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#21
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SpaceShipOne and reentry heat
David Given wrote:
How about fitting the shuttle out with a lifeboat? Stick it somewhere in the cargo bay. If a shuttle gets sufficiently damaged that it can't reenter, you use the capsule to get the crew down. Ships usually (certainly not always) take some time to sink. In a Columbia-type scenario, you don't know you have a serious problem until it hits hard and fast. Even if it could survive having the orbiter come apart around it, you need at least enough time to get to such capsules. Something like this is why the B-58 and B-70 bombers had enclosable ejection capsules for the crew, which were also their normal seats. And even in the B-70 that went down after a mid-air collision, one of the crew still didn't make it out. And, of course, a 'lifeboat' capsule takes weight/volume away from possible payloads.... -- You know what to remove, to reply.... |
#22
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SpaceShipOne and reentry heat
Henry Spencer wrote:
The fast answer is that there is no crossover point: aerodynamic braking is *always* a lot cheaper in mass than doing the same braking with rocket fuel. That's not quite 100% true when you start examining specialized situations, but for normal reentry from orbit you can take it as given. That depends very heavily on the ratio between delta V and rocket exhaust velocity, as you know. Currently rocket exhaust velocities are so much lower than useful delta vees that this ratio is almost always greater than 1, which is definitely the wrong side of that exponential to be on. But in the long term, if we have rockets with very high exhaust velocities then this ratio could be much lower, and the behavior would be more linear with respect to delta V and mass ratio. High performance gas core nuclear thermal rockets can just about reach the performance levels to do multiples of Earth-to-LEO delta V, NSWR or Orion would certainly have that capability with reasonable mass ratios. |
#23
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SpaceShipOne and reentry heat
In article ,
David Given wrote: There are people here with more technical knowledge than I, but I do know that the shuttle's main engines aren't restartable. (Do they even have on-board fuel tanks?) Not for the main engines. Which, as you note, are not restartable. (There is no fundamental reason why they couldn't be, but there is no requirement for it, and so various details of setup for engine start are handled with the help of ground equipment.) How about fitting the shuttle out with a lifeboat? Stick it somewhere in the cargo bay. If a shuttle gets sufficiently damaged that it can't reenter, you use the capsule to get the crew down. It's been proposed many times. It presents some problems of physical layout, its mass puts a considerable dent in the payload capacity... and note that it wouldn't have saved Columbia's crew, since they didn't know something was badly wrong until too late. (Nor is there any plausible scenario where they would have. Suspicions about TPS damage were focused on the tiles, not the RCC leading edge, and no plausible imaging -- from the ground or from elsewhere in space -- would have been at all likely to notice a small dark hole in a black surface.) Depending on whether the capsule had its own thruster system, you would get the choice of putting the shuttle onto a reentry trajectory and then bailing out, or leaving the shuttle on orbit and just returning in the capsule. You'd want the capsule to do its own maneuvering, partly so that entering it and separating wouldn't be time-critical operations, partly to cover cases like the orbiter being unable to do its own deorbit burn. This isn't that big a deal; a deorbit burn isn't large. ...You'd also have to outfit the shuttle with an automated station-keeping facility using the OMS; you wouldn't want it to accidentally fall on someone. The orbiter will be dead and uncontrolled within days anyway: when its fuel cells run out of reactants, it loses power. (What's the lightest-weight way of getting a single human down from orbit? Could you build something like an orbital parachute? If so, would that be more appropriate than a combined capsule?) There have been various proposals for "orbital bailout" kits. But a shared capsule is probably better: it keeps the crew together, it can serve as shelter or boat, it simplifies providing sizable amounts of survival gear and electronics, it greatly simplifies cases where someone is injured. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
#24
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SpaceShipOne and reentry heat
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#25
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Shuttle lifeboat, on-orbit inspection for Criticaility One failures
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#27
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SpaceShipOne and reentry heat
LRW wrote:
I'm just an average person with an English degree, so I'm unfamiliar with the science and physics and space craft reentry, so this is likely a very stupid question. But it's my uneducated understanding that returning space craft, like all objects entering our atmosphere, super-heat from the friction of falling through our atmosphere. Which is why all crafts from Apollo to the space shuttles must have carefully crafted heat shields and enter at a VERY narrow angle to prevent either burn-up or "skipping" off the atmosphere. Now, correct me if I'm wrong, but didn't the commercial SpaceShipOne reach the very edge of the atmosphere? Doesn't it also need the observe the same careful considerations for reentry? If not, why? In simple "Physics for English Majors" language. =) Thanks! Liam The dirty little secret is that 100 km as "the edge of space" is a complete fiction. Earth's atmosphere extends upwards for hundreds of kilometers. SpaceShipOne didn't reenter the atmosphere, it never left! The significance of 100 km is that it taken as the division between the homosphere, where the main composition of the air is a constant, and the heterosphere, where the composition changes with altitude. If reentry heating calculations begin at 150 km (http://www.ucsusa.org/documents/CM_apF-J.pdf), then that should be considered the real edge of space for reentry purposes. One last thing: the heating isn't caused by "friction". If that was the case, skydivers would burn. The heating is caused by hypersonic objects supercompressing the air before it could move out of the way. The supercompression heats up the air and the heat is transferred to the object via radiation. |
#28
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SpaceShipOne and reentry heat
Mike Miller wrote:
Daniel Walker wrote in message ... I imagine the answers will involve the excess fuel needed, manoeuvring, restartable engines, etc., so at what point (what altitude) does the weight of thermal protection tiles beat the extra fuel needed for an orbital craft? I know there's lots of variables involved, but wondered if anyone had considered this? Yes, it's been considered. Getting to orbit typically requires about 90-95% of the vehicle to be fuel. In other words, for every kilogram put in orbit, 9 to 19 kilograms (or more) of fuel are required to put that kilogram into orbit. Hmm. 200Kg payload to 100Km. (SS1). Assume an ISP of 250. Should be good for a few kilos to LEO, with a three/four stage solid of modest performance. |
#29
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SpaceShipOne and reentry heat
Jake wrote:
One last thing: the heating isn't caused by "friction". If that was the case, skydivers would burn. The heating is caused by hypersonic objects supercompressing the air before it could move out of the way. The supercompression heats up the air and the heat is transferred to the object via radiation. It's caused by the gas going through a shock, not (just) by the gas being compressed. As the mach number increases, the density jump across a shock does not increase unboundedly, but approaches some finite limit. The *temperature* increases without limit, however. (There is also some heating from turbulence.) Entropy increases across a shock; it's an non-isoentropic process, unlike compression of gas in an apparatus where the motion of the gas is very subsonic, like your typical air compressor. In this sense a shock is like friction, which is also an entropy-creating process. Paul |
#30
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SpaceShipOne and reentry heat
In article ,
Ian Stirling wrote: Hmm. 200Kg payload to 100Km. (SS1). Assume an ISP of 250. Should be good for a few kilos to LEO, with a three/four stage solid of modest performance. According to XCOR's site, one use for the Xerus (as well as taking a pilot and single paying passenger to 100 km) is to launch approx an 10 kg satellite. -- Bruce |
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