Rocket engine

Rocket engines (also rocket engines ) are drives that produce opposite to the drive direction of the driving force ( thrust) by expelling mass support. Because they do not thereby accrete matter from the outside and accelerated discharge again, they work regardless of the environment, so even in a vacuum.

The work of the rocket engine is the reaction principle (see also reaction propulsion ) in the third Newtonian axiom based. The higher the velocity of the expelled mass of the support, the more effective is the engine speed and the greater the possible change in "Delta V" of the missile. Rocket engines are, inter alia as a drive of carrier rockets, space vehicles or aircraft used. Are widely used rocket engines for the military, where they are used as the drive of ballistic missiles or reactive projectiles (such as from rocket launchers ) or for driving reactive torpedoes.

There are various types of rocket engines and numerous efforts to reduce the required resources of rocket engines (see aerospike ).

Theoretical effects that can be noticed in a rocket engine were presented in 1903 by Konstantin Tsiolkovsky with the Rocket Equation. Later Hermann Oberth came regardless to the same findings.

• 2.3.1 Thermo-electric drive
• 2.3.2 Electromagnetic drive
• 2.3.3 The electrostatic drive

Technology

Most (but not all) of rocket engines are internal combustion engines: They heat by combustion of a fuel with oxidizing agents a supporting mass (usually the / combustion product (s) ) in a combustion chamber at very high temperature and let the energy product of the process in gaseous form through an opening to escape. The liberated in the ( exothermic ) combustion thermal energy and the resulting pressure in the combustion chamber is converted into kinetic energy as it exits (acceleration) and thus generate thrust by the reaction principle. The specially shaped outlet opening of the combustion chamber is called the nozzle, it serves to increase the exit velocity ( resulting in higher shear strength ) as well as to increase the internal pressure in the combustion chamber (in favor of the combustion process ). A nozzle type frequently used is the Laval nozzle. The nozzle must be cooled, which is achieved either by coating or by means of internal cooling lines through which the fuel flows. Ideally, they relax the beam up to the ambient pressure; under reduced pressure or, for practical reasons (length and weight) is not possible, the design of the nozzle is therefore a compromise and design of the drive.

An essential characteristic of rocket engines is the specific impulse, which describes the efficiency of the drive as the ratio between pulse and spent fuel mass. He has - in SI units - the unit m / s and is, for example, in a solid engine at 2450 m / s, a liquid engine such as the Space Shuttle in 4444 m / s

As more components are frequently added for containers carried supplies, fuel pumps and cooling systems.

A rocket lost during the operating period of its rocket engine at ground ( it is noted that at a constant shear, therefore, the acceleration increases). In a chemical rocket engine, the fuel consumption is very high, so this effect is much more significant than that of a nuclear rocket engine, which heats the gas ejected by a nuclear reaction. Still less fuel electric rocket engines, which includes, for example, the ion drive.

The rocket engine is so far the only propulsion, which makes it possible to operate space. To speed up within our solar system is often also the swing-by way of fuel savings. Discussed ( proposed and under development located ) alternatives to rocket propulsion in space, drives without reaction mass such as solar sails, firing mechanisms with a railgun and more; There are many speculations on drives with antimatter or wormholes.

See also: drive methods for space travel.

Rocket engines are used in military aviation to start. In some cases they are also used in automobiles in order to achieve such velocity records. Also, there are applications in the hobby, model and toys: Here compressed air rockets and missiles are often used water.

Types of rocket engines

There are several groups and many types of rocket engines:

• Chemical rocket engines Solid rocket
• Liquid rocket ( monopropellant, bipropellant )
• Hybrid rocket

• Ion thrusters, thermal arc thrusters, Resistojet, etc.

Today's most widely used rocket engines are models with chemical reactions to produce the required energy. There are a number of models that have been proposed so far only theoretically or are under development.

