emptying them of oxidizer is a good enough option that I am satisfied with (at least for cases when not using Sigma Dimensions to make everything bigger), because real nuclear engines would likely use liquid hydrogen, which has low density so a large tank would not be able to store as much fuel as a comparably-sized tank for hypergolic fuels. It really just means that the fuel for NTRs necessitates bigger tanks, which I think is an acceptable tradeoff for the high efficiency.
Alternatively, I would not mind having stock fuel switching capabilities; i.e. the ability to have an option for fuel tanks (similar to the stock texture switching) that changes which type of fuel they contain. There is a mod for this kind of functionality (Interstellar Fuel Switch), and I am of course a strong advocate of just using mods if they will enrich your gameplay experience, but even so it would be nice to see that kind of feature in stock.

[Tonka Crash]

@Snark wrote a mod specifically for this problem.

 

[Behemot]

2 hours ago, septemberWaves said:

because real nuclear engines would likely use liquid hydrogen

Since we don't really have an idea what is the idea behind the nuclear engines in KSP, I don't think so  Besides, they do not differentiate hydrogen, kerosine, hydrazine or anything else, it's all just liquid fuel/oxidiser, the one and only. Although having the remote fuel tank near the launch pad suggest it may be hydrogen, but who knows.

 

As for the engines themselves, I think that weighting 3 tons and only producing 60 kN is trade-off just enough. On the contrary, limited offer of tanks at the moment does not allow full use of them, or at least not really an elegant one. It can move 50ton vehicle just about enough in space using four of them, but the engines themselves than weight 12 tons of that. So far I am using the medium Mk3 tank and 2,5m capsule directly on top of it, but a combination of small Rockomaxx tanks plus the adapter would likely yield the same effect (due to better aerodynamic shape, not being so bulky etc.).
 

Tonka Crash: thanks for the tip! that solves it for now…hopefully devs will implement this in at least some partial form, I think this is pretty much basic stuff you would expect so as such, it should get in the game sooner or later
 

Edited January 23, 2019 by Behemot

[bitzoid]

Some rockomax or s3 sized tanks would certainly be nice. Currenly I usually add a large Mk3 Fuselage and 4 or 8 radially attached subassemblies consisting of 1 structural pylon, 1 nerv, 1 NCS Adapter and 1-3 Mk1 liquid fuel tanks depending on the mass of the payload. During the mission one can decouple half of the nervs once some fuel has been drained. Here is a comparison between different numbers of Mk1's for 50t payload. They can offset the extra weight of more engines but of course adding weight that needs to be lifted to orbit.

 

The x-axis is the number of radially attached nervs. Lines are dv in vacuum (scale right), shaded area is acceleration (scale left). I think for 50t I would go for two Mk1 tanks.

Anyway, the problem with this approach is that usually some of the pylons float next to Mk3 tank or clip way in. If it was a dedicated round tank, this would be much prettier and easier.

 

[Behemot]

It depends on your staging: I prefer to at the very most, get to Kerbin orbit with preceding stages and than while out of atmosphere, take use of the NERVs as it has best fuel efficiency so it is also lighter to actually get into space. However, the smallest usable punch for such ~50ton load (that includes the engines and fuel) seems to be about 3 of them as maneuvering around gets slightly painful with less of them. Getting any reasonable acceleration takes too much time…with 4 NERVs, accelerating to close Mun encounter with clockwise orbit takes about 2,5 minutes real-time burn. With just 2 engines that would be (considering the lower weight) something like 4,5 minutes.

If I'd try to endevour for going into deep space with this, I could let it run on the background and go to do something else  However that is nice idea with the decoupling of the engines. It is true that fuel crossfeed works nice for some time now (I still remember when you had to put the fuel pipes over pretty much every adapter and so, these days I have not used single piece of fuel piping for years) so that may actually work very well. I can already see use for some other adapters than just TVR-400L Stack Quad-Adapter.

 

Actually, another suggestion for devs comes right from this: I see pretty much no coupling adapters from Extra Large (Kerbodyne), 3.75m parts to anything else  (rather than the Kerbodyne ADTP-2-3 tank), are there going to be some in some of the next releases? If not, possibly you can think about that? Some bi, tripple and quad Rockomaxx adapters come to mind right away, and, obviously, also quintiple, sextuple, septuple (at least) small (1.25) sized attachment nodes. When at that, just making larger nuclear engine would also be nice, lol.
 

