The process of forcing an object to move. The word is also used to refer to the entire system of engines for achieving propulsion in the context of flight vehicles.
While the machinery is complex, the principles of operation are common to most propulsion systems. According to Newton’s second law of motion, the net force exerted on an object is equal to the rate of change of its momentum. According to Newton’s third law of motion, every action (of a force) produces an equal and opposite reaction. For flight in the atmosphere, air is used as the working fluid whose momentum is changed by the propulsion system. The reaction to the resulting force acts on the propulsion system and drives the aircraft forward.
Since momentum is the product of mass and velocity, designers can choose to produce a given increase of momentum by either accelerating a large mass of fluid per second through a small change in velocity, or accelerating a smaller mass of fluid through a large increase in velocity. For flight at low speeds, it is more efficient to do the former. For example, helicopters and propeller-driven airplanes use large rotating blades to capture a large amount of air and accelerate it through a relatively small change in velocity. For flight at high speeds, turbojet and ramjet engines, which usually have small intake areas, add heat to the captured air. This heat is then converted to the work done in accelerating the air through a large velocity change, leaving hot jets of air behind. In effect, a force is exerted on the air by the engine to accelerate it backward from the aircraft. The reaction to this force acts on the engine and hence drives the aircraft forward.
The same principle applies to rocket propulsion, in the atmosphere or in outer space. Rockets generate gas at high pressure by burning chemicals, and this gas escapes at high speed through a nozzle. The reaction to the force used in doing so accelerates the rocket. The key idea is that the engine and the propellant gases are pushing against each other: no other medium is needed to be pushed. In the early days of rocket flight, several experts, including editorials in The New York Times, sneered at rocket pioneer Robert H. Goddard for his insistence that rockets could thus work in the vacuum of space, but today such flight is taken for granted.
Early aircraft propulsion systems used piston engines to drive propellers. The revolving blades of the propeller are like rotary wings, producing a force and accelerating the air encountered within the large area swept by the blades. Propellers were termed pusher or puller props, depending on whether they were mounted behind or ahead of the wings. Propellers are highly efficient as propulsion for slow-flying aircraft. Today many short-range aircraft and general aviation aircraft are powered by turboprop engines, where the engine uses the gas turbine principle, but the power generated is used to drive a propeller. For flight at more than about half the speed of sound (Mach 0.5), the speed at the tips of the blades exceeds the speed of sound, and shocks form, generating unacceptable levels of noise and drag.
Renewed interest in propeller-driven aircraft comes from the idea of continuously flying airplanes in the upper atmosphere using solar power to drive a motor and propeller. The National Aeronautics and Space Administration (NASA) Solar Pathfinder demonstrated ascent to over 80,000 feet using wings covered with solar panels. The energy absorbed from the Sun during the daytime can drive the vehicle to such high altitudes that it can glide all night without coming down too low. Thus automatic, continuously flying aircraft can be propelled using solar power.
The earliest evidence of rocket usage is from China, where black-powder rockets stabilized with bamboo poles, perhaps with multiple stages, were used in the twelfth century. The South Indian king Tippu Sultan of Mysore used iron-cased rocket-powered projectiles with 2,400-meter range from 1780 to 1799 in order to protect his nation from British invaders. Using rockets captured from India, Britain’s William Congreve developed solid rockets with a 3,000-yard range, used against Napoleon’s forces in Bologne in 1806, and in the War of 1812 against the United States. Russia’s Konstantin Tsiolkovsky (1857-1935) developed the idea of multistage rockets to escape Earth’s gravity in a 1903 paper titled “Isslyedovanye mirovykh prostranstv ryeaktivnymi priborami” (“Exploration of Space with Reactive Devices,” 1957) discussing the use of liquid oxygen and liquid hydrogen. American Robert H. Goddard (1882-1945) registered a patent in 1914 for the design of a rocket combustion-chamber nozzle and propellant feed system. He published “A Method of Reaching Extreme Altitudes” in 1919 through the Smithsonian Institute, and conducted experiments with liquid-oxygen and gasoline propellants between 1920 and 1940. In Germany, Hermann Oberth published Die Rackete zu den Planetenräumen (1923; the rocket into interplanetary space) and Wege zür Raumschiffart (1929; the road to space travel). During World War II, air-launched rocket-powered unguided missiles were used, followed by Russian use of rockets in artillery barrages, and the German V-1 and V-2 ballistic missiles which were launched into Britain. After the war, with German rocket engineers inducted into American and Soviet research organizations, the missile race accelerated. On October 4, 1957, the Soviet Union’s Sputnik became the first artificial satellite of Earth, and by 1969, Apollo 11 had taken two men to walk on the Moon and return to Earth.
