Propulsion Technologies

Summary

The field of propulsion deals with how aircraft, missiles, and spacecraft are propelled toward their destinations. Subjects of development include propellers and rotors driven by internal combustion engines or jet engines, rockets powered by solid- or liquid-fueled engines, spacecraft powered by ion engines, solar sails or nuclear reactors, and matter-antimatter engines. Propulsion system metrics include thrust, power, cycle efficiency, propulsion efficiency, specific impulse, and thrust-specific fuel consumption. Advances in this field have enabled humanity to travel worldwide in a few hours, visit space and the moon, and send probes to distant planets.

Definition and Basic Principles

Propulsion is the science of making vehicles move. The propulsion system of a flight vehicle provides the force to accelerate and balance the forces opposing the vehicle’s motion. Most modern propulsion systems add energy to a working fluid to change its momentum and thus develop force, called thrust, along the desired direction. A few systems use electromagnetic fields or radiation pressure to develop the force to accelerate the vehicle. The working fluid is usually a gas, and the process can be described by a thermodynamic heat engine cycle involving three basic steps. First, work on the fluid to increase its pressure. Second, heat or other forms of energy should be added at the highest possible pressure. Third, allow the fluid to expand, converting its potential energy directly to useful work, or to kinetic energy in an exhaust.

In the internal combustion engine, a high-energy fuel is placed in a small closed area and ignited by compression. This produces expanding gas, which drives a piston and a rotating shaft. The rotating shaft drives a transmission whose gears transfer the work to wheels, rotors, or propellers. Rocket and jet engines operate on the Brayton thermodynamic cycle. In this cycle, the gas mixture is compressed adiabatically (no heat added or lost during compression). Heat is added externally or by chemical reaction to the fluid, ideally at constant pressure. The expanding gases are exhausted, with a turbine extracting some work. The gas then expands out through a nozzle.

Background and History

Solid-fueled rockets developed in China in the thirteenth century achieved the first successful continuous propulsion of heavier-than-air flying machines. In 1903, Orville and Wilbur Wright used a spinning propeller driven by an internal combustion engine to accelerate air and develop the reaction force that propelled the first human-carrying heavier-than-air powered flight.

As propeller speeds approached the speed of sound in World War II, designers switched to the gas turbine or jet engine to achieve higher thrust and speeds. German Wernher von Braun developed the V2 rocket, originally known as the A4, for space travel, but in 1944, it began to be used as a long-range ballistic missile to attack France and England. The V2 traveled faster than the speed of sound, reached heights of 83 to 93 kilometers, and had a range of more than 320 kilometers. The Soviet Union's 43-ton Sputnik rocket, powered by a LOX/RP2 engine generating 3.89 million Newtons of thrust, placed a 500-kilogram satellite in low Earth orbit on October 4, 1957.

The United States' three-stage, 111-meter-high Saturn V rocket weighed more than 2,280 tons and developed more than 33.36 million Newtons at launch. It could place more than 129,300 kilograms into a low-Earth orbit and 48,500 kilograms into a lunar orbit, thus enabling the first human visit to the moon in July 1969. The reusable space shuttle weighs 2,030 tons at launch, generates 34.75 million Newtons of thrust, and can place 24,400 kilograms into a low-Earth orbit. In January 2006, the New Horizons spacecraft reached 57,600 kilometers per hour as it escaped Earth's gravity. Meanwhile, air-breathing engines have grown in size and become more fuel-efficient, propelling aircraft from hovering to supersonic speeds.

How It Works

Rocket. The rocket is conceptually the simplest of all propulsion systems. All propellants are carried on board, gases are generated with high pressure, heat is added or released in a chamber, and the gases are exhausted through a nozzle. The momentum of the working fluid is increased, and the rate of increase of this momentum produces a force. The reaction to this force acts on the vehicle through the mounting structure of the rocket engine and propels it. Rocket propulsion may result from chemical rockets using bipropellant or solid fuel. Some spacecraft use air-breathing engines. Satellites often use electric propulsion like Hall-effect or ion thrusters.

Jet Propulsion. Although rockets certainly produce jets of gas, the term jet engine typically denotes an engine in which the working fluid is mostly atmospheric air. Hence, the only propellant carried on the vehicle is the fuel that releases heat. Typically, the mass of fuel used is only about 2 to 4 percent of the mass of air the vehicle accelerates. Types of jet engines include the ramjet, the turbojet, the turbofan, and the turboshaft.

