Propulsion
A wheel pushes on a road. A propeller pushes air or water. A rocket throws away mass that it carried onboard. A solar sail exchanges momentum with light.
Each produces an external force by interacting with something beyond the vehicle. The useful first question is what receives the opposing momentum. The next is where the energy comes from.
This article follows the major mechanism families across land, water, air, and space. The animations explain their workings; they are schematic views rather than performance simulations or construction drawings.
Force changes momentum
For a vehicle and everything around it, momentum is conserved. A propulsion system changes the vehicle's momentum by transferring momentum to the ground, a fluid, exhaust, light, or another external environment.
For a simple steady jet, the axial thrust contains momentum-flow and pressure terms:
This assumes a one-dimensional control volume with a uniform exit pressure and a suitable ambient-pressure boundary. The velocities are relative to the engine along its axis. More complicated flows need a fuller balance.
A rocket has no incoming atmospheric stream. An air-breathing engine does. That difference matters when comparing exhaust speed and thrust. Propulsion fundamentals, NASA.
Wheels, tracks, and feet
A driven wheel pushes backward on the road through its contact patch. The road pushes forward on the wheel. Static friction can provide that force while the tire rolls without slipping.
Tracks distribute contact over a larger region. Walking and running alternate contact through feet. They require an external reaction force; spinning a wheel freely in space does not propel its center of mass.
An electric motor or combustion engine may power the wheel. That changes the energy conversion and drivetrain, while the final propulsive interaction remains contact with the ground. Rolling motion, OpenStax.
Propellers, fans, and rotors
A propeller is a set of rotating wings. Aerodynamic forces on the blades change the momentum of the surrounding air. The reaction produces thrust on the assembly. Propeller thrust, NASA.
A helicopter rotor can direct that force upward. A drone uses several rotors to distribute and control it. A ducted fan surrounds the rotating assembly, changing the surrounding flow and structural arrangement.
Marine propellers apply the same momentum principle to water. The fluid's density and its susceptibility to cavitation introduce different engineering constraints.
A large mass of fluid receiving a modest velocity change can produce thrust differently from a small mass receiving a large change. Their required power and losses determine which is useful in a particular operating regime.
What turns the shaft?
Piston engines turn combustion-driven reciprocating motion into shaft rotation through a crank mechanism. The propeller then turns that shaft power into fluid thrust.
Turboprops use a gas turbine to drive a propeller. Turboshafts prioritize shaft power for a separate load, such as a helicopter transmission and rotor system.
Electric drives supply torque through an electric motor. Batteries, fuel cells, or generators can provide the electrical energy. An electric aircraft propeller still produces thrust by accelerating air.
These are different power paths to a propulsor. A hybrid drivetrain combines energy-conversion paths; it does not introduce a new way to violate momentum conservation. Turboprop explanation, NASA.
For the torque-producing part, see How electric motors work. Propeller performance also depends on the blades, inflow, and operating point, beyond the motor alone.
Turbojets and turbofans
A turbojet takes in air, compresses it, adds energy through combustion, and expands the hot flow. A turbine extracts enough shaft work to drive the compressor; the remaining flow exits through the nozzle.
A turbofan adds a fan that moves air around the hot core. The turbine powers that fan through shafting. Both the core stream and the bypass stream can contribute thrust.
The turbine does not create energy. It extracts work from the flow, which supports the compression machinery. Fuel supplies energy, while the engine also processes a much larger amount of incoming air.
Bypass flow is part of the propulsion system even though it does not pass through the combustor. The split between streams changes how much fluid is accelerated and by how much.
NASA explains the architectures separately: turbojet and turbofan.
Ramjets, scramjets, and pulsed combustion
A ramjet uses forward motion and its inlet to raise the incoming air's pressure. It has no rotating compressor. The flow through its combustion region is subsonic, even when the vehicle itself is supersonic.
A scramjet keeps supersonic flow through combustion. Its name means supersonic-combustion ramjet. Mixing and burning must happen while the incoming flow continues rapidly downstream.
These engines require incoming air and an appropriate flight condition. They do not provide a general-purpose standstill substitute for a turbine engine. Ramjet principles, NASA.
Pulsejets and detonation-based concepts change how combustion proceeds through time. They still produce propulsion through momentum exchange and pressure forces.
