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Gas Generator Cycle for Rocket Engines - Variants


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Hello everybody!


The gas generator cycle is widely used on rocket engines, but some variations are still possible and desireable. For instance, the Falcon launcher burns the derived flux with much kerosene and little oxygen to limit the turbine's temperature, but the resulting soot complicates the reuse of the engine.


One alternative, similar to Ariane's Viking, would add water to a tuned mix of RP-1 and oxygen, injected in two steps. 107:10:34 results in 873K=600°C; expansion from 50bar to 5bar gives 1133m/s. Good solution with one pump more.

If accepting additional propellants, (yuk!) hydrazine or MMH, hydrated for 600°C, give 2004m/s and 1696m/s - not fully useable by a turbine. Methylamine (yuk as a volatile amine, not carcinogenic like hydrazines) gives 400°C and 1552m/s if its decomposition is even. Hydrogen peroxide, 84% for 600°C, gives 1099m/s but can detonate.


I consider here a solid gas generator. It starts more easily, and mobile inlet vanes at the turbine can control the generator's pressure hence "burning" speed through the pressure exponent. Blowing upside makes debris less critical.




One could add gelled water to smokeless powder. For instance 123:70:30 water, nitrocellulose and nitroglycerine gives 873K=600°C and 1125m/s. Expect some particles from the gel in the gas.

One solid that makes moderate temperatures and no soot is guanidine: pure, it gives 917m/s and 569K=296°C - supposedly too little for a stable "flame". Guanidine may also be brittle. It's a deliquescent and caustic base that melts at +50°C. I take -56kJ/mol from a single source.




Here are some additives to make a hotter and more stable "flame" and to make the block tougher - hopefully... Only experiment can tell. The mix ratio here is the soot limit.




Guanidine tetramethyl, EDA (ethylene diamine) and DETA (diethylene triamine) are liquids. The hope is that they can dissolve or impregnate guanidine, evaporate partially if needed, and leave a shock-proof gummy thing that "burns" better.

PE (polyolefine) would just cover guanidine grains, to bind them together and as a moisture barrier. Maybe the polyolefine is first dissolved in a small alkane that evaporates. Other polymer candidates, like varnishes, must bring little oxygen, and resist the strong base.

If guanidine reacts with formaldehyde (or with ethylene glycol) like melamine does, and the water can be removed, then some "di-condensate" (isomers) shall raise guanidine's melting point and the "flame" temperature. In contrast, the cross-linked polycondensate macromolecule could be formed at the surface of guanidine grains, as a moisture barrier, flame improver, and binder or binder primer.

Nitroguanidine brings heat and performance; combine with the previous ones if possible.


Pumping 1t of RP-1 and oxygen to 100bar, with 74% 79% 88% efficient pumps, turbines and injectors, needs 37kg of gas at 1000m/s, which still blows at 750m/s after expanding from 5bar to 1bar. A solid gas generator adds only an envelope; at 700MPa for 50bar and 8200kg/m3, the envelope weighs some 4kg.

Marc Schaefer, aka Enthalpy



Despite its N-N bond, Aminoguanidine (=Pimagedine) isn't toxic like Hydrazine, Mmh and Udmh are: test persons ingested 300mg/day when it was developed as a drug.
1720kg/m3 (wow!) and bp=+261°C, mp unknown.




All are corrosive, caustic and deliquescent. Guanidine is worryingly volatile, but Amino and Diamino-guanidine little so. They bring more hydrogen for the same carbon, and much heat of formation, which I estimate to +60 and +177kJ/mol for the solids at 298K. Their hotter recomposition would make some soot instead of methane, worse at moderate pressure, but blends can avoid it and achieve an excellent gas speed.




The blends with Guanidine are still volatile: consider coating them as previously suggested. More seducingly, tiny additions of water or ammonia suffice to avoid soot and, given their affinity for the strong polyamine, must build a solid blend. In the list, 700°C is the fuzzy limit of nickel alloys, but the molybdenum TZM alloy (which could improve the turbine of any pumping cycle) could reach 1100°C and obtain 30% more power, saving gas generator mass or improving the main chamber's pressure.