Propulsion System Metrics. The thrust of a propulsion system is the force generated along the desired direction. Thrust for systems that exhaust a gas can come from two sources. Momentum thrust comes from the acceleration of the working fluid through the system. It equals the difference between the momentum per second of the exhaust and intake flows. Thrust can also be generated from the product of the area of the jet exhaust nozzle cross section and the difference between the static pressure at the nozzle exit and the outside pressure. This pressure thrust is absent for most aircraft in which the exhaust is not supersonic, but it is inevitable when operating in the vacuum of space. The total thrust is the sum of momentum thrust and pressure thrust. Dividing the total thrust by the exhaust mass flow rate of the propellant gives the equivalent exhaust speed. All else being equal, designers prefer the highest specific impulse, though it must be noted that there is an optimum specific impulse for each mission. LOX-LH2 rocket engines achieve specific impulses of more than 450 seconds, whereas most solid rocket motors cannot achieve 300 seconds. Ion engines exceed 1,000 seconds. Air-breathing engines achieve very high values of specific impulse because most of the working fluid does not have to be carried onboard.

The higher the specific impulse, the lower the mass ratio needed for a given mission. To lower the mass ratio, space missions are built up in several stages. As each stage exhausts its propellant, the tank and its engines are discarded. When the propellant is gone, only the payload remains. The relation connecting the mass ratio, the delta-v, and specific impulse, along with the effects of gravity and drag, is called the rocket equation.

Propulsion systems, especially for military applications, operate at the edge of their stable operation envelope. For instance, if the reaction rate in a solid propellant rocket grows with pressure at a greater than linear rate, the pressure will keep rising until the rocket blows up. A jet engine compressor will stall, and flames may shoot out the front if the blades go past the stalling angle of attack. Diagnosing and solving the problems of instability in these powerful systems has been a constant concern of developers since the first rocket exploded.

Applications and Products

Many kinds of propulsion systems have been developed or proposed. The simplest rocket is a cold gas thruster, in which gas stored in tanks at high pressure is exhausted through a nozzle, accelerating (increasing momentum) in the process. All other types of rocket engines add heat or energy in some other form in a combustion (or thrust) chamber before exhausting the gas through a nozzle.

Solid-fueled rockets are simple and reliable and can be stored for a long time, but once ignited, their thrust is difficult to control. An ignition source decomposes the propellant at its surface into gases whose reaction releases heat and creates high pressure in the thrust chamber. The surface recession rate is thus a measure of propellant gas generation. The thrust variation with time is built into the rocket grain geometry. The burning area exposed to the hot gases in the combustion chamber changes in a preset way with time. Solid rockets are used as boosters for space launches and storable missiles that must be launched quickly on demand.

Liquid-fueled rockets typically use pumps to inject propellants into the combustion chamber. The propellants vaporize, and a chemical reaction releases heat. Typical applications are the main engines of space launchers and engines used in space, where the highest specific impulse is needed.

Hybrid rockets use a solid propellant grain with a liquid propellant injected into the chamber to vary the thrust as desired. Electric resistojets use the heat generated by currents flowing through resistances. Though simple, their specific impulse and thrust-to-weight ratio are too low for wide use. Ion rocket engines use electric fields or, in some cases, heat to ionize a gas and a magnetic field to accelerate the ions through the nozzle. These are preferred for long-duration space missions in which only a small thrust level is needed but for an extended duration because the electric energy comes from solar photovoltaic panels. Nuclear-thermal rockets generate heat from nuclear fission and may be coupled with ion propulsion. Proposed matter-antimatter propulsion systems use the annihilation of antimatter to release heat with extremely high specific impulse.

Pulsed detonation engines are being developed for some applications. A detonation is a supersonic shock wave generated by intense heat release. These engines use a cyclic process in which the propellants come into contact and detonate several times a second. Nuclear-detonation engines were once proposed, in which the vehicle would be accelerated by shock waves generated by nuclear explosions in space to reach extremely high velocities. However, international law prohibits nuclear explosions in space.

Ramjets and Turbomachines. Ramjet engines are used at supersonic speeds and beyond, where the deceleration of the incoming flow is enough to generate very high pressures adequate for an efficient heat engine. When the heat addition is done without slowing the fluid below the speed of sound, the engine is called a scramjet or supersonic combustion ramjet. Ramjets cannot start by themselves from rest. Turbojets add a turbine to extract work from the flow leaving the combustor and drive a compressor to increase the pressure ratio. A power turbine may be used downstream of the main turbine. In a turbofan engine, the power turbine drives a fan that works on a larger mass flow rate of air, bypassing the combustor. In a turboprop, the power is taken to a gearbox to reduce revolutions per minute, powering a propeller. In a turboshaft engine, the power is transferred through a transmission, as in the case of a helicopter rotor, tank, ship, or electric generator. Many applications combine these concepts, such as a propfan, a turboramjet, or a rocket-ramjet that starts as a solid-fueled rocket and becomes a ramjet when propellant consumption opens enough space to ingest air.

