Jake McGuire wrote

(Henry Spencer) wrote in message
...
In article ,
The tricky part, really, is the oxidizer. Fuels are generally cheap and
straightforward. There are about four reasonable choices of oxidizer --
LOX, peroxide, N2O, and WFNA -- each with its own set of disadvantages.
LOX and peroxide both require meticulous cleanliness, LOX is cryogenic,
high-grade peroxide is costly and hard to get, N2O requires high pressure,
WFNA is highly corrosive.

If the high pressure of N2O is a pain, is there any reason why people
couldn't use NBP N2O? It's not hugely cryogenic - minus 90C vs
minus 180C for LOX, and with a higher specific heat of vaporization it
should boil off much more slowly.

Or is the general theory that if you're going to go to the hassle of
using moderately cryogenic nitrous that you may as well bite the
bullet and use LOX for the increased performance?

Yep. N2O (nitrous oxide, or "laughing gas") isn't that great an oxidiser in
rocket applications, although it's by no means useless. Gives lots of gas,
but it's not energetic enough to give really high exhaust temp's. Good for
cars, but not that good for rockets. Afaik no-one has ever used it in a
space rocket.

It is used in some homebrew/ hobbyist designs though, especially hybrid ones
which use high pressure oxidiser tanks and solid fuel, often perspex. Mostly
they use high pressure tanks for pressure-fed delivery anyway, even the
cryoliquid designs. We are talking homebrew here.

If you want to get some practice in cryoliquid rocketry it might well be a
good (safer, and easier to get and use than lox) way to start. Be careful
not to spill it though, it's not like liquid N2, even a drop will scar.
Clean! Fresh air!! Read lots before you even _think_ of starting!!!



Liquid N2O4 aka NO2 (dinitrogen tetroxide or nitrogen dioxide, same
substance) was fairly widely used as a first stage oxidiser, eg on Titan/
Delta. It's main advantage is that it is storable (and hypergolic with MMH
etc). It doesn't need high pressure, but it has many toxicity, environmental
and regulatory problems, which make it expensive in the US, and it's not
"easy-to-handle" either. If you start pouring it in the open air you will
likely die.

It isn't much used for first stages in the US these days, though it is still
used in first stages by the Russians and Chinese and in some Ariane's, and
it's used by everybody everywhere in lots of upper stages, eg the Shuttle's
RCS and OMS engines to name but two from many.


Of the other oxides of nitrogen, NO (nitric oxide) is cryogenic and
corrosive and toxic and not as good as N2O4, so it's not much use. N2O3
(dinitrogen trioxide) is liquid at -25C and a possible candidate, but N2O4
is better both in performance and temperature stability, though mixtures are
sometimes used (known as MON). N2O5 might just be interesting, but it's a
solid, and not that stable. NO3 is unstable. I can't think of any more
offhand. The peroxides etc are far too dangerous. All nitrogen oxides
(except N2O, which is only mildly toxic) are very toxic.




I don't know of any other liquid oxidisers in use that Henry hasn't
mentioned, save fluorine/ flox, which is ridiculously dangerous and
unfriendly.

Conc. HNO3 is less agressive than WFNA or RFNA, but less powerful and it
doesn't ignite easily or burn well. You could try 90% HNO3/ 10% H2SO4 with
vinyl isobutyl ether fuel (if you can get it) like the German Wasserfall
rocket used, it's hypergolic. And therefore dangerous.

30% (not 30 vol) H2O2 is sometimes available fairly cheaply, but I doubt it
would burn at all. You can distill it at low pressure to concentrate it, but
it's not simple to do.


There are a lot of solid oxidisers, perchlorates (NOT chlorates) are my
personal favourite for performance, after good-ol'-fashioned nitrates, as in
black powder.

There's also a guy in England who traps gaseous oxygen in foamed plastic
fuel at 3000 psi, then uses the hardened foam as a solid monopropellant. I
don't know how good it is as yet, but it might eventually be comparable to
eg lox/kerosene except for the extra chamber weight. Interesting idea
though. I think he's patented it.

--
Peter Fairbrother