Contents: Ollie Eisman Re: Ion propulsion / solar mirror Steve Abrams Re: Ion propulsion / solar mirror Steve Abrams Sails too flimsy???? Roger Arnold Tether propulsion criticism Mike Matthews Tether propulsion rebuttal [long but good --Marc] Paul Dietz Tether propulsion Mike Matthews Re: Tether propulsion Paul Dietz Re: Tether propulsion ------------------------------------------------------------ From: SPACE EXPLORATION Date: Mon, 3 Oct 88 11:56:21 MDT To: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU, space-tech@cs.cmu.edu Subject: Re: Ion propulsion / solar mirror Marc Ringuette writes: > I think an ion propulsion experiment would be a super payload for > a small satellite. It's simple enough to be feasible, but of course > an actual design that can survive launch and work robustly is no easy > trick. I like it. Maybe we could combine a slightly larger scale ion drive with a solar sail. First the satellite would be put in as high a LEO as possible. The ion drive would kick in and move the sat. up to a higher orbit. When drag is less a concern, Steve's solar sail could be deployed from the satellite. ====-Sat-==== <-- deploy solar sail (on to moon?) / / / / / / <-- using ion drive over many months / / Sat --------------------------------- LEO #1 ------------------------------ just a thought Ollie ------------------------------ Date: Wed, 5 Oct 88 01:42:05 CDT From: sedspace@doc.cc.utexas.edu (405986289 abrams) To: space-tech@cs.cmu.edu Subject: Re: Ion propulsion / solar mirror >From Ollie on Mon, 3 Oct 88: >First the satellite would be put in as high a LEO as possible. The >ion drive would kick in and move the sat. up to a higher orbit. >When drag is less a concern, Steve's solar sail could be deployed from the >satellite. The key is not necessarily a very high, circular LEO, but a highly eccentric (e>0.50) LEO with an increasing apogee....this improves the effects of the sail and engine. I'm not the first to suggest attaching sails to satellites. Dr. Robert Forward has advanced a plan, and is currently improving it, to relieve the burden on equatorial GEO. The problem is that the stable area in the sphere of orbits centered at Earth's center is a relatively thin, equatorial strip. Outside this strip, their orbits tend to try to align themselves with Earth's rotational plane. Since they move around, the benefit of being in GEO is lost. However, Forward wants to put sails on the sats to provide a restoring force to allow sats to orbit in GEO outisde that equatorial strip. He explains it all in one of his papers this year: "Light-Levitated, Geostationary, Cylindrical Orbits: Correction and Expansion"...I should have an electronic copy available within the next two weeks if anyone wants it. For that matter, I expect to also have his paper, "Grey Solar Sails" that is an important look at the specifics of radiation pressure. Steve ------------------------------ Date: Wed, 5 Oct 88 01:40:56 CDT From: sedspace@doc.cc.utexas.edu (405986289 abrams) To: space-tech@cs.cmu.edu Subject: Sails too flimsy???? On Fri, 30 Sep 1988, Marc Ringuette posted some ideas just to bait me, I believe... >Solar sails >=========== >It seems clear to me that a solar sail that would give effective propulsion >is just too thin and fragile to be sent up from earth. It may be possible >to use a more robust material, figure out a deployment scheme, and make >small adjustments to an orbit. I don't think trying that kind of deployment >from a tiny package is practical. Well, there are solar sails and there are solar sails. I agree that a gossamer sail (20 grams per square kilometer) isn't feasible, but a 100-150 nm sail certainly is (for that matter, Reynold's Wrap would probably do for a small sat). After all, we're not out to set a speed record, just get the job done. Deployment, on the small scale I envision, is greatly simplified from the larger sail deploymentsmetal films wrinkle and this is usually the sticky part of the deployment. NASA came up with rolling up the sail blades (in their heliogyro design) and deploying them by spinning the sail to let centrifugal force deploy the sail. That's OK, I guess, but I think there are other options. The most practical, to date, is to affix the thicker film to a self-deploying structure. I saw several of these demonstrated at MIT last year. Made out of coffee-straws, they were bound and deployed with two strings. Undeployed, they