Examples of Jet Propulsion: From Heron's Aeolipile to Modern Rockets

Jet propulsion is a method of movement in which a body is pushed forward by ejecting a mass — gas, water, or hot exhaust — in the opposite direction. The reacting stream drives the body the other way, exactly as Isaac Newton's third law of motion predicts: every action produces an equal and opposite reaction. This single principle links the steam toy of an ancient Greek engineer, a swimming octopus, an artillery shell, and a spacecraft leaving Earth.

Newton's cart was not the world's first jet engine. Examples of jet propulsion were observed and studied by scientists long before Newton's experiments and right up to the present day:

• Hero's aeolipile,
• the Chinese rocket,
• sea creatures,
• the falling cat,
• the waterjet boat,
• firearms,
• the muzzle brake,
• the rocket,
• the rocket-propelled shell,
• self-guided aircraft.
• the spacecraft.

What jet propulsion is: definition and principle

Jet propulsion generates thrust by accelerating a working fluid backward, so the vehicle is driven forward by the equal and opposite reaction. The core relationship comes from the conservation of momentum: thrust equals the mass flow rate of the ejected fluid multiplied by its exhaust velocity, plus any pressure difference at the nozzle. This is why the faster and heavier the ejected stream, the greater the push on the body that expels it.

Newton's Third Law of Motion as the basis of jet propulsion

Newton's third law is the physical foundation of every jet-propelled device, from a squid to a rocket engine. When a body pushes a stream of matter one way, that matter pushes back on the body with equal force in the opposite direction. Because the reacting mass carries momentum away, the vehicle gains an equal momentum forward — no external surface to push against is required. You can explore this rule further through Newton's second law, which quantifies how force, mass, and acceleration relate in the same system, and see many more everyday cases in this collection of Newton's third law examples.

Newton's cart and the history of the discovery

Newton's cart was a teaching demonstration showing that a body can propel itself by throwing mass backward, but the principle it illustrated had been at work in nature and in human inventions for far longer. The examples below trace that history — from the first steam turbine to the modern spacecraft — and show that jet propulsion is one continuous idea appearing across physics, biology, and engineering.

Examples of jet propulsion in history and in nature

Jet propulsion appears wherever a stream of fluid is ejected to create thrust. The following historical inventions and living organisms each rely on the same reaction principle, differing only in the working fluid and the way it is expelled.

Hero's aeolipile — the first steam jet turbine

The first steam jet engine was built about eighteen hundred years before Newton's experiments by the remarkable inventor Hero of Alexandria, an ancient Greek engineer; his device became known as Hero's aeolipile.

Who Hero of Alexandria was

We know little about Hero of Alexandria. He was the son of a barber and a pupil of another famous inventor, Ctesibius. Hero lived in Alexandria roughly two thousand one hundred and fifty years ago. In the device he invented, steam from a boiler heated by a fire passed through two tubes into an iron sphere.

How Hero's aeolipile was built and worked

The tubes also served as the axis around which the sphere could rotate. Two other tubes, bent into the shape of an "L", were attached to the sphere so that steam could escape from it. When the fire was lit under the boiler, the water boiled, the steam rushed into the iron sphere, and from there it shot out with force through the bent tubes.

The sphere then spun in the direction opposite to the escaping jets of steam — this happens in accordance with the reaction principle that Newton later formalized. Hero's aeolipile can be called the world's first steam jet turbine, and it is the direct ancestor of every modern turbine engine that turns expanding gas into rotation.

The Chinese rocket — an ancient jet engine

Even earlier, many years before Hero of Alexandria, China invented a jet engine of a somewhat different design, now called the firework rocket.

Firework rockets should not be confused with their namesakes, the signal flares used in the army and navy and fired on public holidays amid the roar of artillery salutes. Signal flares are simply pellets pressed from a substance that burns with a colored flame. They are fired from large-caliber pistols — flare guns.

