Gordon Fecyk

[1.9] Explodium Breathing Engines v1.6.0 beta: "Jet" engines for use on Eve [01 MAR 2020]

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1.5.0 Release for KSP 1.8.0

An updated Explodium Breathing Engines part pack is now available for KSP 1.8.0, from GitHub and SpaceDock. Confirmed to work on Ferram Aerospace Research Continued for KSP 1.8.0; the Razorforce Seven can still fly!

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@Gordon Fecyk, have you ever considering adding a hydrogen breathing variant?  It would be the same basic concept as your explodium breathing engines, but would work in hydrogen atmospheres.  Obviously gas giants would be one place they'd work, but I got thinking about this specifically for the planet Nara in JNSQ.  Nara has a hydrogen-helium atmosphere with a surface pressure of 40 atm.  At that pressure obviously none of stock rocket engines will work (not even Eve Optimized Engines).  So a jet engine that takes in hydrogen from the atmosphere and burns it with an onboard oxidizer might be the perfect solution to allow launches from Nara's surface.

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38 minutes ago, OhioBob said:

have you ever considering adding a hydrogen breathing variant?

It's been brought up once or twice. The chemistry of hydrogen is quite different, certainly a lot more explosive, with a low heat value almost five times that of 'explodium.' So in theory, higher efficiency.

I think I'd want to redo the math for a hydrogen variant, and have these engines and intakes switch modes. I'd also want to double check the density at datum levels of Jool (both versions) and Nara to make sure there's even enough of it to pull through a jet engine. For all of the pressure, there isn't as much of it. As an aside, I had other plans for Nara when the time came.

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Separate engines or mode-switchable ones?

I was thinking about the engineering effort it would take to build one of these things, then build a hydrogen variant, and then possibly build one that could switch combustion modes. I'd end up with an engine that would need to operate over an extremely wide range, from the comparably low power of hydrocarbon vapour to the stupidly high power of hydrogen, and not explode going one way or just going cold the other.

Assuming no one would want to take a single craft to say, both Saturn and Titan in RSS, or both Lindor and Hyugen in JNSQ, it makes more sense to me to make two sets of engines. But is there a gameplay case for a single set of parts that could do both?

--

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@Gordon Fecyk, if the two modes are different enough that it's unrealistic to engineer both into a single part, then I say make them separate parts.  I personally wouldn't expect that I could fly the same craft in two entirely different atmospheres like Titan and Saturn.  But that's just my two cents, for what it's worth.

Have you given any thought to how you'd place a constraint on the engine so that it works only in hydrogen atmospheres?

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16 hours ago, OhioBob said:

Have you given any thought to how you'd place a constraint on the engine so that it works only in hydrogen atmospheres?

I'd create new intakes that use ModuleResourceHarvester to harvest hydrogen, so they only work if it's a resource in the atmosphere. That's how the current explodium intakes work, so the engines don't work on Duna or even on Kerbin. And since the chemistry will be different I can make engines with better efficiency than the current ones.

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6 hours ago, Gordon Fecyk said:

I'd create new intakes that use ModuleResourceHarvester to harvest hydrogen, so they only work if it's a resource in the atmosphere. That's how the current explodium intakes work, so the engines don't work on Duna or even on Kerbin. And since the chemistry will be different I can make engines with better efficiency than the current ones.

OK, I see how you did that with ExpVapour.  I notice you already include support for several planet packs.  I presume you'll add JNSQ?  If not, we can always do it on our side.

For your information, here are the planets that I envision having methane in their atmospheres:
GPP - Otho (~1%), Gauss, (~2%), Catullus (~2%), Tarsiss (~8%)
JNSQ - Lindor (1%), Huygen (4% average, ~6% near surface)

And Eve has an unidentified mix of heavy hydrocarbons (similar to kerosene):
JNSQ - Eve (about 0% to 2% in troposphere)

These are the ones having hydrogen atmospheres:
GPP - Otho (~87%), Gauss (~83%), Nero (~91%), Catullus (~73%)
JNSQ  - Jool (90%), Lindor (86%), Nara (80%)

For JNSQ the abundances are stated in the planet configs.  For GPP I'm estimating based on mean molar mass.

