Date: Wed, 7 Sep 1988 19:59-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: "~/st/lists/stdigest" Subject: Space-tech digest #1 Contents: Intro message Paul Dietz: EM Launchers ------------------------------ Welcome! This is the first message to the space-tech digest mailing list. If you know of others who would like to join, please pass the word. I'm expecting more to join when my message comes out in Space Digest; I'll send new arrivals the messages they've missed. ================ Addresses to use ================ To send mail to the maintainer (currently Marc), use space-tech-request@cs.cmu.edu To send mail to everybody on the space-tech list, use space-tech@cs.cmu.edu You shouldn't need to send mail directly to the digest list, but if you want, you can send mail to space-tech-digest-request@cs.cmu.edu You can't post directly to the digest; I'll be doing that. ================ What's the plan? ================ The plan is that somebody with a discussion topic can send a question, idea, or writeup to the list. At the moment the list contains about fifty people, so it's not a huge overhead. The discussion will go from there; just send mail to space-tech@cs.cmu.edu and it will go to everyone. However, if you want to ask simple questions, it may be best to mail directly to the person involved, and summarize interesting answers to the list. We'll see how it goes. Steve Abrams suggested that we include calculations and references wherever possible. Sounds like a good idea, if we can manage it. More soon. ------------------------------------------------------------------------ | Marc Ringuette | mnr@cs.cmu.edu | I'll be stretching my mouth | | CMU Computer Science | 412-268-3728 | to let those big words come | | Pittsburgh, PA 15213 | | right out. [P. Gabriel] | ------------------------------------------------------------------------ ------------------------------ Date: Fri, 2 Sep 88 16:17:30 EDT From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: Electromagnetic Launchers I haven't received anything from this list yet, so I thought I'd submit a summary of some ideas on earth-based electromagnetic launchers (hereafter EMLs). Comments welcome. Motivation and Payloads ----------------------- First, why from Earth, and not the Moon? Isn't a lot more energy required? Yes, but unless there's a sophisticated infrastructure in place, the cost of power on the Moon will be far higher (the same for labor & capital). The Earth is also a better source of refined materials (rather than just raw rock). Volatiles are much more abundant here. What payloads could be launched? They would have to be acceleration resistant, so no people or fragile equipment could be launched. However, bulk materials are acceptable, including water, metal ingots, storable liquid propellants, some foods, bulk plastic, and so on. Most any material that might be obtained from extraterrestrial sources can be launched by EML in a more refined form. The launch vehicles themselves will be made mostly of useful materials, probably steel, which can be reused in space. Orbits ------ Any EML launching from the Earth's surface will require that the vehicle traverse the atmosphere. The trajectory cannot be horizontal, since atmospheric heating would be excessive. It need not be exactly vertical; an angle of 60 degrees increases the amount of atmosphere encountered by only 15 percent (= 1 - (1/sin 60)). I am not considering schemes where the EML is levitated above the atmosphere (Launch Loops or whatever); these seem too complicated and fragile. Clearly, it is preferable to put the EML on a mountain, this can reduce the amount of atmosphere to be traversed by perhaps half. An EML cannot fire directly into a stable earth orbit, since the vehicle will be placed in an orbit with perigee beneath the earth's surface. So, the vehicle must undergo some change in velocity to place it into a stable orbit. One can imagine active systems (the projectile has a kick motor) or passive systems (the vehicle hits a mass catcher). For a number of reasons I don't consider the second scheme to be viable, at least for earth-based EMLs. I'll concentrate on active systems. Direct launch into LEO does not appear to be feasible. Either the EML launches onto a trajectory with apogee a few hundred km up, in which case the kick motor must do most of the work, or the orbit is higher, in which case the kick motor must kill the vertical velocity component. EMLs do much better launching to high orbits. This is because velocity changes can be made far from the Earth, where they provide much more angular momentum (the moment arm is much longer). Launching to high circular orbit is more feasible than launching to LEO, since orbital velocity drops off as r^-1/2. Even for large orbits a delta-V of 2 km/sec or so is needed. Easier targets are highly eccentric earth orbits (HEEOs). These orbits don't have a lot of angular momentum (at most twice that of LEO), yet they allow the vehicle to change its velocity at great distances from earth (at apogee). Consider launching to an orbit with apogee at 20 earth radii (from the earth's center) and perigee at low altitude. The angular momentum of such an orbit will be just about 1.4 (in canonical units where radius of the earth = 1 and the period of an orbit of radius 1 is 2 pi). So, a launcher at an angle of 60 degrees provides 1/2 the angular momentum. The kick motor at the top must supply the rest: a velocity change of 1 / (2 x 20), or about 200 meters per second. Launching to an orbit with apogee at 40 radii means the delta V is only 100 m/sec. Will such a large orbit be stable? I don't know. Tides from the sun and moon will exert torque, and if the angular momentum decreases the payload will reenter. If the angular momentum increases the perigee will be raised, which might be nice. The mean radius of the moon's orbit is about 60 earth radii. A little simulation might help resolve this. Getting into orbit is only half the problem. The vehicle must also rendezvous