Here are the first four space-tech excerpts ... about 1500 lines. ------------------------------ Subject: Space-tech excerpt: EM Launchers [270 lines, fall '88] Introduction: ============= An Electromagnetic Launcher (EML) is a device which uses electrical power to fire a projectile into orbit. It can be in the form of two parallel rails (a "rail gun") or many rings in a line (a "mass driver" or "coaxial EML"). The actual EML technology is challenging, but it's nearly here. An EML for launch to a highly eccentric orbit will require around 11 km/s muzzle velocity. Our rule-of-thumb values are a launcher a kilometer long and having accelerations of 6000 g's. The biggest challenge, however, may not be the launcher, but rather shielding the projectile for its flight through the atmosphere. Condensed discussion from the space-tech list: ============================================== From: dietz@cs.rochester.edu (Paul Dietz) Subject: Electromagnetic Launchers The Launcher ------------ 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. [ Note on launcher technology: I was looking through the '84 IEEE Transactions on Magnetics like Paul suggested, and it was encouraging! There were several pre-SDI projects on rail guns and coaxial EML's, and my guess is that the problems will be solved. Some of the concentration at that time was on power sources and control questions; future work will have to deal with current densities and bursting forces. -- Marc ] Payloads and Shielding ---------------------- Is 6000 gravities 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! 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 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. 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. [ Wow, I had no idea how nasty the environment in the nose is! This is clearly one of the toughest problems to solve. Wild stuff. -- Marc ] Orbits ------ 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). Direct launch into low-earth orbit (LEO) does not appear to be feasible. 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, but even 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. Consider launching to an orbit with apogee at 20 earth radii (from the earth's center) and perigee at low altitude. 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 about 200 meters per second. Launching to an orbit with apogee at 40 radii means the delta V is only 100 m/sec. -- Paul F. Dietz ------------------------------ [ It's worth emphasizing: highly eccentric orbits are FAR cheaper to reach than circular orbits, because the delta-v you must bring along with you is minimized. The whole point of an EML is so you don't have to bring along all those thrusters. Therefore, all our thoughts from here on assume that you start by launching into a HEEO, then go on from there. -- Marc ] ------------------------------ [ I omit a discussion of how to adjust what orbit you end up in. Send mail if you want the whole thing. I'll sum up as follows: Problem - An eccentric orbit has a very short launch window, once per day. Solution - You have to make a burn at the top anyway, so make the burn a little before or after apogee, and you can widen the launch window. It works out that you can change the angle of the orbit by 15 degrees in either direction by doubling the amount of delta-v you allow. This is probably enough flexibility for most purposes. -- Marc ] ------------------------------ From: Marc.Ringuette@CS.CMU.EDU Subject: How much Delta-v? I have a couple of rough numbers on how much delta-v we'll need to have, in order to get into a highly eccentric orbit. My rule of thumb is that 500 m/s is enough in almost all circumstances. It is possible to reduce that to 200 m/s in some cases, or to need as much as 1000 m/s. The smaller delta-v's can result from choosing a more highly eccentric orbit, or by angling the launcher to impart part of the angular momentum; but I think it's pushing it to expect less than 500 m/s in the normal case. The 1000 m/s numbers come about when the apogee needs to pushed the full 30 degrees from the "natural" apogee of the EML at the time of launch. How much fuel will we have to carry, then? That depends on the specific impulse of the fuel. ====== Typical values of specific impulse are: hydrazine 230 s solid propellants 290 s oxygen 445 s - Dr. Gary D. Gordon, "Spacecraft Technology", 1982. The exhaust velocity Ve is related to the specific impulse Is by: Ve = Is g, where g is the gravitational acceleration, 9.8m/s/s. -- Jim Van Zandt ====== From Jim's numbers, I would guess we can carry 25 percent of the payload as fuel and engine, and achieve 500 m/s delta-v. I think this is quite realistic. ------------------------------ From: Marc.Ringuette@CS.CMU.EDU Subject: What can we do with an EML? 1. Send supplies and construction materials into low earth orbit. LEO is probably the main destination for just about everything, but it's not trivial to get there. See below. 2. Do something useful in HEEO. If we have acceleration-resistant experiments, we can do them right there. This doesn't sound likely. 3. Boost things into geosynchronous orbit. All of a sudden, it is much cheaper to get acceleration-resistant payloads into GEO. What do we want to do there? [ 4. Use HEEO as a staging area for planetary misssions. See Paul Dietz's post, below. -- Marc ] Aerobraking =========== The most promising way of getting from HEEO to LEO is to aerobrake into an elliptical orbit with an apogee at the altitude of LEO, then boost into a circular LEO. Of course, the thing enters the atmosphere at the same speed it left it, which is over 10 km/s. But aerobraking can be done in stages, in the outer atmosphere, so it should be much easier. The boost after aerobraking isn't bad. I worked it out, and in order to boost from an elliptical orbit with perigee at radius 6500 km and apogee at 7000 km, into a circular orbit of radius 7000 km, it takes 430 m/s of thrust. So if we can get into such an orbit (in which most of the orbital velocity is already present) then it's quite reasonable to bring along a solid booster to do the rest. [ Note - to sum up, this means a projectile with about 700 m/s of thrust on board, and a VERY good heat shield, can be put in LEO by a ground-based EML. This is good news! Of course, it's not easy to make this work, but it's a promising thought. -- Marc ] Alternative: ion propulsion =========================== We could possibly get from HEEO to a low circular orbit using solar power and ion propulsion. Since the payload is in a stable orbit, we have lots of orbits in which to do the change. However, if aerobraking turns out to be easy, there's no reason to do it this way. [ Our space-tech group also briefly discussed ion propulsion. Send