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| [[File:SpaceShipOne schematic.png|thumb|350px|Hybrid [[rocket motor]] detail of [[SpaceShipOne]] ([http://scaled.com/projects/tierone/data_sheets/html/ox_tank.htm more information])]]
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| 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.<ref>{{cite web | url = http://www.astronautix.com/lvs/gird09.htm | title = GIRD-09 | publisher = Encyclopedia Astronautix | accessdate = 2009-04-24}}</ref>
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| Hybrid rockets exhibit advantages over both [[Liquid-fuel rocket|liquid rockets]] and [[solid rocket]]s especially in terms of simplicity, safety, and cost.<ref>{{cite web | url = http://www.spg-corp.com/space-propulsion-group-resources.html | title = Hybrid Rocket Propulsion Overview | publisher = Space Propulsion Group, Inc.}}</ref> 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|specific impulse(<math>I_{sp}</math>)]] performance of hybrids is generally higher than solid motors, and roughly equivalent to [[hydrocarbon|hydrocarbon-based]] liquid motors. <math>I_{sp}</math> as high as 400s has been measured in a hybrid rocket using metalized fuels.<ref>{{cite web | url = http://www.spg-corp.com/News_12.php | title = A Brief History of Hybrid Rocket Technology | publisher = Space Propulsion Group, Inc.}}</ref> Hybrid systems are more complex than solids, but the [[PEPCON disaster|significant hazards]] of manufacturing, shipping and handling solids offset the system simplicity advantages.
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| ==Basic concepts==
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| [[File:Hybrids big.png|thumb|300px|Hybrid rocket propulsion system conceptual overview]]
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| In its simplest form a hybrid rocket consists of a [[pressure vessel]] (tank) containing the liquid [[rocket propellant|propellant]], the [[combustion chamber]] containing the solid [[rocket propellant|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.
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| Generally the liquid propellant is the [[oxidizer]] and the solid propellant is the [[fuel]] because solid oxidizers are [[Ammonium Perchlorate Composite Propellant#Environmental and other concerns|problematic]] and [[rocket propellant#Advantages_2|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]].
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| Common oxidizers include gaseous or liquid [[oxygen]] or [[nitrous oxide]].
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| Common fuels include [[polymers]] such as [[polyethylene]], [[cross-link]]ed [[rubbers|rubber]] such as HTPB or liquefying fuels such as paraffin wax.
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| ==Properties==
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| Hybrid rocket motors exhibit some obvious as well as some subtle advantages over [[liquid-fuel rocket]]s and [[solid-fuel rocket]]s. A brief summary of some of these is given below:
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| ===Advantages compared with bipropellant liquid rockets===
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| * Mechanically simpler - requires only a single liquid propellant resulting in less plumbing, fewer valves, and simpler operations.
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| * Denser fuels - fuels in the solid [[phase (matter)|phase]] generally have higher density than those in the liquid phase, reducing overall system volume.
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| * Metal additives - reactive metals such as aluminium, [[magnesium]], [[lithium]] or [[beryllium]] can be easily included in the fuel grain increasing [[specific impulse|specific impulse(<math>I_{sp}</math>)]], density specific impulse, or both.
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| ===Advantages compared with solid rockets===
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| * Higher theoretical <math>I_{sp}</math> is possible.
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| * Less explosion hazard - Propellant grain more tolerant of processing errors such as cracks.
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| * More controllable - Start/stop/restart and throttling are all achievable with appropriate oxidizer control
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| * Relatively safe and non-toxic oxidizers such as [[liquid oxygen]] and nitrous oxide can be used
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| * Can be transported to site in a benign form and loaded with oxidizer remotely immediately before launch, improving safety.
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| ===Disdvantages compared with APCP solid rockets===
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| * A solid is inherently simpler with no moving parts; valves, pumps, regulators, hoses, solenoids, etc.
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| * A solid once developed, reliably has the same results over and over.
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| * A solid can respond to a launch command in a fraction of a second after sitting decades at the ready.
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| * The claim a solid has an explosion hazard is speculative and contrary to experience, and the fact is, it is a slow burning solid that is not even an explosive as defined by Federal Law 18 U.S.C. § 841(d). {{cn|date=January 2014}}
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| * Lower hybrid (<math>I_{sp}</math>) in practice is most common, by about 20-30%.
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| * Hybrids typically suffer from combustion stability issues (water hammer equivalent) resulting in a rough ride.
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| ===Disadvantages of hybrid rockets===
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| Hybrid rockets also exhibit some disadvantages when compared with liquid and solid rockets. These include:
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| * 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.
