What is the main difficulty in creating a "space-plane"?

Question

What is the physics problem that doesn't allow us to fly a suitably-manufactured plane directly out of the Earth's atmosphere into outer space? Of note is that the space shuttle can enter the atmosphere from space, but it must launch and exit the atmosphere like a rocket. Is it primarily due to gravity and speed?

Answer 1

An airplane relies on "Lift" in order to gain altitude. Once a plane reaches the upper levels of earth's atmosphere there is no air left to provide any lift. Think of an airplane as being somewhat like a submarine. Once the submarine reaches the surface of the water it can't go any higher because there is no water to lift it. The best the submarine can do is float on the surface. It's the same with a plane. Once it reaches the top of the atmosphere it can't go any higher. To escape earth's atmoshere you need a rocket, which doesn't rely on lift.

Answer 2

I like mark s answer... good analogy : )

Answer 3

Shhhhhhh....

http://en.wikipedia.org/wiki/Blackstar_(...

http://en.wikipedia.org/wiki/Aurora_airc...

Answer 4

It is not cost effective though possible through modern tech but it just costs to much

Answer 5

a plane relies on the net force created on it's wings in the upward direction. This net force is created becuase the wings create a pressure difference in the air above the wing and the air below the wing. There is more pressure on the underside of the wing, so the plane is pushed upwards. As the plane gets higher, the atmospheric pressure outside lessens, and so less of a force can be produced to lift the relatively heavy plane upwards. It works to use rockets as a force creator however, because the force created by the rockets is (almost) independant of the height above earth's surface.

Answer 6

Before addressing your specific question, let's review some of the characteristics of rocket flight, to establish a baseline, shall we?

Rockets takeoff vertically, for the sole reason of putting as much of the drag causing atmosphere behind them as fast as possible. Once they are in the rarefied upper atmosphere, the acceleration can be done more easily (since aerodynamic drag is reducing) and the rocket will start flying in a gradually more horizontal direction, to gain orbital velocity. The initial nearly vertical flight phase requires a huge amount of thrust, so much that it is mainly (and in some case exclusively) provided by oversized engines that are very heavy and are jettisioned, along with their empty fuel container (be it tankage for liquid propergols, or outer casing for solid rocket boosters).

It may be worth mentioning that rocket engine efficiency is dictated by the pressure of the hot gas exhaust, and that the nozzle expansion (the overall width of the nozzle relative to the "neck", close to the combustion chamber) needs to be adapted to the prevailing conditions (smaller expansion for low altitude, because of higher ambiant atmospheric pressure) and maximum expansion for vacuum operation for greater efficiency.

Now, assume that we'd have a "space-plane". What conditions would it have to operate in? From take-off, atmospheric pressure, and close to orbit, near vacuum. What nozzle expansion should be used? A variable one would be desirable, but mechanism to do so would add to the weight.

The there is the issue of fuel. Since rockets shamelessly throw away major components (lower stages or solid rocket boosters--thay be recoverable/recyclable, mind you, but that still means that they are thrown overboard and do not ride all the way up) would a spaceplane required to keep all its gear attached until it reaches orbit?

Remember that rocket bring their fuel and their oxydizer along; and an ideal spaceplane could be made lighter by having only the oxydizer required for the final phase of the journey, and use atmospheric air as oxydizer while it is abundand enough. But this brings some additonal interesting probelms: should one use two engines: some sort of jet engine for atmospheric flight, and rockets for orbital insertion (hence have useless rockets at the start and useless jets at the end); or should one devise hybrid engines (that may be very complex to design, could be less efficient as they would be some sort of compromize, and could end up being still quite heavy)?

It can also be noted that the wings of the space shuttle orbiter are large enough to allow the returning ship and payload to glide back (i.e. excluding the fuel used for take-off, and that fuel is about 90% of the takeoff weight). If instead we would be counting on wings large enough to lift the whole system from take-off, we'd be dealing with pretty large wings (again quite heavy), and those wings have to work well at low speed (of takeoff) and at Mach 25.

So there you have it: it is hard from a propulsion point of view, from a structural point of view, and from system integration point of view. Very gifted and crafty people are working on the problem, but a solution will not appear overnight.

But they keep trying, and keep the spirit alive.