[Xurkitree]

I'm using Aircraft Plus's fuel tanks for my LF needs, but then using a massive part mod just for the tanks isn't really good.

[Nebbie]

I really would like some kind of variant switcher for fuel simply because the current state of having duplicated fuel tank parts for spaceplanes that only vary in what fuel they carry is silly.

[KerikBalm]

Yes, a fuel switcher would be nice... but I'd actually like to see a liquid hydrogen resources specifically for rapiers, LV-Ns, and perhaps the wolfhound because its Isp is so high.

I was about to comment on adaptors for 3.75m to 2.5... but I just saw that they added fuel to the Kerbodyne ADTP-2-3 in 1.6.. Bravo

[lajoswinkler]

Yes, you can reduce the oxidizer to zero and that gives you half empty tank.

But then again, LV-N should not even use LiquidFuel. It should have its special fuel. One of those things that are not difficult, would make the game richer, but things that are being neglected.

[Nebbie]

5 hours ago, lajoswinkler said:

Yes, you can reduce the oxidizer to zero and that gives you half empty tank.

But then again, LV-N should not even use LiquidFuel. It should have its special fuel. One of those things that are not difficult, would make the game richer, but things that are being neglected.

That would make spaceplanes a lot more complicated because you couldn't share fuel between the LV-N and jet engines.

[lajoswinkler]

11 minutes ago, Nebbie said:

That would make spaceplanes a lot more complicated because you couldn't share fuel between the LV-N and jet engines.

LV-N is not something designed for spaceplanes in the first place.

However, LF could be used as a different mode of operation of LV-N, at a great expense of I_sp.

[Fraston]

6 hours ago, lajoswinkler said:

Yes, you can reduce the oxidizer to zero and that gives you half empty tank.

But then again, LV-N should not even use LiquidFuel. It should have its special fuel. One of those things that are not difficult, would make the game richer, but things that are being neglected.

This game is not about being realistic, it’s about having fun. Play it with mods, if you are so dogmatic with your opinion.  Use interstellar fuel switch and make the NERV burn hydrogen, or a custom fuel, or whatever. 

Video games revolve around fun factor, not nessicarily realism. Realism also doesn’t always mean fun in my book.

[lajoswinkler]

15 minutes ago, Fraston said:

This game is not about being realistic, it’s about having fun. Play it with mods, if you are so dogmatic with your opinion.  Use interstellar fuel switch and make the NERV burn hydrogen, or a custom fuel, or whatever. 

Video games revolve around fun factor, not nessicarily realism. Realism also doesn’t always mean fun in my book.

Simple RTG decay is not realistic. It's just more realistic than cheating. The principle is realistic. Full realism would model thermoelectric module failure and whatnot.

And you're dead wrong about what the game is all about. Founding developers wanted to make something as close to reality they could, without it requiring complex calculations and writing scientific papers on mission planning. That's why we have patched conics instead of "go to orbit" button, and tracking solar panels that don't work in shadows. That's why we have realistic principles of engines. The whole game is built around reasonably realistical principles and it has only increased as years went by. We got a thermodynamic heat model. Realistic? No, not completely, but it's damn close to idealized state.

PB-NUK was supposed to decay, I remember it has been discussed years ago, but it was forgotten because it's a minor part made early in development.

Let's toss it all for those few that can't do it. Wah wah.

Dogmatic, right. That's why Nertea's mods have almost half a million downloads on just one mod sharing site. Come on.

[MrSystems]

Yesterday I started to work on a .cfg that added an LH2 option to all the stock fuel tanks and switched the Nerv to run on LH2. It worked, but...

LH2 is much less dense than RP-1 (~8.8%) and LOX (~6.2%). More than half of the wet mass of a stock fuel tank filled with LH2 is the dry mass of the tank (a Rockomax X200-16, normally 1 ton of tank + 8 tons of LF/OX, became 1 ton of tank + 0.6 tons of LH2, for example).  As noted in this paper, "the mass advantage of the liquid hydrogen fuel will result in a mass advantage for the fuel system only if the liquid hydrogen tank and insulation mass is a small fraction of the hydrogen mass." Thus it is impractical unless/until some modder with skills superior to my own develops a suite of custom LH2 tanks that are volumetrically-appropriate but have much less structural mass per volume than the stock tanks.

