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#11
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Maglev assisted launch SSTO
Henry Spencer wrote: In article , Roger Stokes wrote: ...Mach 5 is a bit fast inside an atmosphere. Even at the top of the highest mountain on Earth, the air will still be thick enough that the vehicle will need a thermal protection system ON THE WAY UP... If the vehicle is SSTO, it will already posess a thermal protection system for reentry at mach 25, which should be only lightly loaded at mach 5-10 for a few 10's of seconds on the way up, even at greater air density. Alas, that doesn't follow. Reusable reentry vehicles do most of their decelerating at extremely high altitude, in very thin air, where thermal loads are modest. Slamming through thick air at hypersonic speed, even briefly, is a far more severe problem. Roughly speaking, heat flux scales with air density times the cube of velocity. (The kinetic energy per kilogram of the incoming air scales with the square of velocity, and how much air you hit per second scales with density times velocity.) At 1/3 of the speed, you get about 1/27th of the heat flux; that's good. But the air density at 8km (highest mountains) is about 1,000,000x that at 100km (typical reentry altitude); that's really bad. The multiplication is left to the student. :-) Cube root(1,000,000) = 100. So the 8 km sled would need to be going 1/100 of typical reentry velocity for same amount of heat? Mach 5 is about 1.7 kilometers/second? And the reentry velocity you're thinking of is about 5.1 km/sec? 1/100 of this speed is .051 km/sec. This is only about 1/6 of mach 1. Hasn't the Concorde flown at mach 1 at lower altitudes and not burned up? I know I'm missing something here. Hop http://clowder.net/hop/index.html |
#12
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Maglev assisted launch SSTO
Henry Spencer wrote: Note that I said "roughly speaking". :-) There are a bunch of complicating factors, mostly notably the fact that only a small fraction of the air's kinetic energy actually shows up as heat at the surface. Just how small that fraction *is*, becomes very important. That's heavily influenced by things like vehicle shape and air flow characteristics (which vary a lot with both speed and density). Changing the velocity by a factor of 100 is too much for this simple approximation; it breaks down. I googled mag lev trains and learned that existing trains achieve a speed of .120 km/sec. The equator speed is about .464 km/sec so the sled might achieve .584 km/sec at lift off. At the mountain top you seem to have a weight savings of about .2 percent. This doesn't seem like a great improvement. The air pressure seems to be about 2/5 of sea level. Would the reduced drag at ignition be a significant advantage? Looking at a globe there seems to be some real estate east of the Andes at the equator. I don't think folks living east of the launch site would be happy. This tentative examination seems to be deflating a fantasy I've enjoyed entertaining in the past. Hop http://clowder.net/hop/index.html |
#13
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Maglev assisted launch SSTO
Henry Spencer wrote:
In article , Roger Stokes wrote: ...Mach 5 is a bit fast inside an atmosphere. Even at the top of the highest mountain on Earth, the air will still be thick enough that the vehicle will need a thermal protection system ON THE WAY UP... If the vehicle is SSTO, it will already posess a thermal protection system for reentry at mach 25, which should be only lightly loaded at mach 5-10 for a few 10's of seconds on the way up, even at greater air density. Alas, that doesn't follow. Reusable reentry vehicles do most of their decelerating at extremely high altitude, in very thin air, where thermal loads are modest. Slamming through thick air at hypersonic speed, even briefly, is a far more severe problem. Roughly speaking, heat flux scales with air density times the cube of velocity. (The kinetic energy per kilogram of the incoming air scales with the square of velocity, and how much air you hit per second scales with density times velocity.) At 1/3 of the speed, you get about 1/27th of the heat flux; that's good. But the air density at 8km (highest mountains) is about 1,000,000x that at 100km (typical reentry altitude); that's really bad. The multiplication is left to the student. :-) I think that this is technically true but misleading. I don't think the 100km typical re-entry altitude is where the heat flux is highest. Do you need any serious thermal protection if you are at orbital speed at 100km (other than because you would need it a few minutes later). At what altitude does a typical re-entry experience its maximal heat flux? The important point remains true, the air density at 60km is about a thousand times that at 8km and a heat flux of 1000 * 1/27 is a no no (orbital speed at 60km is surely a severe heat flux). On a related topic, does any one know if Columbia was far from its maximal heat flux when it started to break up? At what altitude during descent does the protective tiles stop being necessary? Alain Fournier |
