Jet propulsion is the process of pushing an object forward by expelling mass in the opposite direction. It works on a principle you already know intuitively: if you’ve ever felt the kick of a garden hose or watched a balloon fly around a room when you let it go, you’ve seen jet propulsion in action. In aircraft, this means accelerating air (and combustion gases) backward out of an engine so the plane moves forward.
The Physics Behind It
Jet propulsion is a direct application of Newton’s third law of motion: every action produces an equal and opposite reaction. A jet engine accelerates a large mass of air rearward. In reaction, the engine, and the aircraft attached to it, is pushed forward. The faster or heavier the stream of exhaust, the greater the forward thrust.
This is the same principle that drives a rocket, a squid squirting water, or a skater pushing off a wall. The only requirement is that something with mass gets thrown one way so the vehicle moves the other way.
How a Jet Engine Works
A modern jet engine converts fuel into thrust through four continuous stages: intake, compression, combustion, and exhaust. Air enters the front of the engine, gets squeezed into a smaller space, mixes with fuel and ignites, then blasts out the back. Each stage feeds the next, and the whole process happens simultaneously across different sections of the engine.
Intake and Compression
A large spinning fan at the front of the engine sucks in enormous quantities of air. That air passes into the compressor, a series of spinning blade-lined discs that squeeze it into progressively smaller spaces. Compressing the air raises its pressure and energy potential, the same way pumping a bicycle tire makes the air inside hotter and more energetic. By the time air leaves the compressor, it may be at 30 to 40 times its original pressure.
Combustion
The compressed air enters the combustor, where fuel is sprayed in and the mixture ignites. This produces extremely hot, rapidly expanding gases. The combustion happens at roughly constant pressure, meaning the energy goes into heating and accelerating the gas rather than just building up more pressure.
Turbine and Exhaust
Those hot gases slam into the turbine, a set of bladed wheels downstream of the combustor. The turbine extracts just enough energy from the gas flow to keep the compressor and fan spinning at the front of the engine. Everything left over, and it’s a lot, accelerates through the nozzle at the rear. The nozzle narrows the flow, speeding it up further, and that high-velocity exhaust shooting backward is what pushes the plane forward.
Types of Jet Engines
Not all jet engines handle air the same way. The differences come down to how much air goes through the hot core versus how much bypasses it.
Turbojets
The turbojet is the simplest design. All incoming air passes through the compressor, combustor, and turbine. This produces very high exhaust velocities and strong thrust relative to engine size, but it burns a lot of fuel doing it. Turbojets dominated early jet aviation and are still used in some missiles and specialized applications where raw speed matters more than fuel economy.
Turbofans
Turbofans add a large fan at the front that pushes a portion of the incoming air around the outside of the engine core. This “bypass” air never gets combusted. It exits the back at a lower speed than the core exhaust, but there’s a lot more of it, and that combination turns out to be far more fuel-efficient. Turbofan engines produce lower fuel consumption per unit of thrust compared to turbojets, which is why virtually every commercial airliner uses them.
The ratio of bypassed air to core air is called the bypass ratio. Older commercial engines like the CFM56 have a bypass ratio around 5.4, meaning about five and a half pounds of air go around the core for every pound that passes through it. Newer engines like the LEAP-1A push that to 10.5, and the next generation of ultra-high-bypass-ratio designs targets 15 or higher. Each jump in bypass ratio means less fuel burned per mile of flight.
Ramjets
A ramjet has no moving parts at all. It relies on the aircraft’s forward speed to ram air into the engine, compressing it without a mechanical compressor. Fuel is injected, combustion occurs, and the exhaust exits through a nozzle. The catch is that a ramjet cannot produce thrust from a standstill. It needs another propulsion source to get the vehicle moving fast enough (typically above Mach 2) for the ram effect to work. Ramjets are used in certain missiles and experimental high-speed aircraft.
What Affects Thrust Output
A jet engine doesn’t produce the same thrust at every altitude and speed. Air density is the key variable. At sea level, where the air is thick, there’s plenty of oxygen to feed combustion and plenty of mass to accelerate. As a plane climbs, the air thins out. The basic relationship is roughly proportional: thrust drops at the same rate as air density. At 35,000 feet, where most airliners cruise, the air is about one-quarter as dense as at sea level.
Speed also matters. As the aircraft moves faster, incoming air arrives at higher velocity, which changes the pressure dynamics inside the engine. For turbofans, the sensitivity to altitude is slightly greater than for pure turbojets, because that big bypass fan depends heavily on having a large mass of air to push. Pilots compensate by advancing the throttle as they climb, but once it’s fully advanced, any further increase in altitude means a decrease in available thrust.
Afterburners and Extra Thrust
Military jets sometimes need bursts of thrust far beyond what the engine normally provides. An afterburner accomplishes this by injecting additional fuel into the exhaust stream behind the turbine and igniting it. This reheating of the exhaust gas produces a dramatic increase in thrust. In testing, afterburners have boosted thrust by roughly 50%, taking an engine from 62 pounds of thrust to 92 pounds in one documented case.
The tradeoff is enormous fuel consumption. Afterburners are so thirsty that they’re used only in short bursts, during takeoff from short runways, in combat maneuvers, or when breaking the sound barrier. Military turbofan designs with low bypass ratios strike a balance, offering better fuel economy than pure turbojets during normal flight while still supporting afterburner use when needed.
Jet Engines vs. Rocket Engines
Both jet engines and rockets work by expelling mass to generate thrust, but they get their oxygen in fundamentally different ways. A jet engine breathes atmospheric air, using the oxygen in it to burn fuel. A rocket carries its own oxidizer onboard, stored separately from the fuel, and mixes the two in a combustion chamber. This is why rockets work in the vacuum of space, where there’s no air to breathe, and jet engines do not.
The practical consequence is weight. Rockets must carry all their oxidizer with them, which makes them enormously heavy at launch. Jet engines get their oxidizer for free from the atmosphere, making them far more efficient for any vehicle that stays within it. A passenger jet carrying its own oxygen supply the way a rocket does would barely get off the ground.
Electric Propulsion on the Horizon
Researchers are working toward electric alternatives to combustion-based jet engines, but the technology is still in its early stages. Engineers at the University of Wisconsin-Madison recently built and tested a 40-kilowatt fault-tolerant motor drive designed for aviation use, a prototype that won a first-prize paper award at a major IEEE conference in 2025. The focus of that work is on reliability: electric motors in aircraft face extreme temperatures, vibration, and power fluctuations, and the system needs to keep flying even if one component fails.
The gap between prototypes and passenger aircraft remains vast. A full-size airliner would need electric motors in the tens or hundreds of megawatts range, and those are still in the design stage. Small electric vertical takeoff-and-landing air taxis could arrive within a few years, but electrically powered airliners are a longer-term prospect. Battery energy density, the amount of energy stored per pound, is the core bottleneck. Jet fuel packs roughly 50 times more energy per kilogram than current lithium-ion batteries, a gap that no engineering trick can fully close with today’s chemistry.