Rocket engine

Image:Rocket-test-chamber-NASA-med.jpg A rocket engine is a heat engine used especially for spacecraft propulsion. Most rocket engines are internal combustion engines (although non combusting forms exist). Rocket engines take their propellant from on-board tanks and form it into a hypersonic jet, thus obtaining thrust from Newton's third law.

Contents 1 Principle of operation 2 Thermal issues 3 Non chemical rockets 4 Types of rocket engines 4.1 Chemical heating 4.2 Electric heating 4.3 Solar heating 4.4 Nuclear heating

Principle of operation

Rocket engines generally produce a high temperature reaction mass, as a hot gas. This is achieved by combusting a liquid or gaseous fuel with an oxidiser within a combustion chamber. The extremely hot gas is then allowed to escape through a high-expansion ratio nozzle. This bell-shaped nozzle is what gives a rocket engine its characteristic shape.

The effect of the nozzle is to dramatically accelerate the mass, converting most of the thermal energy into kinetic energy. Exhaust speeds as high as 10 times the speed of sound at sea level are not uncommon.

For optimum performance hot gas is used because it maximises the speed of sound at the throat-for aerodynamic reasons the flow goes sonic ("chokes") at the throat, so the highest speed there is desirable. By comparison, at room temperature the speed of sound in air is about 340m/s, the speed of sound in the hot gas of a rocket engine can be over 1700m/s.

The expansion part of the rocket nozzle then multiplies the speed of the flow by a further factor, typically between 1.5 and 4 times, giving a highly collimated exhaust jet. The speed ratio of a rocket nozzle is mostly determined by its area expansion ratio—the ratio of the area of the throat to the area at the exit. Larger ratio nozzles are more massive and bulkier, but they are able to extract more heat from the combustion gases, which become colder and lower in pressure, but also faster.

A significant complication arises when launching a vehicle from the Earth's surface as the ambient atmospheric pressure changes with altitude. For maximum performance it turns out that the pressure of the gas leaving a rocket nozzle should be the same as ambient pressure; if lower the vehicle will be slowed by the difference in pressure between the top of the engine and the exit, if higher then this represents pressure that the bell has not turned into thrust. To achieve this ideal, the diameter of the nozzle would need to increase with altitude, which is difficult to arrange. A compromise nozzle is generally used and some percentage reduction in performance occurs. To improve on this, various exotic nozzle designs such as the plug nozzle, stepped nozzles, the expanding nozzle and the aerospike have been proposed, each having some way to adapt to changing ambient air pressure and each allowing the gas to expand further against the nozzle giving extra thrust at higher altitude.

Thermal issues

The reaction mass's combustion temperature is often far higher than the melting point of the nozzle and combustion chamber materials. Materials technology can place an upper limit on the exhaust temperature of chemical rockets. Rockets can use ablative materials that erode in a controlled fashion, or very high temperature materials, such as graphite, ceramics or certain exotic metals. Alternatively, rockets may employ cooling systems to prevent the nozzle material itself becoming too hot. Regenerative cooling, where the propellant is passed through tubes around the combustion chamber or nozzle, and other techniques such as curtain cooling or film cooling, may also be employed to give essentially unlimited nozzle life.

Non chemical rockets

Rockets emitting plasma can potentially carry out reactions inside a magnetic bottle and release the plasma via a magnetic nozzle, so that no solid matter need come in contact with the plasma. Of course, the machinery to do this is complex, but research into nuclear fusion has developed methods, some of which have been used in speculative propulsion systems.

Image:H1Rocket.JPG Image:Twin Linear Aerospike XRS-2200 Engine.jpg

Types of rocket engines

Rocket engines that could be used in space (all emit gases unless otherwise noted):

Chemical heating

• Solid rocket
• Hybrid rocket
• Monopropellant rocket
• Bipropellant rocket
• Tripropellant rocket
• Dual mode propulsion rocket

Electric heating

• Resistojet rocket (electric heating)
• Arcjet rocket (chemical burning aided by electrical discharge)
• Pulsed plasma thruster (electric arc heating; emits plasma)

Solar heating

• Solar thermal rocket

Nuclear heating

• Nuclear thermal rocket (nuclear fission energy)
• Radioisotope rocket/Poodle thruster (radioactive decay energy)
• Antimatter catalyzed nuclear pulse propulsion (fission and/or fusion energy)
• Gas core reactor rocket (nuclear fission energy)
• Fission-fragment rocket (nuclear fission energy)
• Fission sail (nuclear fission energy)
• Nuclear salt-water rocket (nuclear fission energy)
• Nuclear pulse propulsion (exploding fission/fusion bombs)
• Fusion rocket (nuclear fusion energy)
• Antimatter rocket (annihilation energy)