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
========== 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
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