Reaction engine
Encyclopedia
A reaction engine is an engine or motor
which provides propulsion (thrust
) by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force".
Examples include both jet engines and rocket engine
s, and more uncommon variations such as Hall effect thruster
s, ion drives, mass drivers and nuclear pulse propulsion
.
s and electric propulsion
drives) some energy must go into accelerating the reaction mass.
Every engine will waste some energy, but even assuming 100% efficiency, the engine will need energy amounting to
(where M is the mass of propellent expended and
is the exhaust velocity)
which is simply the energy to accelerate the exhaust.
Comparing the rocket equation (which shows how much energy ends up in the final vehicle) and the above equation (which shows the total energy required) shows that even with 100% engine efficiency, certainly not all energy supplied ends up in the vehicle - some of it, indeed usually most of it, ends up as kinetic energy of the exhaust.
Interestingly, if the
is fixed, for a mission delta-v, there is a particular 
that minimises the overall energy used by the rocket. This comes to an exhaust velocity of about ⅔ of the mission delta-v (see the energy computed from the rocket equation). Drives with a specific impulse that is both high and fixed such as Ion thrusters have exhaust velocities that can be enormously higher than this ideal, and thus end up powersource limited and give very low thrust. Where the vehicle performance is power limited, e.g. if solar power or nuclear power is used, then in the case of a large 
the maximum acceleration is inversely proportional to it. Hence the time to reach a required delta-v is proportional to 
. Thus the latter should not be too large.
On the other hand if the exhaust velocity can be made to vary so that at each instant it is equal and opposite to the vehicle velocity then the absolute minimum energy usage is achieved. When this is achieved, the exhaust stops in space and has no kinetic energy; and the propulsive efficiency is 100% all the energy ends up in the vehicle (in principle such a drive would be 100% efficient, in practice there would be thermal losses from within the drive system and residual heat in the exhaust). However in most cases this uses an impractical quantity of propellant, but is a useful theoretical consideration.
Some drives (such as VASIMR
or Electrodeless plasma thruster
) actually can significantly vary their exhaust velocity. This can help reduce propellant usage and improve acceleration at different stages of the flight. However the best energetic performance and acceleration is still obtained when the exhaust velocity is close to the vehicle speed. Proposed ion and plasma drives usually have exhaust velocities enormously higher than that ideal (in the case of VASIMR the lowest quoted speed is around 15000 m/s compared to a mission delta-v from high Earth orbit to Mars of about 4000m/s).
For a mission, for example, when launching from or landing on a planet, the effects of gravitational attraction and any atmospheric drag must be overcome by using fuel. It is typical to combine the effects of these and other effects into an effective mission delta-v
. For example a launch mission to low Earth orbit requires about 9.3–10 km/s delta-v. These mission delta-vs are typically numerically integrated on a computer.
Different reaction engines have different efficiencies and losses. For example rocket engines can be up to 60-70% energy efficient in terms of accelerating the propellant- the rest is lost as heat primarily in the exhaust, but also a small amount lost as thermal radiation.
This is because the useful mechanical energy generated is simply force times distance, and when a thrust force is generated while the vehicle moves, then:
where F is the force and d is the distance moved.
Dividing by length of time of motion we get:
Hence:
where P is the useful power and v is the speed.
Hence you want v to be as high as possible; and a stationary engine does no useful work.
Motor
Motor is a device that creates motion. It usually refers to an engine of some kind. It may also specifically refer to:*Electric motor, a machine that converts electricity into a mechanical motion...
which provides propulsion (thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
) by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force".
Examples include both jet engines and rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s, and more uncommon variations such as Hall effect thruster
Hall effect thruster
In spacecraft propulsion, a Hall thruster is a type of ion thruster in which the propellant is accelerated by an electric field. Hall thrusters trap electrons in a magnetic field and then use the electrons to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the...
s, ion drives, mass drivers and nuclear pulse propulsion
Nuclear pulse propulsion
Nuclear pulse propulsion is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project Orion by DARPA, after a suggestion by Stanislaw Ulam in 1947...
.
Propulsive efficiency
For all reaction engines which carry their propellant onboard prior to use (such as rocket engineRocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s and electric propulsion
Electric propulsion
An electrically powered spacecraft propulsion system is any of a number of forms of electric motors which spacecraft can employ to gain mechanical energy in outer space...
drives) some energy must go into accelerating the reaction mass.
