A hybrid rocket is a rocket with a rocket motor which uses propellants in two different states of matter - one solid and the other either gas or liquid. The hybrid rocket concept can be traced back at least 75 years.
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 rocket motors, but unlike solid rocket motors, hybrid rocket motors can be shut down easily and the thrust can be controlled with a simple throttle. The theoretical specific impulse() performance of hybrids is generally higher than solid motors, and roughly equivalent to hydrocarbon-based liquid motors. as high as 400s has been measured in a hybrid rocket using metalized fuels. Hybrid systems are more complex than solid ones, but the significant hazards of manufacturing, shipping and handling solid rocket motors offset 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 Hydroxyl-terminated polybutadiene (HTPB) or paraffin wax allows for the incorporation of high-energy fuel additives such as aluminium, lithium, or metal hydrides.
Advantages compared with liquid rockets
- Mechanically simpler - requires only a single liquid propellant resulting in less plumbing, fewer valves, and simpler operations.
- Denser fuels - fuels in the solid phase generally have higher density than those in the liquid phase, reducing overall system volume.
- Metal additives - reactive metals such as aluminium, magnesium, lithium or beryllium can be easily included in the fuel grain increasing specific impulse(), density specific impulse, or both.
- Combustion instabilities - Hybrid rockets do not typically exhibit high frequency combustion instabilities that plague liquid rockets.
- Propellant pressurization - One of the most difficult to design portions of a liquid rocket system are the turbopumps. Turbopumps design is complex as it has to precisely and efficiently pump and keep separated two fluids of different properties in precise ratios at very high volumetric flow rates, often cryogenic temperatures, and highly volatile chemicals while combusting those same fluids in order to power itself. Hybrids have far less fluid to move and can often be pressurized by either blow-down or pressurized feed systems which would be prohibitively heavy in a liquid rocket.
- Cooling - Liquid rockets often depend on one of the propellants, typically the fuel, to cool the combustion chamber, nozzle, and bell due to the very high heat fluxes and vulnerability of the metal walls to oxidation and stress cracking. Hybrid rockets have combustion chamber that are lined with the solid propellant which shields it from the product gases. Their nozzles and bells are often graphite or coated in ablative materials similarly to solid rockets. The design, construction, and testing of liquid cooling flows is complex and the system is prone to failure.
Advantages compared with solid rockets
- Higher theoretical - Is possible due to limits of known solid oxidizers compared to often used liquid oxidizers.
- Less explosion hazard - Propellant grain more tolerant of processing errors such as cracks. Propellant grain cannot be ignited by stray electrical charge.
- Fewer handling and storage issues - Ingredients in solid rockets are often incompatible chemically and thermally. Repeated changes in temperature can cause distortion of the grain. Antioxidants and coatings are used to keep the grain from breaking down or decomposing.
- More controllable - Stop/restart and throttling are all easily incorporated into most designs. Solid rockets rarely can be shut down easily and almost never have throttling or restart capabilities.
- Can be transported to site in a benign form and loaded with oxidizer remotely immediately before launch, improving safety.
Disadvantages of hybrid rockets
Hybrid rockets also exhibit some disadvantages when compared with liquid and solid rockets. These include:
- Oxidizer-to-fuel ratio shift ("O/F shift") - with a constant oxidizer flow-rate, the ratio of fuel production rate to oxidizer flow rate will change as a grain regresses. This leads to off-peak operation from a chemical performance point of view.
- Low regression-rate (rate at which the solid phase recedes) fuels often drive multi-port fuel grains. Multi-port fuel grains have poor volumetric efficiency and, often, structural deficiencies. High regression-rate liquefying fuels developed in the late 1990s offer a potential solution to this problem.
- Compared with Liquid based propulsion, re-fuelling a partially or totally depleted hybrid rocket would present significant challenges, as the solid propellant cannot simply be pumped into a fuel tank. This may or may not be an issue, depending upon how the rocket is planned to be used.
For a well-designed hybrid, O/F shift has a very small impact on performance because is insensitive to O/F shift 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 research and development.
Generally, well designed and carefully constructed hybrids are very safe. The primary hazards associated with hybrids are:
- Pressure vessel failures - Chamber insulation failure may allow hot combustion gases near the chamber walls leading to a "burn-through" in which the vessel ruptures.
- Blow back - For oxidizers that decompose exothermically such as nitrous oxide or hydrogen peroxide, flame or hot gasses from the combustion chamber can propagate back through the injector, igniting the oxidizer and leading to a tank explosion. Blow-back requires gases to flow back through the injector due to insufficient pressure drop which can occur during periods of unstable combustion. Blow back is inherent to specific oxidizers and is not possible with oxidizers such as oxygen or nitrogen tetroxide unless fuel is present in the oxidizer tank.
