Date: Thu, 8 Sep 1988 21:55-EDT From: space-tech-request@cs.cmu.edu To: "~/st/lists/stdigest" Subject: Space-tech Digest #2 Contents: Bob Summersgill - EML's background info. Marc Ringuette - EM Launchers (discussion) ------------------------------ 08 Sep 88 19:25:54 EDT Date: Thu, 8 Sep 1988 18:59 EDT From: Bob Summersgill Subject: EML's background info. To: Space-tech Hi- In response to deitz re: EML's and with respect to Steve who wants references: The subject of EML has been discussed at SSI (The Space Studies Institute) since at least as early as 1981. In the conference procedings of that year (Space Manufacturing IV, available from AIAA), Henry Colmb (sp?) has a paper and again in 1985 (Space Manufacturing V) Stuart Hirsch (sp?) also has an article dealing with the economics of such a project. I find a few problems with EMLing that I'm not sure (in my quick read) have adequately been addressed. First is the problem of punching through several miles (75-600 depending on point of veiw) of atmosphere. At velocities necessary to obtain orbit, you have problems similar to re-entry concerns of current technology. Some form of heat sheild is required. An ablative is of course the obvious solution to an expendable launcher. I believe it is Henry Colmb's article above which suggests a telephone pole shaped projectile with an ablative nose. Slipping off of true, would of course cause serious problems. Also the simple need to have a vehicle to launch and deliver a payload to orbit seems counter to the reason that EMLing was first suggested for study at SSI. The vehicle/payload ratio in the Mass Driver is zero giving the maximum efficiency in terms of getting to orbit. In EML you seem to be fighting the rational for Mass Drivers while exploiting the efficiency. EML seems to make more sense as a first stage (refences numerous including Heinlein's 'The Man Who Sold The Moon'). I am not convinced that EML makes sense in light of some other methods I've seen for Earth-to-orbit launching. No, I don't recommend chemical rockets. -rjs ------------------------------ Date: Thu, 8 Sep 1988 20:25-EDT From: Marc.Ringuette@CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: EM Launchers (discussion) [ Administrative note: As far as I know, the mailing list is in good shape, so go to it! You can send to the group by sending mail to "space-tech@cs.cmu.edu" // Marc ] The rest of this message is an email discussion between Paul Dietz and me, which follows on from his earlier posting to space-tech. Paul includes some good info on atmospheric drag. We also discuss how to catch the mass at the top of its trajectory, thus eliminating the need for a complete vehicle to be launched each time. -- Marc ============================================================================= Discussion on EM Launchers, 9/4/88 to 9/8/88. Participants: dietz@cs.rochester.edu (Paul Dietz) mnr@cs.cmu.edu (Marc Ringuette) Summary: [Continues from Paul's post to space-tech, 9/2/88.] Marc and Paul discuss questions on what orbits to choose, mass catchers, atmospheric drag, and required accelerations. Paul has some numbers and references on the drag problem. Marc and Paul agree that mass catchers, if they are used at all, should probably be small OMV's in this case. ------------------------------ From: mnr@cs.cmu.edu (Marc Ringuette) I have comments on 3 issues. 1. Orbits. Your calculations make sense. Question: - A mass catcher sounds like a good idea (say, an Orbital Maneuvering Vehicle (OMV) with a bag of sand in a circular orbit; after impact it reboosts to maintain its position). The only problem I can see is rendezvous, but that sould be OK. What difficulties do you see? 2. Drag in the atmosphere. This is where you differ from conventional net "wisdom", which says horizontal launch is fine. We need some better figures on atmospheric drag. Think you can get any? Did you do any calculations to rule out horizontal launch? 3. How long does the launcher need to be? SSI should be able to give us good figures. It probably depends on things like current densities in the magnets. I'm hoping less than a kilometer will be feasible. Required length varies inversely with the max acceleration. ------------------------------ From: dietz@cs.rochester.edu (Paul Dietz) To take your questions in order... There are several problems with a mass catcher. The first problem is the very high impact speed. In the lunar scheme, mass hits the catcher at only a few hundred m/sec. Launching to low earth orbit, the impact speeds are at least an order of magnitude higher, so impact energies are at least two orders of magnitude higher (per unit payload mass). The second problem is accuracy. Material launched from a lunar mass driver is carefully tweaked after release to get lateral velocity errors down to perhaps a millimeter per second. This can be done because the trajectory is horizontal, and because there is no atmosphere to exert forces. Launching from the earth, I don't see how an unguided projectile could