neal@lynx.uucp (Neal Woodall) (08/29/89)
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), >do exist and have undergone vacuum chamber testing. That they have >not been used for main thrust on a spaceprobe has more to do with >senseless conservativism than with any real technical challenge. 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!). These things don't give very high accelerations, but they can run continuously for *months*.....thrust all the way! Accelerate for half the trip, decelerate for the other half. How do the thrust forces compare to the chemical thrusters used on deep-space probes of today? I know that chemical rockets give more acceleration, but they can only burn for short periods of time. 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?? 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. >(Some people are distrustful of letting a probe use such "new and >unproven technology" -- of course it wouldn't be such a big risk if we >had a larger exploratory program than we do, but as it is, losing one >probe would be catastrophic to the program). Is this the real reason? Why the hell cannot our country do with two fewer B2 bombers, and give that money to JPL for probes, including a full program of electric-ion engine probes to various parts of the solar system? All of the JPL people on TV the other night were saying that this is the last Neptune mission in their lifetimes (even some of the younger ones said this).....if we had an working electric-ion system, we could send probes to all points in our system! Hell, a manned Mars mission would be fast with an ion thruster! Neal
terry@utastro.UUCP (Terry Hancock) (08/29/89)
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 ************************** **************************
dietz@cs.rochester.edu (Paul Dietz) (08/29/89)
In article <4271@utastro.UUCP> terry@astro.UUCP (Terry Hancock) writes: > 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. I don't know what planet you're from, but here on Earth cesium occurs naturally as Cs-133, which is most certainly *not* radioactive. A reason to use mercury is its higher atomic mass. This means that (for the same ionic charge) you can use a higher voltage to reach the same exhaust velocity. Higher voltages are good because current density through an ion engine is limited by space charge effects, and this limit is strongly dependent on voltage (assuming constant separation between the accelerating grids). The more current, and the higher the atomic mass, the higher the thrust per unit engine area. > 50 lbs thrust is TOTAL B.S. for an (electrostatic) ion drive, >certainly any one tested. The specifications for the 30-cm are: Perhaps he was thinking of a magnetoplasmadynamic (MPD) engine. > 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. Looking in an issue of the NY Times I had sitting around, I find that mercury was listed at $275 for a 76 pound flask (look on the commodities page), or about $3.62/lb. In other words, the cost of putting the mercury into orbit is about a thousand times the cost of the mercury itself. Paul F. Dietz dietz@cs.rochester.edu
henry@utzoo.uucp (Henry Spencer) (08/29/89)
In article <6091@lynx.UUCP> neal@lynx.UUCP (Neal Woodall) writes: >... Why the hell cannot our country do with two fewer >B2 bombers, and give that money to JPL for probes, including a full >program of electric-ion engine probes to various parts of the solar system? Because Congress isn't willing to move funds around like that. If you feel this is unfair... have you talked to *YOUR* Congressthing about it lately? If not, why not? >... Hell, a manned Mars mission would be fast with an ion thruster! Well, let's not get carried away; the thrust of the things is pretty low. But yes, it would help. Which is why the Soviets are working on nuclear- electric propulsion for their Mars mission. -- V7 /bin/mail source: 554 lines.| Henry Spencer at U of Toronto Zoology 1989 X.400 specs: 2200+ pages. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
henry@utzoo.uucp (Henry Spencer) (08/29/89)
In article <4271@utastro.UUCP> terry@astro.UUCP (Terry Hancock) writes: >* 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... Perfectly legitimate cancellation; the problem is that the *unit* is silly. Use exhaust velocity instead -- not only is it expressed in sensible units, it's physically meaningful to boot. -- V7 /bin/mail source: 554 lines.| Henry Spencer at U of Toronto Zoology 1989 X.400 specs: 2200+ pages. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr) (08/30/89)
In article <6091@lynx.UUCP>, neal@lynx.uucp (Neal Woodall) writes: | Is this the real reason? Why the hell cannot our country do with two fewer | B2 bombers, and give that money to JPL for probes, including a full | program of electric-ion engine probes to various parts of the solar system? | All of the JPL people on TV the other night were saying that this is the | last Neptune mission in their lifetimes (even some of the younger ones said | this).....if we had an working electric-ion system, we could send probes | to all points in our system! Hell, a manned Mars mission would be fast | with an ion thruster! I'm not sure that an ion thruster would have enough thrust for a manned Mars mission, but consider how small the solar system gets with constant acceleration... 3 billion miles at .01ft/s/s in 651 days would get another look at the far planets pretty quickly. Not a bad deal to Mars, either, assuming a slowdown for orbit and a distance of 250 million miles (I had to assume something), I get about 266 days. If we could convince the politicians that this was a good idea it could happen before the end of the century, based on what I read about the state of the technology. And if it could be launched by a cheap rocket instead of the shuttle the cost would be pin money. The probes would be cheap enough to send out a bunch and live with a failure rate of (name it). ___________________________________________________________________ | | | I'm off on vacation, expect replies Sep 18 or 25 depending on | | weather in Utah. | |___________________________________________________________________| -- bill davidsen (davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen) "The world is filled with fools. They blindly follow their so-called 'reason' in the face of the church and common sense. Any fool can see that the world is flat!" - anon
neal@lynx.uucp (Neal Woodall) (08/30/89)
In article <4271@utastro.UUCP> terry@astro.UUCP (Terry Hancock) writes: >The most powerful Ion drive designed and built (to my >knowledge) is the 30-centimeter-diameter thruster developed at >NASA Lewis Research Center: >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. Any reason why the eletric-ion thruster must use mercury as the reaction mass? Is it possible to develop a thruster that would use a less expensive and dangerous material for reaction mass? Is mercury used simply because it has a high atomic weight? Neal
neal@lynx.uucp (Neal Woodall) (08/30/89)
In article <1989Aug29.124837.23692@cs.rochester.edu> dietz@cs.rochester.edu.UUCP (Paul Dietz) writes: >A reason to use mercury is its higher atomic mass. This answers one of the questions I posted just minutes ago. Thanx. >>50 lbs thrust is TOTAL B.S. for an (electrostatic) ion drive, >>certainly any one tested. >Perhaps he was thinking of a magnetoplasmadynamic (MPD) engine. Please, some details!! Perhaps we can start a thread on "exotic" thrusters which would be good for the next generation of planetary probes. I am interested in any engine that allows continuous thrust for long periods of time, and will probably be of very high specific impulse. I have no info on the MPD engine you speak of...... Neal