As Henry writes, all the big guys are comfortable with hydrazine, and so
are some of the little guys - e.g. Stellar Exploration, which just put
the CAPSTONE 12U cubesat into lunar orbit with a hydrazine propulsion
system. And of course SpaceX has been using MMH/NTO almost from the
start. They all have good reason for this. Hydrazine is about as
well-performing and well-behaved a storable monopropellant as you will
find, ditto the fuel component of a storable biprop system, in every
respect except cost and toxicity. And it's not
OMG-we-spilled-a-teaspoonful-evacuate-the-building-immediately toxic.
Most of the integration hassles for hydrazine come from launch ranges
and other facilities using one-size-fits-all requirements for propellant
safety, and those are driven by the horrifically toxic N2O4. But, A: we
know how to work within those restrictions and B: the major ranges will
likely impose the same one-size-fits-all rule on your preferred "green"
propellant; we've already seen that in places. If you're running your
own facility, you can establish sane rules and procedures for whatever
propellants you use, but it's not clear that sane procedures for
hydrazine alone would be any worse than sane procedures for peroxide,
N2O, or whatever. Just different, because one size does not in fact fit
all.
So if you're going to "transition space propulsion to a new industry
standard", you need to offer something beyond "less toxic than
hydrazine". You need to offer either substantially better performance,
or you need to offer good-enough performance while being *substantially*
easier to work with than hydrazine.
I could maybe make a case for peroxide in this context, though that
would be easier without that pesky slow autodecomposition. For N2O, I'm
not seeing it.
The tank pressure for an uncooled system is going to have to be about
1000 psi or so, which is annoyingly high for a propellant. Much more
troublesome than the ~300 psi of a traditional hydrazine system, and
we're now starting to see small pump-fed propulsion systems with a tank
pressure of ~30 psi.
The performance as a monopropellant will be underwhelming, and I don't
think anyone has yet found a catalyst that will trigger reliable, smooth
decomposition at room temperature.
Potential for a BLEVE is worrisome, and that's going to raise a lot of
eyebrows when you start asking for all those rideshares that you thought
were going to be so easy to get because of your non-toxicity.
Potential for forming detonable mixtures with traces of organic
contaminants are also worrisome, and ditto the rideshare issues that raises.
And that's just off the top of my head. There are probably niches where
N2O makes sense, but I doubt it is going to displace hydrazine.
John Schilling
On 4/4/2023 1:25 PM, Henry Spencer wrote:
On Tue, 4 Apr 2023, Lou G wrote:
I feel like we are far from the end of the in space propellant wars.
Yeah, unfortunately none of the oxidizer choices is really entirely satisfactory. Fuels are a minor problem by comparison (unless you really insist on a monopropellant, in which case again there's nothing that's really completely satisfactory).
No-one wants to deal with hydrazine anymore, and people are afraid it will get banned in the EU
No-one except the guys who are already set up to deal with it, who don't really understand what the fuss is about, because they're so used to hydrazine's hassles that they can't imagine what it would be like to use something better. ("The shit a man never sees is the shit he's standing in." -- Paul Kavanagh.) (The LLNL piston-pump guys, who switched from hydrazine to peroxide for their experimental work, were surprised and impressed by just how much easier it made everything -- for example, they could do quick tests on the lab bench, instead of having to go out to a hazmat test site every time.) Lots of people are still in that category.
Mind you, although hydrazine has a lot of things wrong with it (starting with being both toxic and carcinogenic, as are its derivatives), the really big problem is its partner, N2O4, which is the one that's really bent on climbing out of the tank and killing you *right away*.
Henry