mail to "space-tech-request@cs.cmu.edu" if you'd like a copy of what we said. -- Marc ] ------------------------------ From: dietz@cs.rochester.edu Subject: The Utility of HEEO vs. LEO Marc asked what do we do with an EML that can launch to HEEO. I think a major use would be as a component in a system to explore the inner solar system. The major cost (in delta-v) of getting to Mars or the near-earth asteroids is the cost of escaping the earth's gravity well. A spacecraft in HEEO can fire its rockets at perigee (where they give the craft the most energy) and enter useful interplanetary transfer orbits with surprisingly small delta-v. In an ideal case, entering a Hohmann orbit to Mars from HEEO requires a delta-v of less than 1 km/sec. In a sense, HEEO is already 3/4 of the way to Mars. For visiting Near-Earth Asteroids (NEA's), delta-v could be equally small. For example, it costs 4.4 km/sec to get to 1982 DB from LEO. Leaving from HEEO, the cost would be about 1.2 km/sec (4.4 - 11 km/sec + 7.8 km/sec). Return from 1982 DB costs .1 km/sec using aerobraking. There is every reason to believe that more accessible asteroids exist. I'm not proposing using spacecraft launched by EML to visit asteroids. Rather, the EML will launch bulk materials to an operations center in HEEO. Things that could be launched include: rocket fuel, shielding material, food, air, water. I imagine launching (perhaps in sections) a stripped-down manned asteroid exploration vehicle to LEO. In LEO, sufficient fuel (brought down from HEEO by aerobraking) is added to boost the vehicle to HEEO. Once in HEEO, a full stock of consumables is added, and the fuel tanks refilled. When the vehicle returns from a journey, it stays in HEEO and is refueled there, using primarily materials launched by EML. ------------------------------ [ end of excerpt ] ------------------------------------------------------------ Subject: space-tech excerpt: CMU Mars Rover [ 200 lines, Jan. '89 ] The CMU Mars Rover Project ========================== The next step in Mars exploration is to send a flexible, mobile robot to Mars to collect and study samples from different areas. The importance of the mission is split between observation, on-site testing of samples, and return of samples to Earth. The requirements for this vehicle are pretty stiff if we are to try one of the more ambitious and more useful of the possible missions. The trickiest part of the problem is to do autonomous motion and sampling. Light takes between 10 and 40 minutes to travel the round trip between Earth and Mars, so a vehicle operated from Earth would be extremely slow. Even worse, NASA's Deep Space Network has other jobs to do, and the rover will spend half its time on the far side of Mars. This virtually requires a vehicle which can move and take samples using on-board computers and Artificial Intelligence techniques, with human intervention only once every few hours. The mechanical design of the vehicle is also difficult. The design is most highly constrained by a very low power budget - a few hundred watts to run a 1-ton vehicle. The CMU rover project is a 3-year project to build a rover which will operate on Earth terrains, and be the prototype for a rover which can 1. travel several hundred kilometers, reliably, over the period of about a year, traversing 1 meter obstacles and ravines 2. take core samples, aim instruments, and perform sampling and experiments as flexibly as possible 3. collect about 5 kilograms of samples and transport them to a return vehicle for return to Earth 4. weigh no more than about a ton 5. operate on about 300 watts of continuous power, supplied by a Radioisotope Thermal Generator (RTG) 6. operate efficiently even when not in communication with Earth The project has three main research areas: Mechanical Design, Sensing, and Control. The first group is building the vehicle, and is headed by Red Whittaker, a mechanical engineer who recently constructed a robotic vehicle to clean up Three Mile Island. The second group, sensing, is headed by Takeo Kanade, a Computer Science professor who has been involved with the NAVLAB autonomous truck. They are using a laser rangefinder and computer vision software to maintain a terrain map on board the rover. The third group is headed by Tom Mitchell and Reid Simmons, who do Artificial Intelligence work. Their group is designing the software to do motion and sampling without human intervention. The Ambler ========== The original proposal from the CMU group had been for a rover with large, soft wheels which could ignore small obstacles. However, the mechanical design group determined that a walking rover could better satisfy the reliability, stability, and power requirements of the mission. A few hundred watts is almost no power at all, so a wheeled vehicle loses because it puts so much power into its ground interactions. A legged vehicle is mechanically more challenging, but is smoother in operation and very energy efficient. The Ambler has six legs, each of which has two joints which move in the horizontal plane and a telescoping z-axis which stays vertical. The horizontal and vertical directions are totally decoupled - the machine always stays level, and the two horizontal joints also stay level at all times. Each of the six legs is attached to a central pole at a different height, so they can move 360 degrees without running into each other. The bulk of the body hangs from the center pole, close to the ground. Here is a picture: [[ Note: the design has changed; this is out of date ]] | |--------------------------------- | | | | | | |--------------------------------- -----------------------------| |^ ^ U | | | | |Shoulder Elbow U -----------------------------| | U U | | U U -------------------------- U U | | U U | Body (with RTG, sampling,| U U | computing, robot arm, | U U | instruments) | U U | | U | | | | | | | | | | | | | -------------------------- | | | | | /_\ /_\ (Side view of the body and the lower two of six legs) (the elbow and shoulder move in and out of the page) ============================================================================= _________ _ /--------- __--_\\_ ------// __--__-- \_\_/ //\ __- --__-- \_\_ // __----\ -- | \_\ -//__---- \\ ______--_| |--- | \\ _-- ____--- -- _-___ \\ / --- \ --__-__ / / \ / --|| / / ------ || / / || / / || || (Top view. 5 legs planted, 1 recovering.) (The leg segments shown here are horizontal; the z-axis goes into the page) ============================================================================= The machine has a reach of about 4 meters and a height of about 4 meters. The laser rangefinder goes on top of the central pole. The Ambler will walk a bit like a crab, with five legs on the ground at all times. When a leg is lifted from