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airplanes and space flight
Hi there,
One thing that I always wondered about space flight is why most agencies (NASA, etc.) do not use a more efficient way of lifting into space. The vertical rocket takeoff seems to use so much energy and does not take advantage of the physics of regular flight. Why would one not use something like a modified commercial airliner (make it airtight and so forth) and then perform a regular take off and fly up to the altitude where the air still supports the lift on the wings (using plain old kerosene) and then once that barrier has been reached utilize a rocket engine to make it the rest of the way. Should this not allow for much greater payloads to be carried since less fuel is needed to get up to 30,000 ft? Plus one could use established procedures such as in-flight refuling at altitude to lessen the need for fuel at take off even more. Just imagine how much could be hauled into space and how much cheaper it would be if one would modify a 747 and use the cargo capacity of such a plane. I realize that this is a little simplistic in its description (put a rocket motor on a 747 and have it lift off), but nevertheless, why not take advantage of wing designs, etc. to get into space. I am sure that there is a very good reason why this has not been done yet, since there are thousands of very smart people working on these problems. I would just like to know what the negatives are to this idea that would make it not feasible to implement. I can't imagine that it would be cost, since they spend a boat load on the shuttle program as it is. Thanks for taking the time to answer this question. Regards, Mitch |
#2
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"Mitch" wrote:
Why would one not use something like a modified commercial airliner (make it airtight and so forth) and then perform a regular take off and fly up to the altitude where the air still supports the lift on the wings (using plain old kerosene) and then once that barrier has been reached utilize a rocket engine to make it the rest of the way. I'm sure you'll get much more detailed answers, but the simplest one is that an aircraft's ~10 miles isn't that much of a head start on the 100+ miles for orbit -- and much more importantly, an aircraft's ~500 mph *really* isn't much of a head start on the 17,000 mph needed for orbit. The idea is seductive, going back to Eugen Sanger in the 1930s; it seems reasonable on the face of it to "evolve" gradually from air travel to space travel. But subsonic, supersonic, and hypersonic flight are very different aerodynamic regimes as far as design for lift, drag and collection/compression of engine air is concerned. And the turbojet that works well from subsonic to Mach ~3 is not the same as the ramjet that might make more sense around Mach 5-6, or the scramjet that looks best above Mach 8. Bottom line: the tempting simplicity of the space plane idea starts to get dauntingly complex as you get into the engineering. |
#3
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Mitch wrote:
Should this not allow for much greater payloads to be carried since less fuel is needed to get up to 30,000 ft? Not *much* greater payloads. There is an improvement. The spacecraft can be smaller, or deliver a slightly larger payload, or get into orbit in a single bound. But 30,000ft and, say, 600mph isn't much of a boost. It's a long, long, long way from orbit. The difficulty of getting to orbit is better described by "17500 miles per hour" than any figure of altitude. Mike Miller |
#4
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Mitch wrote:
Hi there, One thing that I always wondered about space flight is why most agencies (NASA, etc.) do not use a more efficient way of lifting into space. The vertical rocket takeoff seems to use so much energy and does not take advantage of the physics of regular flight. Why would one not use something like a modified commercial airliner (make it airtight and so forth) and then perform a regular take off and fly up to the altitude where the air still supports the lift on the wings (using plain old kerosene) and then once that barrier has been reached utilize a rocket engine to make it the rest of the way. Should this not allow for much greater payloads to be carried since less fuel is needed to get up to 30,000 ft? Plus one could use established procedures such as in-flight refuling at altitude to lessen the need for fuel at take off even more. Not really. The amount of fuel saved by starting out at 10Km is quite small. And now, you've got wings, tail, landing gear, jets, ... to carry too. Just imagine how much could be hauled into space and how much cheaper it would be if one would modify a 747 and use the cargo capacity of such a plane. I realize that this is a little simplistic in its description (put a rocket motor on a 747 and have it lift off), but nevertheless, why not take advantage of wing designs, etc. to get into space. Because unfortunately when you do the numbers, it does not make sense. There are arguments for launching stuff from modified aeroplanes, but that's the only case that makes some sense. At best, you need a dry mass of around 10%, in order to get into orbit, with a single stage, and then you have little or no payload. With todays technology, wings, jet engines/... are simply too heavy. |
#5
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Mitch wrote:
Hi there, Why would one not use something like a modified commercial airliner (make it airtight and so forth) and then perform a regular take off and fly up to the altitude where the air still supports the lift on the wings (using plain old kerosene) and then once that barrier has been reached utilize a rocket engine to make it the rest of the way. Gaining altitude and actually getting outside of the atmosphere is easy. The space shuttle does that in under a minute. The whole rest of the flight is gaining speed. So when you see the shuttle sitting on the launch pad and you look at that huge tank of fuel, don't make the mistake of thinking, "wow, it takes all that fuel to get up into space." The truth is, it doesn't take much fuel at all to get into space. It takes A LOT of fuel to get up to speed. Your 747 idea is a way of using wings to gain altitude. That would be a good idea if altitude was the problem. To actually use an aircraft to launch a spaceship (the way Burt Rutan's Spaceship One worked), you need to have a mothership that can lift the spaceship *and* the fuel needed to gain orbital speed. So, imagine an airplane carrying a big heavy tank of fuel like the shuttle's external tank. The shuttle's main fuel tank holds 800 tons of fuel. That's pretty heavy. The largest aircraft in the world is the An-225, and it can only lift 275 tons. You'd need a much larger airplane to carry enough fuel to get up to orbital speed. |
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Hello,
that's exactly what the X Prize Foundation was intended for: http://www.xprizefoundation.com/default.asp The problem with just converting say a plane is that you need a helluva lot of energy to overcome the earths gravity well, the second is security, hence the X Prize. regards dc |
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#8
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#9
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Mikko wrote: Why is that speed needed? So you don't hit the ground when gravity pulls you down. Orbital velocity for a given altitude could be defined as, "Moving fast enough so that when gravity pulls you down far enough to hit the ground, you've moved far enough sideways that you miss the horizon." When you're at an altitude of 200-300 miles (like the shuttle or International Space Station), you only have minutes to scoot sideways far enough to miss the horizon. Earth's gravity is only barely diminished at an altitude of 200-300 miles, so you're falling quickly. For example, it only takes 22-23 minutes for the shuttle or ISS to fall far enough to hit the core of the Earth. The shuttle and ISS need to get about 4000 miles sideways in 22-23 minutes to miss the core, mantle, crust, and atmosphere, or they're going to leave a pile of metallic confetti on the ground. That calls for a high velocity. In fact, the magical number is 17500mph. But also gravity gets smaller when you get more away from the earth? How high does one have to go to have only half of gravity? About 1650 miles, deep in the inner Van Allen radiation belt. What if someone built a 30,000 ft high tube, similar to magnetic trains - electrical magnets around it. Then you could just put metallic cargo inside - without any engine or fuel, and shoot it up. And then it would fall back down and land near the launch point. Well, unless you fired it at escape velocity, in which case the cargo would just drift away from Earth, never to return. Most commercial launchers like to have their satellites stay near Earth, where the customers are. Mike Miller |
#10
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Op Wed, 17 Aug 2005 19:19:28 +0000, schreef Mikko:
Why is that speed needed? In simple terms: because otherwise the whole damned spacecraft falls down, back to earth. It needs speed to constantly "escape" the tug of gravity. It needs more speed the closer it is to the source of gravity, and less speed if it can fly an higher orbit. Very nice explanation he http://en.wikipedia.org/wiki/Orbit |
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