Every engine will waste some energy, but even assuming 100% efficiency, the engine will need energy amounting to
(where M is the mass of propellent expended and
is the exhaust velocity)which is simply the energy to accelerate the exhaust.
Interestingly, if the
is fixed, for a mission delta-v, there is a particular that minimises the overall energy used by the rocket. This comes to an exhaust velocity of about ⅔ of the mission delta-v (see the energy computed from the rocket equation). Drives with a specific impulse that is both high and fixed such as Ion thrusters have exhaust velocities that can be enormously higher than this ideal, and thus end up powersource limited and give very low thrust. Where the vehicle performance is power limited, e.g. if solar power or nuclear power is used, then in the case of a large the maximum acceleration is inversely proportional to it. Hence the time to reach a required delta-v is proportional to . Thus the latter should not be too large.On the other hand if the exhaust velocity can be made to vary so that at each instant it is equal and opposite to the vehicle velocity then the absolute minimum energy usage is achieved. When this is achieved, the exhaust stops in space and has no kinetic energy; and the propulsive efficiency is 100% all the energy ends up in the vehicle (in principle such a drive would be 100% efficient, in practice there would be thermal losses from within the drive system and residual heat in the exhaust). However in most cases this uses an impractical quantity of propellant, but is a useful theoretical consideration.
Some drives (such as VASIMR
Variable specific impulse magnetoplasma rocket
The Variable Specific Impulse Magnetoplasma Rocket is an electro-magnetic thruster for spacecraft propulsion. It uses radio waves to ionize and heat a propellant and magnetic fields to accelerate the resulting plasma to generate thrust...
or Electrodeless plasma thruster
Electrodeless plasma thruster
The electrodeless plasma thruster is a spacecraft propulsion engine. It was created by Mr. Gregory Emsellem based on technology developed by French Atomic Energy Commission scientist Dr Richard Geller and Dr...
) actually can significantly vary their exhaust velocity. This can help reduce propellant usage and improve acceleration at different stages of the flight. However the best energetic performance and acceleration is still obtained when the exhaust velocity is close to the vehicle speed. Proposed ion and plasma drives usually have exhaust velocities enormously higher than that ideal (in the case of VASIMR the lowest quoted speed is around 15000 m/s compared to a mission delta-v from high Earth orbit to Mars of about 4000m/s).
For a mission, for example, when launching from or landing on a planet, the effects of gravitational attraction and any atmospheric drag must be overcome by using fuel. It is typical to combine the effects of these and other effects into an effective mission delta-v
Delta-v
In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....
. For example a launch mission to low Earth orbit requires about 9.3–10 km/s delta-v. These mission delta-vs are typically numerically integrated on a computer.
Cycle efficiency
All reaction engines lose some energy- mostly as heat.Different reaction engines have different efficiencies and losses. For example rocket engines can be up to 60-70% energy efficient in terms of accelerating the propellant- the rest is lost as heat primarily in the exhaust, but also a small amount lost as thermal radiation.
Oberth effect
Reaction engines are more energy efficient when they emit their reaction mass when the vehicle is travelling at high speed.This is because the useful mechanical energy generated is simply force times distance, and when a thrust force is generated while the vehicle moves, then:
where F is the force and d is the distance moved.
Dividing by length of time of motion we get:
Hence:
where P is the useful power and v is the speed.
Hence you want v to be as high as possible; and a stationary engine does no useful work.
Types of reaction engines
- Rocket-like
- Rocket engineRocket engineA rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
- Electric propulsionElectric propulsionAn electrically powered spacecraft propulsion system is any of a number of forms of electric motors which spacecraft can employ to gain mechanical energy in outer space...
, including VASIMR
- Rocket engine
- Airbreathing
- turbojet
- turbofanTurbofanThe turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
- Pulsejet
- RamjetRamjetA ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
- ScramjetScramjetA scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...
- Liquid
- Pump-jetPump-jetA pump-jet, hydrojet, or water jet, is a marine system that creates a jet of water for propulsion. The mechanical arrangement may be a ducted propeller with nozzle, or a centrifugal pump and nozzle...
- Pump-jet
- Rotary
- aeolipileAeolipileAn aeolipile , also known as a Hero engine, is a rocket style jet engine which spins when heated. In the 1st century AD, Hero of Alexandria described the device, and many sources give him the credit for its invention.The aeolipile Hero described is considered to be the first recorded steam engine...
- aeolipile
- solid exhaust
- mass driverMass driverA mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to make electromagnets. Sequential...
- mass driver