Propulsion

Unless you are a glider enthusiast, you value propulsion as the means of keeping missiles and aircraft in the air. The thrust vector overcomes drag and gravity and maintains the speed necessary for lift generation. It is usually directed parallel to the vehicle’s centerline, although helicopters and the V-22 Osprey display their individuality by thrusting in other directions as well. For our simulations we deal only with body-fixed propulsion systems whose thrust vector is essentially in the positive direction of the body Is axis, possibly slanted by a fixed angle.

You will be surprised how far just basic physics will take us in modeling missile and aircraft propulsion. However, you should not bypass the solid foundations laid in the classic book by Zucrow7 and the excellent textbook by Cornelisse et al.8 Some recent and up-to-date compendiums were published by AIAA for missile propulsion,9 hypersonic airbreathers,10 and aircraft propulsion.11 Even the control book by Stevens and Lewis12 has some useful information on turbojet engine modeling.

Most missiles are rocket propelled with the oxidizer carried onboard. Some supersonic missiles use the oxygen of the air for combustion in their ramjet or scramjet propulsion units. The air is captured by the inlet, retarded and com­pressed, fuel is injected and ignited, and the mixture exhausted through the noz­zle. No rotary machinery is employed. Aircraft and cruise missiles, on the other hand, employ rotating compressors and turbine machinery for propulsion. Based on simple physics, I will derive the thrust equations for rockets, turbojets, and combined-cycle engines.

Newton’s second law will serve us well, both for missile and aircraft propulsion. In each case the time-rate-of-change of momentum generates the propulsive thrust. We first derive the thrust equation for rockets.