How firework rockets differ from signal flares

The Chinese rocket is a cardboard or metal tube closed at one end and filled with a gunpowder composition. When the mixture is ignited, a stream of gas bursts out at high speed from the open end of the tube, forcing the rocket to fly in the direction opposite to the gas jet — the same momentum exchange that drives a modern rocket engine.

How the Chinese rocket was built

Such a rocket can launch without the help of a flare gun. A stick tied to the rocket's body makes its flight steadier and straighter, acting as a simple stabilizer just as fins do on later projectiles.

Sea creatures: jet propulsion in animals

Jet propulsion is also found in the animal kingdom. Cephalopods — cuttlefish, octopuses, and squid — have neither fins nor a powerful tail, yet they swim as well as any other sea creatures. These soft-bodied animals have a fairly capacious sac or cavity in the body. Water is drawn into the cavity, and then the animal forcefully expels it. The reaction of the ejected water drives the animal in the direction opposite to the jet.

Cuttlefish, octopuses, and cephalopod mollusks

Cephalopods are the most refined natural jet swimmers. A squid draws water into its mantle cavity and drives it out through a narrow, steerable funnel; by aiming the funnel, the animal can dart forward or backward. Flying squid can even reach speeds that launch them clear of the water for short glides. Scallops clap their shells to squirt water and hop across the seabed, showing that even a simple mollusk exploits the same reaction thrust.

Breathing and movement: the dual function of the jet

In many aquatic animals the same jet serves two purposes at once — respiration and locomotion. A squid or octopus passes water over its gills to extract oxygen and then expels that same water through the funnel to move, so every breath doubles as a propulsive stroke. Dragonfly larvae draw water into and out of a rectal chamber to breathe, and by expelling it sharply they shoot forward, using the exhaled water as a means of escape.

Comparing propulsion efficiency among sea creatures

Jet propulsion is powerful but not always the most efficient way to swim, and its economy varies widely among organisms:

• Squid and cuttlefish — fast acceleration and quick bursts, but relatively high energy cost compared with fish that use undulating fins.
• Salps and tunicates — gelatinous filter feeders that pump water through the body, combining feeding with gentle, continuous jet locomotion.
• Jellyfish — pulse their bells to push water backward; they are among the most energy-efficient swimmers because they recapture some of the fluid's motion between pulses.
• Sea hares and scallops — use jetting only in short bursts, mainly to escape predators rather than for sustained travel.

The differences also depend on scale: at the small sizes and low Reynolds numbers of larvae and salps, water feels comparatively "thicker", so the fluid dynamics of a jet behave very differently than they do for a large, fast squid.

The falling cat and the dispute over Newton's laws

The most curious display of self-orientation was demonstrated by an ordinary cat. About a hundred and fifty years ago the well-known French physicist Marcel Deprez declared:

"Do you know, Newton's laws are not entirely correct. A body can move by means of internal forces, without leaning on anything or pushing off from anything." "Where is the proof, where are the examples?" the listeners protested. "You want proof? Very well. A cat that has accidentally fallen from a roof — there is your proof! However the cat falls, even head down, it will always land on all four paws. Yet the falling cat leans on nothing and pushes off from nothing, and turns over quickly and deftly. (Air resistance can be neglected — it is far too small.)"

Indeed, everyone knows this: cats, when they fall, always manage to land on their feet.

Cats do this instinctively, but a person can do the same thing deliberately. Divers leaping from a platform into the water can perform a complex figure — a triple somersault, turning over three times in the air and then suddenly straightening out, stopping the rotation of the body, and diving into the water along a straight line.

The same movements — without interaction with any external object — can sometimes be observed in the circus during the performances of acrobats and aerial gymnasts.

The falling cat was filmed with a motion camera, and then frame by frame on the screen the researchers examined what the cat does while flying through the air. It turned out that the cat rapidly rotates a paw. The rotation of the paw causes a counter-motion — a reaction of the whole body — and it turns in the direction opposite to the motion of the paw.

