In a message dated 7-31-01 7:45:13 PM, [EMAIL PROTECTED] writes:
<<Similarly, Deep Space 1
has wonderfully demonstrated the use of low-cost, autonomous, ion-drives to
explore the Outer Planets. A great success - yes? Yet we STILL have to
spend billions of $ to develop a fast drive for a Pluto-Kuiper Express
Mission to get a probe there by 2020? Somebody please explain to me what's
going on.... Are we learning ANYTHING from these precursor missions?
>>
Deep Space 1 did not demonstrate ion drives "to the Outer Planets", it
demonstrated ion drives for inner solar system exploration
(asteriods/comets). I guarantee that NSTAR-type ion propulsion will be used
again for other inner solar system missions. It probably will not be used for
PKE type missions. It won't be used for Europa Orbiter. JPL has done some
studies on NSTAR ion engines for PKE. Unfortunately, it only pays off (on a
Delta 4 class LV) with 10 - 15kW of solar power, 3 to 4 NSTAR ion engines,
and inner solar system gravity assists, of which the opportunities are
limited . You're only able to thrust out to about 4 AU, then coast, and then
you're only roughly matching the trip times of a Jupiter gravity assist
mission. That's a lot of extra bucks for a limited expansion of mission
flexibility.
Ion propulsion is almost useless for a Europa Orbiter mission because most of
the delta-V is at Jupiter, in a major gravity well, at a significant distance
from the sun. Ion propulsion makes a lot more sense for outer planet missions
when you go to non-solar energy (the n-word) and crank up the power a bit. Or
if you use aero-capture technologies to replace the delta-V at the outer
planet, and use higher power ion propulsion in the inner solar system to give
you a fast outbound trip. Those are the kind of interesting capabilities that
the advanced propulsion technology investment budget item may get you. For
example, a 10 kW ion engine (NSTAR is 2.3 kW), running off solar power in the
inner solar system, with aerocapture at the outer planet, can get you re
spectable Titan Orbiters and Neptune Orbiters off of Delta 4/Atlas 5 class
LVs. And no, the technology would not cost billions!
It really doesn't do a heck of a lot for PKE, but PKE is a one-of-a-kind
mission at this point. It may be useful for missions to Europa other than an
orbiter, that would depend on specific trajectory approaches.
Interestingly, low power ion propulsion might allow you to delete a Jupiter
gravity assist on a PKE mission and still retain comparable trip times. You
would need to double or triple the radioisotope power source to get a few
hundred watts to run a small ion thruster. And it only pays off for very low
mass spacecraft, in the <300kg regime. But launch off a Delta 4/Atlas 5 can
get you there reasonably without the gravity assist. Of course, the
radioisotope sources to do this (with the requisite mass) don't exist today.
The ARPS (advanced radioisotope power system) and Stirling engine (power
conversion) projects get you there, with payoff dependent on length of time
for technology development.
So please don't dismiss the accomplishments of Deep Space 1, or declare NASA
incompetent. NSTAR ion propulsion is just not a do-everything engine. Nor
were the first few chemical rocket engines ever used.
Scott
==
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