The simplest rocket engine has a propellant grain of fuel and oxidizer in solid form, ignited at one end. As the solid melts and vaporizes due to the heat, the chemical reaction starts, releasing much more heat. The hot gases reach high pressure in the combustion chamber and exhaust through a nozzle, reaching high velocities. Rocket designers shape the propellant grain (the shape of the interior core of the solid propellant) in various ways to tailor the rate at which the solid material is consumed, thus predetermining how the thrust will vary with time. In general, the thrust of a solid rocket cannot be controlled once it starts, aside from releasing the pressure and thus stopping the combustion: most modern solid propellants do not burn unless the pressure is several atmospheres.
Liquid propellants are stored in one or more tanks, and pumped into the combustion chamber, where the pressure is usually much higher than in the storage tanks. While liquid rockets are more controllable, the pumps often pose failure risks; however, the lack of control of the solid rocket is also a disadvantage. Hybrid rockets use a bi-propellant, where the liquid propellant is metered to flow over a solid propellant grain.
The performance of a propulsion system is characterized by its specific impulse (Isp), which is the thrust developed per second, per unit weight of the propellant consumed, at the standard value of Earth’s gravitational acceleration, and expressed in units of seconds. The specific impulse of solid-fueled rockets is limited to about 270 seconds. Liquid-fueled rockets using storable fuels are limited to about 250 seconds. Rockets with cryogenic fuels such as liquid oxygen and liquid hydrogen reach 390 to 450 seconds. Proponents of nuclear thermal propulsion hope to achieve an Isp of 825-925 seconds. Electrothermal propulsion, where the propellant gas is heated by an electric arc, promises 800 to 1,200 seconds; electromagnetic acceleration, 5,000 seconds; and ion propulsion, 10,000 seconds.
High Isp does not tell the whole story, since the higher Isp systems usually required heavy machinery, and produce very small amounts of thrust. The specific impulse of engines in space is proportional to the exhaust velocity of the propellant gas. For a given addition of momentum per unit mass, hydrogen, having the lowest molecular weight, provides the highest specific impulse. An efficient type of rocket engine is the solar-hydrogen engine used in orbit transfer vehicles shuttling between low-Earth orbit and geosynchronous Earth orbits. Here solar energy is focused by a collector to heat hydrogen, which then flows out at high speed through a nozzle.
A heat source is crucial to propulsion, and one which generates the most heat with the least expenditure of fuel weight would produce the highest specific impulse. Nuclear reactions satisfy this criterion, but the weight of the shielding needed for the reactor, and the consequences of a crash, have limited their use in flight propulsion. The slow neutron reactors used in ships and submarines proved to be too heavy for use in aircraft, while other designs, which could heat air to high temperatures quickly, operated at temperatures too high for available materials and posed extreme radiation hazards. In the 1950’s, an Aircraft Nuclear Propulsion (ANP) project led to several advanced designs for nuclear-powered intercontinental bombers, but none appear to have been flight-tested. Project Pluto, a secret project conducted in Nevada, developed a nuclear-powered ramjet supersonic cruise missile. Small nuclear reactors have been used in deep-space probes such as the Galileo mission, and it is expected that missions to other planets, such as an exploration of Jupiter’s atmosphere, will require nuclear propulsion to provide the required specific impulse. Proposed nuclear thermal rockets will heat propellant gas (hydrogen) through the coolant channels of a solid-fuel reactor core at about 3,000 degrees Kelvin, and expand hydrogen through a nozzle.
Ionized gases are accelerated to high exhaust velocity using electromagnetic fields in engines used to produce low thrust, available for station-keeping orbit corrections over long durations on spacecraft. The Boeing 702 Xenon Ion Thruster claims an Isp of 3,800 seconds and thrust of 165 million newtons (by comparison, the Saturn V at liftoff produced over 33 million newtons). The weight of the system required to produce the electromagnetic field has restricted the usage of ion propulsion to low-thrust applications, perhaps until superconducting electromagnets become available for use in such systems.
