Physics

Rocket and Jet Engines and their Differences



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The aircraft piston engine approached its limits of horsepower and mechanical complexity during World War Two. The development of rocket and jet engines enabled enormous advances in aviation and aeronautics.

Rocket and jet engines were both developed with military aviation in mind (although original goals were more noble). Each engine burns fuel and oxygen and provides vastly more thrust than conventional piston engines. This is where the similarities end.

Jet engines, known as gas turbines or turbo fans, ingest atmospheric air as their source of oxygen. The benefit of not carrying a supply of oxygen is also a handicap. These engines suffer from oxygen starvation at high altitudes and reduced performance in hot weather. The fuel, normally JP-4, is really a type of kerosene that contains very little water contamination and is relatively economical.

Although very expensive, jet engines are quite simple and incredibly reliable. The spinning blades you notice on those monstrous GE and Rolls Royce engines are the first stage of the compressor. These blades ingest massive quantities of air and hurl it back into the next compressor section. Each stage being smaller, the air pressure and temperature increases dramatically. This hot air combines with fuel in the burning chamber, exits as extremely hot gas, and spins additional sets of blades on its way out. These blades turn the front compressors, perpetuating the process. Tens of thousands of pounds of thrust can be produced. Some of a jet engine's thrust is from the kinetic energy of exiting hot gasses. The turbo fans, however, produce most of the thrust.

Some military engines have an additional "afterburner" section for even more, albeit crude and expensive, thrust. Raw fuel is simply burned with the exiting hot gasses. An F-15 Eagle will gulp several gallons of fuel per second in full afterburn! An afterburner is not unlike a rocket, obtaining extra thrust purely by the kinetic energy of hot gas.

Even the best jet engine cannot produce high enough thrust to pry a spacecraft from the grip of earth's gravity. Only a rocket engine can produce millions of pounds of thrust. The unfortunate drawback is that thrust of this magnitude is partially required for the purpose of lifting its own oxygen supply. A rocket engine cannot obtain free oxygen on its way up. This eliminates the altitude restriction placed on jet engines and enables spacecraft to leave the atmosphere altogether.

All of a rockets thrust is produced by the kinetic energy of hot exhaust gasses exiting the nozzle. Unlike a jet engine, these gasses do not have to pass through turbines. Vastly higher temperatures and performance are obtained. Like the jet's afterburner, rocket engines have a voracious appetite for fuel.

Two basic types of rocket engines are most common liquid and solid fuelled. The Space Shuttle demonstrates both solid and liquid fuelled rockets. The two long rockets mounted on each side contain solid fuel and operate independently. The main Shuttle engines burn liquid fuel, most of which is stored in the large external tank.

The solid rockets (boosters) have no moving parts and are economical. One drawback is there can be no throttle control or means of aborting. (Some systems use a solid fuel/liquid oxidizer to enable throttling.) Once they are ignited they will burn until all the fuel is expended. As evidenced by the loss of the Space Shuttle Challenger, many engineers find them to be too risky for manned flights.

The main Space Shuttle engines, like Saturn rockets of the Apollo missions, burn liquid oxygen and hydrogen. These have the luxury of throttle control and can be stopped and restarted as required. They are much more complex and expensive. A malfunction using this volatile fuel is dramatic and deadly. Many other liquid propellants are used, including kerosene. Compared to jet engines, the safety record of rockets is dismal. Most failures are caused by the need to make every component as light as possible. The lightest possible weight and reliability do not go hand in hand.

The end goal of both jet and rocket engines is the production of thrust for aircraft, spacecraft and missiles. Technological advances continue to blur the distinctions. In addition to combining both engines on a single craft, countless hybrids exist and more are on the drawing boards. Two exciting examples are ramjet and scramjet engines.

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