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Re: Arcjet Motor



>The bipropellant motors used so far on hamsats are only burn duration
>controlled. They do not provide vernier (thrust angle) or throttling
>cacpability. I am not sure if the kick motor burn can be aborted from
>the ground... any experts on that aspect?

Throttling capability greatly increases the complexity of the engine,
reducing reliability and increasing weight.  A fixed thrust engine is a
much more practical solution for unmanned spacecraft, since it does not
have to guarantee propellant/oxidizer mixture at anything but 100%, and the
controller does not have to command anything but "on" (100%) or "off" (zero
thrust) or manage fuel/oxidizer pumps.  With fixed thrust you can use
ullage and helium pressurization to handle the fuel and don't have to deal
with large subassemblies of moving parts which may fail on-orbit for any
number of reasons.  Remember, getting to LEO costs $10K/pound, and GTO only
costs more ..

>The flip side of the coin is that modern solid monopropellant motors are
>extremely consistent in performance once ignited (note Space Shuttle).
>Ignition circuits can be "fail safed" to prevent the ignition of only a
>part of a cluster. Even in the event of a non balanced firing, a large
>enough satellite mass and spin rate may prevent a "tumble." I'd be
>interested to hear from those that can do the math in that area, given a
>bird about the mass/moments of AO40.

Take a look at NASA's PAM-A and PAM-D motors used for GTO/GEO transfers
from the STS cargo bay.  The commercial sats are spun up on a spin table
before release specifically to cancel out imbalanced propellant burn in the
PAM, and have to be despun when they reach their intended orbit.  There is
significant engineering involved with SRM maneuvering, for all the reasons
you mentioned and many more, and quite frankly, an MMH/NO4 hypergolic
propellant engine is by far the simplest and most rugged propulsion system
you can get.  I think the 400N motor and the arcjet would both have worked
just fine if the still-unknown "event" hadn't taken out critical hardware
on board AO-40.  I know an SRM would have been far more trouble if it had
fired, say, up-orbit at apogee, which would very likely have dropped AO-40
right in the ocean.  With a hypergolic engine, you can at least shut it off
if it misfires, at least if the He system depressurizes ..

>With regard to the article on particle emissions being a danger to the
>Shuttle, etc. NASA will certainly let the industry know if there needs
>to be some curb on orbital SRM's (solid rocket motors) emissions. My
>interpretation of that news article suggests that they are concerned
>about the really big SRM's being used as upper stage motors on some
>vehicles.

Again, AFAIK, the PAM-A and PAM-D are still solid propellant engines,
unless they have switched to liquid propellant and I didn't hear about it.
Any burn outside of LEO, i.e. above 500nm altitude or so, isn't going to
affect manned operations very much.  If we ever start making regular lunar
transits, that may change, but for now, I doubt people will care very much
about SRM burns at apogee because of exhaust debris.  Even at LEO, I would
be much more worried about screws, bits of insulation, jettisoned booster
and interstage structures, and so on, than I would be about propellant
residue ..


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         Bruce Bostwick  mailto:lihan@ccwf.cc.utexas.edu
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