- Hard starts - An excess of oxidizer in the combustion chamber prior to ignition, particularly for monopropellants such as nitrous oxide, can result in a temporary over-pressure or "spike" at ignition.
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. Liquid-fuel rockets typically have TNT equivalencies calculated based on the amount of fuel and oxidizer which could realistically intimately combine before igniting explosively; this is often taken to be 10–20% of the total propellant mass. For hybrids, even filling the combustion chamber with oxidizer prior to ignition will not generally create an explosion with the solid fuel, the explosive equivalence is often quoted as 0%.
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. SpaceShipOne, the first private manned spacecraft, was powered by SpaceDev's hybrid rocket motor burning HTPB with nitrous oxide. However, nitrous oxide was the prime substance responsible for the explosion that killed three in the development of the successor of SpaceShipOne at Scaled Composites in 2007. The Virgin Galactic SpaceShipTwo follow-on commercial suborbital spaceplane uses a scaled-up hybrid motor.
SpaceDev was developing the 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. SpaceDev was acquired by Sierra Nevada Corporation in 2009, becoming its Space Systems division, which continues to develop Dream Chaser for NASA's Commercial Crew Development contract. Sierra Nevada also developed RocketMotorTwo, the hybrid engine for SpaceShipTwo. On October 31st 2014 SpaceShipTwo was lost, initial speculation suggests that its hybrid engine exploded in testing and killed one test pilot and seriously injured the other. 
U.S. Rockets manufactures and deploys hybrids using self-pressurizing nitrous oxide N2O and HTPB as well as HTP and HTPB. The High Test Hydrogen Peroxide H2O2 86% and Hydroxyl-terminated polybutadiene (HTPB) and aluminum hybrids developed by U.S. Rockets produce a sea level delivered specific impulse (Isp) of 240, well above the typical 180 of N2O-HTPB hybrids. In addition to that, they are self-starting, restartable, have considerably lower combustion instability making them suitable for fragile or manned missions such as Bloodhound SSC, SpaceShip Two or SpaceShip Three. The company has successfully tested and deployed both pressure fed and pump fed versions of the latter HTP-HTPB style. Deliverables to date have ranged from 6 inch to 18 inch diameter, and development units up to 54 inch diameter. The vendor claims scalability to over 5 meters diameter with regression rates approaching solids, according to literature distributed at the November 2013 Defense Advanced Research Projects Agency meeting for XS-1.
Organizations working on hybrids
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. Stanford University is the institution where liquid-layer combustion theory for hybrid rockets was developed. The SPaSE group at Stanford is currently working with NASA Ames Research Center developing the Peregrine Sounding rocket which will be capable of 100 km altitude. Engineering challenges include various types of combustion instabilities.
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.
Rocket Lab sells hybrid sounding rockets and related technology.
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.
Copenhagen Suborbitals, a Danish rocket group, has designed and test-fired several hybrids using N2O at first and currently LOX. Their fuel is epoxy, paraffin wax, or polyurethane. The group eventually moved away from hybrids because of thrust instabilities, and now uses a motor similar to that of the V-2 rocket.
Several universities have recently experimented with hybrid rockets. Brigham Young University (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.
The WARR student-team at the Technical University of Munich has been developing hybrid engines and rockets since the early 1970s. Using acids, oxygen or nitrous oxide in combination with polyethylene or HTPB. The development includes test stand engines as well as airborne versions, like the first German hybrid rocket Barbarella.
University of Brasilia's Hybrid Team has extensive research in paraffin wax/nitrous oxide hybrids having already made more than 50 tests fires. Hybrid Team is currently working liquefied propellant, numeric optimization and rocket design
Many other universities, such as Embry-Riddle Aeronautical University, Purdue University, the University of Michigan at Ann Arbor, the University of Arkansas at Little Rock, Hendrix College, the University of Illinois, Portland State University, and Texas A&M University have hybrid motor test stands that allow for student research with hybrid rockets. Boston University's student-run "Rocket Propulsion Group", which in the past has launched only solid motor rockets, is attempting to design and build a two-stage hybrid sounding rocket to launch into sub-orbital space by 2015.
Florida Institute of Technology has successfully tested and evaluated hybrid technologies with their Panther Project.
A United Kingdom-based team (laffin-gas) is using four N2O hybrid rockets in a drag-racing style car. Each rocket has an outer diameter of 150mm and is 1.4m long. They use a fuel grain of high-density wound paper soaked in cooking oil. The N2O supply is provided by Nitrogen-pressurised piston accumulators which provide a higher rate of delivery than N2O gas alone and also provide damping of any reverse shock.