Of course you can assume higher hydrocarbon abundances if necessary to make the engines work.  These are just the percentages I used when I did the atmospheres.

Edited by OhioBob

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So I was doing the math...

...and it appears engines breathing hydrogen and burning oxidizer might actually be less efficient than ones breathing hydrocarbon vapours.

Back at the ExV Wiki I described burning RP-1 with a lot of ambient O2 and got a pretty decent low heat value per RP-1 unit, which I'm treating as having a direct relationship to specific impulse - the hotter, the more efficient. I would compare this heat value to that of burning hydrocarbons in the air with onboard O2, and I got a 45% reduction in heat value, which I equated to a 45% reduction in specific impulse so I could keep the thrust levels the same as stock jets.

I also compared this to burning hydrogen with ambient O2 and got a low heat value about 2.5 times that of RP-1. This is where I figured hydrogen could be more energetic, and more efficient, than RP-1.

What I hadn't considered was the reverse problem, of ambient H2 and burning that with onboard O2. Jets are very efficient in reality because a little fuel goes a long way, and because you don't have to carry the heavier oxidizer around. The reverse problem actually requires a less explosive ambient fuel to be more efficient. Or so it seems.

My original C2H6 math went like this:

2 kmol C2H6 = 60 kg, 5 kmol O2 = 160 kg
60 kg C2H6 + 160 kg O2 -> 176 kg CO2 + 108 kg H2O + 3114 MJ energy
0.375 U C2H6 + 1 U O2 -> 1.375 U exhaust + 97.31 MJ energy

Compared to KSP's RP-1 analogue which produced about 215 MJ energy per KSP mass unit, this was about 45% less efficient, which is where I got the ISP values from.

The same math with ambient hydrogen seems to go like this:

2 kmol H2 = 4 kg, 1 kmol O2 = 32 kg
4 kg H2 + 32 kg O2 -> 36 kg H2O + 479 MJ energy
20 kg H2 + 160 kg O2 -> 180 kg H2O + 2395 MJ
0.125 U H2 + 1 U O2 -> 1.125 U H2O + 74 MJ

74 mega joules, even less efficient than 'explodium' with the same engine. 34%. 

This might get us out of the depths of Jool, or off the surface of Nara, but not much further.

Have I missed something?

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5 minutes ago, Gordon Fecyk said:

Have I missed something?

I don't know much about jet engines, but I do know rocket engines.  I presume it's basically the same.  I'll do some math to see what I can come up with.

 

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@Gordon Fecyk, I've done some calculations, but I went about it very differently than you.  I just made some basic assumptions about the environment and engines, from which I computed the specific impulse.  Maybe this will help.

Spoiler

Environmental Conditions
Air pressure:  1 atm
Air temperature:  200 K
Air composition:  90% H2 + 10% He, by volume

Engine Specifications
Oxidizer:  liquid oxygen
Air-oxidizer ratio:  1.4, by mass
Overall compression ratio:  40

Calculations
Combustor pressure:  40 atm
Combustor temperature:  1000 K
Overall specific impulse:  318 s
Specific impulse, LOX only:  763 s

Specific impulse is "ideal" without any losses.  Obviously a good bit of energy will go into spinning the turbines, but I don't have an answer on how much would be lost.

Lowering the air-oxidizer ratio (more oxidizer) increases the combuster temperature and overall specific impulse, but lowers the LOX-only specific impulse.

I haven't done any calculations for hydrocarbon.  I can do so if you think it would be helpful.

I can also change the environmental conditions or engine specs if you want to see what effect that has.