fit into a coffee can; deployed, they formed parabolic, satellite-dish shapes with 1-2 square meters of area. They were deployed by pulling a string. This would work for us (taking, by my calculations, some 15 years to reach the Moon), but it doesn't finesse the problem. Alternatively, put a shape memory alloy frame around it...as I've suggested. This isn't impractical as the stuff's commercially available now. Add heat and...presto! Finally, we don't have to use metallic films. The more I research conducting polymers, the more I like them. For one thing, they can be folded without severe wrinkling; they are very light -- 0.4 g/cm^3 as compared to 2.7 g/cm^3 for aluminum; and they're novel. This, too, isn't futuristic stuff. Also, the polymer films would be relatively easily-made in space. The polymerization process can be well controlled to yield appropriate thicknesses -- how do you think they made the 1 nm polymer spheres in the Shuttle that are now sold by the National Bureau of Standards? Continued polymerization causes the spheres to grow together (although, if let go for too long, it ends up looking like cottage cheese) into a film. The film can then be doped by simply exposing the film to a gaseous dopant and...voila...a conducting polymer (that's fairly reflective; polyacetylene doped with iodine has reflectances around 60% at 500 nm -- this can be increased by higher doping levels). The whole process could be automated in a GAS-can-sized package or the film can be made on Earth. This could then be connected with a very small, simple, GAS-can- sized ion thruster. If initally launched in a pretty eccentric orbit with a high enough perigee, judicious use of the ion thruster and sail combination could get the package to the moon in five years or so...it's slow, but can get the job done. >Mirror for solar power >====================== >How about getting far more power per kilogram by shipping up a deployable >mirror (similar to a small solar sail, but with less stringent requirements)? >Perhaps Steve's idea of using a Phase Reversal Zone Plate (PRZP) to focus >the light would be the most effective. >The tricky problems would probably be > (a) the deployment mechanism, and > (b) solar cells that won't overheat in that much sunlight. A mirror would help (and, of course, would be providing an additional component of force whether you chose to use it or not -- what stringent requirements???) by focusing sunlight, but why settle for a few tens of watts? Wouldn't solar thermal power yield 200-300 watts if, for no other reason than more of the spectrum would be utilized rather than the relatively small regions used by solar cells? As I suggested, PRZPs were to be used to focus the light *transmitted* by very thin films (5-15 nm for aluminum). They are *diffraction* devices, not reflection devices, so they can only be used on transmitted light. You could, however, vary the index of refraction of your mirror in the same pattern (which would depend on whether the mirror was spherical or parabolic) as a PRZP that would introduce the desired pi/2 phase shift. The idea would be to get all reflected to be *in phase* at the focal point. >The focal point won't be exact because sunlight certainly isn't >monochromatic; The foci of PRZPs are situated longitudinally from the sail and normal to the sail's plane...they are a function of wavelength. The problem with the lack of monochronicity is that you get some losses due to imperfect phase-matching -- the various wavelengths are still phase-shifted by pi/2, but the wavelengths are different. However, this can be overcome by a two layer PRZP film that acts like an optical analog of a "bandpass" filter. I believe that the formula for white light losses is losses=1-sin((pi/2)(delta/lambda)), where delta is the range of wavelengths and lambda is the "characteristic" wavelength. I usually choose lambda to be abound 480-500 nm to coincide with the peak solar power output. >So what do you think of sending up an ion propulsion setup, and having >a little satellite that can boost itself into a higher orbit? >It would be fun to send it farther, but communications could become >much trickier. Can we handle it? I think it sounds OK, but the communications problem is why I suggested laser communications... Steve ------------------------------ From: telesoft!roger@ucsd.edu (Roger Arnold @prodigal) Date: Fri, 30 Sep 88 14:07:03 PDT To: space-tech@cs.cmu.edu, sedspace@doc.cc.utexas.edu Subject: Re: Tether propulsion > Does anyone know of any good references for the earlier suggestions to > boost a satellite's orbit via a current-carrying tether? Don't know of any good ones; the one's I've seen have been obviously flawed, and I haven't kept a record of them. > The area > enclosed by the current-carrying tether would have to be pretty > extensive in order to generate enough of a magnetic moment to interact > with Earth's relatively weak magnetic field. Area enclosed? A "perceptive misimpression"? It seems to reflect an implicit awareness that current flow is always in a closed loop; failure to appreciate the consequences of this is the major flaw in the proposals that I've seen. But as to the misimpression: in the proposals, the tether does not enclose any area. It's a linear conductor, spewing negative charge/attracting positive charge at one end and doing the opposite at the other. The proposals assume that the tether can be analyzed simply in terms of the force on the tether from the cross product of the current in the tether and the earth's magnetic field. Garbage! > Has anyone generated any > numbers for this? I'm getting ready to sit down to do this (I wanted > to earlier, but pre-Shuttle enthusiasm has kept my phone ringing), so > I'll let ya' know. If you can do that, you're either a lot sharper than I am, with a lot of time (and probably computer resources) to work on the problem, or you're using an invalid model--like the authors of the proposals. To get a valid answer, you have to include a model of ion species density and distribution in near earth orbital space, and figure in space charge effects both from the ion emittor/collectors at the ends of the tether, and from the interaction of the ion return current with the earth's magnetic field and with the field from the current-carrying tether. I gave up on trying to model the system analytically. At best, the current carrying tether is a novel form of reaction engine, that imparts a small velocity increment to a relatively large mass of plasma in the region that its orbit passes through. At worst, it will generate drag by entraining plasma in a circulating current that nullifies the earth's magnetic field in the region of the tether. The only two ways I can see to get a real handle on what would happen are experimentally--orbit an experiment and see--or through low level simulation on a supercomputer. If you can do better, though, have at it. > [deleted] > Steve - Roger ------------------------------ Date: Tue, 4 Oct 88 23:22:04 GMT From: matthews%asd.span@Sds.Sdsc.Edu (Michael C. Matthews) To: space-tech@cs.cmu.edu Subject: Re: Tether propulsion (VERY LONG) [I apologize for going into data dump mode again on this... I'll try to keep it under a couple of hundred lines in the future.] In his article of Friday, 30 Sep 88, Roger Arnold (telesoft!roger@ucsd.edu) writes: >> Does anyone know of any good references for the earlier suggestions to >> boost a satellite's orbit via a current-carrying tether? > Don't know of any good ones; the one's I've seen have been obviously > flawed, and I haven't kept a record of them. >> The area >> enclosed by the current-carrying tether would have to be pretty >> extensive in order to generate enough of a magnetic moment to interact >> with Earth's relatively weak magnetic field. > Area enclosed? A "perceptive misimpression"? It seems to reflect > an implicit awareness that current flow is always in a closed loop; > failure to appreciate the consequences of this is the major flaw in > the proposals that I've seen. But as to the misimpression: in the > proposals, the tether does not enclose any area. It's a linear > conductor, spewing negative charge/attracting positive charge at one > end and doing the opposite at the other. The proposals assume that > the tether can be analyzed simply in terms of the force on the tether > from the cross product of the current in the tether and the earth's > magnetic field. Garbage! Neglecting for the moment Mr. Arnold's use of inflammatory terms such as "obviously flawed" and "garbage", I would like to address some "perceptive misimpressions" that he seems to have. I quote from _Tethers in Space Handbook_, pp. 2-29 - 2-36 (see my previously posted bibliography), [a discussion on electrodynamic tether power generation]: ] The discussion of electric power generation