Is it possible to use compressed air to get a ramjet engine up to speed?

Question

A ramjet engine is useless until it reaches atleast 400mph.

Answer 1

The idea of a ramjet is as an air-breathing vehicle capable of sustaining high speed within the atmosphere, be more efficient than a rocket. Hence it is designed without the need for compressor in the first place. Adding compressed air will defeat the purpose of the original design. It becomes like a rocket!

All it needs is a booster to achieve the initial spped for the ramjet to operate. And there are various means to achieve this.

Answer 2

Possible? Sure. Practical, no.

Why?

Well, the whole point of this is to make something that flies,
right? That means that the engine needs to produce enough
energy to keep it in the air - and in order to produce enough
oxygen long enough and fast enough, you have to have a
really big tank with very strong (and thus heavy) walls.

It turns out that it should be more efficient to get the craft
in flight and up to speed then to "simulate" it with compressed
oxygen.

Answer 3

Your question eplains why we don't use ram jets. What makes more sense? The compressor of a turbojet or a giant CO2 bottle?