 

Edited by OhioBob

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Posted (edited)

For comparison, here are some calculations for a nitrogen-methane atmosphere:

Spoiler

Environmental Conditions
Air pressure:  1 atm
Air temperature:  200 K
Air composition:  94% N2 + 6% CH4, by volume

Engine Specifications
Oxidizer:  liquid oxygen
Air-oxidizer ratio:  10, by mass
Overall compression ratio:  40

Calculations
Combustor pressure:  40 atm
Combustor temperature:  1045 K
Overall specific impulse:  128 s
Specific impulse, LOX only:  1408 s

 

Edited by OhioBob

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10 hours ago, OhioBob said:

I don't know much about jet engines, but I do know rocket engines.  I presume it's basically the same.

You did pretty good from what I see on your posts here. It looks like you confirmed what I discovered, just finishing the math right down to the specific impulse numbers. You ended up with half the ISP using hydrogen as you did using methane for LOX-only.

I can put together a test version and see what happens.

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Posted (edited)

@Gordon Fecyk, while I can do the math, I'm far from an expert on combustion in jet engines.  What the math tells me is the more fuel rich we run the engine, the greater the Lox-only Isp (but the lower the overall Isp).  I took the LOX concentration down to the point where the combustion temperature was about 1000 K (I was afraid to go any lower than that).  What concerns me is if that's really enough oxygen to support combustion, particularly in the second case.  With the air-oxidizer ratio that I used, the mixture of gases in the combustor ends up being, 86.6% N2, 5.5% CH4 and 7.9% O2, by volume (mole).  That's a very low oxygen concentration.  I did the math like all that oxygen is burning with the methane, but would that happen in real life?  Could that small amount of oxygen find and react with all the methane before being discharged from the engine?  If in practice the combustion is incomplete, then my numbers are wrong.  Maybe more oxygen has to be added to assure adequate combustion.  And if that's the case, then the Lox-only Isp would plummet.
 

Edited by OhioBob

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Posted (edited)

I just went the other extreme and added twice as much oxygen as needed to fully oxidize the methane.  Hopefully that's enough to assure complete combustion.  The results are far different.

Spoiler

Environmental Conditions
Air pressure:  1 atm
Air temperature:  200 K
Air composition:  94% N2 + 6% CH4 by volume

Engine Specifications
Oxidizer:  liquid oxygen
Air-oxidizer ratio:  3.5542 by mass
Overall compression ratio:  40

Calculations
Combustor pressure:  40 atm
Combustor temperature:  1319 K
Combustor O2 concentration:  19.4% by volume
Overall specific impulse:  142 s
Specific impulse, LOX only:  646 s

In this case the mixture of gases in the combustor are, 75.8% N2, 4.8% CH4, and 19.4% O2 by volume.  Those numbers look much more like I'd expect from a jet engine operating on earth.

If we were to flip the numbers around and assume we were injecting fuel into an nitrogen-oxygen atmosphere, then we'd have:

Air-fuel ratio:  35.335, by mass
Specific impulse, fuel only:  5155 s

Those numbers seem reasonable to me.

 

Edited by OhioBob

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The case of the hydrogen engine is different than the methane engine in that there is a great abundance of fuel.  The injected oxygen is not going to have any problem finding plenty of hydrogen to react with.  But if the oxygen concentration is too low, is it possible that the engine could actually blow itself out and not sustain combustion.  I don't know the answer to that.  With the numbers I used, the oxygen concentration in the combustor is only 4% by volume.

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I just discovered that I use GOX in my last couple calculations instead of LOX.  I've fixed it.

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2 hours ago, OhioBob said:

I did the math like all that oxygen is burning with the methane, but would that happen in real life? 

What I've been doing here, is finding out what the overall heat value was from each reaction, coming up with a percent comparison to what KSP jet fuel (RP-1?) did, and then adjusting the stock engines' ISP based on the percentage difference. I figured the stock jet engine efficiency was based on the type of jet and its intended role. The Goliath, for instance, is far more efficient than the other jets and exceeds the ideal ISP I came up with for RP-1 to O2. The other engines were less efficient than the ideal ISP, and any deviance from the ideal ISP I could explain as inefficiencies in each design, or less-than-optimal combustion products (carbon monoxide, for instance) that would produce less energy or no energy. 