A chemical rocket engine

A chemical rocket engine works ( unlike some other engines ) is completely independent of its environment. It is an internal combustion engine as the air-breathing jet engine, but in contrast to that not rely on oxygen in the air as the oxidizing agent: It will be carried along all the necessary equipment, such as the oxygen required for combustion of the fuel. The missile can therefore work in a vacuum.

The following three forms of chemical engines are common and differ in the storage condition of the equipment:

Solid engine

See main article solid rocket engine.

The fuel tank is also the combustion chamber. A distinction between the front burner in which the cylindrical fuel block burns from the end (constant circular internal surface) and the central burner in which a fuel passage of a cylindrical, star-shaped, or else a prismatic cross-section extends through the entire length of the propellant block and said burns from the inside ( burning surface in the form of a prism sheath, depending on the channel cross-section results in a trajectory of burning surface area ). Front burner for a long time to develop a low thrust, central burner for much shorter time, a very high thrust; so-called boosters are therefore usually designed as a central burner.

Due to the consistency of the fuel, various properties can be derived. It requires no tanks, supply lines or control valves, because the reaction mass is already in the combustion chamber. Due to the solid consistency of the fuel that is easy to store in the rocket and transported under less risk. Therefore, military rockets are almost always designed as a solid rocket. Another advantage of solid rocket is the high achievable thrust. The disadvantages, however, include the poor regulation of the thrust force and the duration of work. Combustion can not be canceled or restarted after ignition.

However, the most important advantage of solid rocket is the high thrust, the solid rocket boosters of the Space Shuttle are with a thrust of up to 14.5 mega -Newton until today the strongest rocket engines at all. The largest U.S. liquid engine of the Saturn V F-1 reached a maximum of about 6.9 mega- Newton.

Liquid engine

See main article: liquid rocket engine.

The structure of liquid rocket engines allows a thrust regulation, long working hours and a relatively favorable reuse. In liquid rocket engines and fuel (if it is not a Monergoltriebwerk ) oxidant outside of the engine mounted. They can be designed wiederzündbar with little extra effort, so that the engine during flight can have several firing stages.

Often these are in the supplies to highly aggressive chemicals or gases kaltverflüssigte. Both must be kept in special corrosion-resistant and insulated tanks, so as to avoid evaporation of the gas or an engagement of the container wall.

Because the fuel must be stored and promoted a liquid propellant rocket in their construction is usually more complicated than a solid rocket. Due to the usually high-energy fuels, temperatures of up to 4000 Kelvin in the combustion chamber, which requires the use of highly heat-resistant materials and an efficient cooling. Cooling can be used on the oxidant and fuel. Due to the high pressure under which the gases are in the liquid form, to the various components on the heat exchanger in order to cool due to the low temperature.

Fuel promotion

In a liquid rocket engine or the fuel against the prevailing pressure must be delivered into the combustion chamber.

• In principle, the pressure gas production, the tanks are pressurized (usually helium or other inert gas. ) This limits the combustion chamber pressure and is therefore only suitable for low-power systems, but increases reliability because fewer parts are needed.
• The pump output allows high pressures and benefits, without the entire tank structure must be designed for the combustion chamber pressure. The disadvantage is the increased complexity of these systems. The pumps running as turbines can be operated with auxiliary fuels or directly with the main fuels, where you meet the following further distinction:

Primary or secondary current

In liquid rocket engines with pump delivery can be made between primary and secondary power engines:

• In the main current engines, the entire fuel to be fed through the (main) combustor. The turbines for fuel production are here either through a heated in the cooling system of the engine fuel component ( expander cycle ) or driven by a signal generated in a pre-combustion chamber working gas ( Staged Combustion Cycle).
• In addition to current engines, the parts of the fuels that are used to operate the turbines Treibstofförderung, not guided by the main combustion chamber. A design of the side-stream engine, the gas generator cycle dar. This is combusted to drive the fuel pump, a part of the fuel in a gas generator, and expanding the operating gas in the main power unit parallel nozzle or fed to the main flow in the divergent part of the main nozzle. A different expression, the topping cycle dar. Here, the fuel stream is divided into two strands. The smaller current flows through the cooling of the engine, drives the turbine, the fuel pump and is supplied to the main flow in the divergent region of the main nozzle.