by tether systems will ] begin with electromagnetic systems in Earth orbit. Consider a vertical, ] gravity-gradient-stabilized, insulated, conducting tether, which is ] terminated at both ends by plasma contactors... ] ] As the tether system orbits the Earth, it cuts across the geomagnetic ] field from west to east at very high speeds (about 8 km/s if deployed ] from the Shuttle...). Due to this motion, the geomagnetic field ] induces an electromotive force (emf) across the length of the tether... ] ] ...In this Earth orbit, the emf acts to create a potential difference ] across the tether by making the upper end of the tether positive with ] respect to the lower end. The emf acts to collect electrons at the upper ] end and drive them down the tether to the lower end, where they are ] emitted when a current is allowed to flow in the tether. ] ] In order to produce a current from this potential difference, the tether ] ends must make electrical contact with the Earth's plasma environment. ] Plasma contactors at the tether ends provide this contact, establishing ] a current loop (a so-called "phantom loop") through the tether, external ] plasma, and ionosphere. Although processes in the plasma and ionosphere ] are not clearly understood at this time, it is believed that the current ] path is [as follows: ] The collection of electrons from the plasma at ] the top end of the tether, and their emission from the bottom end, ] creates a net-positive charge cloud (or region) at the top end, and a ] net-negative charge cloud at the bottom. The excess free charges are ] constrained to move along the geomagnetic field lines intercepted by ] the tether ends, until they reach the vicinity of the E region of the ] lower ionosphere, where there are sufficient collisions with neutral ] particles to allow the electrons to migrate across the field lines ] and complete the circuit. ] ] To optimize the ionosphere's ability to sustain a tether current, the ] tether current density at each end must not exceed the external ] ionospheric current density. Plasma contactors must effectively ] spread the tether current over a large enough area to reduce the ] current densities to the necessary levels... ] Note that for thrust generation, rather than power generation, the current flows in the opposite direction (negative charges collect at the top, and positive charges at the bottom). The key point here, of course, is that THE IONOSPHERIC PLASMA COMPLETES THE CURRENT LOOP. The important question, then is "What is the current capacity of the ionospheric plasma?", for a given tether/plasma contactor configuration. Mr. Arnold seems to assume that that capacity is zero, a view that does not hold with current (pardon the pun) thinking on this matter. The issues of which Mr. Arnold writes are of concern, but the basic electrodynamic tether characteristics CAN be validly modelled "in terms of the force on the tether from the cross product of the current in the tether and the earth's magnetic field", if plasma-coupling losses are appropriately modelled as impedances: ] To optimize the ionosphere's ability to sustain a tether current, ] the tether current density at each end must not exceed the external ] ionospheric current density. Plasma contactors must effectively spread ] the tether current over a large enough area to reduce the current ] densities to the necessary levels. Three basic tether system ] configurations, using three types of plasma contactors, have been ] considered up to this point. They are (1) a passive large-area ] conductor at both tether ends; (2) a passive large-area conductor ] at the upper end and an electron gun at the lower end; and, (3) a ] plasma-generating hollow cathode at both ends. ] ] ...[several paragraphs of discussion on the three methods, with ] the resulting conclusion that the hollow cathode, a device which ] emits a neutral plasma (argon) at very low mass flow rate, is by ] far superior]... ] The basic equation of the current loop (circuit) is: ] Vind = IR + Vlow + Vup + Vion + Vload ; ] where ] Vind = emf induced across the tether ] I = tether current ] R = resistance of the tether ] Vlow = voltage drop across the space charge region around ] the lower plasma contactor ] Vup = voltage drop across the space charge region around ] the upper plasma contactor ] Vion = voltage drop across the ionosphere ] Vload = voltage drop across a load. ] ] This