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What would happen if the pilot of a pressurized rocket-powered airplane-shaped vehicle attempted to take off in a vacuum? I would expect the plane to just taxi no matter what the pilot did; is this correct?

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It would depend on how high the pilot could raise the nose. The wings aren't doing anything and you might as well remove them. If a rocket were just rolling along the ground with no atmosphere, well it's a rocket car and you might as well take it to Bonneville Salt Flats and try to set a record with it. Without an atmosphere, the speed of the car would only be limited by rolling friction.

To get it to lift off the surface, you would have to tilt the thrust line enough that the vertical component of thrust created was enough overcome gravity. You could do this, say with the thing tilted at 45 degrees, with another lateral rocket under the nose to lift it. At 45 degrees your 100,000 lb rocket engine would be making just under 71000 lbs in the vertical plane, and same on the horizontal plane. If the machine weighs, say, 50000 lbs, it'll take off.

Of course, you can stand it straight up and get ALL of the thrust working to oppose gravity, and do away with the need for a runway at the same time, so why even bother with a horizontal takeoff? Just go straight up, like the Lunar Module on the Moon, or any other ballistic rocket.

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