A hybrid rocket is a rocket with a rocket engine which uses propellants in two different states of matter - one solid and the other either gas or liquid. Hybrid rockets are not a new concept but were conceived at least 75 years ago.
Hybrid rockets exhibit advantages over both liquid rockets and solid rockets especially in terms of simplicity, safety, and cost. Because it is nearly impossible for the fuel and oxidizer to be mixed intimately (being different states of matter), hybrid rockets tend to fail more benignly than liquids or solids. Like liquid rockets and unlike solid rockets they can be shutdown easily and are simply throttle-able. The theoretical specific impulse(Isp) performance of hybrids is generally higher than solids and roughly equivalent to hydrocarbon-based liquids. Isp's as high as 400s have been measured in hybrid rockets using metalized fuels. Hybrid systems are slightly more complex than solids, but the significant hazards of manufacturing, shipping and handling solids more than offsets the system simplicity advantages.
In its simplest form a hybrid rocket consists of a pressure vessel (tank) containing the liquid propellant, the combustion chamber containing the solid propellant, and a valve isolating the two. When thrust is desired, a suitable ignition source is introduced in the combustion chamber and the valve is opened. The liquid propellant (or gas) flows into the combustion chamber where it is vaporized and then reacted with the solid propellant. Combustion occurs in a boundary layer diffusion flame adjacent to the surface of the solid propellant.
Generally the liquid propellant is the oxidizer and the solid propellant is the fuel because solid oxidizers are problematic and lower performing than liquid oxidizers. Furthermore, using a solid fuel such as HTPB or paraffin allows for the incorporation of high-energy fuel additives such as aluminum, lithium, or metal hydrides.
Advantages of hybrid rockets
Advantages compared with bipropellant liquid rockets
Advantages compared with solid rockets
Disadvantages of hybrid rockets
Hybrid rockets also exhibit some disadvantages when compared with liquid and solid rockets. These include:
For a well-designed hybrid, O/F shift has a very small impact on performance because Isp is insensitive to O/F near the peak. In general, much less development work has been performed with hybrids than liquids or solids and it is likely that some of these disadvantages could be rectified through further investment in R&D.
Generally, well designed and carefully constructed hybrids are very safe. The primary hazards associated with hybrids are:
Because the fuel in a hybrid does not contain an oxidizer, it will not combust explosively on its own. For this reason, hybrids are classified as having no TNT equivalent explosive power. In contrast, solid rockets often have TNT equivalencies similar in magnitude to the mass of the propellant grain. Liquids typically have TNT equivalencies calculated based on the amount of fuel and oxidizer which could realistically "pool" in the combustion chamber before igniting explosively. Because the oxidizer/fuel combinations used in liquids often contain more energy than TNT per mass, the TNT equivalent is calculated by multiplying the mass accumulated in the chamber times the ratio of energy densities.
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 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 and the synthetic rubber HTPB. SpaceShipOne, the first private manned spacecraft, is powered by a hybrid rocket burning HTPB with nitrous oxide.
Space Propulsion Group was founded in 1999 by Dr. Arif Karabeyoglu, Prof. Brian Cantwell and others from Stanford University to develop high regression-rate liquefying hybrid rocket fuels. They have successfully fired motors as large as 12.5 in. diameter which produce 13,000 lbf. using the technology and are currently developing a 24 in. diameter, 25,000 lbf. motor to be initially fired in 2010.
Orbital Technologies Corporation (Orbitec) has been involved in some US government funded research on hybrid rockets including the "Vortex Hybrid" concept.
Environmental Aerospace Corporation (eAc) was incorporated in 1994 to develop hybrid rocket propulsion systems. It was included in the design competition for the SpaceShipOne motor but lost the contract to SpaceDev.
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 is where liquid-layer combustion theory for hybrid rockets was developed. A group at Stanford is currently developing the Peregrine Sounding rocket which will be capable of 100km altitude.
Many other universities, such as the University of Michigan at Ann Arbor, the University of Arkansas at Little Rock, Hendrix College, 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.
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.
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.
Published - July 2009
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