with a space station. If the space station is in LEO, one can use the following scheme. Launch into HEEO with the same inclination as the space station's. Wait for regression of nodes to make the orbital planes coincident (this might take up to two months for typical space station orbits). Aerobrake down to LEO and enter a phase-matching orbit, then rendezvous. One has to arrange the HEEO so that the vehicle will be near perigee when the orbital planes coincide. Aerobraking might be done over many orbits to reduce heating. To reach a space station in HEEO, put the space station in polar orbit and launch the payloads into polar HEEO, doing a plane change at apogee. Ideally, one would put the launcher at the pole, but high latitude would also work. Payloads in HEEO can also be redirected onto orbits intersecting the moon. Directing plastic-carrying vehicles to impact near a colony may be a good way to supply the colony with volatiles. The Launch Vehicle ------------------ Anything shot out of an EML will experience thousands of gees of acceleration. The vehicles had better be rugged. They should also be cheap; ideally, we should not have to reuse them. Since the vehicle will have to use on-board rockets for course corrections and raising the perigee, we will have to know its position and orientation in space. It is likely a good idea to move guidance functions off of the vehicle -- gyros and accelerometers are expensive and sensitive. Position can be determined using position determination satellites. In space, one does not have errors introduced by refraction in the ionosphere, so high accuracy should be possible using a Geostar-like system. Measurements with an accuracy of 1 meter over 1000 seconds should enable one to compute velocities to within a few millimeters per second. To determine the orientation of the vehicle, we can illuminate the vehicle with microwave beams from a number of satellites. The vehicle measures phase differences between antennas on its periphery. We don't need very accurate measurements; errors of perhaps a few degrees or so around each axis are ok. Inaccuracies lead to errors in delta-V, but these errors can be measured and corrected. The vehicle doesn't need much maneuvering capability -- total delta-V of only a few hundred meters per second. For simplicity, one should probably use a pressure-fed liquid monopropellant, like hydrazine, or even just compressed gas. The engines need not have high thrust; the perigee-raising maneuver can take hours. For a vehicle with a mass of a few tons this requires a thrust of O(100) newtons. The launch vehicle must have a high ballistic coefficient (ratio of mass to cross-sectional area) in order to keep aerodynamic deceleration tolerable. Just what the limit is depends on the drag coefficient of the vehicle at extreme hypersonic velocities. I don't know what C_d is at 11+ km/sec. I guess one should make the vehicle have at least one kilogram of mass per square centimeter of cross section (the column density of air at sea level). For a 10 cm radius vehicle, this gives a mass of 300 kilograms. If the vehicle has an average density of 5 grams/cm^3 it would be at least two meters in length -- an aspect ratio of 10-1. Probably the vehicle should be larger, perhaps 1000 kilograms, and have a lower average density. The Launcher ------------ The major expense of the launch system is probably the EML itself. The kinetic energy of a 1000 kilogram vehicle at escape velocity is 63 gigajoules, or about the energy of 15 tons of high explosive. Quite a bit to deliver quickly. If the launcher is arranged nearly vertically, it must be at most a few km long (running up a mountain side). Perhaps you could bury or submerge the lower end, but that seems complicated and expensive. To reach 11 km/sec in 1 km requires an average acceleration of over 6000 gravities. Perhaps a way to reduce the acceleration the EML has to supply is a "ski-jump" launcher. A long, low acceleration EML is placed horizontally. At the end, a ramp diverts the vehicle from horizontal to a 60 degree angle. The vehicle is suspended above the ramp by repulsive magnetic levitation. The centripetal acceleration supplied by the ramp is high, but no high power storage or switching circuitry is needed: power is supplied by the vehicle's motion. Also, the force is applied laterally, so it can be spread over a larger area (remember, the vehicle is long and narrow). I chose 60 degrees as the angle because this cuts the vertical size of the ski jump in half compared to one diverting the vehicle 90 degrees (with the same radius of curvature). Repulsive maglev normally required the vehicle to have a magnet that induces currents in the track. It probably is uneconomical to put super- conducting magnets in the vehicle, especially if it isn't reusable, so some scheme using currents induced in normal conductor coils in the vehicle is probably required. I hope this doesn't require excessive currents in the vehicle coils. If it is necessary for the vehicle to use SC coils in the horizontal part, perhaps they can be placed in a sabot that is detached and decelerated before the ramp (as in a mass driver). Military Applications --------------------- The EML + guidance system described here would have military applications. Aside from being able to launch payloads into space, it can be used for nonnuclear intercontinental bombardment. The kinetic energy of a vehicle would greatly exceed its own mass in high explosive, so by simply guiding vehicles to impact on enemy targets one could do a great deal of damage, with no risk of pilots being shot down and captured. If projectiles are long-lived they could be stored in HEEO. Equiped with terminal guidance systems they would make potent weapons against industrial targets, surface ships, airfields and perhaps against missile silos. However, vehicles that had been deorbited at apogee would be obvious hours or days before impact. Paul F. Dietz dietz@cs.rochester.edu ------------------------------