Hybrid rocket engine

See hybrid rocket.

In hybrid rocket engines, both solid and liquid fuel components are used. The solid propellant, the liquid fuel is supplied regulated, which allows an improved control over the working speed and duration than in pure solid-fuel engines.

Such an engine is approximately used in SpaceShipOne, a privately developed rocket, the first private enterprise a human into space carried in 2003 ( more than 100 km altitude). The mixture of solid combustion chamber and a simple liquid - oxidant ( nitrous oxide ) proved to be particularly economical.

Control systems

There are a number of ways to influence the thrust vector of a rocket engine:

• Beam deflection (e.g., by thrusters or fold ) was used in the early designs, such as the A4
• Swing of the push beam ( by turning the engine with combustion chamber ) is the method most commonly used
• Asymmetric combustion (eg injection of secondary fuel in the jet thrust ).

Fuels

See rocket propellants.

When the engines referred to here a wide range of fuels has prevailed until today. In the chemical fuel systems, a distinction is generally either by the type of fuel in solid, liquid or hybrid fuels or on the number of participating in the combustion process reactants in monergol, diergol or triergol.

Solar thermal rocket engine

A solar thermal power, solar Orbit Transfer Vehicle, SOTV, to go from LEO according to GEO, is in development. Two inflatable parabolic focusing solar radiation on a block of graphite is passed through the hydrogen which is thereby heated to approximately 2400 Kelvin.

Electric rocket engine

See: electric rocket engine.

Electric propulsion systems for space use electrical energy to generate thrust. Since they produce only low thrust, they can not be used for launchers, but come so far only on satellites and probes used.

Pros and cons for electric drives are:

• Very high specific impulse possible ( exit velocity of the fuel ).
• Comparable low thrust level feasible, a start from the surface is therefore not possible purely electrically.
• Low shear leads to precise attitude control maneuvers, eg for observation satellites.
• The performance is limited by the electric power that can be provided by the spacecraft.
• Short-term changes in direction are difficult because of the long burn time with low thrust.

On the basis of the various constructions and methods to generate thrust, the electric drives are further distinguished. The classification is done on the principle in which a) electro-thermal, b ) electrostatic and c ) electromagnetic drives. Depending on the type of electrical energy will continue to distinguish between solar-electric and nuclear electric systems.

Thermo-electric drive

See: thermal arcjet.

The ( gaseous ) fuel is heated by a resistance heater or an electric arc and then accelerated through a nozzle. The high temperatures in a comparable thrust can be generated, however, is limited by the low mass flow. As fuel used gases with low molecular mass, such as hydrogen or ammonia. The efficiency of the electrical energy to heat is relatively small.

Engines with resistance heating, are those referred to as Resistojets with arc heating as Arcjets.

The recoverable shear of an electro- thermal drive is low on some 100 mN. The outflow velocity is typically in the range of 10,000 to 30,000 m / s

Electromagnetic drive

See: Magneto Plasma Dynamic Drive.

The fuel and the support material is heated as the electro-thermal drive means of resistance heating or by an electric arc. However, an electromagnetic drive accelerates the generated plasma ( ≈ 10,000 K) in a magnetic field ( Lorentz force ), rather than with a nozzle.

The recoverable shear of an electromagnetic actuator has been poor and is in the mN range. For this electrical power in the kW range are needed.

The effectiveness of plasma thrusters depends on the fuel used. Most fuels are used with low molar mass, such as hydrogen, its storage, however, is expensive.

