Why dont jet engines work without a spiner in the center of the blades

[fhawes]

Just trying to complete my physics coursework on bloodhound ssc. Could anyone help to answer the question: why don't jet engines work in supersonic air without a spinner in the centre of the blades ?

Thanks.

 

[etudiant]

The problem is that the center of the blades is fixed, so it acts as an air dam. It is bad enough as a drag source at subsonic speeds, but when supersonic, the shock wave will play havoc with your inlet performance. So either the inlet slows the incoming air to subsonic and the engine face is left naked, as in the Tornado for instance, or the engine is part of the inlet and needs the spinner as part of the air flow management.
Inlet aerodynamics are a huge part of high speed flight. Read the discussions on the SR 71 and its J58 engine, the inlet did most of the work in propelling that airplane.