equation simply states that the emf induced across the tether ] by its motion through the magnetic field is equal to the sum of all ] of the voltage drops in the circuit. The IR term in the equation is ] the voltage drop across the tether due to its resistance (according ] to Ohm's Law)... ] ] The voltage drop across the space charge region at each tether end ] is caused by the impedance of that region. The voltage drop across ] the ionosphere is likewise due to its impedance. The problem with ] these equations is that the impedances of the charge regions around ] the tether ends are complex, nonlinear, and unknown functions of the ] tether current. The impedance of the ionosphere has also not been ] clearly determined. Although some laboratory studies have been ] performed, and estimates made, detailed flight tests will have to ] be performed before these quantities can be clearly determined. ] ] It has been calculated that the ionospheric impedance should be on ] the order of 1-20 ohms. The highest impedances of the tether system ] are encountered at the space charge sheath regions around the upper ] and lower plasma contactors. Reducing these impedances will greatly ] increase the efficiency of the tether system in providing large ] currents. Data exist which indicate that plasmas released from hollow ] cathode plasma contactors should greatly reduce the sheath impedance ] between the contactors and the ambient plasma surrounding them. Data ] from one study of hollow cathodes predict Zlow (electron emitting end) ] to be on the order of 20 ohms, and Zup (electron collecting end) to be ] on the order of 10-100 ohms. Studies of [Plasma Motor-Generator] ] systems with hollow cathode plasma contactors, on the other hand, have ] indicated that there is a nearly constant voltage drop of 5-20 volts ] at the tether ends, independent of tether current (reference - Dr. James ] McCoy, NASA/Johnson Space Center). Therefore, for the PMG model, the ] voltage across the tether is simply reduced by 20 volts to account for ] the voltage drop at both tether ends. Although processes in these ] plasmas and in the ionosphere are not well understood and require much ] continued study and evaluation through testing, preliminary indications ] are that feasible tether and plasma-contactor systems should be able ] to provide large induced currents. Remember that the ionospheric plasma is NOT in orbit with the tethered satellite, and the satellite sweeps THROUGH the plasma at orbital speed. >From page 28 of _Guidebook for Analysis of Tether Applications_, by Joseph A. Carroll: ] Motion of the tether through the geomagnetic field causes an EMF in the ] tether... The motion also causes each region of plasma to experience ] only a short pulse of current, much as in a commutated motor. In essence, each end (plasma contactor) of the tether spews a "sheet" of charged plasma (see figure 1 below), which follows the curvature of the magnetic field lines toward the magnetic pole, to the E region of the lower ionosphere, where collisional diffusion of charge completes the circuit between the upper and lower field lines. >From Mr. Arnold again: > The only two ways I can see to get a real handle on what would happen > are experimentally--orbit an experiment and see--or through low level > simulation on a supercomputer... Of course, you're absolutely right, and experimentation is exactly what we're doing. The Tethered Satellite System (TSS) is a joint project between the United States and Italy, and TSS-1, an electrodynamic mission, is scheduled for flight on STS-45 (31 Jan 1991). From "Tethered Satellite System Science Interfaces", presented by Dr. N. Stone in December, 1987 [also the source of Figure 1]: ] TSS-1 SCIENCE OBJECTIVES SUMMARY ] ] * The physics of steady-state electrodynamic tether operations ] * Characteristics & Phenomena of high voltage plasma sheath ] * Plasma oscillations & instabilities ] * Anomalous ionization ] * Cross-field current flow ] * Field aligned current drive phenomena ] * Hydromagnetic waves ] * Double layers ] * Electrodynamic tether current collection characteristics ] * Effects of tether voltage ] * Effects of plasma conditions ] * Effects of lighting conditions ] * The physics of time-varying electrodynamic tether operations ] * VLF & ULF wave generation ] * VLF & ULF propagation characteristics through ionosphere to ground ] * Tether impedance ] * Electrodynamic tether-orbiter system charging time constant ] * Investigation of fundamental processes in space plasmas ] * Plasma expansion phenomena ] * Critical velocity ionization phenomena ] * Neutral gas-magnetoplasma interactions ] * Tether mechanics ] * Tethered satellite system dynamics ] * Dynamic noise in tethered satellite systems These data will, of course, be vital to any practical application of electrodynamic tethers in the future, but until then, we have to go on what we know so far. We have to use something to do the control systems interactions and dynamics analyses for this mission, and the method I've outlined here is what we're using, because it does not appear at this time that the effects Mr. Arnold describes will drive the behavior of the system. --------------------------------------------------------------------------- NOTE TO NET (if there's anybody still reading :-) : I may get flamed for this, but I thought it would be nice if we could get away from having to use ASCII graphics to draw pictures. So, I decided to try sending a figure to the net as a BinHexed, packed, PICT file for a Macintosh. If (a) not enough people have access to Macs for displaying these pictures, (b) I get a lot of complaints about wasting so much net bandwidth for (what I admit is) a relatively simple drawing, or (c) somebody comes up with some better standard for sending illustrations with technical messages, I won't do this again. Please tell me what you think! --------------------------------------------------------------------------- Figure 1: ========= Format: Macintosh PICT file created by MacDraw, packed with Packit III and converted with BinHex 4.0 Instructions: Extract the "garbage" :-) between the "cut here" lines into a Macintosh text file and use BinHex 4.0 or later to convert it to a Macintosh binary file. Use Packit to unpack it to a PICT file, then use your favorite object oriented picture editor (MacDraw, SuperPaint, etc.) to read it as a PICT file. ---------------------------- cut here ------------------------------------- (This file must be converted with BinHex 4.0) :'80SBA*RC5"6D'9PG#"(C@pYCA4bH5j`DA3!8%P8)&"*9#!!N!3,p`#3",cr8%e K0#!%"!L&c5'MX695fB45+D+KF&R8-PZ6AL`@%XUJ+aZKpYhVFb'BDS+rlALm*dq 3!12[4[0%MY+66E-d%eVbFdia@GE@BLH4P0jdAhT,eUEq!aFrF6['"McVfX3rkDp IY)29X(SkK0ZFdXX$Rqp`0B4qa,MNKKiSLhZ0pMfIN`,9,SiVRq2CI%0XJeGZ9+' (J[08!3Q&3M3N&b`LUfjaaFTPa#rj!JVFEB1SP%T1-)*DEH$'p"D#pl*GT,"Lk#L cKK[NBl0[2jC!"ir-q2G1hfRck`k2,6lK%pb$lqN*h`ehTj+D@b-N(idbCF`L@hk SGr8*9L#ZpKBkd2eY&QSa-j1M(`!8bJd+4,NUF4*3JGA3G9MCS[Z,Vr`PF`(e`Ve q3j8[FDXK`Gq`'R1EPApl)LIl52"%1j,ZD234K9U*AL95b['SPLbT@SDbI'5bTLI V`UQGU%$R2``+5LK$(B$l!IC!`&B!#Jd*!["J#"9%e9Cr8mdI@@!!S$[J$[2R[ji lcjlpZZklNdI@@SDRQK5C3#Q&-!5hq9Dh[DlFVLCAdP-6*P54!&-+B!P[mUd+BVP @T#dAi4%!5qZ)%4!!N!4!!*!*!8`#Q!%*FU8bJbXT*3!3PSC9U@KP@TD'9D!"%5N 3!*!%,i68,FQC)!J35pJRG!"fM+)`6K+1b!#33JGah`#@KP@XG21fK!J%Y$+Y3#q fleI&pYhUq9Nb(B,D6ZB5rLPEHI(a!EZ3!lUXGFIbf5L1dFF)`(ZpVBmAI(`V1bC D1)($PVR(#L#UJEaiZq6F*&+BJ%XL%C%(C)%!(M$+-Tl$li6@eNc[&rNH%LP-3#@ 4#-L$XN#!$aK`J+8iq,r)rP"j0`N8TL!5b)4N3GNJ3!H--L8PUTRH,r)m*&+BJ%X L%C%(C)%!(M$K!8TamAq4r+$#1Z28Zm*&+BJ%Ue#-L$XN$)(M"l%"N!$[%M(IeP5 L@BUmQFXa9*fl+e*DRLrb2K,UA6X(M%V,U9"k&c5BR(#GFGDQFmAB5+8a!*9U%C% (C)#Q$aKN(m`XG+08UPCLV`ClhIYGTiR9RDLY59[q*@m#bl9*q*qq,r)m*&+BJ%U e#-L#5JA)2''3!&b1T)`%qdPQ+["R[Gqeq,i4N3GA!&b$aKFRhEPTiR9RDLY59[q *@m#bl9*q*qq,r)q%9!kIB2'*@+JY%HiiC*LFF*-adUCcaG`Lh*Kf$aL9Pe)f23Z +1&11%A*2bBI&haL9T)5qSkL@J2-ieNN8U1bQ6aGmBPBk+2kC584eMM@A!ljE#+2 aGmBPCEdF)@HR91XFDb0(8("Tfr&haL9Lj5f4d%@imcM@2JXiF%R6mAI'*@,di#k MJ6V('XF!+)Bj4cr&haL9R54p%E5p$c10BU4RiSNa[&haL9Pk4kM2NXdHCaV-iCk 6dCjiZq-5X8kG-9SZ4jR'XX`HCR*FI&hcr(1JER0-12FjTKae,,H4EM4F*G"ESmN ZMG&bLf5k0dBi44SYQk-ET-d8D6"(Nh4h%8k60ZR45DSTdZk6"ZMT*HdQVG1JQ#P l6RTGfk@@PP*JYdY&'GTC5,C1Hh4`5-k4REG)dPm4R5CD,CZR16QTI'k-E*`6QTR *PYdBe6)R"ZRG5edb)SdcQkFa+dYGZPNSm8V4r44YdYY'FSm@kG4)`M18aNIfk1D Q1NB5CTM0dG"(XQ1Q!5CYdS8J8Hb+'Q!ETC!!P+3+B&&$ESce*8T4R#B&ZM'JmdP 4c%C`h4bcXMc4Qk1Bh5hPR(C4cdCZh5abaLcNGp(2ESiBG%XChfkA8HJk+2&& #ETeabKk)l52&ZPLR,(+6(4fQk@)2`jD5%afkGiX-IL8T)ETBCeb`d4+@k1$,HGF l+)h5i&N&[&`GPZMm(0,),#&`h4M-jJjTbL`Qk-C(11B@#FTZPJ&PR1(i@#h6PR4 ,,,%(ih5VaFR4,(,%ETB*ESZ5K,(ET9aRKETC*3YdFSFNc`jaC,G,2(6(*(51Fh4 b#iMTLj(5ETbKabiM0KFYdHK#1115-fESaL-e)4m$NYdGXAM04cKm0dG%kS[2(1E TIaecUP@RYdX)U`GFkT9VG+V'D&@&QR9ETBa94CVG(%Nb1598-dETb"TdLC(*ESZ "TdQkhF`"@,G,S,JHUK$QTMGU"HRN2Rq1G![a8e5mHjQQA38XYj&11-+*(N+G*JM