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Mission to Alpha Centauri
I did a few calculations on what it might take to do a flyby of Alpha
Centauri, so, like a fool, I propose to rush in (to this newsgroup) where angels fear to tread. I read that Deuterium-Helium3 fusion would give an exhaust velocity of about 0.1 c if there was 100% efficient conversion of energy to thrust (neglecting the fact that no one knows how to do this). My thought was that a 2-stage intersteller rocket design could potentially be used as follows (and if I've reinvented someone else's idea I apologise): stage 1: stage 2 41 tons stage 1 rocket 410 tons DHe3 fuel 13500 tons stage 2: Instrument probe 1 ton 2nd stage rocket 10 tons DHe3 fuel 30 tons I chose v/ve = 1 for convenience. Stage 1 would accelerate stage 2 to 0.1c, stage 2 would accelerate the instrument probe to 0.2c which would pass alpha centauri about 20 years after launch. The telemetry would arrive back about 24 years after launch. Instructions for detailed investigation would be beamed back to stage 1 which could then manouver for further investigations on it's flyby 40 years after launch. Though the technology probably won't exist for decades or centuries, and though the above masses for engines etc are sheer guesswork, I wondered what "blue sky" interstellar vehicle proposals or mission outlines exist in the research community that are more than just sceince fiction. Also, has anyone considered the size and communications capability needed for a low-relativistic payload, and the data that could be gained in a flyby that would last about 24 hours (neglecting how the payload would be accelerated). |
#2
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Mission to Alpha Centauri
In article ,
Roger Stokes wrote: I read that Deuterium-Helium3 fusion would give an exhaust velocity of about 0.1 c if there was 100% efficient conversion of energy to thrust... My thought was that a 2-stage intersteller rocket design could potentially be used as follows... Though the technology probably won't exist for decades or centuries, and though the above masses for engines etc are sheer guesswork, I wondered what "blue sky" interstellar vehicle proposals or mission outlines exist in the research community that are more than just sceince fiction. See, in particular, the BIS's "Project Daedalus" in the late 1970s, which proposed a two-stage D-3He fusion rocket for a Barnard's Star flyby mission. Some of the details of their design probably would not work, in hindsight, but they explored the issues in considerable depth. (Unfortunately, the Daedalus report is not exactly easy to find unless you have a good library handy.) There has been quite a bit of speculation in similar veins since. Also, has anyone considered the size and communications capability needed for a low-relativistic payload, and the data that could be gained in a flyby that would last about 24 hours (neglecting how the payload would be accelerated). Yes. :-) Size depends on what you want to do, but you'd want something fairly substantial. If memory serves -- my copy of the report isn't handy at the moment -- Daedalus specified a 100t payload, which included a variety of sub-spacecraft. Communications is not a big problem, given large dishes and ample power, and given that data can trickle back to Earth over a period of several years. (The probe's own decision-making has to be entirely autonomous anyway, since by the time it's close enough to get new data, there is no time to consult Earth about how to proceed.) And the fast flyby is definitely a problem, but probably a manageable one. This was one of the reasons why Daedalus had sub-probes, so that it could do close flybys of all planets in the target system simultaneously. -- MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer first ground-station pass 1651, all nominal! | |
#3
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Mission to Alpha Centauri
"Roger Stokes" writes:
I did a few calculations on what it might take to do a flyby of Alpha Centauri, so, like a fool, I propose to rush in (to this newsgroup) where angels fear to tread. I read that Deuterium-Helium3 fusion would give an exhaust velocity of about 0.1 c if there was 100% efficient conversion of energy to thrust (neglecting the fact that no one knows how to do this). My thought was that a 2-stage intersteller rocket design could potentially be used as follows (and if I've reinvented someone else's idea I apologise): [...rest deleted...] You've re-invented the British Interplanetary Society's "Project Daedalus," modulo your choice of target star (the BIS chose Barnard's Star, since back then it was suspected to have at least one planet). See: http://www.angelfire.com/on2/daviddarling/Daedalus.htm http://www.geocities.com/TelevisionCity/2049/DAEDALUS.HTM http://members.nova.org/~sol/solcom/stars/barnards.htm -- Gordon D. Pusch perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;' |
#4
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Mission to Alpha Centauri