Everything happens in strict accordance with Newton's laws, and it is precisely thanks to them that the cat lands on its feet. The same thing happens in every case where a living creature, without any apparent cause, changes its motion in the air by redistributing angular momentum internally.

The waterjet boat

Inventors had the idea: why not borrow from the cuttlefish its method of swimming? They decided to build a self-propelled vessel with a water-jet engine. The idea was certainly feasible. True, there was no certainty of success: the inventors doubted whether such a waterjet boat would turn out better than an ordinary propeller-driven one. An experiment had to be made.

They chose an old tugboat, repaired its hull, removed the screw propellers, and installed a pump-jet in the engine room. This pump drew in water from overboard and forced it out astern through a pipe in a powerful jet. The boat moved, but it still traveled more slowly than a propeller steamer.

This is easily explained: an ordinary propeller spins astern unobstructed, with only water around it; the water in the pump-jet was set in motion by almost exactly the same kind of screw, but it turned not in open water but inside a tight pipe. Friction of the water jet against the walls arose.

Friction weakened the pressure of the jet. The steamer with the water-jet drive traveled more slowly than the propeller one and consumed more fuel. Nevertheless, builders did not abandon such vessels: they turned out to have important advantages. A ship equipped with a propeller must sit deep in the water, otherwise the propeller will uselessly churn the water or spin in the air.

Therefore propeller steamers are afraid of shallows and rapids and cannot navigate shallow water. Water-jet steamers, on the other hand, can be built shallow-draft and flat-bottomed: they need no depth — wherever a boat can pass, a water-jet steamer can pass too. The first water-jet boats in the Soviet Union were built in 1953 at the Krasnoyarsk shipyard. They were intended for small rivers where ordinary steamers cannot sail. The same idea reappears in modern marine engineering as the pump-jet, valued for shallow water and maneuverability.

Firearms and recoil

Engineers, inventors, and scientists studied jet propulsion especially diligently with the appearance of firearms. The first guns — all sorts of pistols, muskets, and hand-cannons — struck the shooter hard in the shoulder with every shot. After several dozen shots the shoulder began to hurt so badly that the soldier could no longer aim.

The first cannons — pishchals, unicorns, culverins, and bombards — leapt backward when fired, so that they sometimes maimed the gunners if the men failed to dodge and jump aside. The gun's recoil interfered with accurate shooting, because the cannon jerked before the ball or shell had left the barrel. This threw off the aim, and the fire became inaccurate.

Artillery engineers began their struggle with recoil more than four hundred and fifty years ago. At first the carriage was fitted with a spade that dug into the ground and served as a firm support for the gun. It was then thought that if the cannon were braced firmly enough from behind, so it had nowhere to roll back, the recoil would vanish. But this was a mistake. The law of conservation of momentum had not been taken into account.

The cannons broke all the props, and the carriages became so loose that the gun became unfit for combat. Then the inventors understood that the laws of motion, like all laws of nature, cannot be remade to suit oneself — they can only be "outwitted" with the help of the science of mechanics. They left the carriage a comparatively small spade for support and set the barrel on "slides" so that only the barrel, not the whole gun, rolled back.

The barrel was connected to a compressor piston that moves in its cylinder exactly as the piston of a steam engine does. But in the cylinder of a steam engine there is steam, while in the gun compressor there is oil and a spring (or compressed air). When the gun barrel rolls back, the piston compresses the spring. Meanwhile the oil is forced through small holes in the piston to the other side of it.

Strong friction arises, which partly absorbs the motion of the recoiling barrel, making it slower and smoother. Then the compressed spring straightens and returns the piston, and with it the gun barrel, to its former position. The oil presses on a valve, opens it, and flows freely back under the piston. During rapid fire the gun barrel moves almost continuously forward and back. In the gun compressor the recoil is absorbed by friction.

The muzzle brake

When the power and range of cannons increased, the compressor alone was no longer enough to neutralize the recoil. To help it, the muzzle brake was invented. A muzzle brake is merely a short steel tube fixed to the end of the barrel and serving as a kind of extension of it.