behind the vehicle, it is moved all the way to the front of the vehicle to minimize the number of footfalls required. The body slides forward using the horizontal joints only, spending energy only on friction losses and ground sinkage. It moves almost like floating on water. The z-axis is used to hoist the body up and down, and to lift each foot for recovery to the next position. The machine moves very slowly (since the limiting factor is the ability to control the motion reliably, not the motion itself). The body averages a few centimeters per second, which is plenty as long as the machine can operate autonomously. The Software ============ Building the Ambler is about half the project. The other half is putting together a software system to reliably (VERY reliably) move the robot from place to place and perform sampling tasks. A terrain map (more or less a contour map of the immediate vicinity of the rover) is maintained by integrating data from the rangefinder. Other information, such as "this is a rock" or "this is black stuff that sticks to your feet" may be attached to the basic map. This allows motion planning to be done accurately. The rover will be controlled by commands from the control center on Earth, such as "go north as long as it's safe" "go back and pick up rock 13 and look at its underside" "follow this path to rock 15 and drill a hole in it, at this angle" "pick up one of those gray pebbles, about half an inch wide" "put this dust in your mass spectrometer" "aim your infrared sensor at anything unusual" The commands won't be in English, but rather will be specified in terms of frames and slots in a knowledge representation system designed by the control group. Realistically, there will probably be a team of geologists fighting over what is most important to do next. Rather than forcing them to do the optimization of exactly what to do, it will be necessary to have a planning system which can do the best thing given a set of goals of varying importance and difficulty. For instance, if one goal is very easy, you might as well do it first rather than a more desirable but much more difficult goal. There are also background goals best monitored by the machine, such as "Never go too near a dropoff" and "Don't point your satellite dish away from Earth." A flexible geometric reasoning system will be a component of the software. It's important for the rover to have the ability to pick up a rock, and also to be able to notice if the rock was dropped by accident. This involves creating a general purpose planner which can generate expectations about what will be true in the world if all went as planned, and to check if this really happened. ============= As of now, January '89, the project has finished its first year. A single-leg testbed is being used to test the leg design and the footfall planning software. The project is funded by NASA, and there is interaction with groups from JPL, TRW, and Martin Marietta. JPL in particular has a parallel project, designing a more conventional wheeled rover for the same mission. Ours is considered to be the more ambitious and high-risk of the two attempts. The results will be evaluated before the possible production of a Mars-ready rover to be launched in approximately 1998. [ end of excerpt ] ------------------------------ Subject: Space-tech excerpt: Electrodynamic Tethers [ 160 lines, Oct. '88 ] 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: Electrodynamic Tethers There were some recent comments and questions about Electrodynamic Tethers. I quote from _Tethers in Space Handbook_ (NASA, 1986), pp. 2-29 - 2-36 [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. ] 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]... ] ] 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, 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. There are some aspects of electrodynamic tethers which can't be predicted purely by theory. 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 be vital to any practical application of electrodynamic tethers in the future. ---------------------------------------------------------------------------- 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 ------------------------------ [end of excerpt] ------------------------------ Subject: Space-tech excerpt: High Velocity Guns [375 lines, early '89] Here is some discussion on high-velocity guns, with the focus being on getting projectiles to orbital velocity using current materials and chemical fuels. Surprisingly, the prospects look good! ------------------------------ From: dietz@cs.rochester.edu I just read an interesting article: "The Distributed Injection Ballistic Launcher" H. Gilreath et. al., JHU APL Technical Digest 9(3), 1988, pp. 299-309. In a conventional gun, pressurized gas is injected once, and expands as the projectile travels down the barrel. As a result, acceleration drops off. The initial pressure is limited by the strength of the projectile and/or the barrel. Ideally, a gun should maintain constant pressure on the projectile. The DIL (approximately) does this by injecting gas behind the projectile from the sides at points along the barrel. This is a fairly old idea; the German V-3 guns in WWII used it (although they were never made operational). Just as a mass driver can be thought of as a linear electric motor, a DIL can be thought of as a linear internal combustion engine. Discrete injection of gas behind a flat-based projectile doesn't work very well. Instead, Gilreath et. al. propose to make the projectile boat-tailed -- that is, make its base be a long cone -- and inject the gas against the boat-tail as the projectile passes. If the boat-tail is sufficiently pointy (small boat-tail angle theta) then the axial velocity the gas must attain is reduced (by a factor of tan(theta)), and the system can operate efficiently even if the projectile is travelling much faster than the speed of sound in the gas. The limit the authors give is about 15 km/sec. The authors say (but do not justify) the mass penalties associated with launching directly to orbital velocities would be very great, due to the need for thermal protection. They suggest using the DIL as a first stage. They do say, however, that "complex electronics packages ... can easily tolerate accelerations of tens of thousands of g." That they say this isn't surprising, since JHU APL developed the first gun launched proximity fuse during WWII, and it tolerated 20,000 g, even though it contained five vacuum tubes. The article has an interesting picture of an extended-barrel 16" gun (conventional, not a DIL) that was operated in Barbados in the 1960s and early 70s. It could launch atmospheric diagnostic probes at 1.6 km/sec, with apogees up to 100 km (at a launch cost of few dollars per kilogram). The gun was used to launch scramjet test vehicles; they failed at launch, but theoretically they could have had a range of up to 3700 km with apogee at up to 1000 km. There is a picture of one test vehicle. It had a mass of 100 kg and burned 3 kg of triethyl aluminum (it is not clear if this vehicle had the stated range). It was designed to withstand accelerations up to 10,000 g, but suffered structural damage to its fins and skin in the test firing. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: attcan!utzoo!henry@uunet.UU.NET > ... "complex electronics packages > ... can easily tolerate accelerations of tens of thousands of g." > That they say this isn't surprising, since JHU APL developed the first > gun launched proximity fuse during WWII, and it tolerated 20,000 g, > even though it contained five vacuum tubes. Said packages do have to be specially built, though. The vacuum tubes for the proximity fuzes (a "fuse" is an electrical safety device) were quite unorthodox designs. They were placed on the axis of the shell to minimize centrifugal force, and the tube elements were made of the thinnest practical wire to get maximum benefit from the square-cube law. The tube envelopes, needless to say, were metal. I'm trying to recall a piece I saw some years ago on the electronics in the Copperhead laser-guided shell. My dim recollection is that they used ring-shaped circuit boards around a central core, supported the boards at both core and outer edge, and otherwise just used careful mil-spec circuit construction. > The article has an interesting picture of an extended-barrel 16" gun > (conventional, not a DIL) that was operated in Barbados in the 1960s > and early 70s. It could launch atmospheric diagnostic probes at 1.6 > km/sec, with apogees up to 100 km... Ah yes, HARP. (High Altitude Research Project.) A joint US-Canada project. The gun was two battleship barrels end-to-end. One hears occasional mutterings that work along those lines may have been continued for a while, on a smaller scale in secret. HARP used fairly straightforward methods: an extra-long barrel, smoothbore (it was actually 16.5 inches when they bored out the rifling, I think) with fin-stabilized projectiles, and subcaliber projectiles (that is, projectiles rather smaller than the gun bore, padded out to full diameter with a light-alloy jacket that falls off on departure from the barrel). These techniques are all standard, on a less ambitious level, for modern tank guns (although smoothbore guns weren't in HARP's day). "Distributed Injection Ballistic Launcher", indeed. :-) It's a booster cannon. (I think we can claim that Heinlein's name for it has priority, by about 40 years.) Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ From: dietz@cs.rochester.edu Following up on my last message about the distributed injection launcher, here are some other concepts I've read or thought about. All involve projectiles accelerated in a tube by gas pressure. Ram Accelerator --------------- In this concept, the space between the wall and the projectile forms an annular ramjet. Before firing, the tube is filled with some kind of gas (either oxidizer, fuel, a mixture or a monopropellant). The projectile front is shaped to compress the oncoming gas; the back is shaped to act as an inside-out nozzle. The projectile might carry fuel or oxidizer (perhaps in solid form). I recently read somewhere (on the net?) a report of a professor and students that built a model ram accelerator. Anyonme remember this? Travelling Charge Gun --------------------- Unlike a conventional gun, in which the charge is burned in the chamber, the charge in a TCG is attached to the back of the projectile and travels down the barrel. As a result, the pressure produced by the burning is applied where it does the most good. When the projectile velocity is high this is much more important than the loss in velocity due to the need to accelerate the charge. The TCG can be thought of as an inside-out solid rocket, where propellant burns inwards and the space between the propellant and the tube wall acts as a nozzle. The projectile could also have a spike projecting backwards to increase thrust. Unlike a conventional solid rocket, the projectile need not have a steerable nozzle or, indeed, any guidance at all. However, the fuel must burn much more quickly. One might also design a multistage TCG, equivalent to a multistage solid rocket. The first stage would help compress the exhaust from the second stage; this clearly isn't possible with ordinary rockets. One might also begin with the stages disconnected, perhaps with some buffer gas between them. This idea leads to... Multistage Light Gas Guns ------------------------- A large piston is accelerated by a conventional gun. It is rammed into a pump chamber filled with hydrogen or helium. The light gas is compressed and heated, and, after rupturing a diaphragm, accelerates a smaller projectile down a tube. The maximum muzzle velocity in a gun is, roughly, proportional to the initial speed of sound in the gas. The speed of sound is sqrt (gamma R T / M) where gamma is the ratio of specific heats, R is the universal gas constant, T is the initial temperature and M the molecular weight. So, it makes sense to use hot, light gases. There are light gas guns in operation that can accelerate gram sized objects to 7 km/sec or more. Naively, I think you could scale up guns while maintaining constant pressures and muzzle velocities. So, increasing the dimensions by a factor of ten would increase the projectile mass by a factor of a thousand. Scaling (for example) some numbers I saw for the Ames light piston gun (muzzle velocity: about 7 km/sec) to a 1000 kilogram projectile would give it a length of half a mile and a barrel diameter of four and a half feet. I'm not saying this would be practical, but it is interesting. A large light gas gun would use hydrogen, which is cheaper and has better performance. I wonder if it would be possible to combine light gas guns with traveling charges, so as to maintain pressure on the projectile at late times. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: dietz@cs.rochester.edu Continuing this thread... In traveling charge guns, the charge burns from the back forwards, not from the sides inwards. The pressure behind the projectile is already high, so using some sort of nozzle probably doesn't make sense. One thing that worries me about gun type launchers is abrasion between the projectile and the barrel wall. I assume this problem is solved by letting some of the gas leak around the projectile, forming a gas bearing. I talked to John Hunter at LLNL briefly. They are building a scale model of a light gas gun launcher. The full scale concept will launch projectiles with masses of several metric tons at 5 - 9 km/sec, at the rate of several per day. Their paper will be presented at a AIAA conference next July. Dr. Hunter said he would send me an abstract, and, if he does not object, I'll send more details to this list when I receive it. I find it encouraging that the professionals are actively investigating this topic. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: Andrew Higgins From: dietz@cs.rochester.edu (Paul F. Dietz) > I recently read somewhere (on the net?) a report of a professor and > students that built a model ram accelerator. Anyone remember this? You may be referring to the article "Impulsive Behavior" by Susan Sutphin in the April 1988 issue of _Space_World_ (Vol. Y-4-292 p. 18). This is certainly not a technical article, but it does give some good background information. A brief summary follows: Students at the University of Washington are working on a chemically propelled mass launcher. The project is headed by Professors Adam Bruckner and Abraham Hertzberg of the university's Department of Aeronautics and Astronautics. The vehicle is similar to the main body of a ramjet used in unguided missles. The vehicle travels through a stationary tube filled with premixed high pressure gaseous fuel and oxidizer. The vehicle carries no primary propellant of its own. According to Bruckner,"The concept is that we can accelerate a vehicle weighing several thousand kilograms up to about 10 kilometers per second using only chemical energy and readily available fuels." The project has produced a small scale model that uses a projectile weighing between 50 and 100 grams and achieves a velocity of 2,400 meters per second. They hope to increase this to 4,000 meters per second before having to move to a different facility. All design work is based on current technology. The university has signed a teaming agreement with Olin Corp. and has received a research grant from Langley Research Center to further the effort. Both Ames and Lewis Research Centers are showing interest. Bruckner, Hertzberg, and Bogdanoff currently have a number of patents pending. Also, according to the April 1988 issue of _Aerospace_America_, high velocity gun launch concepts were debated at the AIAA/Defense Advanced Research Projects Agency lightsat conference in Monterey CA, and at a similar conference at Utah State Univ. Someone may want to look into these. -- Andrew J. Higgins | Illini Space Development Society 404 1/2 E. White St apt 3 | a chapter of the National Space Society Champaign IL 61820 | at the University of Illinois phone: (217) 359-0056 | P.O. Box 2255 Station A e-mail: ahiggins@uxe.cso.uiuc.edu | Champaign IL 61820 ------------------------------ From: Ted Anderson I was the one who posted a note on the RAM Accelerator. The paper I have was published at the _37th meeting of the Aeroballistic Range Association_, Quebec, Canada, 9-12 September, 1986. The authors are A. Hertzberg, A.P. Brucknet, and D.W. Bogdanoff all from the Aerospace and Energetics Research Program, University of Washington, Seattle, WA 98195. On the issue of scaling up a light gas gun I'll mention that a John Hunter at LLNL was working on exactly this at least as recently as May. I have a draft of a paper he was preparing but I don't know what it's publishability status is. He determined that the system does indeed scale well. Ted Anderson ------------------------------ From: dietz@cs.rochester.edu > The paper I > have was published at the _37th meeting of the Aeroballistic Range > Association_, Quebec, Canada, 9-12 September, 1986. The paper has now appeared in a journal: AIAA Journal, 26(2) (Feb. 1988), pages 195-203. It is slightly different from the conference version, but does not report the most recent experiments that reached 2.4 km/s. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: dietz@cs.rochester.edu I've received copies of some recent papers on the ram accelerator. The most interesting is: "The Ram Accelerator: A Chemically Driven Mass Launcher" P. Kaloupis and A. P. Bruckner. AIAA-88-2968. Presented at the 24th Joint Propulsion Conference, Boston, July 11-13, 1988. The paper describes the components of a system that can place a 2000 kg vehicle into LEO. The details are apropos to the discussion in this mailing list about gun based launchers. The launch vehicle has a diameter of .76 m and is 7.5 m long. It is first accelerated by a methane/air gun to 700 m/s. The ram accelerator contains nine different gas mixtures at 33 atmospheres (the gas mixtures tailored to have the correct properties for different ranges of projectile speed). The mixtures range from 0.5 CH4 + O2 + 3 CO2 (.7 to 1.1 km/s) to 8 H2 + O2 (7.2+ km/s). Peak acceleration is < 1000 g. The vehicles are made of graphite epoxy and have a total structural mass of 625 kg. The graphite epoxy is coated with carbon-carbon ablator. This heating is apparently not as bad as I had feared. Total mass loss for a 9 km/s launch velocity due to atmospheric heating (starting at 4000 m altitude) is only about 38 kg for a 16 degree angle of elevation, about 20 kg for 22 degrees. Velocity loss ranges from 10% (30 deg.) to 20% (16 deg.). It is claimed that mass loss from ablation decreases as muzzle velocity increases, because although the heat load is higher, the vehicle is not exposed to the heating for as long a time. It is also claimed that the heating is largely convective, not radiative. In-tube ablation due to passage through the propellant gas is less than 1 kg; ablation in the combustion zone, less than 3 kg. Unfortunately, the vehicle is aerodynamically unstable. They'd better add fins, I think. For a 9 km/s launcher, the launch tube is 5.1 km long and is made of 41,700 tonnes of AISI 4340 steel. The paper talks about orbital maneuvesrs. Solid rocket motors are ruled out because they could not withstand launch, and have insufficient performance. Instead, they propose using nitrogen tetroxide and monomethylhydrazine, pressurized by a gas generator using hydrazine. Isp = 297 sec, thrust = 10,000 newtons. Hardware mass of the propulsion system is approximately 200 kg. The vehicle is aerobraked down to LEO in one pass at 30-50 km without the use of special aerodynamic devices. This apparently does not present heating problems. Mass fraction to LEO ranges from 19% (8 km/s launch velocity, angle 22 deg) to 43% (10 km/s, 18 deg). However, they have apparently not addressed the problem of matching orbital planes, perhaps because they think waiting for precession to match planes is too time consuming. I find it encouraging that low launch angles lead to acceptable ablation. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ [ end of excerpt ] ------------------------------------------------------------ Space-tech excerpt: Mars mission ship design [500 lines, fall '89] ------------------------------ From: al@questar.QUESTAR.MN.ORG (Al Viall) Setting aside long discussions on logistics of a Mission to Mars (cross your fingers NOW), I would really like varied input on what you might think the ship for a Mars Mission would look like and how it would operate.(you may now uncross your fingers)<---heh!! Let's us possibly assume that the world powers could get together long enough for such an undertaking, since we all know that such a project will take much more than just one entity to do this. Such a ship would most likely be designed to incorporate some sort of artificial gravity for the two-year journey there and two-years back. But details, lets hear details.......... ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU Certainly you should have two capsules on a cable 100m to 1000m in length, spinning to provide between .1 and 1 G. My guess is, say, a 500m cable providing 1G might be most practical. That's no sweat. Probably one capsule will contain the people and life support, and the other one will contain random stuff to balance the weight (although exact balance doesn't matter). As much as possible can be left in at the center axis to reduce load on the cable. The astronauts would probably have a small electrical cable-climbing winch to transport one of them up the cable to the center, to get more supplies or do maintenance. [ Aside: ring-shaped structures are unnecessary and not as flexible as just two masses on a cable, spun up to speed after the mission starts. ] [ Aside #2: why waste all that microgravity time? Set up some tended experiments at the hub, and possibly have workspace for 1/3 of the crew so you can work there in shifts. ] Ion propulsion is a likely engine setup. Its practicality might depend on the power requirements once the mission has reached Mars. The bigger the power requirements at the destination, the more power-generating capability you're going to bring along, which would be available to fire reaction mass out the back end of the vehicle in transit. I'm very fond of ion propulsion and would try to design the mission using it. If that's done, then the ion propulsion unit and solar collectors would sit at the center axis, in the null gravity region. I don't have figures on the quantity of supplies you'd need, but probably some effort to recycle air and water would be made, and perhaps growing plants would be brought along. ------------------------------ From: dietz@cs.rochester.edu I wonder if it would be possible to ameliorate the bad effects of microgravity by *sleeping* in one gee. I imagine a small radius centrifuge with ordinary beds. Perhaps if properly strapped in (so the head cannot move) Coriolis forces would not be important. Perhaps the same centrifuge could be used for exercise, if the crew are careful not to turn their heads. The downside would be that the crew would have to move between one and zero gee every day. They would have to be selected for resistance to space sickness. Paper studies I've seen point towards MPD (MagnetoPlasmoDynamic) rather than ion engines. Also, in a large vessel a nuclear reactor is better than solar arrays, I think, if only because the solar arrays would be severely degraded spiralling up through Earth's radiation belts. My personal opinion is that while it will be possible to visit Mars, and perhaps even set up a base, using chemical or fission-electric propulsion, real exploration and colonization will require high power fusion rockets. ------------------------------ From: dietz@cs.rochester.edu (Paul F. Dietz) Marc asked: how does an MPD thruster work? A coaxial MPD thruster consists of two concentric electrodes. The inner electrode is a rod, the outer a cylinder. The inner electrode may be longer or shorter than the outer. A gas is introduced between the electrodes at one end. A strong current passes from one electrode to the other through the gas. The currents in the electrodes produce a magnetic field which circles the inner electrode. The Lorentz (JxB) force propels the ionized gas down the barrel. You can think of this as a railgun that accelerates a plasma rather than a solid object. Beyond the electrodes, the plasma can continue to carry current. If properly designed, the magnetic field can be such that the JxB force has an inward directed component (pinch force), which helps reduce the lateral expansion of the plasma. In continuous mode, an MPD thruster needs megawatts of power to operate at high efficiency. If lower average power (and thrust) is desired, you can operate the thruster in a quasisteady mode (plateaus of current with low duty cycle) or in a pulsed mode (even shorter pulses). Solid-fueled MPD engines have been investigated for use in stationkeeping of satellites, since they can produce strong, short pulses of thrust (good for maneuvering rotating satellites) and are smaller than ion engines. ------------------------------ From: Korac MacArthur I think that the tether idea would not be as good as a rotating cylinder. The cylinder would allow for cargo to be stored at the walls, serving as a rad hard shelter (especially if it was water and fuel tanks, metal parts,etc). Even better for control would be the two cylinder design (opposite rotation) so an outer framework would not spin, and allow good null-gee areas as well as a stable platform for course correction thrusters instead of stopping the rotation or figuring how to time the thrusters right. Admittedly, two cylinders and a framework would take longer to build, but if you can't do it right the first tim e... I'd hate to have a tether snap, then all the parts go their separate (off course) ways. At worse, a more solid hulled ship would still be on course if anyone could fix it by the time the injection burn had to be made. ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU > I think that the tether idea would not be as good as a rotating cylinder.... Eek! You're comparing apples and oranges. A tether would add little weight to the system, and can be an add-on to most Mars mission scenarios. A cylinder would change everything, be extremely heavy, and make any mission many times more expensive. The gains are dubious also. Structural strength just isn't an important factor in a planetary mission; null-gee areas should be available regardless; and shielding would be very inefficiently placed at the rim of a rotating structure where it would require even greater structural strength. ===== The amount of thrust required to spin up a rotating structure is something to check; the velocity required of the capsules is the square root of the desired acceleration times the radius. For a 500m tether (250m radius), the capsules must be going 50 m/s relative to the center to produce 1 g at the outside. That's not too much compared to the thousands of m/s needed for the mission. ===== How about the shielding question, though? Should my Mars scenario include a well-shielded "box" for the astronauts to hide in during solar maxima? Maybe someone could check on that. ------------------------------ From: John Roberts Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. A system of two pods of equal mass connected by a cable and spinning in free-fall is very simple to describe, and I am willing to believe that it will perform as stated. When these conditions are altered, however, I am concerned that several problems might arise that would reduce the usefulness of the approach. Some of the possibilities: + Pods of unequal mass: presumably the two pods would move in circular paths at different distances from the center of mass, which would not be halfway along the line. The lighter pod would experience higher acceleration (not necessarily a problem). + Shifting masses: moving masses up and down the connecting cable would perturb the motion of the pods and the cable, for instance setting up oscillations which could last for hours or longer. + Center pod halfway along cable: I believe this would allow conditions in which the three pods were no longer in a straight line. Depending on the relative masses and the magnitude of perturbing forces, significant deviations could arise, including permanent imbalances and long-term oscillations. As a simple example, with unbalanced outer pods, the contents of the center pod would "slosh" around in small circles several times per minute. + Thrust applied while spinning: Any thrust applied while the system was spinning would have to be carefully calculated to avoid perturbing the internal motion of the system. I'm not saying that the system is impractical, but that the calculations to prove that it is practical are not trivial. (By the way, does anybody know of a computer program that can simulate the motion of a system such as this?) ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU On the weight issue: I got some sales literature from DuPont about Kevlar cables. Kevlar is roughly equivalent to steel cable of the same size, but has about 1/6 the weight. They quote 2.76E9 N/m/m tensile strength for Kevlar 29. A cable which can hold 100T at 1 gravity is 14 sq. cm. in cross section, assuming a 4x safety factor. At 1.44 g/cc, a 500m cable to hold 100T weighs 1T. I believe this is an actual practical figure, since they use this stuff to anchor oil rigs. That is, a 500m cable to hold up against 1 gravity can hold 100 times its own weight; since there are two capsules, that makes .5% of the mass of the system. The 500m length is arbitrary, but the rest are pretty solid. ========== John Roberts mentions some interesting questions... - shifting masses: if this is a problem, and to save energy, use a funicular setup with an hourglass-shaped crank near the axis. The crank would look like this: ------- ------- | ------- -------| | | | | ------ ------ | | | | | | | | ||||||||||| | | | | | | | | ------ ------| | | | | | | | | ------- | ------- | | | | ------- | ------- | | | weight | at | top | | | | | | | person at bottom Cute, huh? It preserves constant tension and costs no energy. - Oscillations: This is something to be dealt with at some point; part of the solution may be some small thrusters that do active damping of oscillations. - Thrust: good question! What's a good way to apply thrust to a rotating system like this? The axis stays mostly fixed, so it isn't possible to always thrust along the axis, unless you can change the axis as quickly as the thrust direction. But how do you apply thrust out of the axis without causing trouble? Perhaps you attach thrusters to every chunk of mass in the system and accelerate them equally... Anyone have an idea? ------------------------------ From: Tero Siili I don't want to spoil anybody's enthusiasm in brainstorming for a Mars mission; however, some people at JPL have been thinking about these things and I assume, that a report has also been published. A rotating spacecraft has been proposed, but looking like three spokes; the spokes are tunnels, NOT tethers (and with three spokes tethers would be complicated, if not impossible). When contemplating artificial gravity solutions, one must keep in mind, that - the positive or preventive effects of artificial gravity have not been demonstrated yet - we don't know, what level of AG - g/2, g/3, g/6 - would be sufficient to prevent adverse effects - tests have indicated, that humans do not feel comfortable, if the rotation rate exceeds approximately 2 rpm; this together with the required gravity level will dictate the diameter of a rotating spacecraft. The open questions must be studied carefully before a humanity embarks on a Mars mission. For these studies a special laboratory will be needed, a Variable Gravity Research Facility (VGRF). This problem has quite recently been studied by the students of the International Space University in Strasbourg, France this summer. A report will most probably be out by January 1990. To give you an idea of the costs, this prerequisite for the Mars mission would cost 30 000 M USD! Concerning radiation, some type of "safe haven" is probably mandatory. If the spacecraft is designed with the spoke concept, every spoke can have its own shelter. ------------------------------ From: Jonathan Leech Re the discussion about artificial gravity, here's a tangentially (sic) related reference: in Nature V340 (31 Aug 1989), pg. 681, "Space Sickness on Earth": "Sir: We report here the surprising aftereffects of prolonged centrifuge runs in which we, the three scientist-astronauts on board the D-1 Spacelab mission, have participated. We think we can simulate the space adaptation syndrome..." To summarize, they found that after several hours of centrifuging at 3G, they felt SAS-like effects upon returning to 1G, and suggest this provides a good way of studying SAS. This is a letter, not an article, so details are sparse. ------------------------------ [ taken from sci.space ] From: shelle@caen.engin.umich.edu (Thomas A Kashangaki) I know of two very complete sources for references and information on Rotating Space Stations. The first is a series of NASA Contractor reports that cover a detailed design of: AN ADVANCED TECHNOLOGY SPACE STATION FOR THE YEAR 2025 The Bionetics Corporation/ NASA Langley Research Center CR # 178345 (and three or four others). This is a very detailed study of all the different technology issues that have to be addressed before a rotating space station is a viable option. It is probably the most complete and up-to date study of rotating spacecraft to date. I am sure Bionetics or Langley would be willing to provide copies of the reports. And the one that I am more familiar with is a two year study performed here at the University of Michigan under the NASA/USRA Universities Advanced Design Program: PROJECT CAMELOT (Circulating Autonomous Mars-Earth Orbital Transport) Senior Design project 1986/87 and 1987/88 University of Michigan Aerospace Engineering Dept. Ann Arbor, MI 48109 These two reports describe a large 20-man spacecraft that would be used in support of a manned Mars base. The reports are quite complete and have good bibliographies and reference lists. Reports can be obtained by writing to Prof. Harm Buning at the above address. ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU I've been thinking about how to do off-axis accelerations of a rotating capsule system. I think I've got it! Check this out: have a triangular arrangement of capsules with cables at high tension connecting each to the other two, stiffening the structure through tension: ------------------------------- \ --_ _-- / \ --_ _-- / \ --_ _-- / \ --_ _-- / \ X / \ | / \ | / \ | / \ | / \ | / \ | / \ | / \ | / \|/ Accelerations off the axis just increase tension on some of the cables. This has the really great feature that all stiffening is done by tension. This allows the use of lightweight cables rather than stiff structural materials which would undoubtedly be much heavier. This reminds me of Buckminster Fuller...I wonder if there's a more efficient arrangement than the one I describe? The parameters can vary, and this may change the configuration. Let's hold the 1 g radial acceleration fixed. If a low-thrust system is used, side acceleration may be limited to, say, .01 g; so the cables at the "edges" of the triangles above needn't be very large. If a high-thrust chemical system is used, it may be impractical to keep the system spinning at all; or maybe some sort of weird pentagon will have sufficient rigidity to handle off-axis accelerations. Hmm. I haven't worked out any numbers, but I'm pretty happy with this triangle deal for low accelerations. Fun! === I'm still not convinced one way or the other on the exact best form of a rotating structure. I'm definitely convinced that cables are better than a fixed structure for any large diameters (which are probably necessary for strength and dizziness reasons). I'm not so sure how to accomplish off-axis accelerations: my triangle idea is one way that seems to hold up under a little analysis; maybe just very small accelerations at the axis of a single-cable system would be adequate; or if the mission requires only a small number of course corrections, perhaps de-spinning the system would avoid the problems of oscillations in the system. Let me know if you think I've missed something. ------------------------------ From: dietz@cs.rochester.edu (Paul F. Dietz) Here is some information from the 1986 NASA Mars Conference on possible mission plans... (1) Conjunction Class Missions The most traditional type. It is relatively low energy, with a long stay time. Outbound 270 Stay 530 Return 209 ------------------- Total 1009 days (2) Opposition Class Missions Very high energy, much shorter trip time, but also shorter stay time. Outbound 254 Stay 20 Return 245 ------------------- Total 519 days An inbound swing-by of Venus reduces the energy requirements of this class of missions, and increases the stay time, at the cost of a slight increase in trip time: Outbound 267 Stay 60 Return 366 ------------------- Total 693 days (3) Low Thrust Transfer Missions These employ low thrust electric rockets or solar sails. They include a coasting portion during the middle of the trip when no thrust is applied. Earth Spiral 52 Outbound 510 Mars Spiral 39 Stay 100 Mar Spiral 23 Return 229 Earth Spiral 16 ------------------- Total 969 Not all of this time need be taken by the crew; the crew could board after the vehicle has spiralled above the van Allen belts. I do not know what acceleration these numbers imply; an ultra low mass solar sail could probably do better. The inbound spiral at Earth could be avoided by parking the vehicle in HEO and returning the crew via OTV. (4) VISIT (Cycler) Orbits The next two concepts use "spaceports" in solar orbit as stepping stones to/from Mars. The VISIT-1 orbit is a 1.25 year orbit that swings close to Earth once every five years while approaching Mars once every 3.75 years. The VISIT-2 orbit is a 1.5 year orbit that approaches Earth once every three years while approaching Mars once every 7.5 years. The orbits would have to be retuned once every 20 years or so, and do not exploit planetary swing-by. (5) Escalator (Cycler) Orbits Unlike VISIT orbits, these orbits (due to Buzz Aldrin) use planetary swingby to rephase the cycler orbit. Basically, it is a more elliptical orbit that passes by the Earth once an orbit and Mars twice an orbit. At Earth, a wingby redirects it on to the next Mars encounter. The orbit's period is about 2 years. It would require a bit of nudging at times, for a total of about 2 km/s over 15 years. The orbit is high energy. ------------------------------ From: sci!daver%gungnir@Sun.COM (Dave Rickel) I haven't seen much yet about environment. It seems pretty clear that you'll want to recycle air and water. I dimly recall something about how bubbling contaminated air through superheated water would break down most of the nasty compounds. Anyway, it seems that life support would probably be some sort of algae to handle 95% of the recycling/detoxification, and some non-biological system (involving heat, electricity, chemicals (hmm, sounds like something you'd use on Godzilla), filtration, or whatever else is handy) to deal with the rest. Any ideas of how good you'd get? How much per man such a system would weigh, how many kg of consumables you'd have to carry per man per day? I've no idea what the state of affairs is regarding long-term life support systems, i suspect that we don't know nearly enough yet to get to Mars and back (i also suspect that something good enough could be put together in five to ten years, if we were really serious). ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU I hope we can discuss some other aspects of the design of a Mars ship: - Is laser communications practical? How much dispersion is there, and if it is low, how do you aim the lasers? - What kind of power systems are good for this kind of mission? Could we use a rotation-stiffened mylar mirror to magnify sunlight to solar cells or a thermionic generator? - How many exercise bikes should you bring along? - What kind of mass is involved in each subsystem you need to bring? Can you think of anything else? ------------------------------ [ end of excerpt ]