In article ,
Roger Stokes wrote: I did a few calculations on what it might take to do a flyby of Alpha Centauri [[...]] I read that Deuterium-Helium3 fusion would give an exhaust velocity of about 0.1 c if there was 100% efficient conversion of energy to thrust (neglecting the fact that no one knows how to do this). My thought was that a 2-stage intersteller rocket design could potentially be used as follows (and if I've reinvented someone else's idea I apologise): [[...]] If you can find a copy, A. R. Martin, editor, "Project Daedalus: The Final Report on the BIS Starship Study" published as a supplement to JBIS, 1978 no ISBN gives 192 pages of detailed information about a similar study done in the early-to-mid 1970s. I believe this JBIS issue is long out of print, but a library with back files of JBIS might have a copy. I don't know of any online copy. It would be nice if someone who has a paper copy could get permission from the copyright holders, then scan it and put it online... Daedalus was an unmanned undecelerated flyby probe to Barnard's Star (distance about 6 light years or so). The design goal was to complete the mission (including data return) within the lifetime of the youngest engineers working on the project, and to use minimal extrapolations from present-day technology. They originally hoped for a 40 year flight time at 0.15c, but their final design went to 50 years at 0.12c. They used a 2-stage D-He3-pellet nuclear-fusion rocket, to be launched from solar orbit out beyond Jupiter. I don't recall the launch mass or mass ratio, but as I recall the final payload mass was 50 tonnes. There was supposedly also some info about a NASA study on interstellar flight concepts in the Feb 1999 issue of Scientific American. Searching http://www.sciam.com/issue.cfm?issueDate=Feb-99 might turn up some tidbits. ciao, -- -- "Jonathan Thornburg (remove -animal to reply)" Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut), Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html "Washing one's hands of the conflict between the powerful and the powerless means to side with the powerful, not to be neutral." -- quote by Freire / poster by Oxfam |
#5
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Mission to Alpha Centauri
Why do you need staging in space?
You need staging for ground-LEO because you need lots of thrust to get going, and the engines to produce that thrust, as well as the tanks to withstand it and accelerate the propellants, are all heavy and useless once you've got your gravity losses down and you want to dump them as soon as possible thereafter. But, short of having REALLY big engines, I don't think you're going to do much about your (solar) gravity losses from LEO-alpha centauri, nor do I think you need to. So how about this: DHe3 fuel 13500 tons rocket 10 tons probe 1 ton Sure, the acceleration is miserable. Yes, you get some gravity losses because most of your thrusting is done well away from the Sun, rather than doing the plunge to Mercury and blasting away there. But you only have one set of engines instead of two, and so you get _better_ mass ratio. For a "tank", I'd use some sort of thin aluminized plastic film that can keep the DHe cold and contain sublimation. As the DHe evaporates, the gas is routed back to the engines where it's either burned or refrigerated. Micrometeoroid impacts and the resulting leaks are a problem. Distance and thin films are the solution. And finally, with an exhaust velocity of .1c, why have such a huge mass ratio in the first place? If you want to get to .2c, you just need a mass ratio of 7.5. Was the .1c Ve a typo? |
#6
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Mission to Alpha Centauri
Iain McClatchie wrote:
Why do you need staging in space? The same reason you would need staging anywhere; it's a method of achieving the necessary mass ratio. Jim Davis |
#7
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Mission to Alpha Centauri
Hello
I have two questions in this regard: 1. How large is the danger of the spacecraft being pulverized by space- debris at these speeds? I would think that at such high speeds even smallest debris (even if it is only a few molecules in size, or even some clouds of gas) would be very dangerous, right? 