Hybrid rocket
From Free net encyclopedia
A hybrid rocket propulsion system is a rocket engine composed of a solid propellant lining a combustion chamber into which a liquid or gaseous propellant is injected so as to undergo a strong exothermic reaction to produce hot gas that is emitted through a De Laval nozzle for propulsive purposes.
These systems are superior to solid propulsion systems in the respects of safety, throttling, restartability, and environmental cleanliness. However, hybrid systems are slightly more complex than solids, and are more expensive and may be heavier.
In its simplest form a hybrid rocket consists of a pressurised tank full of oxidiser leading into a valve and from there into a combustion chamber lined with a fuel and on into a conventional rocket engine. To light the engine the valve is opened permiting oxidiser to reach the fuel, and an ignition source is supplied to start the combustion. The combustion burns along a hole through the long axis of the chamber called a 'port' and then out through the rocket nozzle producing thrust.
Common oxidizers include gaseous or liquid oxygen and nitrous oxide.
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Disadvantages of hybrids
The mixture control in hybrid rockets is less accurate than either solid rockets engines or bipropellant rocket engines. As the burn progresses, the port through the combustion chamber widens. This increases the surface area that is subject to combustion, and increases the burn rate, but it also increases the volume of fluid that can move from the injector through the engine. This changes the mixture ratio adversely, and most hybrids become oxidiser rich towards the end of the burn; impairing the average specific impulse.
Hybrid safety
Generally, well designed and carefully constructed hybrids are very safe. However, explosions have occurred; usually due to:
- pressure vessel failures- if the combustion chamber burns through, the relatively large combustion chamber will catastrophically fail.
- blow back- nitrous oxide tanks particularly have been known to blow-back and cause a monopropellent detonation (combustion instabilities are one of the main causes of this phenomena, particularly towards the end of a burn when the tank pressure is low, ensuring sufficient pressure drop across the injectors is necessary at all times.)
- hard starts caused by an excess of oxidiser in the combustion chamber prior to ignition, particularly monopropellants like nitrous oxide.
The fuel itself is generally inert and cannot normally explode; and unlike solid rockets, cracks are safe unless material falls and blocks the nozzle, dependent on the physical properties of the material (rubberised fuel will pass through without exploding the chamber).
Propellant combinations in rocketry are sometimes given an 'explosive equivalence'. This, multiplied by the weight of propellant gives an equivalent weight of TNT that would give the same effect as a crash of the vehicle. The explosive equivalent of hybrid rockets propellant is often taken to be 0, whereas liquids are 10-20% and solids are around 10%.
Organizations working on hybrids
The Reaction Research Society (RRS), although known primarily for their work with liquid rocket propulsion, has a long history of research and development with hybrid rocket propulsion.
Several universities have recently experimented with hybrid rockets. BYU, the University of Utah and Utah State University launched a student-designed rocket called Unity IV in 1995 which burned the solid fuel hydroxyl-terminated polybutadiene (HTPB) with an oxidizer of gaseous oxygen, and in 2003 launched a larger version which burned HTPB with nitrous oxide. Portland State University also launched several hybrid rockets in the early 2000's. Many other universities, such as the University of Arkansas at Little Rock and the University of Illinois, have hybrid motor test stands that allow for student research with hybrid rockets.
SpaceShipOne, the first private manned spacecraft, is powered by a hybrid rocket burning HTPB with nitrous oxide. The hybrid rocket engine was manufactured by SpaceDev. SpaceDev partially based its motors on experimental data collected from the testing of AMROC's (American Rocket Company) motors at NASA's Stennis Space Center's E1 test stand. Motors ranging from as small as 1000 lbf (4.5 kN) to as large as 250,000 lbf (1.1 MN) thrust were successfully tested. SpaceDev purchased AMROCs assets after the company was shut down due to lack of funding.
There are a number of hybrid rocket motor systems available for amateur/hobbyist use in high-powered model rocketry. These include the popular HyperTek systems and a number of 'Urbanski-Colburn Valved' (U/C) systems such as RATTWorks, Skyripper Systems,West Coast HybridsandPropulsion Polymers. Recently Contrail Rockets have introduced a hybrid motor system offering thrust equal or greater to equivalently sized solid rocket motors.
All of these systems use nitrous oxide as the oxidiser, and with the exception of Contrail, which uses HTPB, a plastic fuel (such as PVC or PolyPropylene). This reduces the cost per flight compared to solid rocket motors, although there is generally more 'GSE' (ground support equipment) required with hybrids.
These motors vary from 'G-class' (up to 160 Ns) to 'M-class' (up to 10240 Ns) with thrust ranging from 60N to over 5000N, and can propel a suitable rocket to altitudes of up to 30,000ft.