Water could be gelled, but I hope a better process can produce the propellant block. A carrier gas would bring vapour or ammonia between the grains of Amino- or Diamino-guanidine where they would settle and soften the surface; the block would be pressed, possibly in the final cartridge; water or ammonia would later diffuse to the depth of the grains to achieve a solid, while a light carrier gas like hydrogen or helium would exodiffuse. The cartridge can then be sealed for storage and be the pressure vessel at the rocket.

This all depends on how the aminoguanidines recompose. Additives can help as usual, and fusible fibres can toughen the block.

Aminoguanidines may serve in liquid blends as well, but more usefully in a staged combustion cycle, so I'll put a word there.

Marc Schaefer, aka Enthalpy

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For a gas generator that recomposes a single liquid, here are mixture attempts based on ethylenediamine (EDA). Solubilities and resulting viscosities are unknown. Solutes use to improve liquid ranges.

Ammonia is dissolved, the minimum that avoids soot at 80bar; unpleasant enough, and more would reduce the performance.

The optional 6% water bring pure ethylenediamine's melting point from +11°C to a -1°C eutectic; more would worsen. Dissolving loses 11cal/g heat of formation but saves ammonia and should raise the flash point from +43°C and reduce the vapour pressure, includig from ammonia. The effect on viscosity is unclear.

Guanidine loses some expansion speed put increases the density, so that more pressure can compensate, and saves ammonia. Aminoguanidine gains expansion speed and recomposition temperature, but needs more ammonia. Diaminoguanidine even more so. Less diaminoguanidine outperforms more aminoguanidine. Nitroguanidine would need much ammonia, more so than diaminoguanidine for the same performance.




The expansion speed is from 80bar to 1.6bar as throttled from 120bar, a good value for graphite-fibre tanks and helium pressure feed that helps start and restart.




Nickel alloys withstand the recomposition temperature, which exceeds EDA's autoignition in air (385°C) but is little for a quick reaction; injection against the gas stream may help - expect hotter regions then. Staged injection favours a stable recomposition. The helium tank is smaller if coupling it thermally with the oxygen tank. The glowplugs can be from a Diesel engine if big enough.

Marc Schaefer, aka Enthalpy

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Hello everybody!


The gas generator cycle is widely used on rocket engines, but some variations are still possible and desireable. For instance, the Falcon launcher burns the derived flux with much kerosene and little oxygen to limit the turbine's temperature, but the resulting soot complicates the reuse of the engine.


One alternative, similar to Ariane's Viking, would add water to a tuned mix of RP-1 and oxygen, injected in two steps. 107:10:34 results in 873K=600°C; expansion from 50bar to 5bar gives 1133m/s. Good solution with one pump more.


If accepting additional propellants, (yuk!) hydrazine or MMH, hydrated for 600°C, give 2004m/s and 1696m/s - not fully useable by a turbine. Methylamine (yuk as a volatile amine, not carcinogenic like hydrazines) gives 400°C and 1552m/s if its decomposition is even. Hydrogen peroxide, 84% for 600°C, gives 1099m/s but can detonate.


I'm not much of an expert in rocketry, so can you explain some of the jargon here? What do you mean when you say that a particular fuel (or rather probably its combustion) "gives a certain temperature and velocity"?


I thought I understood that the temperature is the temperature you reach in the turbine due to the decomposition of the fuel (a temperature which is limited, so you don't break the turbine)? But what then do you mean with that velocity? What is going that fast, and where? And wouldn't that velocity depend on some more design parameters that you did or did not introduce here?

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Oops, you're right.




Combustions use to be too hot for turbines. Typically over 3000K with liquid oxygen, while nickel alloys work at 700°C. A main combustion chamber can have its walls cooled by a propellant flown through a jacket, but at a rocket turbine, this would be too complicated (it's done at aeroplane engines to gain ~100K operating temperature).