2. If it is going at high speeds like these, does Einsteins relativity affect the flight- time? I mean if the probe is on its way for so many years at such high speeds. How much longer would that make the journey appear to us on earth? Or the other way round, how much shorter would it appear on the spacecraft? Or wouldnt there be any effect at all? CU Elmar "Roger Stokes" schrieb im Newsbeitrag ... I did a few calculations on what it might take to do a flyby of Alpha Centauri, so, like a fool, I propose to rush in (to this newsgroup) where angels fear to tread. I read that Deuterium-Helium3 fusion would give an exhaust velocity of about 0.1 c if there was 100% efficient conversion of energy to thrust (neglecting the fact that no one knows how to do this). My thought was that a 2-stage intersteller rocket design could potentially be used as follows (and if I've reinvented someone else's idea I apologise): stage 1: stage 2 41 tons stage 1 rocket 410 tons DHe3 fuel 13500 tons stage 2: Instrument probe 1 ton 2nd stage rocket 10 tons DHe3 fuel 30 tons I chose v/ve = 1 for convenience. Stage 1 would accelerate stage 2 to 0.1c, stage 2 would accelerate the instrument probe to 0.2c which would pass alpha centauri about 20 years after launch. The telemetry would arrive back about 24 years after launch. Instructions for detailed investigation would be beamed back to stage 1 which could then manouver for further investigations on it's flyby 40 years after launch. Though the technology probably won't exist for decades or centuries, and though the above masses for engines etc are sheer guesswork, I wondered what "blue sky" interstellar vehicle proposals or mission outlines exist in the research community that are more than just sceince fiction. Also, has anyone considered the size and communications capability needed for a low-relativistic payload, and the data that could be gained in a flyby that would last about 24 hours (neglecting how the payload would be accelerated). |
#8
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Mission to Alpha Centauri
"Iain McClatchie" wrote in message om... And finally, with an exhaust velocity of .1c, why have such a huge mass ratio in the first place? If you want to get to .2c, you just need a mass ratio of 7.5. Was the .1c Ve a typo? My rocket equation physics is undergraduate level. If I recall, the 0.1c ve is actually 0.09c, and I posted late at night after three beers so (moderator permitting) if you believe that 0.2c per stage is achievable I would be very interested to hear the rationale. :-) starting beer #3 now... --Roger |
#9
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Mission to Alpha Centauri
"Roger Stokes" wrote:
Also, has anyone considered the size and communications capability needed for a low-relativistic payload, and the data that could be gained in a flyby that would last about 24 hours (neglecting how the payload would be accelerated). The flyby would necessarily last days, effectively, regardless of the speed. Communications would be fairly straightforward at those distances (up to ~5 ly). What you want is big mirrors and lasers. We can just about make 6m mirror space-based telescopes now, and there's quite a lot to suggest that 10 or 20m shouldn't be too much of a hassle. Then, you hook up your high power lasers and/or sensitive photometers to your mirror (for transmit or receive). You should be able to get decent data rates with fairly modest technology that way. Up to 5 ly, kilobit/s rates would be easy, megabit/s and gigabit/s would be moderately challenging but not terribly so. Your probe is going to need a big telescope anyway, so it might as well do double duty. The real difficulty would probably be science collection planning. The only good way to do that is to create a collection plan (i.e. observation plan) based off the most recent science data, but with a two-way comm-lag of up to almost a decade, that's a non-starter, so the craft is going to need some quite impressive smarts. As for observations, the obvious, and necessary, choice is high-resolution imagery. The target system is certain to have many targets spread around it which are worthy of interest (planets and whatnot) and the probe is certainly not going to closely flyby *all* of them, so it needs to observe most of them from several AU away, so you want to bring as big a telescope as you can (luckily, you've already got one! how convenient). Also, you'll want to stretch out the science collection as much as possible, so that you can collect useful data for the longest duration, that means you need to expand the start and stop points out as far as possible, and that in turn means you need to be able to collect good data at as far a distance away as possible. Reasonably, the flyby encounter is going to occur over years and transition through different data collection modes, from low-rate data collection out far away, to jumpin' all over the place working as fast as you can at closest approach, back to low-rate data collection again, with intermediate steps in between appropriate to the distance from the system. |
#10
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Mission to Alpha Centauri
"Iain McClatchie" wrote in message om... Why do you need staging in space? Sure, the acceleration is miserable. The beer wore off :-) Very low acceleration would prolong trip time so I assumed a certain minimum acceleration of (say) 0.1g would be needed to actually be going at 0.2c for most of the trip. This would require rigid fuel tanks, and increase the mass of the vehicle. What values of space fuel tank mass/volume ratio are achievable today - depends on acceleration, and type and volume of fuel I know but any figures? |
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