What I do have that a) works and b) is practical for in-game rocket design is the following, which you can save as a .cfg in your own private/custom folder in GameData. It:

• Uses B9PartSwitch and ModuleManager to provide LF-only options for all the stock and Making History tanks
• Does the same for the Missing History tanks
• Provides an LF/OX option for the Mk0 Liquid Fuel Fuselage
• References every part by name, so it will not affect parts from other mods, neither for good nor for ill
• Doesn't change the Nerv at all

Even under the "atomic rockets use liquid fuel" simplification this is still a bit too easy, because RP-1 is less dense than LOX and it shouldn't be a 1:1 substitution, but this does make it a lot easier to put together a Nerv-powered rocket.

(I'd have used Snark's SimpleFuelSwitch instead of writing my own, except I already am running B9PartSwitch.)

//// Allows switching between LFO and LF-only for all stock, Making History, and Missing History fuel tanks
////   Requires B9PartSwitch
////   Written by Chad Tucker under the MIT Licence, 2019
////   'MrSystems' on forum.kerbalspaceprogram.com
////   for help see https://wiki.kerbalspaceprogram.com/wiki/CFG_File_Documentation


//default B9 configs result in costs and masses higher than stock parts have

B9_TANK_TYPE
{
  name = stockLFO
  tankMass = 0 // this adds to part mass, which is why it was heavier with default configs
  tankCost = 0 // this adds to part cost, which is why it was more expensive with default configs
  RESOURCE {
    name = LiquidFuel
    unitsPerVolume = 0.55
  }
  RESOURCE {
    name = Oxidizer
    unitsPerVolume = 0.45
  }
}
B9_TANK_TYPE
{
  name = stockLF
  tankMass = 0 
  tankCost = 0 
  RESOURCE {
    name = LiquidFuel
    unitsPerVolume = 1
  }
}


// stock parts of type LFO
//  note: Size3To2Adapter_v2 is a version 1.6 part
@PART[fuelTankSmallFlat|fuelTankSmall|fuelTank|fuelTank_long|Rockomax8BW|Rockomax16_BW|Rockomax32_BW|Rockomax64_BW|Size3SmallTank|Size3MediumTank|Size3LargeTank|fuelTank3-2|miniFuelTank|fuelTank4-2|fuelTank2-2|fuelTank1-2|adapterMk3-Mk2|adapterMk3-Size2Slant|adapterSize2-Mk2|adapterSize2-Size1|adapterSize2-Size1Slant|adapterSize3-Mk3|toroidalFuelTank|externalTankCapsule|externalTankRound|externalTankToroid|mk2_1m_Bicoupler|mk2_1m_AdapterLong|mk2SpacePlaneAdapter|Size3To2Adapter_v2]:AFTER[B9PartSwitch]
{
  MODULE {
    name=ModuleB9PartSwitch
    moduleID=fuelSwitch
    baseVolume = #$/RESOURCE[LiquidFuel]/amount$
    @baseVolume += #$/RESOURCE[Oxidizer]/amount$
    switcherDescription = Fuel Type
    switcherDescriptionPlural = Fuel Types
    switchInFlight = FALSE

    SUBTYPE {
      name = LF/Ox
      tankType = stockLFO
      allowSwitchInFlight = FALSE
    }

    SUBTYPE {
      name = Liquid Fuel
      tankType = stockLF
      allowSwitchInFlight = FALSE
    }  
  }
}

// stock parts of type LF-only (could add spaceplane parts if desired)
@PART[miniFuselage]:AFTER[B9PartSwitch]
{
  MODULE {
    name=ModuleB9PartSwitch
    moduleID=fuelSwitch
    baseVolume = #$/RESOURCE[LiquidFuel]/amount$
    switcherDescription = Fuel Type
    switcherDescriptionPlural = Fuel Types
    switchInFlight = FALSE