bESc0-Y(Nh5ddIdbfk+K&3MqPh5B0dQDB"&3h4p)TN`#Fp,Zh6'5U%8cG(p-#P8) YNjlG&'PQ*J@kCb-b5c%bd@cG-Y(-4Q6G&XPY)jLCbCEG1HM`TE6G,ZPb4i88DCc G*JPp5j0dH5GT,kMqLMETG%G41dh4Q+6&(86'4rESaJPq5BYd9I4f8[b2T-CZLY& !4f89#2TZMUSqd8",9480dZb3!!Ml5pTDVG(ZN4)"&T,fh4BL*%5d%@QkASkb)M- %Y"ZLU,`GC1+-`ET4#i,`R"1,G*dUXA#8HR"ZM`$YP9T@P(YdF1FSGY*LPEG&)9F FT)`NaET'4B"9b+!NBESq5i&J*!SS$G*jhLi*%5"ESU5kRH35)h4eKbbkPi3ETL+ %FXj"H'k+#G3S5rR)ET81kG3FJ[lG-q4SlT9ij$G+11J4SZ"9lG,5(F1J@+A"ZLV `Yah#hPLYdISam@ik"Efk-q+0'2R-(3ET4JiT4T'MQ0dHSq"a6SNDESc`kBq"ER4 ESiahcTPj&Zh4Tjhaa5mYdk4'Cia$LYdF)-#-aaaLETfb0M!GiFGZNb,@)f-KhQk +8S#eL0M*ZR*&%8!lT'fk98FNGeZMX(E&+FN86G,Q1-01f+9ZLL(''YeZeLR4j#L +-8)9SqZe![6b(-)*!2a8e5mHjQQA3V&%28@NQka4$e-Y&,!J!!rR5+JUk1a!EZk 3!kfJ,R0-12FjTKakcN$e(65EV1328Y&&,#45QN#9DK'4"*3&JH-1%eGEQe3Va9+ +qU$@TeDUI+bT1hDZpcUa52H[)Se"ckY5+4qeUPCL9+am[HGZfI8L[&@p[&m)b)1 VJ#bJmBF,kqh#e*pDT8LYmV+jpkV@1YReG5ILR9DNVRiR9RDPU9q,r)rM&!EZN!2 UY)+Bk2&+2+%ii89F,BH,NH,[Mi9NE+-,L1k4QFF*E)T5h2&hbEK)T6%!PN3M)Jl *!c"i`b$2a`#6AaIj(K)T6%!PN3M)Jl*!c"i`iC8iNraIj(mS2*Z%LP-3#@4#-L$ XN!!c"i`R%iA'-N`I&rNH%LP-3#@4#-L$XN$-(M$K61*2mAq4r+$bEK)T6%!PN3M )Jl*!c"i`b'SV6"mAq4i5+8a!*C%)b)1b3-`H-1&-iNraIj(mS2*Z%LP-3#@4#-L $XN$-(M$)FY*Y-(aIj(K)T6%!PN3M)Jl*!c"i`i8cL6r&rNIbJmQi5+8a!*C%)b) 1b3-`H--Kh8[+B2Lrb2#45Q)",)K'4"f5"Q$aK`TR%Rq,r)rP"j0`N8TL!5b)4N3 GNJCJmBC$TT9+B2Lrb2#45Q)",)K'4"f5"Q$aK`TR%Rq,r)rP"j0`N8TL!5b)4N3 GNJCJmBC#V%CLQ$i[mM`N8TL!5b)4N3GNJCJmBF+Ca*rLrb2j3H6F*&+BJ%XL%C% (C)'B2''3!-qLL6"mAq4i5+8a!*C%)b)1b3-`H-1&-iNraIj(mS2*Z%LP-3#@4#- L$XN$-(M$)6"+06"mAq4i5+8a!*C%)b)1b3-`H-1&-iNraIj(mS(mHK80(B@d*&+ BJ%Ue#-L$XN#q"i`R$)-H(P'&428'GU6Y(,+e@SPH*9,+mDL@,+RLrb2#45Q)"+Y 3M)Jl*!Bm(M$JpV2M$-eLU9e*fMPPDV85[%UPPH04,&P6aIj(`TILKl"ia+abbm$ P*h$2Ca`TIMPc(UH,Z%H%MAB2'*@BbAi951HMY((#Fdkq"ZALl#45QN#9DK'4"f5 "9SH-(X2[)Ck(!mh#Ue%Va+TCAM85aC8`eUTC2M*C8a2f[)U*pT,-9DKV(bXa+kN l2I0DRR[GCDRLrb2j-YLX3&"&EQ3dE3!!!: ---------------------------- cut here ------------------------------------- ---------------------------------------------------------------------------- Disclaimer: I only work | Mike Matthews for Lockheed, I don't | Lockheed Engineering & Sciences Company, Inc. speak for them. I don't | Avionics Systems Department speak for NASA either, | Flight Control Systems Section for that matter. Any | Tether Dynamics Group opinions expressed here | Houston, Texas are mine alone and are, | MATTHEWS%ASD.SPAN@STAR.STANFORD.EDU therefore, TRUTH. | matthews@cup.portal.com ------------------------------ [ Good info, Mike. Well done. But I give a hearty "bah, humbug" to your graphics format. Macintoshes, of all things. -- Marc ] ------------------------------ Date: Wed, 5 Oct 88 09:19:17 EDT From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: Tether propulsion An amusing application of an electrodynamic tether would be to run a rocket. If the exhaust velocity of the rocket is sufficiently low, the thrust can exceed the drag on the tether, and the vehicle can be accelerated. This doesn't violate conservation of energy; one is extracting kinetic energy from the reaction mass and concentrating it in the rest of the vehicle. I imagine JPL is looking at using tethers for probes around Jupiter and Saturn. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Wed, 5 Oct 88 14:42:29 GMT From: matthews%asd.span@Sds.Sdsc.Edu (Michael C. Matthews) Subject: Re: Tether Propulsion To: space-tech@cs.cmu.edu X-ST-Vmsmail-To: SDSC::"space-tech@cs.cmu.edu",MATTHEWS > An amusing application of an electrodynamic tether would be to run > a rocket. If the exhaust velocity of the rocket is sufficiently low, > the thrust can exceed the drag on the tether, and the vehicle can > be accelerated. This doesn't violate conservation of energy; one is > extracting kinetic energy from the reaction mass and concentrating it > in the rest of the vehicle. I'm sorry, Paul, but I don't follow this at all. Are you talking about an electric rocket or chemical rocket? Are you talking about using the tether for power generation, to run an electric rocket? If so, then yes, it does violate conservation of energy. Remember that any power generated by an electrodynamic tether comes at the expense of the kinetic energy of the satellite. Of course, if you meant something else, please let me know. > I imagine JPL is looking at using tethers for probes around Jupiter > and Saturn. Indeed they are (or at least somebody at NASA is)! The powerful magnetic fields and relatively thick upper atmospheric plasmas of these gas giants (especially