This is a big difficulty at rocket turbines. One answer is to detune the propellant ratio there. Easy with hydrogen, difficult with hydrocarbons: more fuel makes much soot before the temperature is bearable, more oxygen tends to burn the metals, the joints, everything.


What's worse, a flame is difficult to stabilize at such a low temperature - one answer is to burn hot and dilute later - and the mixture and flame can be heterogenous, with some spots dangerously hotter, or with hotter periods over time.


So a single propellant - which avoids the uncertainties of mixing - the produces naturally a mild temperature would be welcome, even more so if it doesn't soot. This is the case with hydrogen peroxide (used from the V2 to the Soyuz) and with hydrazine, but both are dangerous. The amines I suggest are caustic, but at least no brutal carcinogens, and hopefully won't detonate.




The speed I indicate is for unhindered gas expansion, in this case from 80bar to 1.6bar because I found such pressures meaningful.


The speed essentially indicates how much mechanical power is available in a mass unit of the produced gas, hence how much of this additional flux is required to rotate the pumps for the main propellants.


In some turbine designs, the gas is completely expanded before the turbine rotor, where only the direction change provides a force, a torque and a power. There, this speed can apply directly.


In many designs, the expansion occurs in part in the rotor, or is done over several stages. Then the speed indication tells a designer if the turbine speed will match easily what the turbine and pump materials survive (that's a big worry with hydrogen) and if it will fit easily what the pumped propellants need. If not, multiple stages may bridge the gap, more or less well.

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True, it's not directly a turbine speed. It can be a gas speed under some circumstances, not uncommon, where the turbine is of Laval type.


It does tell, through m*V2/2, how much work is extractible from this secondary flow - before the turbine introduces its losses.


It also tells very much how the turbine(s) must look like. For instance, if one expands hydrogen (heated to 600°C by a little bit of oxygen) from 100bar to 5bar, it acquires 2792m/s, while a flat Inconel disk explodes around 460m/s, and an oxygen centrifugal pump needs 120m/s - by seeing such figures, a turbopump designer grasps that the turbine will be inefficient even with several stages.

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Ammonium dinitramide "ADN" (NH4)+[N(NO2)2]- is a fashionable solid oxidizer:
H4O4N4, 1810kg/m3, Hf298=-148kJ/mol but dissolving adds 36kJ/mol, mp+93°C. Solubility in water is 3.57kg/kg.

It's a seducing replacement to ammonium perchlorate in solid grains and a discussable replacement for hydrazine and tetroxide - I'd switch to electric propulsion or to oxygen. It looks good here as an auxiliary gas generator to rotate a turbopump.

It brings H, O, N so its recomposition makes sootless gas, at 1800°C so the turbine demands dilution - hence as a liquid propellant.


Here stoechio amounts of fuels are added in water. All fuels (ammonia, diaminoguanidine, EDA) gave me the same performance since the gas is mainly vapour: safety and practical consideration (melting point) can decide.


These mixes make hotter gas than those based on EDA for the same performance, but they would achieve temperatures fitting (hypothetical) molybdenum turbines, and be then more efficient. They have a lower vapour pressure and are also denser.


Now a fuel-richer mix produces methane and some hydrogen in addition to vapour. The expansion speed improves, but the propellant is volatile and possibly viscous, provided so much ADN dissolves. Water and ammonia prevent soot here.


The previous mixes are safer.


The previous injector scheme may keep useful.

Other solid oxidizers are allegedly better
but I got solid data for ammonium dinitramide.

Marc Schaefer, aka Enthalpy

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In gas generators that recompose a liquid, added ethylenediamine and ammonia have drawbacks (toxic vapours, viscosity, melting point) hard to guess on the paper. A more seducing option is to combine the dinitramide with a cation heavier than ammonium.