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| * 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.<ref>{{cite web | url = http://science.nasa.gov/headlines/y2003/28jan_envirorocket.htm | title = Wax Hybrids | publisher = Science@NASA | accessdate = 2009-06-01}}</ref>
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| * 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.
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| For a well-designed hybrid, O/F shift has a very small impact on performance because <math>I_{sp}</math> is insensitive to O/F shift near the peak.
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| 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]].
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| ==Hybrid safety==
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| Generally, well designed and carefully constructed hybrids are very safe. The primary hazards associated with hybrids are:
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| * '''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.
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| * '''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.
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| * '''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.
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| 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 rocket]]s often have TNT equivalencies similar in magnitude to the mass of the propellant grain. [[Liquid-fuel rocket]]s 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%.
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| == Operational hybrids ==
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| 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.<ref>{{cite news| url=http://www.huffingtonpost.com/2009/11/30/virgin-galactic-spaceship_n_373978.html | work=Huffington Post | first=Bianca | last=Bosker | title=Virgin Galactic SpaceShipTwo getting ready for test flights ahead of space tourism | date=2009-11-30}}</ref><ref>http://news.softpedia.com/news/Spaceship-Test-at-the-Mojave-Desert-Test-Area-Kills-2-61171.shtml</ref> The [[Virgin Galactic]] [[SpaceShipTwo]] follow-on commercial suborbital spaceplane uses a scaled-up hybrid motor.
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| 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]].
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| U.S. Rockets <ref>http://v-serv.com/usr/crr457mm.htm</ref>manufactures and deploys hybrids using self-pressurizing nitrous oxide N<sub>2</sub>O and HTPB as well as HTP and HTPB. The High Test Hydrogen Peroxide H<sub>2</sub>O<sub>2</sub> 86% and Hydroxyl-terminated polybutadiene (HTPB) and aluminum hybrids developed by U.S. Rockets produce a sea level delivered specific impulse (I<sub>sp</sub>) of 240, well above the typical 180 of N<sub>2</sub>O-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<ref name="USR Hybrid">Video of an 18" diameter self-starting and ending HTP-HTPB hybrid near Garlock, CA. [http://v-serv.com/usr/motors/images/18in/HTP-HybridFiring01.mov], October 17, 2009. Accessed December 31, 2013.</ref> and deployed both pressure fed and pump fed versions of the latter HTP-HTPB style. To date, no deaths or injuries have occurred in the development of this style of hybrid. 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.
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| == Organizations working on hybrids ==
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| [[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.<ref>{{pdf|[http://www.stanford.edu/dept/aeroastro/aeroastro/50th/posters/peregrine.pdf Peregrine rocket poster (2008)]}}. Stanford University </ref> Engineering challenges include various types of combustion instabilities.<ref>{{pdf|[http://aa.stanford.edu/students/media/posters2012/waxman.pdf Peregrine rocket poster (2012)]}}. Stanford University</ref>
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| [[Orbital Technologies Corporation]] (Orbitec) has been involved in some US government funded research on hybrid rockets including the "Vortex Hybrid" concept.
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| Environmental Aerospace Corporation (eAc)<ref name="EAC hiome">EAC Company home page. [http://www.hybrids.com/ ], Accessed December 31, 2013.</ref> 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.
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| [[Rocket Lab]] sells hybrid sounding rockets and related technology.
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| 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.
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| [[Copenhagen Suborbitals]], a Danish rocket group, has designed and test-fired several hybrids using N<sub>2</sub>O at first and currently LOX. Their fuel is epoxy, paraffin, or polyurethane.<ref>[http://copenhagensuborbitals.com/boosters.php Copenhagen Suborbitals] HEAT booster development and tests, with photos and video. Accessed 2010-06-03</ref> The group eventually moved away from hybrids because of thrust instabilities, and now uses an motor similar to that of the V-2 rocket.
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| 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.
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| The WARR<ref>[http://www.warr.de/raketentechnik WARR]</ref> 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 (rocket)|Barbarella]].
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| [[University of Brasilia]]'s Hybrid Team has extensive research in paraffin/nitrous oxide hybrids having already made more than 50 tests fires. Hybrid Team is currently working liquefied propellant, numeric optimization and rocket design
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| 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 at Urbana-Champaign|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",<ref>[http://www.burocket.org/ "Rocket Propulsion Group"], Boston University</ref> 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.<ref>[http://www.burocket.org/overview.php "Rocket Propulsion Group >> Overview"] Boston University</ref>
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| [[Florida Institute of Technology]] has successfully tested and evaluated hybrid technologies with their Panther Project.
