Chemical Forums

Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Enthalpy on June 10, 2018, 09:51:28 AM

Title: Rocket oxidisers
Post by: Enthalpy on June 10, 2018, 09:51:28 AM
Hello everybody!

The choice of a rocket odixiser is even narrower than for rocket fuels:
This is what survived from thousands of investigated miracle oxidisers, so new claims meet scepticism. It shouldn't prevent trying: aeroplanes flew after many disappointments.

The PhD thesis of Martin Rahm is a source of data for nitramides:
Ammonium dinitramide NH4N(NO2)2 page 14/87
Trinitramide N(NO2)3 page 64/87, he synthesized it.
Title: Re: Rocket oxidisers
Post by: Enthalpy on June 10, 2018, 09:53:14 AM
The only composition used in solid launcher stages is ammonium perchlorate, aluminium and polybutadiene. Progress results from graphite fibre casings and from more powder volume mixed in the prepolymer.

One promising solid oxidiser, ammonium dinitramide, has already a production plant. Replacing the perchlorate, it would emit no HCl. It would gain interesting 12s if even more powder volume could fit in the prepolymer than now, so the gain in real life is unclear to me.

This is very far from liquids' performance, sorry. Claims by the proponents base on the "impulse density" which is little relevant for space transport.
Title: Re: Rocket oxidisers
Post by: Enthalpy on June 10, 2018, 09:53:39 AM
One other oxidiser is trinitramide. Detected in minute concentration in solution, so bigger amounts will probably show drawbacks as a propellant, say if they detonate. But the compound can have uses for synthesis or as a test for a theory - or as a publication before being forgotten.

The heat of formation is only estimated by software: +203kJ/mol for the solid at 298K, according to Montgomery cited by Rahm.

The mix would beat perchlorate by 26s, nice. As trinitramide contains no fuel, less oxidiser is needed in the prepolymer, and less aluminium too as it happens, so the solid volume fraction is no barrier.

The proponents suggest that trinitramide, whose stability at room temperature is uh-doubtful, could make cryogenic solid propellants. My answer: if accepting cold, we could mix liquid oxygen with powder of methane or ethylene and freeze everything in a graphite fibre casing. Performance unbeatable by any nitramide whatsoever.
Title: Re: Rocket oxidisers
Post by: Enthalpy on June 10, 2018, 09:58:15 AM
For the synthesis of trinitramide, I've already suggested atomic nitrogen in NO2
whose most probable result is nitrogen, oxygen and their mundane compounds, plus oxidized tantalum. Worth a try?

Here is a different attempt, with NX3 in NO2.

While NO2 reacts easily with X2, bonds are stronger in NX3, in kJ/mol according to
   243     F-NF2
   188     F-NO2
   158     F-F
   381     Cl-NCl2
   142     Cl-NO2
   243     Cl-Cl

so light shall break X-NX2 to let the fragments react with NO2. Advantageously, the compounds can be cold and diluted.

One unknown is the fragile O2N-N(NO2)2 bond. A software estimate is 110kJ/mol, less than mean 168kJ/mol for I-N in NI3. This favours a big halogen in NX3, alas, and the wavelength should excite the trihalide rather than the nitramide if this is possible, and pass through XNO2.

The only NX3 spectra I have
are for NF3 (absorbs too little) and NCl3, which absorbs better than NO2 at 254nm (8*10-19cm2/mol). NBr3 and NI3 should be more sensitive to this wavelength.

So the procedure would be: cool and evacuate the reactor, fill it with NO2 and a carrier gas, switch the lamp, sublimate NX3, separate the lab debris by sedimentation, analyze.

Marc Schaefer, aka Enthalpy