    SUBTYPE {
      name = Liquid Fuel
      tankType = stockLF
      allowSwitchInFlight = FALSE
    }  

    SUBTYPE {
      name = LF/Ox
      tankType = stockLFO
      allowSwitchInFlight = FALSE
    }

  }
}

// Making History parts of type LFO
@PART[Size1p5_Size0_Adapter_01|Size1p5_Size1_Adapter_01|Size1p5_Size1_Adapter_02|Size1p5_Size2_Adapter_01|Size1p5_Tank_01|Size1p5_Tank_02|Size1p5_Tank_03|Size1p5_Tank_04|Size1p5_Tank_05|Size3_Size4_Adapter_01|Size4_EngineAdapter_01|Size4_Tank_01|Size4_Tank_02|Size4_Tank_03|Size4_Tank_04]:NEEDS[SquadExpansion/MakingHistory]:AFTER[B9PartSwitch]
{
  MODULE {
    name=ModuleB9PartSwitch
    moduleID=fuelSwitch
    baseVolume = #$/RESOURCE[LiquidFuel]/amount$
    @baseVolume += #$/RESOURCE[Oxidizer]/amount$
    switcherDescription = Fuel Type
    switcherDescriptionPlural = Fuel Types
    switchInFlight = FALSE

    SUBTYPE {
      name = LF/Ox
      tankType = stockLFO
      allowSwitchInFlight = FALSE
    }

    SUBTYPE {
      name = Liquid Fuel
      tankType = stockLF
      allowSwitchInFlight = FALSE
    }  
  }
}

// Missing History parts
@PART[Size1p5_Size2_Adapter_02|adapterSmallMiniTall|Size1p5_Size2_Adapter_02|Size3_Size2_Tank]:NEEDS[MissingHistory]:AFTER[B9PartSwitch]
{
  MODULE {
    name=ModuleB9PartSwitch
    moduleID=fuelSwitch
    baseVolume = #$/RESOURCE[LiquidFuel]/amount$
    @baseVolume += #$/RESOURCE[Oxidizer]/amount$
    switcherDescription = Fuel Type
    switcherDescriptionPlural = Fuel Types
    switchInFlight = FALSE

    SUBTYPE {
      name = LF/Ox
      tankType = stockLFO
      allowSwitchInFlight = FALSE
    }

    SUBTYPE {
      name = Liquid Fuel
      tankType = stockLF
      allowSwitchInFlight = FALSE
    }  
  }
}

 

 

[putnamto]

or just the ability to fill a tank completely full of lf, or ox at your discretion.

[KerikBalm]

On 1/31/2019 at 12:01 AM, Nebbie said:

That would make spaceplanes a lot more complicated because you couldn't share fuel between the LV-N and jet engines.

On 1/31/2019 at 12:14 AM, lajoswinkler said:

LV-N is not something designed for spaceplanes in the first place.

However, LF could be used as a different mode of operation of LV-N, at a great expense of I_sp.

Well, speaking of that... The rapier engine is based on the Sabre engine in development, while the LV-N is based on the NERVA which was developed in the 60's and 70's. Both of these engines are designed to use liquid hydrogen as fuel. The Isp of the high end jet engines only makes sense if the liquid fuel is hydrogen:

https://en.wikipedia.org/wiki/Specific_impulse#Larger_engines

Using the J-58 of the SR-71 as a stand-in for the whiplash (turbo ramjet, which could describe with reasonable accuracy what the J-58 is) has an Isp of 1900 (I've seen 2100 in other sources). In game its 4000 Isp - about double real life

The EJ200 of the Eurofighter is a reasonable stand-in for the panther, with a wet Is pof about 2100, and a dry of about 4,500, in game its 4000 and 9000, again, about double real life.

Only the GE CF6 (as a goliath analogue) gets similar Isp  and only at sea level, not at high speed and altitude cruise- where it is again about half

The sabre: https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

in contrast should get 3,600 Isp, to the rapier's 3,200 Isp... it uses LF

The way to get all these engines to have the right Isps is to say they run on LF....

So the jet engines and Nervs could easily use the same propellant.