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, HyperTek, West Coast Hybrids, Contrail Rockets, and Propulsion Polymers. All of these systems use nitrous oxide as the oxidizer and a plastic fuel (such as Polyvinyl chloride(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.
In Italy one of the leading centers for research in hybrid propellants rockets is CISAS (Center of Studies and Activities for Space) "G. Colombo", University of Padua. The activities cover all stages of the development: from theoretical analysis of the combustion process to numerical simulation using CFD codes, and then by conducting ground tests of small scale and large-scale rockets (up to 20 kN, N2O-Paraffin wax based motors). One of these engines flew successfully in 2009.
In Taiwan, hybrid rocket system developments began in 2009 through R&D projects of NSPO with two university teams. Both teams employed nitrous oxide/HTPB propellant system with different improvement schemes. One team (NCKU) added 50 percent of paraffin in the solid grain for boosting the regression rates. The other team (ARRC/NCTU) incorporated innovative mixing enhancement devices to push the overall combustion efficiency towards the theoretical value. This team takes full advantage of high-fidelity simulations and experimental works for very cost-effective developments. Several hybrid rockets have been successfully launched so far, reaching altitudes of 10~20 km. Their plans include attempting 100~200 km altitude launch to test nanosatellites by the end of 2014, and developing orbital launch capabilities for nanosatellites in the long run.
In 1953 Pacific Rocket Society (est. 1943) was developing the XDF-23, a 4" x 72" hybrid rocket, designed by Jim Nuding, using LOX and rubber polyall called "Thiokol". They had already tried other fuels in prior iterations including cotton, paraffin wax and wood. The XDF name itself comes from eXperimental Douglas Fir from one of the first units.
Korey Kline of Environmental Aeroscience Corporation (eAc) first fired a gaseous oxygen and rubber hybrid in 1982 at Lucerne Dry Lake, CA, after discussions on the technology with Bill Wood, formerly with Westinghouse.  The first SpaceShipOne hybrid tests were successfully conducted by Kline and eAc at Mojave, CA.
American Rocket Company fired the first very large hybrids and tailored N2O and HTPB hybrids to government uses with limited adoption due to combustion instability and low Isp.
In popular culture
|This section does not cite any references or sources. (December 2009)|
An October 26, 2005 episode of the Television show MythBusters entitled "Confederate Rocket" featured a hybrid rocket motor using liquid nitrous oxide and paraffin wax. 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 was built by Senior members of the United Kingdom Rocketry Association (UKRA) and 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 faulty explosive bolts between the Robin and the external tank and the Robin subsequently crashed into the ground and seemed to have exploded soon after. In fact this explosion was added for dramatic effect as neither Reliant Robins nor hybrid rocket motors explode in the way depicted.
- "GIRD-09". Encyclopedia Astronautix. Retrieved 2009-04-24.
- "Hybrid Rocket Propulsion Overview". Space Propulsion Group, Inc.
- "A Brief History of Hybrid Rocket Technology". Space Propulsion Group, Inc.
- "Wax Hybrids". Science@NASA. Retrieved 2009-06-01.
- Bosker, Bianca (2009-11-30). "Virgin Galactic SpaceShipTwo getting ready for test flights ahead of space tourism". Huffington Post.
- Video of an 18" diameter self-starting and ending HTP-HTPB hybrid near Garlock, CA. , October 17, 2009. Accessed December 31, 2013.
- PDF. Stanford University
- PDF. Stanford University
- EAC Company home page. , Accessed December 31, 2013.
- Copenhagen Suborbitals HEAT booster development and tests, with photos and video. Accessed 2010-06-03
- "Rocket Propulsion Group", Boston University
- "Rocket Propulsion Group >> Overview" Boston University
- Skyripper Systems, dead link
- West Coast Hybrids, dead link
- Contrail Rockets
- Propulsion Polymers, dead link
- April 1954 Popular Mechanics magazine "With the amateur - but serious - rocketeers out on the Mojave desert, it's Fourth of July the year around. By Shep Shepherd. pp. 81-85.
- California Rocketry magazine Korey Kline test fired a gaseous oxygen and rubber hybrid in 1982 with witness Jerry Irvine and with collaboration of Bill Wood.
- eAc photo gallery Gallery of photos from the first successful SpaceShipOne static test with Korey Kline of eAc and Burt Rutan of Scaled Composites.
|Wikimedia Commons has media related to Hybrid rocket engines.|
- Developing and testing of a 2kN hybrid rocket engine (German)
- Hybrid Rocket Engines, AspireSpace, membership-based organisation (in UK) (particularly see technical papers)
- Portland State Aerospace Society paraffin hybrid links
- Hybridrocket, Private page (German)
- WARR, students developing hybrid engines (German)
- Hit09 S.r.l Company - Advanced Solution for Aerospace Application