Aucbvax.4326 fa.space utzoo!decvax!ucbvax!space Thu Oct 8 04:13:51 1981 SPACE Digest V2 #8 >From OTA@SU-AI Thu Oct 8 04:07:41 1981 SPACE Digest Volume 2 : Issue 8 Today's Topics: Energy in Space Energy in Space Energy in Space Letter writing campaigns Bussard ramjet speed limit ---------------------------------------------------------------------- Date: 7 Oct 1981 17:30:47 EDT (Wednesday) From: Mike Lease Subject: Energy in Space To: Dietz at USC-ECL, SPACE at MIT-MC Cc: mlease at BBNP Reply to: MLEASE at BBN-UNIX The problem with fossil fuels is that they are in limited supply, not that there is a temporary shortage of them. I agree that the cost of an SPS system would be steeper than the cost of a "50KW diesel motor/generator"; but the SPS may well be usable a century from now, whereas the fuel for the generator will in all probability have been used up by then. Of course, I am not taking into account wear and tear on the SPS -- anyone have any guesstimates on that issue? Anyway, what we need to do is begin searching for renewable sources of energy while we still have non-renewable ones left to enable us to do so, and SPS is one of the sources we should investigate. Mike Lease ------------------------------ Date: 8 October 1981 00:19-EDT From: Robert Elton Maas Subject: Energy in Space To: DIETZ at USC-ECL cc: SPACE at MIT-MC I agree. If we try to simply build an SPS now from Earth materials, and assemble it in space, it'll be too expensive (capital intensive). We have to first establish industry in space, so we can process lunar and asteroid/comet materials, and build an SPS when that industry is sufficiently well along that building an SPS becomes cost-effective. Meanwhile, after we establish industry, but before we can afford SPS, we can do energy-intensive materials production (aluminum mostly) in space and send the finished product to Earth. There's so much titanium in moonrocks that maybe we'll also extract titanium and send it to Earth, in fact that might be cost-effective before aluminum is, simply because the Earth-price of titanium is so much higher. I suspect it's fairly easy to run simple aluminum and titanium extraction by remote control, moreso than running SPS-manufacturing/assembling devices. Summary: (1) Assays of lunar-polar and asteroid for hydrogen&carbon (2) Lunar-polar remote-control water extraction (3) Lunar-polar remote-control aluminum&titanium-extraction (4) Catching an asteroid and r-c extracting carbon&iron ... I hopw we get funding for (1) soon. ------------------------------ Date: 8 October 1981 01:21-EDT From: Jerry E. Pournelle Subject: Energy in Space To: DIETZ at USC-ECL cc: SPACE at MIT-MC 1. Space solar power systems may or may not be economical for delivery of power to Earth; but they seem very useful for delivery of power IN SPACE. Industrial, experimental, and, yes, military systems in space will need power; someone will get it and use it; it matters little whether they speak Japanese, Swahili, Russian, or American, except possibly to US citizens. >From my viewpoint it is easier to learn to get along with US than to learn Russian. 2. 90% of the resources available to mankind are not on Earth. Someone will get them. If not us, who? If not now, when? 3. We kill some 50,000 people a year with burining oil and coal. Nuclear, I grant you, appears to be far safer and healthier, but it is also politicially and economically impossible. True, it is economically impossible for diabolical reasons: those who hate it do things which run up the costs and then say it is uneconomical, the moral equivalent of those who cut off a person's legs and then castigate him for not making a living; but it is still economically impossible. What, then shall we use? Shall we drain America first? We ship $50 billion a year off to foreign lands for oil each year (perhaps more; I use the lowest figure I have ever heard); the whole SPS project is only $100 billion, ALL OF WHICH IS SPENT IN THE US, and which is very likely to return us a very great deal in fallout technological benefits; which I have not heard that investment in security forces for Saudi Royalty does for us. ------------------------------ Date: 8 October 1981 01:47-EDT From: Jerry E. Pournelle Subject: Letter writing campaigns To: FFM at MIT-MC cc: SPACE at MIT-MC Bush is more than 50% probable chance to be President within 10 years; he is known to favor space; he gets little mail. JEP ------------------------------ Date: 8 Oct 1981 00:28:19-PDT From: decvax!utzoo!henry at Berkeley Subject: Bussard ramjet speed limit Since I haven't seen anybody else demolish the myth about the 0.17c speed limit on Bussard ramjets, here goes. The article in Asimov's was not the first recognition of the problem; contrary to what was said in the article, this problem has been known for years, but has not been trumpeted out loud because there is a straightforward fix. It is reasonably obvious to anyone who does a real analysis of the Bussard ramjet instead of relying on analogy from "ordinary" jets. (In the following, all velocities etc. are with respect to the ship.) The purported speed limit works as follows: thrust from the expulsion of fusion-engine exhaust is counterbalanced by intake drag from fast-moving incoming fuel hitting the ramscoop field, so net momentum transfer to the gas stream works out to be zero. With some simplifying assumptions, the speed limit equals the engine exhaust velocity; it is easy to set an upper bound on this based on the reactions involved. The mistake is to assume that the kinetic energy of the incoming fuel is necessarily converted to heat or some other useless form. WRONG! Suppose instead we decelerate the incoming protons against an electric field. The momentum IS transferred, but the energy is stored as potential energy. We use that stored energy to further accelerate the outgoing exhaust by letting the protons fall down the other side of the same electric-field potential hill. Other variations are possible, but the principle remains the same: use the kinetic energy rather than wasting it. There is still some momentum transfer, because the same amount of kinetic energy does not mean the same amount of momentum at different velocities. The exhaust acceleration is less effective at producing momentum because it is applied to already fast-moving material. But this is a lesser effect; barring losses, there is always a nonzero net momentum transfer to the gas stream (unless relativity introduces some subtle complication at extreme velocities; not my specialty). Of course there WILL be losses, and integrating this with a ramscoop may be a lot of fun, but those are problems of technology, not fundamental physics. In any event, the ramscoop is the EASY part of a Bussard ramjet, by current thinking: the HARD part is getting a decent reaction rate out of a fusion reaction burning ordinary hydrogen. Building a ramscoop is a formidable engineering problem, but a fast proton-proton reactor involves nasty difficulties of fundamental physics. It may be necessary to go to an internal energy source, either ordinary fusion (the "ram-augmented rocket" scheme) or antimatter. The latter is the more interesting: antimatter-heated rockets have rather (!) high performance themselves, and adding "free" reaction mass makes it even better. To sum up: the near-c Bussard ramjet is not impossible, it's just complicated, difficult, and not quite the way it was visualized. Henry Spencer ------------------------------ End of SPACE Digest ******************* ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.