Ramjet/scramjet's......how do you set them up!

Just a quick question to some of the techheads out there....How do you set up the fuel tp air ratios/curves etc on engines such as the SR 71's and the "hypothetical" PWDE of the Aurora as these engines operate as a ramjet/scramjet and need a supersonic airflow before they will operate.

Is this correct (I am a Aircraft Electrical engineer not a engine man) and how would they acheive this whilst ground running as I have setup conventional engine control units for the Tornado which has reheat curves etc, etc but this is for a conventional engine not one that changes in flight!

There must be a way of doing it or am I just being a bit thick!!!!!!

Sv...out!!!

The fuel to air ratio is basically determined by the throttle setting, and it basically works the same in conventional engines, ramjets, scramjets, and PDEs.

The big problem that is different between subsonic burners and supersonic burners is mixing the fuel and air before combustion. Techniques that work quite well in subsonic burners are stupendously bad in supersonic burners, and much research has been done on different techniques for scramjet or dual-cycle applications.

If you are asking how they run ramjets or scramjets on the ground (or anytime at relatively slow speeds) well they typically just don't. They are horribly inefficient so some other propulsion system must be used to get them up to speed first. For testing purposes they can pump large amounts of air through them on the ground, but they don't have a magical way of making them work at slow speeds, they just have to design around this major drawback of those particular types of engines.

Pulse Detonation Engines (PDEs) don't have this particular limitation and that's why they are of major interest to propulsion engineers. In theory, a PDE can operate from standing still all the way up to hypersonic speeds, but of course there are many design challenges along the way.