MadSci Network: Engineering

Re: Why do space vehicles take off vertically?

Date: Mon Dec 21 16:41:27 1998
Posted By: David Ellis, Researcher, NASA Lewis Research Center
Area of science: Engineering
ID: 914176958.Eg

Aerodynamic lift is very efficient and cheap for travelling at subsonic and even slightly supersonic speeds. No one is proposing replacing a 747 with a rocket anytime soon. Things get much more difficult as you increase speed.

Using wings and a horizontal take-off and landing is not a bad idea. NASA and others have and are investigating it. Commercially the Pegasus system uses a combination of jet (mother aircraft) and rocket (Pegasus) to lift small loads into orbit. NASA's Hyper-X program also is looking at using horizontal take-off. The Natio nal Aerospace Plane (NASP) originally envisioned using a winged vehicle but later changed to a lifti ng body design because the net contribution of the wings was negative.

You see, wings do carry a price. An efficient wing at low speed tends to be long, thick and relatively straight. As the speed increases, thinner wings that are swept back become more efficient. If you have seen pictures of the F-14 or other variable sweep wing aircraft, you see that they change go back as the speed increases. A spacecraft cannot afford the weight of the sweep wing mechanism, so a single wing configuration which is most efficient at a single speed would have to be chosen. At all other speeds the wing would not be operating at peak efficiency and may actually create more drag than lift.

Wings of any configuration become dead weight that must be carried to orbit after the atmosphere becomes too thin to provide lift. They also represent a severe heating problem upon re-entry. The leading edges of the space shuttle's wings become white hot during re-entry.

In the end, it will require careful engineering to balance the cost of wings in terms of weight and drag against their benefit of lift. At the present time, vertical lift using all rocket power is the easiest and most econmical way to achieve Earth orbit.

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