Date: Sun, 6 Nov 1988 12:43-EST From: space-tech-request@cs.cmu.edu To: "~/st/lists/stdigest" Subject: Space-tech Digest #13 Contents: Marc Ringuette Solar Dynamic Power Paul Dietz Re: Solar Dynamic Power Paul Dietz Returning From Asteroids Christopher Welty A new topic for debate [ rockets vs. planes ] Steve Abrams Re: Public Domain Orbital Mech Code Paul Dietz Colloidal Electrostatic Engines ------------------------------------------------------------ Date: Tue, 1 Nov 1988 22:03-EST From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Solar Dynamic Power Solar Dymamic sounds like it should be by far the best way of getting good power-to-weight ratios in a small spacecraft. Put out a big mirror, and you can have a very big temperature difference to run a heat pump or steam engine. I'd like to discuss whether this is really true, and what the best kind of generator is. It should take advantage of the big temperature at the focus of the mirror while requiring as little mass as possible. However, I don't feel inspired right now to go out and hit the books. Anybody want to start us off? -- Marc ------------------------------ Date: Wed, 2 Nov 88 16:08:08 EST From: dietz@cs.rochester.edu To: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU Cc: space-tech@cs.cmu.edu Subject: Solar Dynamic Power I was looking at some information on nuclear electric power systems. The designs I saw used thermoelectric or thermionic generators. Typically, the converter, radiator + structure ran about 10 kg/kWe (adding the reactor + shielding roughly doubles this). The input temp. to the converter is 1400 K. The radiator operates at 775 K. The efficiency of the converter is from 9 to 13 percent. Heat is carried in heat pipes; would actively pumped coolant make the system lighter? A helium turbine would probably be a lot more efficient, if less reliable. Someone should give us data on the space station system. Whatever happened to the SSI study of solar dynamic powersats? They contracted out to someone to see what fraction of the things could be made from ET resources. Was the answer unfavorable? Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Wed, 2 Nov 88 10:17:26 EST From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: Returning From Asteroids I was thinking some more about returning mass from asteroids. Suppose we can easily extract water from a class C NEA. There are asteroids for which putting the water onto an earth-intersecting trajectory is not difficult -- delta V of a few hundred m/sec. However, we still have to aerobrake on arrival at Earth. That means going to below escape velocity in one pass through the atmosphere. A conventional aerobrake for this task would be massive, perhaps 25% of the mass of the cargo. We want to minimize the mass of the components that have to be sent to the asteroid, since that's where we find most of the delta-v. One could leave the aerobrake in HEEO for reuse in the next mission. We then go to LEO by repeated passes using a lightweight (1-2% cargo mass) metal shield. It might be a better idea to use transpiration cooling. We have lots of asteroidal water available. Cool the heat shield with some of it and expel the steam forward to buffer the shield against the shock heated air. I speculate that the heat shield could be made much less massive in this case, with some modest (10%?) water consumption. Extending this idea, perhaps a sufficiently cheap supply of water in LEO would make it easier to return massive objects to earth, again using transpiration cooled heat shields. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Wed, 2 Nov 88 21:26:50 EST From: weltyc@fs3.cs.rpi.edu (Christopher A. Welty) To: space-tech@cs.cmu.edu Subject: A new topic for debate Here's one I'd like to see some talk about: What do various people out there think about the value of `flying' or `launching' into space? What I mean is, if we here on the world renknowned space-tech list were to really put our heads together and build a launch vehicle (I'll supply the coffee and boloney (bologna) sandwiches) in order to get us to our privately owned and operated space station, how would we do it? Build a rocket? Build a spaceplane? Some hybrid like the shuttle? I'm talking about NOW. Go to it guys. ===== Christopher Welty --- Asst. Director, RPI CS Labs weltyc@cs.rpi.edu ...!njin!nyser!weltyc ------------------------------ Date: Thu, 3 Nov 88 12:00:26 CST From: sedspace@doc.cc.utexas.edu (405986289 abrams) Posted-Date: Thu, 3 Nov 88 12:00:26 CST To: space-tech@cs.cmu.edu Subject: Re: Public Domain Orbital Mech Code I, too, would be interested in suchg code. If, however, we cannot find any as yet...perhaps this group might contain enough people to divvy up the code and write such a program. What does the group think? Such a program should be able to handle low and high thrust propulsion systems, gravity assit maneuvers, etc...perhaps, even, defining launch position/ velocity vector and arrival position/velocity vectors with a time span to yield an optimal course. Is this feasible for a destop computer? I should think so. I'll try to look into it further and post something of my feelings this weekend... Steve ------------------------------ Date: Sat, 5 Nov 88 13:34:30 EST From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: Colloidal Electrostatic Engines I was thinking about using ion engines to retrieve material from near earth asteroids. It seems to me that what you really want is an efficient electric engine with a lower Isp that can use asteroidal materials (perhaps after some processing). Ion engines have Isp = 3000 or more, and pretty good efficiency (70-80%). I want a rocket with an Isp of perhaps 600, but still with high efficiency to minimize the cost of the power supply. There was some work years ago (early 60s) on collodial electrostatic engines. The idea is to use small colloidal particles instead of ions in an electrostatic rocket. The implications: - The mass/charge ratio is much higher, so for a given potential the exhaust velocity is lower. - Much less energy is needed to ionize the reaction mass, since the charge/atom ratio is small. Also, less energy is lost to line radiation from excited atoms. - Current density in an ion engine is limited by space charge effects. At constant Isp, current density is proportional to m/e, and mass flow density (in mass/(time x area)) is proportional to (m/e)^2. At constant m/e, current density is proportional to Isp^3. These both assume the accelerating gap's dimensions are unchanged; current density is proportional to (gap size)^-2. I imagine some scheme where asteroidal material is processed into very fine particles, say 100,000 amu. The particle receive one unit of charge each and are accelerated through a potential of several tens of thousands of volts. If necessary, the particles could be made larger and given more charge, and the accelerating potential increased, although if it is too large arcing might occur. The key problem here is finding a way to make colloidal particles of uniform mass and charge without making ions (which would parasitically carry much of the current if they were present in quantity). Making very small particles might be difficult. We might go to larger particles. The electric field at the surface of a spherical particle of radius r at charge Q is 9x10^9 Q/r^2 (in mks units). If we charge the sphere to the field emission limit (10^10 v/m for perfectly spherical positively charged particles) then a particle of radius r and density rho has a mass/charge ratio of 3.8 r rho kg/coul. If r = 10^-5 m and rho = 2.5e3 kg/m^3, m/Q = 2.5e-3 kg/coul, and to reach an exhaust velocity of 6000 m/sec requires a potential drop of 450,000 volts. I think it more likely that we cannot reach 10^10 v/m, and higher accelerating potentials are needed. We'd need some sort of multistage linear accelerator. I wonder if variations in the mass/charge ratio of the particles would interfere with the operation of the accelerator, or if the principle of phase stability would fix small errors. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ End of Space-tech Digest #13 *******************