REM%IMSSS@SU-AI.ARPA (Robert Elton Maas) (10/31/86)
B> Date: Fri, 10 Oct 86 10:22:46 EDT B> From: ST401385%BROWNVM.BITNET@wiscvm.wisc.edu B> To: space digest <space-incoming@s1-b.arpa> B> Subject: Specific Impulse B> Yes, but with an ion drive, one is typically more concerned B> about getting the most performance out of the ENERGY source, which B> is usually the limiting factor instead of the reaction mass. B> A fixed voltage ion drive will give a fixed energy per unit charge, E. B> E=1/2 mv**2, so momentum transfer (mv) is SQRT (2mE). B> To optimize this per unit reaction mass, we want the smallest molecular B> weight possible. To optimize this per unit ENERGY, we want the B> HIGHEST molecular weight possible. That's why mercury or cesium is B> typically used. I don't believe this. In deep space, you have months to reach your target. During that time, your solar collector or atomic pile can generate more energy than you need for your delta-vee, but the only fuel you can carry with you is what you could launch, which is limited by your launch booster. (Assuming you don't have a space station with re-fueling center, which would allow a different strategy of accumulating fuel from lots of launches to power a single deep-space probe.) Therefore if there were no engineering problems you would pick the lightest ion so you could pack the largest number of molecules in your ion-fuel tank for a given Earth-launch mass. But there are indeed engineering problems: Hydrogen requires pressure containers whereas mercury is a compact liquid at normal temperature and pressure. Other atomic materials are like Hydrogen, or solid. Mercury is the only atomic liquid available. Compounds would have to be broken into parts (they'd break apart anyway if you tried to use them as ion fuel) and you'd have to get rid of the parts you're not using or else have equipment to handle all the parts with their different particle masses and different ionization characteristics, a big hassle equipmentwise. I have no experience in this area, this is just brainstorming, would like to hear from an expert to judge our difference of opinion.
chiaraviglio@husc2.UUCP (lucius) (11/02/86)
My understanding of the way that ion drives work is that they
accelerate the ions out backwards electrostatically, a procedure that is not
temperature-dependant, and thus will lose no more energy in unuseable heat for
large-molecular-weight substances than small-molecular-weight substances.
Therefore, other factors are the considerations in choosing a propellant.
--
-- Lucius Chiaraviglio
lucius@tardis.harvard.edu
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