...Why should a rocket fuel for use on Mars be designed to have a flash point greater than room temperature on earth?...
Wouldn't specific impulse be of greater importance? Would it be better to pick an oxidiser first and determine performance properties when used in combination?
Thanks for your interest!
The flash point
is useful for safety on Earth. It's unimportant once on Mars, sure. But rocketry is already dangerous enough that single hazard sources should be prevented where possible.
The choice to my eyes, after progress among other options, boiled down to
- Liquid oxygen and a small hydrocarbon, both pressure-fed
- Or Mon-33
, preferably with a storable hydrocarbon, used in my original pumping cyclehttp://www.scienceforums.net/topic/60205-pumping-cycles-for-rocket-engines/page__gopid__629135#entry629135
corresponding to the third drawing there.
Presently, I've concluded that I prefer Mon-33 (33%wt NO dissolved in 67% N2
). It's toxic and the cycle needs a pump, but
- Being storable, it needs neither a permanent active cooling nor a fragile vacuum thermal insulation in the Martian atmosphere;
- Isp is 30s better than pressure-fed oxygen-methane;
- Tanks are lighter.
The Isp combined with lighter tanks lifts Martian samples or a crew from Mars to low Martian orbit in a single stage which previously de-orbits and lands
the full craft (aided by parachutes). This is much harder with pressure-fed oxygen-methane.
The pumping cycle promises to start easily, and an igniter for Mon-33 looks reliable as wellhttp://forum.nasaspaceflight.com/index.php?topic=27308.msg849905#msg849905
Both solutions, Mon-33 and LOx, combine well with the attitude thrusters.
Putting all arguments together, I believe the cycle with Mon-33 is a more reliable global solution despite toxicity.
-----Several fuels combine
well with Mon-33 and my pumping cycle. Some of them more efficient:
+3s, mp=-100°C Tetramethyldiaminobutane
+5s diazaspiroheptane with N,N' methyl or cyclopropyl
+5z azetidine with N-methyl or cyclopropyl
+6s, mp=-83°C Azetidine
+7s 1,3-diazetidine with N,N' methyl or cyclopropyl
All are amines likely to ignite upon contact
with Mon-33 but too slowly for a thruster. Only dangerous.
Azetidine is a volatile flammable amine; hazards for most others are little known, liquid range neither.
The limited performance improvement would optimize masses a little bit but not the number of stages at a Martian descent-ascent module, making the alkane a clear choice.
The rest of the trip shall use hydrogen-oxygen everywhere possible, clear. Including the return leg, since tanks are easy to insulate in vacuum, and I describe a permanent cooler therehttp://saposjoint.net/Forum/viewtopic.php?f=66&t=2051
moving part on Tue Jan 12, 2010 - cycle on Sun May 16, 2010 - fabrication of the heat exchanger on Wed May 19, 2010