[Enthalpy]

Hello imaginative chemists and everyone!

========== Gas generator rocket cycle

Rockets engines need a big chamber pressure so the nozzle transforms much combustion heat into gas kinetic energy. Light tanks at low pressure usually carry the liquid propellants that many-MW turbopumps bring to the injection pressure. In the "gas generator cycle", a few % of the pumped propellants feed an auxiliary chamber that provides gas to the turbine.

The rudimentary rocket turbines accept some +600°C, normally obtained from a very reducing mix ratio as designers didn't dare hot oxygen. With most fuels, huge proportions of soot are sent to the turbine, inefficient and hampering the engine reuse. The Viking-Vikas engine instead adds water to a balanced combustion: no soot, bigger expansion speed from the same temperature, but the engine has one pump more.

The main and auxiliary chembers need igniters. Hypergolic pairs of propellant ease this, but they must ignite swiftly upon contact to avoid accumulations in the chamber before ignition, and as you guess, everyone wants to phase out Nto (dinitrogen tetroxide) and hydrazine, Mmh (monomethyl-), Udmh (unsymmetrical dimethyl-). One method for liquid oxygen and hydrocarbons injects trimethylgallium and trimethylaluminium, stored under pressure and pyrophoric near 1000t propellants, oops.

The turbopump must also start despite the turbine exploits the pressure created by the pump it moves. Usually, the oversized igniters provide gas, and the pressures build up gradually. Sometimes this fails.

========== Pressure-fed auxiliary propellants

I proposed instead to store the auxiliary propellants under pressure in separate tanks
  scienceforums or see the appended diagrams.
Valves then start and restart the turbopump. Small tanks of graphite fibres (or more recent) add very little mass, obvious choice.

The main propellants must be efficient, but now the auxiliary propellants can differ and be hypergolic. Then they reliably double as igniters for the main chamber, and possibly as an attitude and orbit control system (Aocs) and as separation thrusters for the rocket stage. The complete stage becomes simpler than with a separate Aocs.

========== Criteria for the auxiliary propellants

• The amounts are smaller and the cost per kg easier.
• No detonation. This excludes even nitromethane for instance.
• As healthy as possible. Get rid of the carcinogens and the poisons.
• Catalysts welcome in the auxiliary chamber or propellants.

Hypergolic ignition is desired but pyrophoric is unwanted. A disaster needs then two leaks instead of one. Even the flash point should please exceed the tank temperature, like +55°C. Separate oxidiser and fuel shall be easier than a monopropellant here. Chemists' knowledge and imagination please jump in!

Liquids at room temperature are preferred. But I could imagine a hot fuel if the tank's material keeps light or the volume is tiny. Maybe a cryogenic oxidizer.

Performance matters, but less at the auxiliary propellants. The limited turbine temperature also reshuffles the ranking: from the same +600°C, water carries more enthalpy and weighs less than other products (hydrazine, no, thanks). Then the propellants that heat the water are in small amount and their nature influence less the performance.

I prefer when one or both propellants bring all the water and their best mix ratio produces gas directly at the turbine temperature, left diagram. This is safer for the turbine and avoids margins, so +700°C are possible and more efficient. Or a separate water injection, center diagram, can improve the igniter, the Aocs, and it ease the propellants choice. The fuel could also be injected in steps to re-heat the auxiliary gas between turbine stages, right diagram, and gain 6s specific impulse so difficult to obtain from the main fuel.
  description - example with figures

========== First auxiliary propellants candidates

Bombardier beetles do what rockets need. They use dilute hydrogen peroxide and dilute hydroquinone, catalase ignites the pre-mix >100x per second despite the products are at +100°C only
  Bombardier beetle - Catalase - Peroxidase - Hydroquinone at Wiki - witpress

I strongly hope that Fe or Mn oxides in the chamber ignite peroxide and hydroquinone swiftly as our products reach +600°C. First estimates want nice 60% water in the mix: peroxide <64% can't detonate.

More thoughts to come about the auxiliary propellants.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Small oops: water dissolves only 59g/L hydroquinone. Forget the 1b carcinogen catechol. But:

• CCCOCCOCCO diethylene glycol monopropyl ether dissolves 30wt% hydroquinone
• It offers mp<-90°C fp=+93°C, wow.
• Completely miscible with water.
• It might react early with hydrogen peroxide, and brings ig=+204°C that hydroquinone lacks. Try it alone too.
• 4mPa×s are imperfect; a diether would improve but I didn't find solubilities. Or add little water.

  Hydroquinones solubilities - Eastman DP solvent at Eastman, thanks!

