Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!cs.utexas.edu!utastro!terry From: terry@utastro.UUCP (Terry Hancock) Newsgroups: sci.space Subject: Re: Where the hell are electric-ion thrusters???? Summary: Where the Hell they are. Keywords: not to mention "Jupiter at Night" and "Saturn at Night", etc, etc! Message-ID: <4271@utastro.UUCP> Date: 29 Aug 89 09:50:41 GMT References: <8908241857.AA02943@fermat.Mayo.edu> <1989Aug25.183710.3054@utzoo.uucp> <4256@utastro.UUCP> <6091@lynx.UUCP> Reply-To: terry@astro.UUCP (Terry Hancock) Distribution: usa Organization: UT AUSTIN Astronomy Department/McDonald Observatory Lines: 123 In article <6091@lynx.UUCP> neal@lynx.UUCP (Neal Woodall) writes: >In article <4256@utastro.UUCP> terry@astro.UUCP (Terry Hancock) writes: > >>1> Depends on what you mean by "current technology" -- Ion >>drives capable of doing this (with the appropriate power source), >Now here is a subject that I have been meaning to bring up lately: >ion thrusters. Where the hell are they? I understand that the US has >had one tested and ready to go for over seven years now. I uses >(mercury, cesium....I cannot remember which) as the reaction mass, and >uses very high voltages to shoot the charged particles out at HIGH >velocities....not much thrust, but an exceedingly high specific >impulse (although I have read that some tested designs will produce >about 50 lbs of thrust!). > The most powerful Ion drive designed and built (to my knowledge) is the 30-centimeter-diameter thruster developed at NASA Lewis Research Center: NASA Lewis Research Center Cleveland, Ohio 44135 It uses electrostatically accelerated mercury (cesium would by bad news, by the way, it's both very reactive, and radioactive), mercury will just give you heavy metal poisoning if you ingest it. 50 lbs thrust is TOTAL B.S. for an (electrostatic) ion drive, certainly any one tested. The specifications for the 30-cm are: Power Required: 2.75 kW Thrust: 0.135 N (0.03 lb) Specific Impulse: 3000 lbf-s/lbm * Thruster Efficiency: 0.71 Design Lifetime: 15,000 hours (at full thrust) All specifications above are for full thrust. The engine is designed to be throttleable over a 4:1 range. Multiple restarts are no problem. This design thruster has never been flown, although it has undergone over 25,000 hours of vacuum chamber testing, with the longest individual test lasting 4000 hours. The data was current in *1977*! (i.e. this is NOT new technology). Oh, by the way, the J-2 engines used on the Saturn V second and third stage had a specific impulse of 421 lbf-s/lbm, the F-1 a specific impulse of 263 lbf-s/lbm. The ion drive is therefore nearly an order of magnitude improvement. >These things don't give very high accelerations, but they can run >continuously for *months*.....thrust all the way! Accelerate for half the > Quite correct. The 30-cm drive was intended for use as a primary propulsion system for interplanetary missions, particularly in the inner solar system (where power can be supplied by solar panels). A nuclear-powered system for outer solar system applications was suggested later. I have already pointed out that appropriately designed missions could use an inner solar system boost phase (using solar panel power) to reach outer solar system targets. >Didn't the US conduct a test called SERT a few years ago? (SERT = Space >Electric Rocket Test) What was the outcome of the test?? > There were at least two vehicles: SERT I and SERT II. Each was used to test smaller, auxillary thrust systems (for attitude control and stationkeeping of geosynchronous satellites). This class of thrusters, I have heard is in service on some European satellites. SERT I and II were both quite successful, as is implied by the current use of the systems they tested. >Also, it seems that an electric-ion thruster is perfect for earth orbit >transfer vehicles....you could move large masses with them, it would just >take awhile. > A very good idea, suggested in several space-colonization plans that I have seen. This is particularly useful for transferring bulk cargo and non-perishables. An "Ion Barge" like this would take two or three months, travelling in a spiral orbit out to the moon, and a similar time back. With a fleet of such barges, a continual supply line could be maintained with relatively little fuel use. The cost effectiveness of this depends also on the expense of mercury (anyone know what Hg costs?), which ought to be fairly rare, given its atomic weight (>Fe). Probably cheaper to use kilos of Hg than kilotonnes of hydrogen, though. ------------------------------------------------------------------- * I would like to take this opportunity to rag on the engineer who decided to cancel 1 lbf with 1 lbm and thereby arrive at seconds as the unit of Specific Impulse. I would also like to state, for those who may have been confused by this @#&^$(!! that: SPECIFIC IMPULSE IS *NOT* MEASURED IN UNITS OF *TIME* !!!!!!! IT IS MEASURED IN *UNIT IMPULSE PER UNIT MASS* OR EVEN *UNIT FORCE PER UNIT MASS-RATE-OF-FLOW* OR EVEN *UNIT VELOCITY* (All of these are equivalent). This causes problems particularly in converting to metric: 1 second = 1 second (same in both systems) 1 lbf = 4.46 N 1 lbm = 0.454 kg therefore: 1 lbf-s/lbm = (4.46 N)(1 sec)/(0.454 kg) = 9.8 N-s/kg (actually the factor is exactly 1 gee in m/s^2 =~ 9.81) This made it extremely hard for me to understand rocket propulsion, and I hope posting it will save someone else that trouble. ------------------------------------------------------------------- ************************** Terry Hancock terry@astro.as.utexas.edu ************************** **************************