Hybrid rocket
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A hybrid rocket propulsion system comprises propellants of two different states of matter, the most common configuration being 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. Hybrid systems are slightly more complex than solids, but the reduction of safety, storage and transport issues may be an offsetting factor.
In its simplest form a hybrid rocket consists of a pressurised tank full of oxidizer leading into a valve, and from there into a combustion chamber lined with a fuel, and on to a conventional rocket nozzle. To light the engine the valve is opened permitting oxidizer 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 or nitrous oxide. Common fuels include ABS plastic or synthetic rubber, although many combustible substances can be made to work.
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[edit] Hybrid safety
Generally, well designed and carefully constructed hybrids are very safe. However, ruptures have occurred due to:
- pressure vessel failures- if the combustion chamber burns through, the relatively large combustion chamber can catastrophically fail.
- blow back- nitrous oxide tanks particularly have been known to blow-back and cause monopropellent deflagration (combustion instabilities are one of the main causes of this phenomenon, 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 oxidizer 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 100%.
[edit] Disadvantages of hybrids
The mixture control in hybrid rockets may be less accurate than either solid rockets engines or liquid fuel rocket engines. As the burn progresses, the port through the combustion chamber widens. This increases the surface area that is subject to combustion, and may increase the rate of fuel evolution. However the oxidizer can tend to blow through the ever widening port without fully reacting with the fuel. This changes the mixture ratio adversely, and most hybrids become oxidizer rich towards the end of the burn; impairing the average specific impulse. Proper design of the oxidizer feed system fuel composition and physical layout of the fuel can try to minimise these issues.
[edit] Organizations working on hybrids
In 1998 SpaceDev acquired all of the intellectual property, designs, and test results generated by over 200 hybrid rocket motor firings by the American Rocket Company (Amroc) over its eight year life. SpaceDev developed and produced all of the hybrid rocket motors for SpaceShipOne. SpaceDev is currently developing SpaceDev Streaker, an expendable small launch vehicle, and SpaceDev Dream Chaser, capable of both suborbital and orbital human space flight. Both Streaker and Dream Chaser use hybrid rocket motors that burn nitrous oxide (N2O) and synthetic rubber (HTPB). SpaceDev's SpaceShipOne rocket motor was the largest of its kind ever produced. 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.
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. Stanford University researches LOX/paraffin hybrid motors. Many other universities, such as the University of Michigan at Ann Arbor, the University of Arkansas at Little Rock, the University of Illinois, and Portland State University have hybrid motor test stands that allow for student research with hybrid rockets. Boston University's student-run "Rocket Team", although in the past has launched only solid motor rockets, has completed several static tests of motors using paraffin and HTPB solid fuels and nitrous oxide as the oxidizer.
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 Hybrids, Contrail Rockets, and Propulsion Polymers. All of these systems use nitrous oxide as the oxidizer and a plastic fuel (such as PVC or PolyPropylene) or a polymer based fuel such as HTPB. 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 'O-class' (up to 40960 Ns) with thrust ranging from 60N to over 6300N, and can propel a suitable rocket to altitudes of up to 30,000ft.
In the February 18, 2007 episode of Top Gear, a Reliant Robin was used by Richard Hammond and James May in an attempt to modify a normal K-reg Robin into a reusable space shuttle. Steve Holland, a professional radio-controlled aircraft pilot, helped Hammond to work out how to land a Robin safely. The craft achieved a successful launch, flew for several seconds into the air and managed to successfully jettison the solid fuel rocket boosters on time. This was the largest rocket launched by a non-government organisation in Europe. It used 6 x 40960 NS O Contrail Rockets motors giving a maximum thrust of 8 metric tons. However the car failed to separate from the large external fuel tank due to a faulty alignment between the Robin and the external tank (10 thousandths of an inch out) and the Robin subsequently crashed into the ground and seemed to have exploded soon after. In fact this explosion was cut in for dramatic effect, as noted above, in any case hybrids do not explode in the way depicted.
At the July 17, 2006 meeting of the Canadian Space Society [1], a hybrid rocket motor using gaseous oxygen and acrylic was demonstrated. The motor was built by Robert Gissing, Daniel Faber, and Luke Stras as part of a CSS space display for the Toronto Aerospace Museum. The usage of an optical grade acrylic rod as both the fuel and the combustion chamber allows for viewing of the combustion process during firing. A video of a static test firing is available at YouTube
[edit] Hybrid Rockets in Popular Culture
An October 26, 2005 episode of the Television show Mythbusters entitled Confederate Rocket featured a hybrid rocket motor using liquid nitrous oxide and paraffin. The myth purported that during the American Civil War, the Confederate Army was able to construct a rocket of this type. The myth was revisited in a later episode entitled Salami Rocket, using hollowed out dry Salami as the solid fuel.