For flight in the atmosphere, the effective specific impulse can be increased greatly by using oxygen in the air as oxidizer, and air as the working fluid: air does not have to be added to the fuel cost or vehicle weight. There are three principles of jet propulsion: heat addition to the working fluid is most efficient if the heat is added at the highest pressure possible; the conversion of heat to work is most efficient if the temperature difference is largest; and the thrust is most efficient in driving the aircraft if the exhaust velocity is close to (but greater than) the flight speed.
In the gas turbine cycle, the working fluid is first compressed, then heat is added at constant pressure, and finally work is extracted from the hot, high-pressure fluid as it expands and flows out. Thus, gas turbine engines incorporate a compressor to increase pressure, a combustion chamber to add the heat through a combustion reaction between the fuel and air, a turbine to extract work and run the compressor, and a nozzle to expand the flow out. For large engines used by commercial aircraft, the optimal value of pressure ratio (between the highest pressure after compression and the outside) is as high as 50. At supersonic speeds, the deceleration of the air at the front of the engine itself raises the pressure substantially; the optimum pressure ratio may be only 7. As Mach number increases beyond 2.5, the need for a mechanical compressor vanishes, and ramjet engines can operate. Here the incoming air is decelerated, so that its pressure increases to such large values that mechanical compressors and the turbines to operate them are not needed.
All other gas turbine engines require compressors to increase the pressure of the incoming air, and turbines which drive the compressor and extract work required to run other components including propellers, rotors, and fans. These turbomachines change pressure through several stages. Each stage has a rotor where work is done on the fluid to change its momentum, and a stator, or counter-rotating rotor, to recover the momentum change and convert it into a pressure change. Turbomachine stages may be centrifugal or axial. In centrifugal stages, air comes in near the axis and is flung out pressurized at the periphery. In axial stages, the flow is predominantly parallel to the axis, with rows of blades successively increasing momentum by swirling the flow and recovering the pressure by reducing the swirl.
The first jet engines were turbojets, where all of the airflow went through the same compressor and combustion chamber. The first jet engine was patented in 1930 by Sir Frank Whittle (1907-1996). The PowerJets Model W.1 engine was first tested in April, 1937, and according to Sir Whittle, “made a noise like an air raid siren,” sending onlookers running for cover. It weighed 700 pounds and produced 860 pounds of thrust, using a double-sided centrifugal compressor. The first British aircraft to use the engine was the Gloster Meteor, a night fighter which first flew in March, 1943, eventually reaching 420 miles per hour. The first jet-powered flight, however, was on a Heinkel aircraft powered by Hans von Ohain’s (1911-1998) axial-compressor turbojet engine in Germany. The first jet fighter took off on July 18, 1942, a Messerschmitt Me-262 fighter piloted by Fritz Wendel of the German Luftwaffe, using a Junkers Jumo 004 turbojet engine producing 2,200 pounds of thrust. Earlier attempts had been made using BMW003 turbojet engines, which used a seven-stage axial-flow compressor and an annular combustion chamber with sixteen burners. Today, centrifugal compressors are used in the turbopumps of rocket engines, while axial compressors are dominant in most aircraft applications. Helicopter turboshaft engines use both centrifugal and axial stages. The thrust-to-weight ratio of modern jet engines has improved to well over 4:1.
Turboprop engines use a small turbine to extract enough work from the hot combustor gases to run the compressor, and a large power turbine to extract most of the work from the air to run a propeller. The propeller is connected through a gearbox to reduce the speed of revolution; this adds considerable weight to turboprop engines. The Soviet Bear long-range bomber used turboprop engines with a pair of counter-rotating propellers on each engine. The design tradeoff between high thermal efficiency (requiring high pressure and temperature) and high propulsive efficiency (requiring a small increase of air velocity from the flight speed) is addressed using bypass or turbofan engines, where a part of the captured air goes through a fan and a nozzle, bypassing the main compressor, combustor, and turbine. The bypass ratio is the ratio of the air bypassing the hot core of the engine to the air which goes through the core and has fuel burned in it. Fighter aircraft turbofan engines use a bypass ratio of approximately 1, while modern commercial aircraft engines, such as the GE90 used on the Boeing 777 and Airbus 340 airliners, use bypass ratios up to 12.
In the 1980’s, propfans or unducted fans were explored to bridge the gap between the propeller and the ducted turbofan engine. Using modern computational aerodynamics technology, large fan blades of complex shape were designed to operate with supersonic tip speeds and large pressure rise across each stage. Some designs had counter-rotating rows of fan blades. To increase the capture area, the blades were left without the outer cowling used by turbofan engines. These engines promised large improvements in fuel efficiency for short-haul aircraft, but encountered severe problems of development cost and noise levels high enough to damage the aircraft structure through sonic fatigue.