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| A United Kingdom-based team (laffin-gas) is using four N<sub>2</sub>O 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 N<sub>2</sub>O supply is provided by Nitrogen-pressurised piston accumulators which provide a higher rate of delivery than N<sub>2</sub>O gas alone and also provide damping of any reverse shock.
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| Also in the United Kingdom the [[Bloodhound SSC]] team have [[The Falcon Project]] led by [[Daniel Jubb]] deploying a fully developed hybrid rocket using HTP and HTPB.
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| 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<ref>[http://www.hypertekhybrids.com HyperTek]</ref> and a number of 'Urbanski-Colburn Valved' (U/C) systems such as RATTWorks,<ref>[http://www.rattworks.net RATTWorks]</ref> HyperTek,<ref>[http://www.skyrippersystems.com Skyripper Systems, dead link]</ref> West Coast Hybrids,<ref>[http://www.westcoasthybrids.com West Coast Hybrids, dead link]</ref> Contrail Rockets,<ref>[http://www.contrailrockets.com/ Contrail Rockets]</ref> and Propulsion Polymers.<ref>[http://www.propulsionpolymers.com Propulsion Polymers, dead link]</ref>
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| 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.
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| 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, N<sub>2</sub>O-Paraffin based motors). One of these engines flew successfully in 2009.
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| ==History==
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| 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 and wood. The XDF name itself comes from eXperimental Douglas Fir from one of the first units.<ref>[http://books.google.com/books?id=Nd8DAAAAMBAJ&pg=PA81&dq=1954+Popular+Mechanics+January&hl=en&sa=X&ei=Q3YzT6TaKu2o0AHgjvW_Ag&ved=0CDMQ6AEwATgK#v=onepage&q&f=true 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.</ref>
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| 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. <ref>[http://www.v-serv.com/crp/CRm/CRm.htm 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.</ref> The first SpaceShipOne hybrid tests were successfully conducted by Kline and eAc at Mojave, CA.<ref>[http://www.hybrids.com/gallery.html eAc photo gallery] Gallery of photos from the first successful SpaceShipOne static test with Korey Kline of eAc and Burt Rutan of Scaled Composites.</ref>
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| [[American Rocket Company]] fired the first very large hybrids and tailored N<sub>2</sub>O and HTPB hybrids to government uses with limited adoption due to combustion instability and low I<sub>sp</sub>.
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| ==In popular culture==
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| {{Unreferenced section|date=December 2009}}
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| An October 26, 2005 episode of the Television show ''[[MythBusters]]'' entitled "[[MythBusters (2005 season)#Episode 40 – "Confederate Rocket"|Confederate Rocket]]" featured a hybrid rocket motor using liquid nitrous oxide and paraffin. The myth purported that during the [[American Civil War]], the [[Confederate States Army|Confederate Army]] was able to construct a rocket of this type. The myth was revisited in a later episode entitled ''[[MythBusters (2006 season)#Salami Rocket|Salami Rocket]]'', using hollowed out dry [[salami]] as the solid fuel.
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| In the February 18, 2007 episode of [[Top Gear (current format)|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 hybrids do not explode in the way depicted.
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| ==See also==
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| * [[Spacecraft propulsion]]
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| * [[Rocket]]ry
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| * [[SpaceDev]]
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| * [[Scaled Composites SpaceShipOne]]
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| * [[Space Propulsion Group]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| {{Commons category|Hybrid rocket engines}}
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| <!-- this link does not down load pdf file weel, so that changed to link below.
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| * [http://www.aspirespace.org.uk/aspireold/TechSeries/Introduction%20to%20hybrid%20design.pdf Aspirespace Introduction to hybrid design]
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| -->
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| * [http://www.hybrid-engine-development.de Developing and testing of a 2kN hybrid rocket engine] {{de icon}}
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| * [http://myweb.tiscali.co.uk/aspirespace/Hybrid%20Engines.htm Hybrid Rocket Engines, AspireSpace, membership-based organisation (in UK)] (particularly see [http://myweb.tiscali.co.uk/aspirespace/TechPapers.htm technical papers])
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| * [http://psas.pdx.edu/HybridLinks Portland State Aerospace Society paraffin hybrid links]
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| * [http://www.c-turbines.ch Hybridrocket, Private page] {{de icon}}
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| * [http://www.warr.de/raketentechnik WARR, students developing hybrid engines] {{de icon}}
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| * [http://www.hit09.com Hit09 S.r.l Company - Advanced Solution for Aerospace Application]
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| {{Spacecraft propulsion}}
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| [[Category:Spacecraft propulsion]]
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| [[Category:Rocketry]]
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