Jupiter) make them excellent targets for electrodynamic tether applications. From _Tethers in Space Handbook_, page 3-125: ] Since Jupiter's magnetic field is about twenty times that of Earth, ] an electromagnetic tether should work well there. Because of Jupiter's ] rapid rotation (period = 10 hours), at distances greater than 2.2 ] Jovian radii from its center, the Jovian magnetic field rotates faster ] than would a satellite in a circular Jovian orbit. At these distances, ] the magnetic field would induce an emf across a conducting tether, and ] the dissipation of power from the tether would produce a thrust (not a ] drag) on the spacecraft/tether system. At lesser distances, the satellite ] would rotate faster than the magnetic field, and dissipation of tether ] power would produce drage (not thrust). Examples of induced tether ] voltages are: -10 kV/km (for drag) in LJO; and +108, 50, 21, and 7 V/km ] (for thrust) at Io, Europa, Ganymede, and Callisto, respectively. ] ] Inside the Jovian magnetosphere, at distances >2.2 Jovian radii, the ] spacecraft could decrease altitude (decelerate) by feeding power from ] an on-board power supply into the tether against the induced emf. Below ] 2.2 radii, power from the tether could be dissipated. To return to ] higher altitudes, the process could be reversed. Note that for orbits >2.2 Jovian radii, this means that a spacecraft gets both power and thrust (higher kinetic energy), at the expense of Jupiter's rotational momentum. This would enable a vehicle to tour the Jovian moons, for example, by being aerobraked (or electrodynamically braked) into an orbit in the vicinity of Io (or lower), and draw its power from a tether while it rides the field into higher and higer orbits. ---------------------------------------------------------------------------- Disclaimer: I only work | Mike Matthews for Lockheed, I don't | Lockheed Engineering & Sciences Company, Inc. speak for them. I don't | Avionics Systems Department speak for NASA either, | Flight Control Systems Section for that matter. Any | Tether Dynamics Group opinions expressed here | Houston, Texas are mine alone and are, | MATTHEWS%ASD.SPAN@STAR.STANFORD.EDU therefore, TRUTH. | matthews@cup.portal.com ------------------------------ Date: Wed, 5 Oct 88 14:13:41 EDT From: dietz@cs.rochester.edu To: matthews%asd.span@Sds.Sdsc.Edu Cc: space-tech@cs.cmu.edu, dietz@cs.rochester.edu Subject: Tether Propulsion >> An amusing application of an electrodynamic tether would be to run >> a rocket. If the exhaust velocity of the rocket is sufficiently low, >> the thrust can exceed the drag on the tether, and the vehicle can >> be accelerated. This doesn't violate conservation of energy; one is >> extracting kinetic energy from the reaction mass and concentrating it >> in the rest of the vehicle. >I'm sorry, Paul, but I don't follow this at all. Are you talking about >an electric rocket or chemical rocket? Are you talking about using the >tether for power generation, to run an electric rocket? If so, then >yes, it does violate conservation of energy. Remember that any power >generated by an electrodynamic tether comes at the expense of the >kinetic energy of the satellite. Yes, I'm talking about driving an electric rocket, and, no, it does *not* necessarily violate conservation of energy. This is surprising and counterintuitive, but true, as the following argument demonstrates: Let P Power produced by tether, acting as a generator F Drag force produced by the tether v exhaust velocity of the electric rocket m' mass flow rate through the rocket e efficiency of the rocket T thrust of the rocket Then, m' v**2 / 2 = P e and T = m' v = 2 P e / v. So, the thrust of the rocket is inversely proportional to its exhaust velocity (at constant power and efficiency). If v < 2 P e / F then thrust exceeds drag. Why does this not violate conservation of energy? The rocket is expelling reaction mass, so the total mass of the vehicle (including remaining reaction mass) is decreasing with time. The kinetic energy of the vehicle is not increasing, just its velocity. This might be an interesting way to reboost the space station using waste water as reaction mass. In effect, one is recovering the energy put into the material when it was launched into orbit. It might also be useful around Jupiter. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ End of Space-tech Digest #10 *******************