An electrostatic drive

See ion propulsion

In electrostatic thrusters thrust is generated by evaporation of the supporting mass, if it is not already in gaseous form, the ionization of the atoms and acceleration of the ions in an electric field. In order to avoid electrical charging of the engine, it is necessary to neutralize the supporting mass behind the acceleration section by adding the removed electrons in the ionization. The thrust force of each ion, and hence the efficiency increases with the mass of the accelerated ions, and therefore, ion thrusters, the ions of relatively heavy elements use. Is used because of its inertness and easy transportability almost always the heavy and expensive noble gas xenon today.

The efficiency of these drives is relatively high, as well as the outflow speed. However, the accessible thrust forces are very small and lie in the mN range.

Nuclear rocket engine

Under nuclear space propulsion systems all drive systems are summarized, which are operated by means of nuclear reactions. Nuclear energy can in principle be generated by nuclear fission or nuclear fusion. The so power densities are achievable by the factor ( fission ), and ( fusion ) is greater than the chemical actuators. At the fusion are still working, such as ITER.

To date, however, only the nuclear fission is technically realized and controlled, and only based on it drive systems have been developed and tested, about 1954-1972 in NERVA. Here are a specific impulse of 8339 m / s was achieved in tests and 9810 m / s seem possible, in contrast with the 4444 m / s current liquid fuel engines such as the Space Shuttle.

For operational use in the sense of a space mission no nuclear propulsion system has so far as they have been published can not be used for ecological or political reasons.

Planned and designed for military use of nuclear rocket engines were not beyond the prototype stage. Developed for the planned intercontinental cruise missile Pluto nuclear ramjet Tory - IIC was tested in 1964 for the second and last time. The corresponding project was completed on 1 July 1964.

Allen nuclear propulsion systems or concepts have in common that the energy generated in the nuclear process is transferred to a supporting mass and this is expanded in a nozzle. The only exception to this rule introduced the concept of the nuclear pulse propulsion dar. This should atomic bombs detonated outside the spacecraft and the momentum of the incident on the spacecraft plasma are used for acceleration, see Orion project.

As of 2003, Prometheus was at NASA back on the project thinking. The goal is a nuclear propulsion, which is to allow probe missions to the middle planet of the solar system, for example for the JIMO program. The electrical energy generated by nuclear power is to be implemented here via an ion drive.

In October 2009, the Russian space agency Roskosmos has announced a program begun in 1954, development of a gas core reactor - resume ( Nuclear Gas Core Reactor NGCR ). The engine uses highly enriched uranium in a plasma gas phase at a pressure of 1000 bar and temperatures up to 70,000 degrees Kelvin. Hydrogen as a fuel to be used, supplemented by alkali metals such as lithium, in order to promote the transfer of energy from the radiation. The project plan should be developed by 2012, the development may last nine years, with a cost of 17 billion rubles (580 million dollars) has been budgeted. The engine should produce the conditions for a manned Mars mission.

Cold gas rocket engine

In cold gas drive is a pressurized gas, usually nitrogen, relaxed from a pressure vessel through nozzles. Due to the low specific impulse of this drive is rarely used. Main area of ​​application is the attitude stabilization of small and inexpensive satellites.

Efficiency of rocket propulsion

We go to determine the efficiency of the assumption that an energy E drives the fuel mass M and the remaining vehicle mass m (structure, payload, etc.) apart. For the interest rate of the rocket after separation is then obtained

Due to conservation of momentum also apply the following relation to the speed vg of fuel:

Used and solved for the energy of the rocket obtained

With the introduction of the mass ratio R ( initial mass / final mass ) and relative to the total available energy is obtained

It is however assumed that all the energy is converted at a stroke into velocity. This is not achievable in practice, but a substantial portion of the energy for acceleration of the unburned fuel is lost. It integrates therefore analogous to the procedure in the Rocket Equation and again obtained from the relation to the total energy

The corresponding function has a clear maximum at a mass ratio of nearly 5, but even then only reached almost 40 %.