For air-breathing flight at supersonic speeds, a supersonic inlet must slow down the supersonic flow with minimal losses due to shock waves, so that the fan, compressor, and combustion chamber can operate at subsonic speeds. Inlets vary in complexity from the normal-shock inlet of the early MiG and Sabre fighters, through the movable spike inlets of the MiG-21 or the SR-71, to the multiple-ramp inlets of the F-15 or Concorde. Hypersonic aircraft use the compression across the shock produced by the aircraft fuselage to decelerate, so that engine-airframe integration is vital to such designs. Instead of varying geometry, supersonic flows can also be decelerated and compressed using heat addition (thermal compression). At the other end, nozzles vary from simple convergent nozzles of subsonic aircraft, to the converging-diverging nozzles of fighters with afterburners, to the rectangular nozzles of modern fighters where the thrust can be vectored for maneuvering or vertical takeoff. High-speed aircraft concepts (NASA’s X-33, Lockheed’s VentureStar, and the Japanese ATREX turboramjet) use the Aerospike or Plug Flow nozzles to enable external variation of the nozzle expansion. Several other types of propulsion devices are being studied by researchers.
In the Mini-Magnetospheric Plasma Propulsion (M2P2) concept developed by Robert Winglee at the University of Washington, jets of heated gas plasma, fired from a spacecraft, interact with the magnetic field generated by the spacecraft to produce a mini-magnetosphere around the craft. The interaction of this magnetosphere with the plasma wind from the Sun (the solar wind) produces forces in a fashion somewhat similar to the interaction of an airfoil shape with flowing air generating lift. This force can be tailored to drive the spacecraft around the solar system at very high speeds. Unlike solar sails, which work better to drive a spacecraft in the inner solar system, M2P2 is seen as an option for travel to the outer planets.
Scientists have long speculated that photons could exert pressure on a spacecraft and drive it to speeds approaching the speed of light. Practical systems for focusing high-power lasers onto spacecraft are not yet in use in space. Experiments by Leik Myrabo of Rensselaer Polytechnic Institute and the U.S. Air Force had succeeded, by the year 2000, in lifting small objects to a height of a few dozen meters using ground-based lasers. In extended forms of this concept, the focused laser beam creates an “aerospike” of heated gas ahead of the vehicle, which helps reduce drag as the vehicle is driven up through the atmosphere by a shock created by expanding air beneath the vehicle.
Scientists hope that in the distant future, power generation by nuclear fusion or matter-antimatter interaction will allow the development of propulsion systems with immense thrust levels and very high specific impulse. For now, such systems remain impractical.
Hill, Philip G., and Carl R. Peterson. Mechanics and Thermodynamics of Propulsion. 2d ed. Reading, Mass.: Addison-Wesley, 1992. Comprehensive textbook on gas turbines and rocket propulsion, suitable for undergraduate engineering students. Hunecke, Klaus. Jet Engines: Fundamentals of Theory, Design, and Operation. Osceola, Wis.: Motorbooks International, 1998. A thorough explanation of jet engine mechanics geared toward practical application. Glenn Learning Technologies Project. NASA Glenn Research Center. (www.grc.nasa.gov/www/K-12/airplane/shortp.html) Expositions of principles, example problems, and animated demonstrations, especially on propulsion. Marshall Brain’s “How Stuff Works.” (www.howstuff works.com/turbine.htm) Concise explanations of a multitude of items in terms of both the systems and their components. NASA-Marshall Space Flight Center. (www.msfc.nasa .gov) This Web site provides colorful artists’ concepts, photographs of current projects, and project information on advanced propulsion concepts. Turner, Martin J. L. Rocket and Spacecraft Propulsion: Principles, Practice, and New Developments. New York: Springer Verlag, 2000. Written by a space scientist for readers without a background in engineering. Covers developments in propulsion systems that may power the next generation of space exploration.
Forces of flight
Robert H. Goddard
National Aeronautics and Space Administration
Turbojets and turbofans
The United States’ first rocket-assisted airplane takes off on August 12, 1941. The Ercoupe plane was fitted with a solid-propellant 28-pound-thrust JATO (jet-assisted takeoff) booster.