#14
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Maglev assisted launch SSTO
"Hop David" wrote in message ... I googled mag lev trains and learned that existing trains achieve a speed of .120 km/sec. Presumably this is because they are designed to compete with traditional steel-rail trains - a ticket can't cost more than a few dollars (or euros). A maglev launcher is competing against a shuttle launch at $500 million a pop - so there is incentive to innovate. |
#15
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Maglev assisted launch SSTO
Hop David wrote
I googled mag lev trains and learned that existing trains achieve a speed of .120 km/sec. The equator speed is about .464 km/sec so the sled might achieve .584 km/sec at lift off. A maglev isn't really the best way to go about it. For non-human cargo which can take a bit of g then a big airgun, using hydrogen instead of air as working fluid, will get you about 2 km/s and be a lot cheaper than a maglev. No carts, just a light, reuseable sabot. I once costed one at UK £4 Bn (approx $6 Bn), but that was a while ago. A slightly cheaper system using air will give about 900 m/s, plus the 468 for Earth's orbit, and be suitable for humans. Even building both together would be a lot cheaper than a maglev. Maglev's are very expensive. -- Peter Fairbrother |
#16
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Maglev assisted launch SSTO
Roger Stokes wrote: "Gordon D. Pusch" wrote in message ... "Roger Stokes" writes: The track would be dug into a groove to direct sonic booms upward so the NIMBYs wouldn't whine. Shock waves don't work that way. Thanks - I guess I was visualizing from the anti-sound walls sometimes seen along freeways. How do shock waves propagate from a supersonic object in a trench? violently |
#17
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Maglev assisted launch SSTO
Peter Fairbrother wrote: Hop David wrote I googled mag lev trains and learned that existing trains achieve a speed of .120 km/sec. The equator speed is about .464 km/sec so the sled might achieve .584 km/sec at lift off. A maglev isn't really the best way to go about it. For non-human cargo which can take a bit of g then a big airgun, using hydrogen instead of air as working fluid, will get you about 2 km/s and be a lot cheaper than a maglev. No carts, just a light, reuseable sabot. I once costed one at UK £4 Bn (approx $6 Bn), but that was a while ago. A slightly cheaper system using air will give about 900 m/s, plus the 468 for Earth's orbit, and be suitable for humans. 900 m/s is about Mach 2.65? I believe the Concorde attains Mach 2.5 so I am venturing to guess Mach 2.65 is plausible. so that totals about 1.4 km/sec. A typical orbit speed is about 9 km/sec? (Mass payload + Mass fuel)/mass payload = e^(Vfinal/Vexhaust)? 4 km/sec is a plausible exhaust speed? e^((9-1.4)/4)=6.7 e^(9/4) =9.5 This seems like quite an improvement! I guess this wouldn't be truly SSTO but actually TSTO where the first stage (aka the big air gun) is ground based and reusable? Hop http://clowder.net/hop/index.html |
#19
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Maglev assisted launch SSTO
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#20
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Maglev assisted launch SSTO
In article ,
Hop David wrote: Looking at a globe there seems to be some real estate east of the Andes at the equator. I don't think folks living east of the launch site would be happy. Indeed, the city of Mecapa is located almost precisely on the equator (longitude 51 W), just inland from the Atlantic coast, where the Amazon river flows into the Atlantic ocean. I can't find Mecapa's population in my atlas, but it's a fairly major city (it's the capital of Amapa state). I don't think its residents, or their representatives in the Brazilian Congress and Senate, would appreciate being downrange of such a launcher. -- -- "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 "Space travel is utter bilge" -- common misquote of UK Astronomer Royal Richard Woolley's remarks of 1956 "All this writing about space travel is utter bilge. To go to the moon would cost as much as a major war." -- what he actually said |
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