So I'm thinking of something like an "asteroid belt survey swarm"
mission, and trying to put together some quasi-realistic numbers:
this is most likely at the "when in doubt, guess" level, but here
goes:
From what I've been able to find (from NASA/Lewis and EAFB papers
from the late 90's on the SPT-140), the total mass is something
like 20kg for a single 5kw thruster system (about 7kg for the
anode block itself, and 13kg for the "power processing unit" -
the DC-DC converter that generates the various thruster power
supplies (main discharge, heater, magnets...) from a common DC bus.
Assume another 20 kg for solar power arrays (~6 kW at 300 W/kg), and
another 40 kg for "everything else": tanks, structure, controls+comms,
science payload... 80 kg total dry mass. Allow another 80kg for xenon
propellant.
Assume we run the thruster at 0.25 N and 21.6 km/sec exhaust velocity
(i.e., Isp = 2.2k sec - per the SNECMA PPS-5000 document, this looks
like the corresponding operating point would be about 520 V at 9.6 A
(~5.0 kW total).
So the thruster would operate for 86.4 k sec. - 1.0 day - per kg of
xenon, for a total operating time of 80 days (6912 ksec).
With an Mi/Me of 2.0, we get a "raw" delta-V of:
ln(2.0) * 21.6 = 14.97 km/sec. If we estimate
the distance traveled during the thrust period
from a standing start, based on the mid-burn
acceleration, we get:
.25 N / 120 kg = 0.0021 m/sec^2
0.5 x .0021 m/sec^2 x (6912 ksec)^2 = 50 e9 m
(50 million km - equivalent to about 1/3 A.U.)
Of course, a practical probe mission wouldn't just thrust straight
out; this is just a ballpark calculation of raw "free space stage
performance"... but it looks like, even on such a mission, we use
up all the xenon before we're experiencing much of a shortage of
sunlight.
For an asteroid survey swarm, I'd envision perhaps three development
stages: the first might be the "energizer bunny" model (just keep
going and going) - a simplistic "outward bound" run for propulsion
and communication development.
Version 2 would be the "redshirts" (Ensign Expendable), with relatively
inexpensive optical and radar mapping payloads, intended to record the
orbits of stuff that's too small to see but big enough to be a
collision hazard; version 3 would have more elaborate science
payloads (laser+spectrograph rigs for determining chemical
composition, etc.)
-dave w