Methyl-, trimethyl- and dimethyl-ammonium dinitramide improve the performance and keep a reasonable pH. Dissolved in water for 700°C or 1100°C, they expand to 1550 and 1850m/s, while from 700°C, added ethanol must thin them, and from 1100°C, gains 100m/s if the mix isn't too thick nor flammable.

Guanidinium isn't so good, but di- or tri-aminoguanidinium nitrate replacing 10% dimethylammonium dinitramide avoids alcohols and keeps 1900m/s from 1100°C. Tri- or di-aminoguanidinium dinitramide must be more soluble and efficient, if practical.

These aqueous gas generators are second to toxic hydrazine but outperform the explosive hydrogen peroxide by far.

Marc Schaefer, aka Enthalpy

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  • 5 months later...

On May 01, 2014, I proposed to recompose mainly ethylenediamine in a gas generator (good for a staged combustion as well) with some dissolved ammonia to avoid soot. Ethylenediamine is a bit volatile and corrosive, so I checked if 1,2,3-triamino-propane (a ligand known as Trap) could replace it. It can, but needs 2-3 times more ammonia than ethylenediamine does, so I doubt the triaminopropane mixture improves the vapour.

Though, triaminopropane in smaller proportion could serve as an antifreeze in ethylenediamine.

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  • 2 months later...

A deplorable accident reminds us that the F-16 (and seemingly others, as toxicity was feared after a Rafale accident) uses hydrazine in its emergency power unit. No compressor: the pressure-fed liquid (could be a solid) decomposes without oxygen to produce gas at a temperature bearable by a turbine.

I proposed here many monopropellants as a gas generator to rotate the pumps of a rocket without the bad hydrazine. A few should hopefully work and would feed the emergency power unit just as well - it's the same task. Upgrades!

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  • 3 weeks later...

sorry for my bad english

and this is properly not the right thread for this type of question ;)


i thinking about a alternative for a rocket turbo pump motor (just thinking)

i shoud be easy to construct, reliable and "cheap"


i ended with a combustion engine :D fed with compressed air (nitrogen/ oxygen mixture) and gasoline of corse


sounds stupid, but i think i coud work.


a very simplified calculation:


e.g. a 1l 4-stroke engine engine shoud breathes @ 10k rpm 5000l air per minute, burning duration 240sec -> 10000l


air can easy compressed to 300bar (as far as i know) -> a ~34l pressure bottle (and small amount of gasoline) should be enough for 240 sec run-time.


for performance tuning of the combustion engine the use of slightly more oxgen in mixture than normal air shoud work to (until the motor burns ;) )




- relatively lightweight

- reliable

- easy to constuct (modification of existing motors)

- easy throttling and restating

- relatively high efficiency ?!


complete nonsense or something that might work ? (e.g. for projects like http://copenhagensuborbitals.com)


some (usefull) respond would be nice!


best regards

Edited by jim beam
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Hi Jim!


Your English works, no worry - and it isn't my mothertongue anyway, so I'm just happy that we share a language.


350 bar is a standard pressure, and you could store pure oxygen, provided you keep much of the CO2 and H2O in the cylinder during exhaust and suck little oxygen at each cycle, to limit the temperature. Similar things are done on torpedoes.


Piston engines are not used at full-size rockets because:


They're too heavy! For instance the RD-180' turbopump on Atlas has 100MW shaft power (135,000 horse power). It weighs under 1 ton and is as tall as a person. This boat


has a 70MW piston engine, cute picture there


that's how turbines and pistons compare... Or just have a look at a compressed car engine: the turbocompressor has about the same power as the rest, yet is tiny.


Turbines are simpler and more reliable than piston engines. That was the basic reason for airliners to switch to gas turbines.


Existing piston engines are too small for a launcher, so reusing a design is no option. For a much smaller rocket, it's better to put pressure in the propellant tanks, or to use an electric motor plus batteries. Electric motors are much lighter than piston engines and batteries are lighter than a pressure tank for air or oxygen. Then, pumps for propellants also are of centrifugal type because of the mass, and an electric motor fits their rotation speed while a piston motor doesn't.

