|
|
Post by ka9q on Nov 22, 2011 7:39:31 GMT -4
Also note that one of the big advantages of hypergols that you didn�t mention is their good temperature range — they are easily storable. Although these fuels are liquids at STP, you did recently point out that N 2O 4 has an uncomfortably high freezing point. It can present a thermal control challenge especially if you expect to be in eclipse for a long time. I suppose if that's an insurmountable problem there's always IRFNA. N 2H 4 also has a high freezing point, and while the methylated hydrazines have much lower freezing points they're not suitable for monopropellant engines. What about N 2O (nitrous oxide)? I'm familiar with its use as an oxidizer in hybrid rockets, especially amateur high power rockets because it is far less toxic and corrosive than the other oxides of nitrogen. Its main drawbacks are lower performance than N 2O 4 and a much higher vapor pressure at 20°C: 58.5 bar. It can be kept liquid at room temperature if you don't care too much about the weight of the tanks but I suspect that's a much bigger deal in a spacecraft than in a low performance amateur rocket. Still, it was used successfully in Space Ship One. I had forgotten that N 2O also has a positive enthalpy of formation, meaning it can be used in a monopropellant thruster with a catalyst though the performance isn't great. Has anyone actually done this? |
|
Bob B.
Bob the Excel Guru?
Posts: 3,072
|
Post by Bob B. on Nov 22, 2011 10:05:15 GMT -4
What about N 2O (nitrous oxide)? I've done some past ISP calculations on N 2O and found that is not a particularly good performer as an oxidizer. It depends on the fuel it's burned with, but generally it is about 3% lower than IRFNA and about 7% lower than N 2O 4. If you're looking for a less toxic alternative, H 2O 2 actually looks better than N 2O in regard to performance. Peroxide seems to be comparable to IRFNA at 85% concentration and comparable to N 2O 4 at 95%. I had forgotten that N 2O also has a positive enthalpy of formation, meaning it can be used in a monopropellant thruster with a catalyst though the performance isn't great. Has anyone actually done this? I've read a paper that promoted the use of N 2O as a monopropellant. It's better than H 2O 2 in this role but not nearly as good as N 2H 4. The real gist of the paper was to call attention to N 2O as a low toxicity alternative in dual mode systems. A dual mode system is one having a bipropellant main engine and monopropellant thrusters for attitude control, where one of the two main engine propellants is also the monopropellant. Typically, hydrazine is used as the monopropellant and fuel; however, N 2O can be used as the monopropellant and oxidizer. Likewise, H 2O 2 can be a monopropellant and oxidizer. N 2O would perform better in the thrusters, but H 2O 2 would perform better in the bipropellant engine. Neither is as good as N 2H 4 |
|
|
|
Post by ka9q on Nov 22, 2011 13:29:12 GMT -4
H2O2 scares me. The stuff is just too damn unstable. Lots of chem labs have had serious accidents by accidentally synthesizing organic peroxides. (Many terrorists do it intentionally.) I think even the Germans, Japanese and Russians who tried to propel submarines and torpedoes with it have largely given up.
While it's easy to dilute with water to a non-toxic concentration, as 90+% rocket fuel it's probably as deadly as any other oxidizer.
Interestingly the FEMA HAZMAT threat-to-life classification for both H2O2 and N2O4 is 3. I would have expected 4 for the latter. Peroxide has a 2 rating for reactivity while it's 0 for N2O4. Naturally both have the OX qualifier.
|
|
|
|
Post by theteacher on Nov 23, 2011 17:53:06 GMT -4
|
|
|
|
Post by ka9q on Nov 23, 2011 19:19:11 GMT -4
That cartoon really has been making the rounds lately. I don't know anyone who hasn't gotten it pretty quickly.
|
|
raven
Jupiter
That ain't Earth, kiddies.
Posts: 509
|
Post by raven on Nov 25, 2011 9:39:16 GMT -4
While the concept of that joke is funny, my sense of comedic timing is enraged.
|
|
|
|
Post by ka9q on Nov 25, 2011 9:59:49 GMT -4
How so?
|
|
raven
Jupiter
That ain't Earth, kiddies.
Posts: 509
|
Post by raven on Nov 25, 2011 11:12:42 GMT -4
You could cut at least three panels and a line of dialogue and make it funnier in the process. Say, cut from the scene of them taking a sip to the chemist standing beside a grave marker. If you still want the "idiot" line, you could have the dead guys ghost looking morose over the grave with the chemist telling him off.
Enraged is probably too strong a word, but it could be punchier.
|
|
|
|
Post by ka9q on Nov 26, 2011 0:38:49 GMT -4
According to Bob B's page, straight N2O4 has a freezing point of -9°C while MON-3 freezes at -15°C and MON-25 at -55°C. That last one is quite an improvement, but what is the effect of all that NO on performance as an oxidizer?
|
|