Its diameter is larger than that of the bore, so it does not in the least hinder the shell from flying out of the muzzle. Several elongated openings are cut around the circumference of the tube's walls.

The powder gases flying out of the gun barrel behind the shell immediately spread outward, and part of them enter the openings of the muzzle brake. These gases strike the walls of the openings with great force, rebound from them, and fly out — but no longer forward, rather slightly sideways and backward.

In doing so they press on the walls forward and push them, and with them the whole gun barrel. They help the carriage spring because they tend to cause the barrel to roll forward. And during the time they were inside the barrel, they pushed the gun backward. The muzzle brake significantly reduces and weakens the recoil.

Other inventors took a different path. Instead of fighting the jet motion of the barrel and trying to quench it, they decided to put the recoil to good use. These inventors created many models of automatic weapons — rifles, pistols, machine guns, and cannons — in which recoil serves to eject the spent cartridge and reload the weapon.

Rockets and rocket-propelled shells

One need not fight recoil at all but can use it: since action and reaction (recoil) are equal in force, equal in right, and equal in magnitude, let the reactive action of the powder gases, instead of pushing back the gun barrel, send the shell forward toward the target.

Thus rocket artillery was created. In it the jet of gases strikes not forward but backward, creating a forward-directed reaction in the shell. For a rocket weapon the expensive, heavy barrel proves unnecessary. A cheaper, simple iron tube serves perfectly to direct the shell's flight. One can dispense with the tube altogether and let the shell slide along two metal rails.

By its construction a rocket shell is like a firework rocket, only larger in size. In its head, instead of a colored Bengal-fire composition, a bursting charge of great destructive force is placed. The middle of the shell is filled with powder, which on burning creates a powerful jet of hot gases pushing the shell forward.

Moreover, the combustion of the powder can last a significant part of the flight time, not just the short interval during which an ordinary shell moves through the barrel of an ordinary cannon. The shot is not accompanied by such a loud sound. Rocket artillery is no younger than ordinary artillery, and may even be older: old Chinese and Arab books written more than a thousand years ago report the combat use of rockets.

In descriptions of later battles a mention of war rockets flashes here and there. When English troops were conquering India, Indian rocketeers with their fire-tailed arrows struck terror into the English invaders who were enslaving their homeland. For the English of that time, rocket weapons were a novelty.

With rocket grenades invented by General K. I. Konstantinov, the brave defenders of Sevastopol in 1854–1855 repelled the attacks of the Anglo-French troops.

The great advantage over ordinary artillery — the elimination of the need to haul heavy cannons — attracted the attention of commanders to rocket artillery. But an equally serious drawback hindered its improvement. The propellant, or "boost", charge could only be made from black powder, and black powder is dangerous to handle. It happened that during the manufacture of rockets the propellant charge exploded and workers died. Sometimes a rocket exploded at launch and gunners perished.

The Soviet designers and inventors eliminated this defect. During the Great Patriotic War they gave the army excellent rocket weapons — the "Katyusha" guards mortars — and invented the RS ("eres"), the rocket-propelled shells.

In quality, Soviet rocket artillery surpassed all foreign models and caused the enemy enormous losses. Defending their homeland, the people were forced to place every achievement of rocket technology in the service of defense.

Jet propulsion of an aircraft

An aircraft jet engine works by drawing in air, compressing it, burning fuel to heat it, and expelling the resulting hot gases rearward at high speed — the reaction driving the aircraft forward. This is the same reaction principle as Hero's aeolipile or a squid, but engineered for continuous, controlled thrust at high altitude and speed. The turbojet, turbofan, turboprop, and turboshaft are the main families of this technology used across aviation.