[Enthalpy]

As an alternative to dissolved hydroquinone for the auxiliary fuel, I propose a tetralin, isotetralin, 1,4-cyclohexadiene, or a mixture. Better, their 1-aldehydes, which lower the mp by 70K, raise the flash point by 70K, and lower the autoignition from +500°C to +200°C. A mixture, preferably eutectic, shall raise the estimated +54°C fp of cyclohexadienealdehyde or lower the estimated -20°C mp of isotetralinaldehyde. The tetralins bring more hydrogen per mass unit than cyclohexadiene.

Are these hydrogen donors as good as hydroquinone? They ship "stabilized by 0.1% hydroquinone" but a team hydrogenated quinones in 24h at +30°C from 1-methyl-1,4-cyclohexadiene
  pubs.acs.org
Synthesis of 1-naphthaldehyde
  Orgsyn
Birch reduction gives 1,4-cyclohexadiene and isotetralin = 1,4,5,8-tetrahydronaphthalene. Catalytic hydrogenation of naphthalene gives tetralin = 1,2,3,4-tetrahydronaphthalene.

I described a sturdy catalyst there, using a honeycomb winding of wire sintered together
  scienceforums
Nickel superalloys resist the +700°C from oxidizer and fuel together, Ni catalyses the hydrogen release while Fe and Mn oxides, preferably by metal deposition on some wires and oxidation, decompose H₂O₂. The catalyst in the mixture works for the bombardier beetle.

Some data sources:
  Benzene - Benzaldehyde - Naphthalene - Naphthaldehyd - Naphthaldehyd at en.wiki, de.wiki, cz.wiki
  1,4-Cyclohexadiene at Nist - sigmaaldrich - it.wiki
  Tetralin at sigmaaldrich

Marc Schaefer, aka Enthalpy

[Enthalpy]

Hydrogen peroxide or oxygen react at room temperature with known compounds. You chemists certainly know more, so please complete!

========== Lophine, lucigenine

Any better at a rocket than the others here under?

========== Oxalates

Oxalate of diphenyl and more reacts with H₂O₂. Maybe not very quickly. Catalysts?

     Diphenyl  Dimethyl  Diethyl  Dipropyl  Diisopropyl  Dicyclpropyl
----------------------------------------------------------------------
bp              Methy-   +185°C    +214°C     +189°C
fp             lating     +76°C  est +105°C  est +80°C
mp    +136°C    agent     -41°C
----------------------------------------------------------------------

Diethyl oxalate has an excellent liquid range, others may ignite better. Asymmetric compounds are welcome, their mixtures too: mix the reactants at production.

========== Luminol

The temporary mix with H₂O₂ is "stable", then traces of iron in blood release O₂ that reacts with luminol to emit light. I hope spraying both on Fe or Mn oxides makes a fast reaction.

========== Tetrakis(dimethylamino)ethylene or TDAE

C10H24N4 200,32g/mol CN(C)C(N(C)C)=C(N(C)C)N(C)C. It reacts with O₂ even from air at RT, hopefully always at a safe pace.

Plain dimethylamino has mp=-4°C, ρ=861kg/m³, fp=+53°C, estim bp=+237°C @1atm. Mixing some ethylmethylamine at the production, maybe some azetidine, must improve the mp and the fp.

"The reaction between TDAE and gaseous oxygen becomes uncontrolled by 70°C and can result in fire or explosion"
  nl.wiki
64% H₂O₂ doesn't detonate alone but provides O₂ at +100°C, with as many moles H₂O that hopefully don't hurt. More reactions will start anyway, and catalysts can help.

I estimate by hand ΔH_f=+39,8kJ/mol (liq 298K), less accurately than usual. This makes TDAE (rather the mixtures) an excellent fuel (if oxygen cools the engine), 2s better than RG-1 "kerosene". If safe and affordable, TDAE can excel at all roles: turbine feed, igniter, Aocs, separation rocket, main fuel for an upper stage. Wow!

More compounds bring electron-rich multiple bonds, like diazetidylacetylene. How safe and efficient remains to see.

========== Oxygen replaces peroxide?

H₂O₂ is dangerous and not storable for long. 64% peroxide is also a less efficient propellant than O₂. Since luminol and Tdae react with O₂ instead, could O₂ warmed around RT start the flame?

If a  catalyst suffices, perfect. I described a sturdy one there
  scienceforums
Sinter a honeycomb coil of superalloy wires together, deposit Fe or Mn or what it takes, oxidise as needed.

If moderately warm oxygen ignites luminol or TDAE, I described glow plugs for a pilot flame, whose heat capacity save electricity, perfect for an Aocs or a lander, there
  nasaspaceflight
Less simple than a catalyst, but the sensitive fuels let consume less electricity than usual ones.

Marc Schaefer, aka Enthalpy