| A hybrid rocket propulsion system | | | | design of the oxidizer feed system fuel |
| comprises propellants of two different | | | | composition and physical layout of the |
| states of matter, the most common | | | | fuel can try to minimise these issues. |
| configuration being a rocket engine | | | | Organizations working on hybrids |
| composed of a solid propellant lining a | | | | In 1998 SpaceDev acquired all of the |
| combustion chamber into which a liquid | | | | intellectual property, designs, and test |
| or gaseous propellant is injected so as | | | | results generated by over 200 hybrid |
| to undergo a strong exothermic reaction | | | | rocket motor firings by the American |
| to produce hot gas that is emitted | | | | Rocket Company (Amroc) over its eight |
| through a De Laval nozzle for propulsive | | | | year life. SpaceDev developed and |
| purposes. | | | | produced all of the hybrid rocket motors |
| These systems are superior to solid | | | | for Paul Allen's SpaceShipOne. SpaceDev |
| propulsion systems in the respects of | | | | is currently developing SpaceDev |
| safety, throttling, restartability, and | | | | Streaker, an expendable small launch |
| environmental cleanliness. Hybrid | | | | vehicle, and SpaceDev Dream Chaser, |
| systems are slightly more complex than | | | | capable of both suborbital and orbital |
| solids, but the reduction of safety, | | | | human space flight. Both Streaker and |
| storage and transport issues may be an | | | | Dream Chaser use hybrid rocket motors |
| offsetting factor. | | | | that burn nitrous oxide (N2O) and |
| In its simplest form a hybrid rocket | | | | synthetic rubber (HTPB). SpaceDev's |
| consists of a pressurised tank full of | | | | SpaceShipOne rocket motor was the |
| oxidizer leading into a valve and from | | | | largest of its kind ever produced. |
| there into a combustion chamber lined | | | | The Reaction Research Society (RRS), |
| with a fuel and on into a conventional | | | | although known primarily for their work |
| rocket engine. To light the engine the | | | | with liquid rocket propulsion, has a |
| valve is opened permitting oxidizer to | | | | long history of research and development |
| reach the fuel, and an ignition source | | | | with hybrid rocket propulsion. |
| is supplied to start the combustion. The | | | | Several universities have recently |
| combustion burns along a hole through | | | | experimented with hybrid rockets. BYU, |
| the long axis of the chamber called a | | | | the University of Utah, and Utah State |
| 'port' and then out through the rocket | | | | University launched a student-designed |
| nozzle producing thrust. | | | | rocket called Unity IV in 1995 which |
| Common oxidizers include gaseous or | | | | burned the solid fuel |
| liquid oxygen and nitrous oxide. Common | | | | hydroxyl-terminated polybutadiene (HTPB) |
| fuels include ABS plastic or synthetic | | | | with an oxidizer of gaseous oxygen, and |
| rubber, although many combustible | | | | in 2003 launched a larger version which |
| substances can be made to work. | | | | burned HTPB with nitrous oxide. Stanford |
| Unconventional fuels such as salami, | | | | University researches LOX/paraffin |
| bagels, or chocolate have been used. | | | | hybrid motors. Many other universities, |
| Hybrid safety | | | | such as the University of Arkansas at |
| Generally, well designed and carefully | | | | Little Rock, the University of Illinois, |
| constructed hybrids are very safe. | | | | and Portland State University have |
| However, ruptures could occur due to: | | | | hybrid motor test stands that allow for |
| pressure vessel failures- if the | | | | student research with hybrid rockets. |
| combustion chamber burns through, the | | | | SpaceShipOne, the first private manned |
| relatively large combustion chamber | | | | spacecraft, is powered by a hybrid |
| could catastrophically fail. | | | | rocket burning HTPB with nitrous oxide. |
| blow back- nitrous oxide tanks | | | | The hybrid rocket engine was |
| particularly have been known to | | | | manufactured by SpaceDev. SpaceDev |
| blow-back and cause a monopropellent | | | | partially based its motors on |
| detonation (combustion instabilities are | | | | experimental data collected from the |
| one of the main causes of this | | | | testing of AMROC's (American Rocket |
| phenomenon, particularly towards the end | | | | Company) motors at NASA's Stennis Space |
| of a burn when the tank pressure is low, | | | | Center's E1 test stand. Motors ranging |
| ensuring sufficient pressure drop across | | | | from as small as 1000 lbf (4.5 kN) to as |
| the injectors is necessary at all | | | | large as 250,000 lbf (1.1 MN) thrust |
| times.) | | | | were successfully tested. SpaceDev |
| hard starts caused by an excess of | | | | purchased AMROCs assets after the |
| oxidizer in the combustion chamber prior | | | | company was shut down due to lack of |
| to ignition, particularly | | | | funding. |
| monopropellants like nitrous oxide. | | | | There are a number of hybrid rocket |
| The fuel itself is generally inert and | | | | motor systems available for amateur |
| cannot normally explode; and unlike | | | | hobbyist use in high-powered model |
| solid rockets, cracks are safe unless | | | | rocketry. These include the popular |
| material falls and blocks the nozzle, | | | | HyperTek systems and a number of |
| dependent on the physical properties of | | | | 'Urbanski-Colburn Valved' (U/C) systems |
| the material (rubberised fuel will pass | | | | such as RATTWorks, Skyripper |
| through without exploding the chamber). | | | | Systems,West Coast Hybrids, Contrail |
| Propellant combinations in rocketry are | | | | Rockets, and Propulsion Polymers. All of |
| sometimes given an 'explosive | | | | these systems use nitrous oxide as the |
| equivalence'. This, multiplied by the | | | | oxidizer and a plastic fuel (such as PVC |
| weight of propellant gives an equivalent | | | | or PolyPropylene) or a polymer based |
| weight of TNT that would give the same | | | | fuel such as HTPB. This reduces the cost |
| effect as a crash of the vehicle. The | | | | per flight compared to solid rocket |
| explosive equivalent of hybrid rockets | | | | motors, although there is generally more |
| propellant is often taken to be 0, | | | | 'GSE' (ground support equipment) |
| whereas liquids are 10-20% and solids | | | | required with hybrids. |
| are around 100%. | | | | These motors vary from 'G-class' (up to |
| Disadvantages of hybrids | | | | 160 Ns) to 'O-class' (up to 40960 Ns) |
| The mixture control in hybrid rockets | | | | with thrust ranging from 60N to over |
| may be less accurate than either solid | | | | 6300N, and can propel a suitable rocket |
| rockets engines or liquid fuel rocket | | | | to altitudes of up to 30,000ft. |
| engines. As the burn progresses, the | | | | At the July 17, 2006 meeting of the |
| port through the combustion chamber | | | | Canadian Space Society, a hybrid rocket |
| widens. This increases the surface area | | | | motor using gaseous oxygen and acrylic |
| that is subject to combustion, and may | | | | was demonstrated. The motor was built by |
| increase the rate of fuel evolution. | | | | Robert Gissing, Daniel Faber, and Luke |
| However the oxidizer can tend to blow | | | | Stras as part of a CSS space display for |
| through the ever widening port without | | | | the Toronto Aerospace Museum. The usage |
| fully reacting with the fuel. This | | | | of an optical grade acrylic rod as both |
| changes the mixture ratio adversely, and | | | | the fuel and the combustion chamber |
| most hybrids become oxidizer rich | | | | allows for viewing of the combustion |
| towards the end of the burn; impairing | | | | process during firing. |
| the average specific impulse. Proper | | | | |