Expert: Paul Soderman - 1/7/2003

Question
Hi Paul!
I was wondering if you could help me - i am thinking of designing a small model ram jet. - about 4" wide and 10-12" long. i am a bit puzzled as to the various designs. Do they have a combustion chamber/flame tube or is the fuel just diffused into the airstream? i was also wondering about the compression of the air - should the compressive cone be inside the jet or should it protrude out of the front? any information would be very helpful - i have searched the internet but i havent been able to find any useful info!
Thankyou very much,
         Scott

Answer
Scott

A ram jet has relatively few, yet critical components.  Since ram jets are designed for supersonic flight, the inlet should have an isentropic spike projecting forward followed by a subsonic diffuser in the inlet.  The flow passes through an oblique shock on the spike and a normal shock at the diffuser inlet and then slows to pass subsonically through the combustion chamber.  Fuel is injected from nozzles upstream of flame holders that prevent flame-out.  The hot flow then passes through a convergent-divergent nozzle for ideal expansion and thrust.  No turbomachinery - very simple.

Jet propulsion books have schematics and thermodynamic cycle equations.  Mine is Jet Propulsion for Aerospace Applications by W. Hesse and N. Mumford (Pitman Publishing 1964), but may be out of print.

Good luck.

Paul