I am working on a 2D platformer game, and I'm trying to introduce a jetpack which would give a shinespark-like ability from Metroid series, i.e. it needs a run-up to a certain speed, at which the engines become operational and can be fired. The catch: I'd like to stay as hard-sciencey as possible, so the alien supertech from Metroid is unavailable.
Considering modern day tech, the operation sequence reminds me of a pure ramjet which needs a good airflow before it can be fired. The problem is, a ramjet needs a run-up to something around 1 mach (or even more), which is too much for a running character (she's not Flash, although augmented to above-human abilities). Its final operational speed is also far too high.
Another possible engine is a pulsejet, especially a pulse detonation engine. It seems really promising: highly advanced, low operational speeds are possible (although less efficient), it seems like compact designs are possible. But it doesn't seem to specifically need a run-up to ignite (maybe the deflagration-to-detonation transition could possibly need it?).
So, what kind of engine could be used in a jetpack, that becomes operational at speeds of, let's say, 10-30 m/s, and allows to achieve operational speed of, say, 0,5 mach, without too much handwaving? Consider the atmosphere composition and pressure close to Earth-like (slight variations are possible as long as it stays breathable for non-augmented humans). It is implied that the device is either a production model or a working prototype, i.e. the need for a run-up should be justified by the engine technology itself, not an artificial restriction (like a broken/non-functioning pre-compressor, for example).