The basics of how an aircraft jet engine works

A jet engine operates on the Brayton cycle: air is compressed, fuel is added and burned at constant pressure, and the hot gas expands through a turbine and out a nozzle. The turbine extracts just enough energy from the exhaust to drive the compressor at the front, and everything left over becomes thrust. Key architecture common to nearly all designs includes:

• Inlet — slows and channels incoming air to the compressor.
• Compressor — multiple stages of rotating and stationary blades that raise the air's pressure.
• Combustor — where fuel is injected and burned continuously.
• Turbine — driven by the exhaust to power the compressor.
• Nozzle — accelerates the exhaust to produce thrust; an afterburner can inject extra fuel here for a large thrust boost at the cost of much higher fuel consumption.

Modern engines are governed by FADEC (Full Authority Digital Engine Control), and their certification follows standards such as FAA Part 33 and EASA CS-E. High-bypass turbofans like the Rolls-Royce BR710/725 and the Williams FJ44 move most of their air around the core rather than through it, which makes them quieter and far more fuel-efficient than early turbojets.

The compression and combustion process

Compression and combustion are the heart of the engine's power. The compressor can raise incoming air pressure many times over before it reaches the combustor, where fuel burns to raise the gas temperature sharply while pressure stays roughly constant. This hot, high-pressure gas then carries the energy that both spins the turbine and forms the propulsive jet. The efficiency of this cycle — how completely the fuel's energy is turned into useful thrust — is measured by thrust-specific fuel consumption (TSFC), a key figure in comparing engines.

Exhaust velocity and engine thrust

Thrust depends directly on how much mass the engine throws out and how fast. Using the momentum equation over a control volume, thrust equals mass flow rate times the change in velocity of the air, plus a pressure term at the nozzle. This explains a fundamental trade-off in propulsion:

• A turbojet ejects a small mass of air very fast — efficient at high, even supersonic and hypersonic, speeds but noisy and thirsty at low speeds.
• A high-bypass turbofan ejects a large mass of air more slowly — much more efficient and quieter for airliners such as the Boeing 787 Dreamliner.
• A propeller, by contrast, moves a very large mass of air at low velocity; its efficiency falls once the helical tip speed approaches the local speed of sound and shock waves form, which is why propeller aircraft are limited to lower cruise speeds. The Wright Brothers' careful propeller research first established these aerodynamic performance curves.
• A rocket engine carries its own oxidizer and ejects exhaust at extreme velocity, so it works even in the vacuum of space; its efficiency is expressed as specific impulse, and its performance follows the Tsiolkovsky rocket equation.

The spacecraft and rocket propulsion

For centuries people cherished the dream of flying through interplanetary space, of visiting the Moon, mysterious Mars, and cloud-covered Venus. Many science-fiction novels, novellas, and stories were written on this theme. Writers sent their heroes into the heavens on trained swans, in balloons, in cannon shells, or by some other improbable means.

All these methods of flight, however, were based on fantasies with no foundation in science. People merely believed that one day they would be able to leave our planet, but did not know how they would manage to accomplish it.

The remarkable scientist Konstantin Eduardovich Tsiolkovsky, in 1903, first gave the idea of space travel a scientific basis. He proved that people could leave the globe and that the rocket would serve as the means of transport, because the rocket is the only engine that needs no external support for its motion.

Therefore the rocket is able to fly through airless space.

In its construction a spacecraft must resemble a rocket shell, only its head will hold a cabin for passengers and instruments, and all the remaining space will be occupied by the store of combustible mixture and the engine.

To give the ship the necessary speed, suitable fuel is required. Powder and other explosives are by no means suitable: they are both dangerous and burn too quickly, failing to sustain prolonged motion.

Tsiolkovsky recommended using liquid fuel: alcohol, gasoline, or liquefied hydrogen, burning in a stream of pure oxygen or some other oxidizer. Everyone acknowledged the correctness of this advice, because no better fuel was known at the time.

The first liquid-fuel rocket, weighing sixteen kilograms, was tested in Germany on 10 April 1929. The experimental rocket flew into the air and vanished from sight before the inventor and everyone present could follow where it had gone. The rocket could not be found after the test.

The next time, the inventor decided to "outwit" the rocket and tied a rope four kilometers long to it. The rocket soared up, dragging its rope tail. It paid out two kilometers of rope, snapped it, and followed its predecessor in an unknown direction.

