That's al fair, and yeah, got my numbers mixed up there, my bad.
I still don't quite understand why they're going for a biprop then, and not
just roll with peroxide as a monoprop. Sure, your specific impulse is
significantly higher, but your system gains a lot of complexity and you're
carrying way more feed-system and pressurant with you.
As for alternatives, there are some nice self-pressurizing, non-lethally
toxic propellant combinations (if you want to roll biprop) like
Nitrous/Ethane, Nitrous/Propylene, etc. I know that nitrous is definitely
not a walk in the park either (Most of my work has been with the stuff, so
I may be a little biased there) and that peroxide offers a significant
density advantage, but you lose a large amount of complexity that you'd
have with a pressure-regulated system. It is of course a bit more boring
though.
On Fri, Jul 14, 2017 at 4:09 AM, Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Fri, 14 Jul 2017, Dion van Strydonck wrote:
Another drawback that HTP has is its rather poor freezing point, as it
melts only just below the freezing point of water at atmospheric pressure,
which is fine for Earth applications but rather annoying for a deep space
mission.
It usually wouldn't be *that* big a problem, because good batteries often
don't like getting much colder than 0degC anyway. Even the ones that will
work somewhat colder often have annoying complications like more restricted
charging rates when cold. Batteries often define the acceptable
temperature range for a spacecraft interior; most everything else will work
over a rather wider range. (Standard "commercial" electronic parts
normally have a 0-70degC range, but almost all now also come in
"industrial" or "automotive" grades, which have considerably wider
temperature tolerance and only slightly higher prices. The electronic
parts in our satellites are mostly industrial or automotive grade.)
having a freezing point that's 50K higher than your hydrazines does seem a
bit challenging from a thermal standpoint.
Check your numbers here -- ordinary hydrazine, often preferred for
spacecraft because it's the only one that makes a good monopropellant (even
spacecraft with biprop main propulsion often have monoprop RCS systems),
freezes at +1.4degC! MMH and UDMH are the ones that freeze much lower, but
they're less often used.
New Horizons, now out beyond Pluto, is entirely hydrazine monoprop.
Cassini, finishing its mission at Saturn, has N2O4/MMH main propulsion, but
a separate hydrazine monoprop RCS.
Especially as the renders they have shown feature nice, shiny white
probes, which loses you even more heat.
You have to be a little careful here, because the lunar surface is not
like open space. During the lunar day -- and basically nothing survives
the lunar night without nuclear heat -- one entire hemisphere of your
surroundings is blistering hot lunar soil, vigorously radiating heat at
you. So a surface like white paint, which would run pretty cold in open
space, may be just what the thermal-control guy ordered.
...it's kinda surprising to go with peroxide when there are (generally
speaking) better alternatives available.
What alternatives, and "better" by what criteria? When you're developing
your own propulsion system, having to do every test at a hazmat site is a
tremendous pain (and a large expense), and that's what hydrazine (and even
worse, N2O4) demands. The LLNL piston-pump guys (Whitehead et al) worked
first with hydrazine and then switched to peroxide, and were much impressed
at how much easier development became.
Henry
