Why can't planes that use external propellers fly supersonic or can they?

Question

For example if a concord plane is fitted with external propellers for its thrust instead of its normal jet engine, assuming the propellers speed could be increased indefinitely will it fly supersonic? if not, why? since both external propellers and internal propellers are blades that pulls air and pushes it backward to thrust the plane forward.

If increasing the propellers speed indefinately can also make planes with external propellers fly supersonic, then why has is it not been made possible?

Answer 1

It seems you have answered your own ?...."why has it not been made possible?"..... BECAUSE none of those things you state are true!

The propulsive efficiency of external propellors falls off dramatically above Mach .90 (tip speed, the vector sum of rotational and forward speed) The fan, compressor and turbine blades of jet engines move at only about M .85 External propellors are close to their structural limits when operated at the speeds they now turn.

Answer 2

Using external propellers to fly at high speed would be inefficient and require much more fuel than a standard jet engine. If someone did design a plane with external propellers to fly at supersonic speeds the cost would be quite a bit higher to operate than a jet of the same size or weight.

Answer 3

I would say that under the conditions you have set forth (propellers speed could be increased indefinitely and assuming that the material that the propellers are made of would not just fly apart) there is no reason a plane could not... In fact, the plane could fly at the speed of light and faster!

Answer 4

Elaborating on Steve's answer, for a prop to propel and aircraft faster than the speed of sound S, it must be spinning faster than S itself, in addition to being exposed to air passing through it greater than S. This creates a shock wave around the prop like the one formed by the plane itself. As a result, most of the engine's energy goes into making a lot of noise (as in sonic boom) and heat (shocks heat the air), instead of driving the plane forward.