So to keep things simple I just correlated heat value with efficiency. This is where that 45% value I came up with came from. I don't know if this would work in reality, but it seems to work for this simulation for game play's sake so these engines don't seem overpowered. If I had my way, I'd modify Advanced Jet Engines to get the real life thrust, but that seems hard-coded to work with ambient oxygen rather than ambient fuel and would take considerable effort to re-code for this.

I tested a hydrogen jet a moment ago with two modified Hades (Juno) engines and intakes, and used the 34% ISP value I came up with here; about 2100 seconds. It seemed to work in stock Jool's atmosphere fine, but as I expected the air densities I encountered were maybe half of those I found on Eve in spite of the tremendous pressure. This meant less mass to pull through the engines, which in turn meant less thrust. I'll confirm with Realistic Atmospheres Jool later today, but it shows promise. At least I can get thrust.

I'll take 34% for now, just to knock out a set of parts for testing.

But thanks so much for doing the math you did, and taking time out for it. 

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Posted (edited)

@Gordon Fecyk, it sounds like you don't need it, but I was already in the process of doing this, so I might as well post it.  This is the hydrogen engine with the oxygen content bumped way up.  I set the LOX/H2 ratio to 4, which is 1/2 the stoichiometric ratio.

Spoiler

Environmental Conditions
Air pressure:  1 atm
Air temperature:  200 K
Air composition:  90% H2 + 10% He, by volume

Engine Specifications
Oxidizer:  liquid oxygen
Air-oxidizer ratio:  0.3055, by mass
Overall compression ratio:  40

Calculations
Combustor pressure:  40 atm
Combustor temperature:  2935 K
Combustor O2 concentration:  18.5% by volume
Overall specific impulse:  378 s
Specific impulse, LOX only:  494 s

That temperature is awfully high, cooling could be an issue.

 

Edited by OhioBob

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Posted (edited)

I wish I had seen this earlier.  This article says that a minimum oxygen concentration to burn hydrogen is 5%, and to burn methane is 12%.  Using those numbers I compute:

Spoiler

Environmental Conditions
Air pressure:  1 atm
Air temperature:  200 K
Air composition:  90% H2 + 10% He by volume

Engine Specifications
Oxidizer:  liquid oxygen
Air-oxidizer ratio:  1.3164 by mass
Overall compression ratio:  40

Calculations
Combustor pressure:  40 atm
Combustor temperature:  1048 K
Combustor O2 concentration:  5% by volume
Overall specific impulse:  323 s
Specific impulse, LOX only:  749 s

Spoiler

Environmental Conditions
Air pressure:  1 atm
Air temperature:  200 K
Air composition:  94% N2 + 6% CH4 by volume

Engine Specifications
Oxidizer:  liquid oxygen
Air-oxidizer ratio:  6.2554 by mass
Overall compression ratio:  40

Calculations
Combustor pressure:  40 atm
Combustor temperature:  1435 K
Combustor O2 concentration:  12% by volume
Overall specific impulse:  149 s
Specific impulse, LOX only:  1083 s

 

Edited by OhioBob

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Gas Giant Version Ready

I put together a beta release of Explodium Breathing Engines for use on stock Eve and Jool. This works with Alien Space Programs with Jool as the home world, or with craft sent to stock Jool. I have support for additional planet packs for both engine and intake types. I created a new atmospheric resource based on OhioBob's data so I wouldn't interfere with Rational Resources or the Community Resource Pack, nor should this require either of these.

This release still needs localization and a different set of textures to identify the "explodium light" versions of these parts. I'd like to try making variants for the tanks using the variants module, though this might not work for every part.

--

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