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  • 6 years later...

I suggested to re-heat the work fluid between two turbine stages at an expander cycle
and the same applies to the staged combustion cycle and the gas generator cycle. The gas generator cycle benefits most because its strong expansion cools much the working fluid.


The red propellant can be an oxidizer like liquid oxygen, pink hydrogen peroxide about 86% for turbine temperature, green n-dodecane, blue a fuel. The pink and green propellants can be pressure-fed for easy start and restart. The amount of green propellant adjusts the ratio of red and blue propellants.

Cycle variants:

  • Coupled shafts, or a single shaft, enable balanced expansion ratios, more efficient.
  • Turbining first for the more bulky propellant consumes minimally more green propellant but leaves slightly more thrust at the dump nozzle. Other reasons can decide.
  • Some green propellant could be added right before the dump nozzle to push slightly more. Other reasons can decide.

Propellants variants:

  • Peroxide (pink) only 70% concentrated can't detonate and can combine with a fuel, preferably hypergolic.
  • The green propellant preferably auto-ignites at low temperature. Linear alkane, long amine...
  • As pink propellant, recomposing amines are as efficient and maybe safer than peroxide, as I described elsewhere - if they recompose where desired. The green propellant is then an oxidizer, maybe nitric acid.
  • The main propellants could make the working fluid, but a bearable temperature often demand a dirty and very inefficient mix.

Other functions:

  • Pressure vessels can host the pink and green propellants. A catalyst can then start and restart the engine by merely opening valves.
  • Derived pink and optionally green propellants can also ignite the main chamber, by contacting an other or over a catalyst.
  • Membranes in the pressure vessels let start the pumps in zero gravity. The dump nozzle can suffice to push the main propellants in the tanks' outlet.
  • The pink and optionally green propellants can also provide attitude and orbit control to an upper stage, even after the main engine stops.

Marc Schaefer, aka Enthalpy


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  • 3 weeks later...

Here's an example of reheated gas generator cycle with peroxide as in the last message.

64% concentration (70% can detonate, my mistake) burns a fuel to heat the decomposition. This combination is as efficient as peroxide dangerously concentrated to achieve alone the same temperature. The fuel in tiny amount can be chosen for practical reasons: safety, contact ignition with peroxide, autoignition temperature, identical to a main propellant.

The turbines are in series hence not developed independently. It worked for the Vinci.


Scalable 1kg/s peroxide plus 30.5g/s Pmdta make 973K=700°C in the auxiliary flux, expansion from unoptimized 80bar to 2bar accelerates to cumulated 1402m/s or 1013kW harvested by a turbine. With reheating, compare:

  1. The first expansion to 20.6bar accelerates to 958m/s or 473kW.
  2. A turbine stage harvests the expansion.
  3. Additional 13.6g/s Pmdta reheat to 973K=700°C.
  4. Expansion to 6.0bar accelerates to 918m/s or 440kW.
  5. A turbine stage harvests the expansion.
  6. Additional 12.7g Pmdta reheat to 973K=700°C.
  7. Expansion to 2bar accelerates to 872m/s or 402kW.
  8. A turbine stage harvests the expansion.
  9. Expansion to 1bar provides some thrust for roll control.

1315kW are available instead of 1013kW but added Pmdta makes the auxiliary flux 1.026* as heavy. At identical auxiliary consumption, 1.266* as much power multiplies the chamber pressure to gain 6s Isp. A single re-heat would gain 4s.

6s is as much as a super-fuel brings, it does carry more payload. Adding a fuel allows a peroxide concentration that doesn't detonate, by itself very useful. If the auxiliary propellants ignite by contact, the engine starts and re-starts easily. A separate turbine to pump the main fuel lets adjust its proportion by the amount of auxiliary fuel. Two turbine stages provide a better speed for the oxygen pump.

At a gas generator cycle for hydrogen, reheating brings only 3s because added oxygen makes the auxiliary flux quite heavier.

Marc Schaefer, aka Enthalpy

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