The pivotal difference between jet and rocket engines is that jets draw the oxygen for combustion from the surrounding air, whereas rockets carry their own oxidiser on board. Because jets rely on atmospheric oxygen, they are powerless in the vacuum of space whereas a rocket's self-contained supply lets it ignite and thrust even where no air exists.
A jet presents two openings - an intake at the front to swallow air and an exhaust nozzle at the rear to expel the hot mixture - while a rocket motor needs only a single exhaust nozzle, having no requirement for an air inlet. The absence of an intake frees the rocket from ‘ram drag’, the retarding force jets must overcome to slow and compress incoming air. The net thrust a rocket produces equals its gross thrust (save for minor back-pressure effects).
Jets burn hydrocarbon fuels with atmospheric oxygen, whereas rockets may employ a wider palette of propellants - liquid hydrogen and oxygen, hypergolic chemicals, or solid composites - all carried aboard. Rocket motors, though the lightest for the thrust they generate, are less propellant-efficient than jets, registering the lowest specific impulse among air-breathing and rocket engines alike.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the difference between a jet engine and a rocket engine?
The difference between a jet engine and a rocket engine is that a jet engine is an airbreathing engine with two openings, an intake and an exhaust nozzle. It gets oxygen to burn fuel from the air. Because it draws atmospheric air through its intake, it has ram drag and its net thrust is less than its gross thrust. Jet engines increase pressure inside the engine to produce thrust, and they carry turbine machinery that compresses the incoming 80% nitrogen-20% oxygen mixture. Jet engines cannot operate in vacuum, and their highest speed is about Mach 3.5.
A rocket engine is a reaction engine with only one opening, the exhaust nozzle. It lacks an air intake and carries its own oxidizer, mostly pure liquid oxygen, so it can travel in the vacuum of space. Rocket engines have no ram drag and their net thrust equals their gross thrust. They produce thrust by increasing internal pressure and ejecting exhaust rearward. Rocket engines carry all components of reaction mass on board, carry 10,500 lb (4,763 kg) of fuel plus oxidizer turbo-pumped to the combustion chamber, and can reach speeds above Mach 3.5.
Which is more efficient: a jet engine or a rocket engine?
Jet engines are usually more efficient than rocket engines. Efficiency is judged by thrust-specific fuel consumption, the metric that records how many grams of propellant each engine needs to deliver one kilonewton of thrust for one second. A modern turbofan in dry mode consumes about 17 g/kN s (0.037 lb/lbf hr). The same engine with afterburner needs roughly 48 g/kN s (0.106 lb/lbf hr). The Space Shuttle main engine demands 225 g/kN s (510.8 lb/lbf hr), so a rocket uses approximately 10 times as much fuel to produce the same thrust. Because a jet can draw oxidiser from the atmosphere while a rocket must carry all of its own, the airplane with a rocket engine does not get far, and in ordinary flight regimes jet engines are far more efficient. Even the best rocket-engine can reach 70%, yet that high chamber efficiency is outweighed by the enormous propellant flow required to overcome the dead-weight oxidiser. Only when the mission leaves the atmosphere - where air is no longer available - does the rocket's ability to travel in the vacuum of space override its poor fuel economy.