This runaway could not be found either. The first successful flight of a liquid-fuel rocket took place in the USSR on 17 August 1933. The rocket rose, flew its allotted distance, and landed safely. All these discoveries and inventions are based on Newton's laws.

Self-guided aircraft and wartime jet innovations

During the war German engineers built several hundred self-guided aircraft: "V-1" flying bombs and "V-2" rockets. These were cigar-shaped projectiles fourteen meters long and one hundred sixty-five centimeters in diameter. The deadly cigar weighed twelve tons — nine of fuel, two of casing, and one of explosive. The "V-2" flew at speeds up to 5,500 kilometers per hour and could climb to heights of 170–180 kilometers.

These instruments of destruction were not distinguished by accuracy and were suitable only for shelling such large targets as big, densely populated cities. The German fascists launched "V-2" rockets from 200–300 kilometers away toward London, counting on the city being so large that something, somewhere, would be hit.

Newton could hardly have supposed that his ingenious experiment and the laws of motion he discovered would form the basis of weapons created out of bestial hatred for people, and that whole districts of London would be turned into ruins. The same era, however, also drove the first practical jet aircraft: Germany's Me-262 turbojet fighter and the rocket-powered Me-163 flew during World War Two, and their engines were the direct ancestors of today's civil turbofans.

How jet propulsion is applied in the modern world

Jet propulsion today powers nearly all fast, long-range transport — from airliners and military jets to helicopters and spacecraft — because no other technology delivers so much thrust so reliably at high speed and altitude. Beyond aircraft, the same reaction principle drives pump-jets in boats, plasma thrusters that accelerate ionized gas electromagnetically for satellites, and experimental future engines.

Jet propulsion in commercial and private aviation

Commercial aviation depends on high-bypass turbofan engines that balance thrust, range, and fuel economy for hundreds of passengers. Airliners such as the Boeing 787 Dreamliner combine efficient engines with lightweight structures to fly intercontinental routes; agencies including IATA and the Regional Airline Association coordinate the standards behind these operations. Private aviation uses the same technology scaled to smaller aircraft categories:

• Light jets — powered by engines like the Williams FJ44, ideal for short regional hops.
• Midsize jets — often using the Pratt & Whitney Canada PW300-series for balanced range and cabin size.
• Large-cabin jets — fitted with engines such as the Rolls-Royce BR710/725 for long international travel.
• Helicopters — driven by turboshaft engines, a jet core that delivers power to a rotor instead of a jet nozzle.

Private charter operators such as GlobeAir and jet-card providers like BlackJet market on-demand access, and their safety is audited against programs and standards including ARGUS, Wyvern, IS-BAO, and the guidance of NBAA. Pilots learning these systems study aircraft engine fundamentals through organizations such as AOPA, the University Aviation Association, California Aeronautical University, and outreach programs like EAA Young Eagles.

Technologies that improve fuel efficiency

Fuel efficiency in jet engines has advanced chiefly by raising the bypass ratio, improving materials, and refining the combustion process. Higher bypass moves more air around the core at lower velocity, cutting both noise and TSFC. Digital FADEC control keeps each engine running at its optimum point, while advances in maintenance, reliability, and mechanical problem diagnosis extend engine life and lower operating cost. Noise reduction — through chevron nozzles and quieter fan designs — has been driven by research at NASA and industry engineers alike.

The future of jet technology and sustainable fuel

The future of jet propulsion points toward carbon-neutral operations built on Sustainable Aviation Fuel (SAF), which can be burned in existing engines with far lower lifecycle emissions. Electric and hybrid propulsion, hydrogen combustion, and plasma thrusters that accelerate charged particles electromagnetically are all under active development for different roles — the last of these already propelling satellites in space. As with every device in this history, from Hero's spinning sphere to a modern spacecraft, these emerging engines still obey the same law that Newton set down: thrust is the reaction to mass thrown the other way.