Aerospace propulsion is the technology that generates thrust to move aircraft through the atmosphere and spacecraft through the vacuum of space. Every propulsion system works on the same core principle: accelerate a mass of material in one direction, and the vehicle moves in the opposite direction. That’s Newton’s third law in action. What changes across different engines is what gets accelerated, how fast it’s ejected, and where the energy comes from.
How Thrust Actually Works
A propulsion system does work on a “working fluid,” some substance it can push against. In a jet engine, that fluid is air mixed with burning fuel. In a rocket, it’s superheated gas from chemical reactions. In an electric thruster, it’s a stream of charged particles. The engine accelerates this material out the back, and Newton’s third law produces an equal and opposite force pushing the vehicle forward.
The amount of thrust depends on two things: how much mass you’re ejecting per second, and how fast it’s moving when it leaves. This is why a massive jet engine burning tons of fuel per hour can produce enormous thrust for takeoff, while a tiny ion thruster producing a whisper of force can still be incredibly useful in space, where even small pushes accumulate over months into tremendous speed.
Air-Breathing vs. Non-Air-Breathing Engines
The most fundamental division in aerospace propulsion is whether the engine breathes air from the atmosphere or carries everything it needs onboard. Jet engines are air-breathing: they pull in outside air, use it as both a working fluid and a source of oxygen for combustion, and blast it out the back at high speed. This makes them efficient for flight within Earth’s atmosphere but useless in space, where there’s no air to breathe.
Rocket engines are non-air-breathing. They carry both their fuel and their oxidizer (the chemical that lets fuel burn), which means they work anywhere, including the vacuum of space. The tradeoff is weight. Hauling your own oxygen supply means rockets need massive tanks, which is why the vast majority of a rocket’s launch weight is propellant.
Inside a Jet Engine
Modern jet engines follow a thermodynamic process that moves air through five stages. First, the inlet slows incoming air and feeds it to the compressor. The compressor, a series of spinning fan blades, squeezes the air to high pressure and temperature. That compressed air then enters the combustion chamber, where fuel is injected and ignited at constant pressure. The resulting hot, high-pressure exhaust blasts through a turbine, which spins and drives the compressor at the front of the engine. Finally, a nozzle accelerates the exhaust back to the surrounding air pressure, producing thrust.
The limiting factor in jet engine performance is heat. The hotter you can run the combustion and turbine stages, the more efficient the engine becomes. Today’s high-pressure turbine blades are made from coated nickel-based superalloys that tolerate temperatures up to about 2,200°F. Ceramic composite systems push that ceiling higher, to roughly 2,577°F, though they’re limited by the melting point of their protective silicon layer. Researchers at the University of Virginia are developing new coatings aimed at reaching nearly 3,300°F, which would significantly boost efficiency.
Solid vs. Liquid Rockets
Chemical rockets come in two main flavors, and each involves real engineering tradeoffs.
Solid rockets pack fuel and oxidizer together into a pre-mixed cylinder. Once ignited, they burn until the propellant is gone. You can’t throttle them, pause them, or shut them down without destroying the casing. What you get in return is simplicity and durability: a solid rocket is easy to handle and can sit in storage for years before firing. They’re commonly used as boosters for their raw, reliable power. A typical solid rocket booster produces a specific impulse (a measure of fuel efficiency) of about 240 seconds at sea level and 280 seconds in vacuum.
Liquid rockets store fuel and oxidizer as separate liquids, pumping them into a combustion chamber where they mix and burn. This design lets you control thrust precisely: throttle up, throttle down, shut off, restart. The cost is complexity and weight from all the pumps, valves, and separate tanks. Propellants are typically loaded just before launch. The SpaceX Merlin engine, burning kerosene and liquid oxygen, achieves about 282 seconds of specific impulse at sea level and 348 in vacuum. Engines burning liquid hydrogen and liquid oxygen, like the RL10, reach up to 450 seconds in vacuum.
Newer Propellant Combinations
Methane and liquid oxygen (sometimes called “methalox”) have emerged as a popular middle ground. SpaceX’s Raptor engine uses this combination to achieve about 330 seconds at sea level and 380 in vacuum. Methane is easier to store than liquid hydrogen, produces less soot than kerosene, and could theoretically be manufactured on Mars from the local atmosphere, making it attractive for future interplanetary missions.
Electric Propulsion for Deep Space
Electric propulsion systems use electrical energy to accelerate charged particles to extremely high speeds. The thrust they produce is tiny compared to chemical rockets, but their fuel efficiency is extraordinary. An ion thruster like NASA’s NSTAR achieves a specific impulse of about 3,100 seconds, roughly ten times better than the best chemical rockets.
Ion thrusters work by stripping electrons from a propellant gas (usually xenon) to create charged ions, then using electrically biased grids at voltages exceeding 10,000 volts to accelerate those ions out the back of the engine. Hall-effect thrusters use a different approach: a magnetic field perpendicular to an electric field traps electrons while accelerating ions to high exhaust velocities.
The practical impact is dramatic. For a hypothetical asteroid mission requiring 500 kg of delivered payload and a velocity change of 5 km/s, a chemical rocket would need about 2,147 kg of propellant. An ion thruster would accomplish the same mission with just 91 kg. That reduction in propellant mass means smaller, cheaper launch vehicles or the ability to carry far more scientific instruments. Electric propulsion has become increasingly common for both satellite station-keeping and deep-space science missions over the past decade.
Nuclear Thermal Propulsion
Nuclear thermal propulsion sits between chemical rockets and electric thrusters on the performance spectrum. Instead of burning fuel chemically, a nuclear reactor heats a propellant (typically hydrogen) to extreme temperatures and expels it through a nozzle. This produces high thrust, like a chemical rocket, but at roughly twice the fuel efficiency. The NERVA engine concept demonstrated a specific impulse of about 850 seconds in vacuum.
NASA considers this technology a key part of its Moon to Mars vision. Faster transit times to Mars would reduce crew exposure to cosmic radiation, increase the payload delivered to the destination, and enable abort scenarios where a crew could divert from Mars and return safely to Earth if something went wrong. In 2025, NASA awarded a contract extension to General Atomics and Standard Nuclear to continue development.
The Energy Challenge for Greener Flight
Conventional jet fuel packs about 43 megajoules of energy per kilogram. That energy density is what makes modern aviation possible: a relatively small mass of fuel carries enormous energy. Replacing it is harder than it sounds.
Today’s best lithium-ion batteries store roughly 0.2 to 0.4 MJ/kg at the pack level, about one-hundredth the energy density of jet fuel. Engineers estimate that batteries would need to reach at least 1,000 watt-hours per kilogram (roughly 3.6 MJ/kg) to replace the propulsion of a conventional single-aisle airliner, and current technology sits between 180 and 300 Wh/kg. That gap makes fully electric commercial aviation a distant prospect for anything beyond short-range, small aircraft.
Hydrogen is a more promising alternative. Its energy density is approximately three times higher than jet fuel by weight (33 to 40 MJ/kg), and it can be used either by burning it directly in a modified jet engine or by feeding it through fuel cells that generate electricity. The catch is volume: hydrogen is far less dense than kerosene, so storing enough of it requires much larger tanks, which reshapes aircraft design from the ground up. Sustainable aviation fuels, synthetic or bio-derived replacements that work in existing engines, offer a nearer-term bridge at a similar energy density to conventional jet fuel (around 43 MJ/kg).