[AeroGav]

On 1/23/2019 at 1:55 PM, septemberWaves said:

I have two, somewhat conflicting, opinions on this. The first is that using large LFO tanks for nuclear engines and simply emptying them of oxidizer is a good enough option that I am satisfied with (at least for cases when not using Sigma Dimensions to make everything bigger), because real nuclear engines would likely use liquid hydrogen, which has low density so a large tank would not be able to store as much fuel as a comparably-sized tank for hypergolic fuels. It really just means that the fuel for NTRs necessitates bigger tanks, which I think is an acceptable tradeoff for the high efficiency.

The counterpoint is that the KSP aerodynamic model generates most drag from fuselage parts rather than wings,  and this tends to scale with tank volume.  You will get extremely high drag penalties operating a spaceplane with LV-Ns inside an atmosphere.

The counter-counterpoint is that you shouldn't be operating LV-Ns inside an atmosphere.    I'd retort that this is a game and whilst there is zero chance of the public accepting such a risk today,  if you assume Kerbals have attitudes of the early 1950s,  the technology itself could be made to work.   

As regards introducing extra fuel types / tank switchers ,  that does sound good.

Nuclear Thermal rockets can run on non cryo fuels, eg.  Ammonia/Hydrazine.   ISP drops to 600 or so,  but that's still quite an improvement on chemical engines that run off room temperature storables.  In reality I suspect that even in the case of a hypersonic waverider spaceplane powered by an NTR,  you'd still use LH2, but again the stock aero model does not like voluminous lifting bodies like Skylon/Venture Star,  so  a  denser propellant that can live in wing parts may be preferable in game.

Note, jet engines can also run off ammonia ,  but since a litre of ammonia only releases half as much heat as the same quantity of kerosene when burned in air,  ISP would have to be reduced further.   Then again,  a litre of ammonia requires less oxygen to burn than a litre of jet fuel, so the engine's high altitude performance might get some minor gains to offset this.

You don't want to be putting cryogens in wing tanks,  because a) high surface area means ludicrous boil off rates  and b) ice formation.

With all this talk about nerfs to jet engines and more restrictions on lv-ns,  it might be worth referring people to Project Timberwind,  a 1980s tech pebble bed NTR.    The TWR was still low compared even with a hydolox engine, but streets ahead of the NTR in game.

Game balance is such a complex subject,  the ripple effect of every change spreads far and wide.  The fact that stage recovery is almost impossible without mods forces people to ssto or disposable,  which makes nerfing engines or adding realism problematic.

[Salmon]

That's what craft file editing is for. You can also make it a subassembly so that you don't have to constantly go into the KSP directory, then the save, and so on.

[Aeroboi]

I'm against emptying fuel tanks. As someone who builds spaceplanes I wouldn''t like carrying the dry mass of half a empty tank.

A fuel switcher with a option for LF only should do.

And people can point there finger at mods for the time being but it is a very simple feature and stock should have it I agree on the original request, there should ultimately be more of anything stock wise.

[klesh]

14 hours ago, Salmon said:

That's what craft file editing is for.

 

What about Enhanced Edition users?

[Martian Emigrant]

On 1/23/2019 at 11:18 AM, Behemot said:

Since we don't really have an idea what is the idea behind the nuclear engines in KSP, I don't think so  Besides, they do not differentiate hydrogen, kerosine, hydrazine or anything else, it's all just liquid fuel/oxidiser, the one and only. Although having the remote fuel tank near the launch pad suggest it may be hydrogen, but who knows.

....

Hi.

Good point. I never thought of "What is it"?

I always imagined it as being kerosene, diesel, jp-4 or something like that.

Like most people I have used LF tanks past Eeloo. I guess that IRL that would make for a tank full of paraffin (Think white candle wax).

Quite unusable.

 

We also don't use ullage motors in the game (To bring the fuel to the bottom end of the tank).

Don't they all use ullage motor? Even a liquefied gas would need to be picked up by the turbo pumps.

As far as I know the Helium is just a non-reactive way to replace the volume lost by removing fuel. Not a way to keep it "Down there".

 

ME

[Salmon]

7 hours ago, klesh said:

 

What about Enhanced Edition users?

It probably wouldn't work, unless you could somehow send the craft file to someone with a laptop. I have no idea how you would get it out of the save folder though.