achieve velocity accuracies of better than (say) 1 m/sec; if the projectile must travel for 1 hour to reach the catcher the catcher must be 7 km across. The third problem is duty cycle. On the moon, the mass catcher can be placed in L2 and can be launched to continuously. A mass catcher in LEO could only be launched to a small fraction of the time, from any given launch site. Low duty cycle makes it harder to amortize the fixed cost of the system. I think horizontal launch from the Earth's surface is out of the question. Suppose the relevant part of the atmosphere is 10 km thick. Horizontally, a vehicle will traverse sqrt(2 x 10 km x 6,400 km) = 360 km of atmosphere before reaching the same altitude (more, actually, since the vehicle will curve down under gravity). You're going to be expending energy travelling at Mach 35+ (if nothing else, heat of compression in the shock wave will dissociate air molecules); that hopeful estimate of C_d = 10**-5 seems unbelievable to me. One can do a reality check by looking at meteors and reentry vehicles. I really should look into a text on hypersonic aerodynamics, though. The length of the launcher is a function of the achievable acceleration, as you said. This depends on the ballistic coefficient of the vehicle. EMLs can be thought of as operating using magnetic pressure. Therefore, for a fixed design (say, SSI's mass driver), the acceleration goes down as the mass per unit cross section of the vehicle goes up. There are limits on magnetic pressure set by mechanical constraints on the launcher: if the magnetic fields are too high the coils (or rails) will experience too large a bursting force. Perhaps these limits can be evaded by a system that applies force at several points along the length of the launch vehicle, but that would require pulsing each coil repeatedly in rapid succession. The other way to get around the problem is to surround the vehicle with a sabot with a much larger cross sectional area; say, a 2 meter diameter sabot around a 20 cm diameter vehicle, reducing the required magnetic pressure by a factor of 100 and field by a factor of 10 (assuming the mass of the sabot is negligible). I carefully left out of the discussion details of the actual linear accelerator, since the merits of the different schemes are unclear to me. I have some attraction to the solenoid quench gun, if only because it is simple. I have a feeling that the SSI mass driver is not the best system for launching material off the moon. The variable pitch induction launcher design from UT Austin, if I recall correctly, can achieve much higher accelerations (to lunar escape velocity in 3 m) and can use aluminum buckets. See the last electromagnetic gun issue of IEEE Transactions on Magnetics for the article. ------------------------------ From: dietz@cs.rochester.edu (Paul Dietz) I looked in the library for texts on hypersonic aerodynamics. I found many books on thermophysics of atmospheric re-entry (obivously interesting to DOD & NASA). One book had an article on earth-based mass drivers: Chul Park (NASA Ames) and Stuart W. Bowen. Abalation and Deceleration of Mass Driver-Launched Projectiles for Space Disposal of Nuclear Wastes. In Thermophysics of Atmospheric Entry, T. E. Horton (ed.), pages 201-225, Progess in Astronautics and Aeronautics, volume 82. AIAA, 1982. The article describes the aerothermal environment faced by vertically launched projectiles. The projectiles are assumed to be launched at 17 km/sec (after drag) so that, if launched at dawn from the equator, they escape the solar system. The vehicles are assumed to have hemispherical noses, and have C_d >= 0.5. The nose shield is made of graphite. At these velocities, the shock heated air reaches 40,000 K (at sea level), and radiates fiercely. The purpose of the heat shield is to sublimate and form an optically thick gas layer to block this radiation. The high pressure of the shock wave means the ablated gas layer can be optically thick. Calculations show the projectile loses about .1 ton of heat shield (for a 1 ton vehicle). Depending on the mass of the vehicle, it loses between .4 (acceptable) and 30 km/sec (unacceptable) of velocity. What does this mean for launch to earth orbit? Naively, I'd expect the shock wave temperature to scale as v**3/4 (since the power in the incoming air increases as the cube of velocity, and radiated power scales as the fourth power of temperature). On the other hand, the lower pressure at 11 km/sec would mean the ablation layer isn't as thick, so more radiation would get through. Also, one could reduce drag by choosing a more pointy shape. It seems clear, though, that horizontal launch from the earth's surface is out of the question. ------------------------------ From: dietz@cs.rochester.edu (Paul Dietz) Thinking some more about horizontal launching from the earth's surface... Perhaps it would be possible to launch at a low angle, but not horizontally. For example, depressing the trajectory 60 degrees from vertical doubles the