Gravity Assist. A spacecraft can be accelerated by sending it close enough to another heavenly body (such as a planet) to be strongly affected by its gravity field. This swing-by maneuver sends the vehicle into a more energetic orbit with a new direction, enabling surprisingly small mass ratios for deep space missions.

Tethers. Orbital momentum can be exchanged using a tether between two spacecraft. This principle has been proposed to efficiently transfer payloads from Earth's orbit to lunar or Martian orbits and to exchange payloads with the lunar surface. An extreme version is a stationary tether linking a point on Earth's equator to a craft in geostationary Earth orbit, the tether running far beyond to a counter-mass. The electrostatic tether concept uses variations in the electric potential with orbital height to induce a current in a tether strung from a spacecraft. An electrodynamic tether uses the force exerted on a current-carrying tether by the magnetic field of the planet to propel the tether and the craft attached to it.

Solar and Plasma Sails. Solar sails use the radiation pressure from sunlight bounced off or absorbed by thin, large sails to propel a craft. Typically, this works best in the inner solar system, where radiation is more intense. Other versions of propulsion sails, in which lasers focus radiation on sails that are far away from the Sun, have been proposed. In mini magnetospheric plasma propulsion (M2P2), a cloud of plasma (ionized gas) emitted into the field of a magnetic solenoid creates an electromagnetic bubble around 30 kilometers in diameter, which interacts with the solar wind of charged particles that travels at 300 to 800 kilometers per second. The result is a force perpendicular to the solar wind and the (controllable) magnetic field, similar to aerodynamic lift. This system has been proposed to conduct fast missions to the outer reaches of the solar system and back.

Careers and Course Work

Propulsion technology spans aerospace, mechanical, electrical, nuclear, chemical, and materials science engineering. Aircraft, space launchers, spacecraft manufacturers, and the defense industry are major customers of propulsion systems. Workplaces in this industry are distributed over many regions in the United States and near many major airports and National Aeronautics and Space Administration (NASA) centers. The large airlines operate engine testing facilities. Propulsion-related work outside the United States, France, Britain, and Germany is usually in companies run by or closely related to the government. Because propulsion technologies are closely related to weapon-system development, many products and projects come under the International Traffic in Arms Regulations.

Students aspiring to become rocket scientists or jet engine developers should take courses in physics, chemistry, mathematics, thermodynamics and heat transfer, gas dynamics and aerodynamics, combustion, and aerospace propulsion.

Machinery operating at thousands to hundreds of thousands of revolutions per minute requires extreme precision, accuracy, and material perfection. Manufacturing jobs in this field include specialist machinists and electronics experts. Because propulsion systems are limited by the pressure and temperature limits of structures that must also have minimal weight, the work usually involves advanced materials and manufacturing techniques. Instrumentation and diagnostic techniques for propulsion systems constantly push the boundaries of technology and offer exciting opportunities using optical and acoustic techniques.

Social Context and Future Prospects

Propulsion systems have enabled humanity to advance beyond the speed of ships, trains, balloons, and gliders to travel across the oceans safely, quickly, and comfortably and to venture beyond Earth's atmosphere. The result has been a radical transformation of global society since the early twentieth century. People travel overseas regularly, and on any given day, city centers on every continent host conventions with thousands of visitors from all over the world. Jet engine reliability has become so established that jetliners with only two engines routinely fly across the Atlantic and Pacific oceans. However, jet engines are not very energy-efficient, which makes them expensive to operate and detrimental to the environment. As such, addressing this problem is a major area of jet engine research in the twenty-first century.

Propulsion technologies are just beginning to grow in their capabilities. In the early twenty-first century, specific impulse values were, at best, a couple of thousand seconds. However, concepts using radiation pressure, nuclear propulsion, and matter-antimatter promised values ranging into hundreds of thousands of seconds. Air-breathing propulsion systems promise specific impulse values of over 2,000 seconds, enabling single-stage trips by reusable craft to space and back. As electric propulsion systems with high specific impulses come down in system weight because of specially tailored magnetic materials and superconductors, travel to the outer planets may become routine. Technology like Pulsar Fusion's Hall-effect thruster uses only small amounts of liquid fuel but reaches speeds up to 30 kilometers per second. Its thrust is generated using electromagnetic fields with high-speed, ionized gas. This speed and practical fuel use supports the continued commercial and scientific space missions. Spacecraft with solar or magnetospheric sails, or tethers, may make travel and cargo transactions to the moon and inner planets routine as well. These technologies are at the core of human aspirations to travel far beyond their home planet.

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