Their combustion cycle is a separate question from whether they ingest air or carry oxidizer. NASA taxonomy, propulsion section.
Moving water
An open propeller acts directly on surrounding water. A waterjet draws water through an inlet, transfers energy through a pump, and discharges it through an outlet.
Oscillating fins use changing shapes and strokes to create hydrodynamic forces and a wake. Their motion differs from continuous rotation, but water still receives the opposing momentum.
A waterjet does not imply a combustion jet engine. It may be driven by a diesel engine, gas turbine, or electric motor. Waterjet principle, Wärtsilä.
Rockets carry their reaction mass
A rocket can work in a vacuum because it does not need the atmosphere for its reaction force. Propellant exits carrying momentum; the vehicle gains momentum in the opposite direction.
Chemical rockets release energy through chemical reactions. Liquid, solid, and hybrid describe how their propellant components are stored and supplied; they share the underlying exhaust-momentum mechanism.
Bipropellant systems use two separately supplied reactants. Monopropellant systems obtain hot gas from a supplied propellant's reaction or decomposition. Cold-gas thrusters simply expand stored gas without combustion.
Those categories describe different systems, rather than a universal ranking. See NASA's in-space propulsion survey and Ethlox, methalox, and other propellants.
Heat can come from another source
A nuclear-thermal rocket uses reactor heat to raise the propellant's energy before expansion. A solar-thermal concept uses collected sunlight. Solar-thermal explanation, NASA. Both still expel reaction mass.
Nuclear-electric propulsion instead converts reactor energy to electricity for an electric thruster. The energy source and the thrust-producing mechanism are different parts of that chain. Space nuclear propulsion, NASA.
Fusion propulsion remains a research family with widely varying concepts. A physical concept, a laboratory experiment, and an operational spacecraft system are different levels of evidence.
Electric propulsion in space
A spacecraft electric thruster uses electrical energy to accelerate carried propellant. This differs from an electric aircraft motor, which turns a propeller that accelerates surrounding air.
A gridded ion thruster ionizes propellant and accelerates ions using electric fields associated with grids. A neutralizer supplies electrons so the escaping beam does not leave the vehicle accumulating charge indefinitely.
A Hall thruster uses electric and magnetic fields in a plasma discharge. Electrons are strongly affected by the magnetic field; an electric field accelerates the ions that form the exhaust.
These thrusters can use little propellant over long operating periods, but they need power, power-processing hardware, and thermal management. Ion propulsion, NASA and electric propulsion mechanisms, NASA.
Other branches include electrothermal devices that electrically heat propellant, electrospray devices that accelerate charged droplets or ions, and electromagnetic plasma thrusters that use current-field interactions.
“Plasma” alone does not identify one propulsion mechanism. The important distinction is how energy enters the propellant and how the resulting flow gains directed momentum.
Wind sails and solar sails
A wind sail exchanges momentum with air. A sailing boat also interacts with water through its hull, keel, and other submerged surfaces. Together, those forces can produce motion across the wind.
A solar sail exchanges momentum with photons. Reflection changes light's momentum, which produces a small force on the sail. It does not require solar-wind particles or onboard exhaust propellant.
For normally incident light on an ideal perfect reflector, the force is:
I is irradiance, A is illuminated area, and c is light speed. Absorption, angle, and real optical properties change the result. Radiation pressure, OpenStax.
The wind-sailing picture follows MIT's sailing explanation; NASA describes a flown solar-sail technology demonstration in ACS3.
Electric-sail concepts interact with solar-wind particles using electric fields. Electrodynamic tethers exchange momentum through currents interacting with an external magnetic environment.
These are different mechanisms from a reflective light sail. NASA surveys these propulsion families.
Why high exhaust speed can mean low thrust
For a stationary ideal rocket with pressure-matched exhaust, suppose all available jet power becomes directed exhaust kinetic energy:
Eliminating mass flow gives:
At fixed power, higher exhaust speed reduces thrust while reducing the propellant needed for a given impulse. This is one reason high propellant efficiency and strong immediate acceleration are different goals.
Real systems lose power and carry power-generation hardware. A complete vehicle comparison includes that hardware, the environment, required acceleration, operating duration, and mission constraints.
The useful map is now consistent: identify the momentum exchange, identify the power source, and account for the machinery connecting them. Different propulsion families solve different versions of that engineering problem.