distance the vehicle must pass through the atmosphere, but allows the launcher to supply cos 30 deg. = 86.6% of the needed angular momentum. If a vehicle can suvive vertical launch at 17+ km/sec, perhaps it can survive this depressed trajectory at 11 km/sec. This depressed trajectory would reduce the delta-V for placement into a 20 earth radii HEEO to less than 100 m/sec. The other trick that might save horizontal launch is some aerodynamic maneuvering in the upper atmosphere to flatten the trajectory. This seems complicated, though, and if you're going to be launching a vehicle with on-board smarts you might as well use my scheme to put it in HEEO. No sense using space-based propulsion to lift mass from LEO to high orbit when it can be put there directly. ------------------------------ From: mnr@cs.cmu.edu (Marc Ringuette) Good stuff, and I mostly agree. But... Mass catchers ============= I'm presuming the catcher isn't hugely massive, but instead is maneuverable, and can intercept the payload and reboost itself into HEEO. Probably the difficulties aren't worth it, but it would allow the boost at the top to come from an engine that didn't have to be thrown up with the payload. Duty cycle probably isn't a problem; the payloads hang up at the top of their trajectories for a long time, and varying the initial angle and velocity is probably possible. Practically, though, it's hard to imagine this being cheaper than just attaching a very durable booster to the payload. Air Resistance ============== That Park & Bowen article sounds interesting, but raises more questions: 1. What was the aspect ratio of their projectiles? 2. Of course, you don't start at sea level. How much easier is it from 15,000 feet? I get the feeling that the drag problem won't just go away, though. I'm especially worried that they had to think up such a snazzy ablative shield; those temperatures are pretty hellacious. Power Requirements ================== I did some very rough calculations on how bad the power requirements are. In terms of raw power, it's not too bad (a minute's worth of power out of a million-kilowatt power plant gives your 63 billion joule energy figure). The actual physics of getting this much force into the payload will be the tough part. Accelerations ============= Do you have a feeling just how bad these accelerations are? I can't imagine 6000 gravities. Do steel or aluminum deform at such pressures? Is there any hope of putting boosters or electronics on board? This seems to be the fundamental limitation of earth-based EM launchers. There's no way you can reduce the accelerations required, just from the basic orbital mechanics of it. As a means of putting raw materials in orbit, though, there's hope. Polar orbits ============ You mentioned polar orbits in your initial message. I'm not clear on why you would want polar orbits. ------------------------------ From: dietz@cs.rochester.edu (Paul Dietz) Mass catchers... I don't think making the catcher maneuverable is a good idea. This will REALLY limit the duty cycle, since the errors in successive payloads aren't going to be the same, and it takes time to maneuver from one to the next. Also, the catcher is going to be heavy (remember, closure velocity is several km/sec, so it will have to be heavily armored), moving it around will consume a lot of reaction mass and/or energy. Drag... Their projectiles had radii up to 20 cm, and lengths up to 10 m. They found 50% of the velocity loss occured below 6 km, while mass loss from the ablator peaks at 25 km. This strange situation occurs because at lower altitudes the dynamic pressure is higher, so the blowing layer of vaporized carbon is more optically dense and blocks radiation from the shock more effectively. Power... There was a survey article in IEEE Spectrum some years back. There may have been more than one. Kolm proposed using a LN2 cooled aluminum storage coil and tapping into the Pacific Intertie for a few minutes. Accelerations... 6000 gees is below the acceleration experienced in guns and some artillery pieces. You might look up the muzzle velocity and length of antitank guns (for example). The bigger the projectile, the lower the acceleration you can tolerate, though, since the projectile is longer and under "gravity" exerts more pressure on its base (the square-cube law in operation). Time to look at a book on interior ballistics. Polar orbits... The reason I proposed polar orbits was because a payload launched from an EML to a point above a pole can put itself into an elliptical orbit in the same plane as the space station. This is not the case for other inclinations except equatorial orbit. ------------------------------ From: mnr@cs.cmu.edu (Marc Ringuette) Polar orbits ============ Oh, I get it. It's MUCH cheaper to change orbital planes at apogee, so you want all the apogees to be at the same place! I still don't like polar orbits, though, especially because if you aerobrake, you end up in nearly the worst possible LEO. ------------------------------ From: dietz@cs.rochester.edu (Paul Dietz) Some additional comments... You asked if 6000 gravities is too high for boosters or electronics. The answer for electronics is "no". Proximity fuses in artillery shells tolerate accelerations of this magnitude or higher -- even with WWII technology. Some SDI schemes have proposed smart projectiles launched at 500,000 gravities in orbiting railguns. Since those contain course correction capability, it must be plausible to have rockets that can withstand those accelerations (albeit very small rockets). I will do more look-up on this subject. I would imagine that as the fuel fraction of the vehicle goes down, it becomes easier to make rockets acceleration resistant, since you can make fuel tanks smaller and more robust (walls inches thick, say, probably manufactured by machining the block(s) of metal that will form the vehicle's body). If the total delta-V required is 200 m/sec, and one is using a monopropellant with Isp of 150 seconds, the fuel fraction is about 15% (what's the Isp of hydrazine?). Making the rockets have low thrust also helps, since nozzles, fuel lines, valves, etc. can be made smaller and therefore stronger. ------------------------------ From: dietz@cs.rochester.edu (Paul Dietz) Let me refine this a bit. You suggested a "bag of sand" with rockets in HEEO. Actually, the delta-V between the catcher and the payload can be quite small (< 100 m/sec), so it's probably better for the catcher to rendevous with and grapple the payload at apogee. The mass of the catcher (an OMV, really) could then be much less than the mass of the payload, so little fuel would be needed for it to maneuver. I worry, though, about trying to catch more than one payload per orbit. They will be spread around by velocity errors, so it would take the OMV a long time (I'd guess on the order of an hour) to move from one to another. What does the catcher do with the payload after grabbing it? I guess it should stash the payload onto a co-orbiting storage rack. The payloads would have to carry fuel to refuel the OMV. After 100 launches (say) the rack, equiped with an aerobrake, is lowered to LEO by the OMV, or, oppositely, is boosted to HEO or out of Earth orbit entirely. The payloads could be pretty dumb. Just put in an acceleration resistant microwave transponder for tracking. No attitude control would be needed. I don't know if grappling an unguided, possibly rotating, object would be easy. Perhaps the OMV could be teleoperated. Some of the payloads would have valves and tanks for carrying fuel, probably nitrogen tetroxide and UDMH. In a 1 day orbit, supposing we catch a one ton payloads on each orbit, one catcher can get about (say) 150 tons/year into HEEO. If current launch costs to HEEO are about $10,000/lb, this is worth about $3 billion. I should subtract out the mass of the fuel for the OMV. Fully used, this system would require several rack/OMVs in different elliptical orbits. As the earth rotates, the launcher would fire to different catchers. The catcher's orbits would have to have periods that were integer multiples or fractions of 1 day, so the catchers would be in the right positions when the payloads reach their apogees. Phase matching would be important; you can't let the catcher/storage rack and the EML get out of sync. It might make sense to put the catchers in polar HEEO's with different orbital planes. That way, full storage racks could change plane at apogee to a common orbital plane and bring all the payloads together. The other thing to do is to rendevous in high circular orbit, or perhaps at one of the earth-moon Lagrange points. I suppose it depends on where you are eventually going: to inject to Mars, for example, you have to be on an orbit so that after accelerating to above escape velocity at perigee the hyperbolic orbit ends up pointing in the right direction. ------------------------------ From: mnr@cs.cmu.edu (Marc Ringuette) OMV's to catch the payloads =========================== I like your small-OMV scenario. For these parameters (on the order of 100 m/s velocity change), it sounds like a good idea. They can be small enough that you can have a bunch of them, and you don't mind if they can only be used once a day. The main reason to bother with all this hassle, I think, is so that you can have an OMV with decent communication and rendezvous capability, rather than having to boost such capability on each launch. If you settle for a dumb booster on each payload, you then need OMV's to collect the payloads for whatever you want to do next. I'm not sure if communications is an issue, hence the next topic: Communications ============== Does anybody know what kind of communications is appropriate for small space vehicles in orbits from 1 to 20 earth radii? What kind of antenna, tracking, and power do you need? How small a package can you use? I'd love to get a better idea on this, since it impacts solar sails and other near space applications as well. ------------------------------ [ end ]