Hybrid
A rubbery fuel
Rocket propellants come in two parts, fuel and oxidizer, which work together to keep an engine burning.
SpaceShipOne burns a material called hydroxy-terminated polybutadiene (HTPB), a common ingredient in tyre rubber, as fuel with nitrous oxide serving as the oxidizer. According to press statements by Scaled, the combination was chosen for SpaceShipOne after a lengthy study into potential engine systems.
For SpaceShipOne, reaching space takes three things: a pilot, a spacecraft, and enough to propellant to rocket away from Earth.
But the fuel in SpaceShipOne's tank is about as exotic as the
spacecraft's design. SpaceShipOne, the first non-governmental crewed
vehicle to reach space, uses a combination of rubber and nitrous oxide
-- also known as laughing gas -- as it's powerhouse
This is a very stable and non-toxic system.
Based in Poway, California, SpaceDev is responsible for refueling SpaceShipOne after each flight as well providing crucial elements of its hybrid rocket engine, a cross between traditional liquid and solid rocket motors.SpaceShipOne was designed by aerospace veteran Burt Rutan and his Mojave, California-based firm Scaled Composites. The craft represents one of 26 teams vying for the Ansari X Prize competition to privately build a vehicle capable of transporting three people 62 miles (100 kilometers) above Earth twice in two weeks, with the winner snagging a $10 million purse.
Rubber-based hybrid engines tend to burn at slower rates compared to solid materials, which can affect the final amount of thrust an engine can produce. Some of that burn reduction can be recovered by SpaceDev's use of pie-like ports for its solid fuel, but additives in the solid fuel could also boost performance.
SpaceShipOne's engine, a composite structure designed by Scaled,
consists primarily of two parts: a tank of nitrous oxide and a cylinder
with the HTPB fuel. Both SpaceDev and Miami, Florida based
Environmental Aeroscience Corporation (eAc) contributed components of
the engine.
There has to be a way to shut off the motor ...
The propellant comes together in an eight-foot combustion chamber, where the liquid oxidizer is converted into a gas, then ignited by small pyros to start the engine. SpaceShipOne pilots can shut down the vehicle's engine by closing a valve though which nitrous oxide enters the fuel chamber.
In conventional rockets, propellant can be pre-mixed -- as in the solid rocket boosters (SRBs) used NASA space shuttle -- or sit in tanks that are filled just prior to launch, like liquid oxygen and hydrogen rockets. In both engine configurations, the are highly volatile and can be toxic to handle.
The fact that the oxidizer and fuel are not molecularly mixed in hybrid engines makes them non-explosive.
The HTPB fuel developed by SpaceDev is non-volatile, making it easier and safer to store than other materials. It is readily available in five-gallon drums and inexpensive, Benson said. Nitrous oxide's tendency to be self-pressurizing also does away with the need for complicated pumps to push it into SpaceShipOne's combustion chamber, he added.
The system also touts a somewhat cleaner way to reach space. Its byproducts -- water vapor, carbon dioxide, carbon monoxide and nitrogen -- are a bit more preferable than the waste produced by shuttle SRBs, which burn ammonium perchlorate and aluminum.
Loading a hybrid engine
To prepare SpaceShipOne for flight, rubber fuel is poured into a fuel
casing and allowed to harden in a pie-like mold before being installed
into the spacecraft's aft fuselage. After a test flight, ground crews
need only replace the fuel casing and attached nozzle, then top off the
nitrous oxide tank to ready the craft for the next trip.
Although SpaceDev's HTPB mixture is a trade secret, it does include some non-HTPB material, none of which is toxic, explosive or environmentally unfriendly.
Lower performance is a downside to hybrid engine systems in general, which is why SpaceDev is currently developing a high-performance version to rival some conventionally propelled rockets.
Engines of the future
The success of SpaceShipOne could increase general interest in hybrid engines, especially for eventual use in space where the hydrazine is currently the prime propellant.
Hydrazine is highly toxic, explosive, and the logistics of loading a spacecraft with the fuel are difficult. There, hybrid fuels could be a viable alternative.
SpaceDev has already begun an in-space effort with the Air Force Research Laboratory to develop a second stage engine to push small payloads around Earth, moving them to higher and lower orbits as needed. The company is also developing its Streaker launch vehicle to use hybrid engines as well.
Parrafin - based Hybrid engine technology
(Based on article published by Stanford Uni)
Paraffin was previously thought to be weak, easily broken and unsuitable for use as rocket fuel. But Cantwell's team found that it is quite strong -- at least twice as strong as conventional solid propellants. The paraffin they use as rocket fuel is the same material used as hurricane candles and sculptor's wax. "Paraffin" is a generic name for a family of simple hydrocarbons with carbon chain lengths ranging from 20 to 40. Different group members are suited to different applications.
Safer, cheaper
Paraffin
Safer, cheaper Paraffin fuel can contribute significantly to making it safer and cheaper to get into space. "If that were accomplished, human access to space would become more routine, and the ability to do scientific studies and commercialize the use of space would also increase dramatically," Cantwell said. For example, scientists could undertake as many missions as necessary to clean up accumulated debris in our near-space environment. Conventional rocket fuels are either solids or liquids, but paraffin fuels are used in a hybrid system combining solid and liquid materials. An oxidizer such as oxygen or nitrous oxide is generally used with all fuel types to aid burning.
Solid fuels include a rubberized material incorporating the oxidizer and other additives such as aluminum or ammonium perchlorate. The fuel-oxidizer composite is dangerous, as it may explode even during shipping and installation. The fuel burns very rapidly in the rocket combustion chamber to generate the rocket propulsion force known as thrust. Once the solid fuel is ignited, the rocket motor cannot be shut off.
This is dangerous because there is no chance to ensure adequate thrust build-up before take-off. Liquid rocket fuels include kerosene and liquefied hydrogen. In these systems, the fuel and oxidizer are held separately in large tanks and then fed into the rocket chamber, where they mix and burn.
Valves are used to regulate fuel and oxidizer flow, and increase, decrease or shut off thrust. This allows more control over the launch process than do solid fuels. The system relies on complex and expensive machinery, however, and is subject to catastrophic fires. Hybrid rocket fuels are considered a safer alternative to traditional solid and liquid fuel systems. In hybrids, the thrust chamber contains only solid fuel. This reduces the potential for devastating fires and explosions. The oxidizer is ignited as it is forced over the fuel surface. Like liquid systems, hybrids can be throttled, but require only one set of valves -- for the liquid oxidizer.
Although hybrids have been in development over the last 50 years, they have not made it into mainstream commercial applications because they did not produce as much thrust as liquid and solid systems. "Hybrid rockets tend to be sort of anemic in their ability to produce thrust," Cantwell said. This is because the fuel burns too slowly, relying on a process limited by the rate at which fuel evaporates and mixes with oxidizer. By contrast, the fuel and oxidizer are forced together in liquid systems and pre-mixed in solid systems.
Birth of a new hybrid
In 1995, the U.S. Air Force began to address this problem with a new type of hybrid fuel -- a simple hydrocarbon, pentane, frozen using liquid nitrogen. The pentane burned three to four times faster than conventional fuels. The Air Force engineers explained their results by saying that less heat was required to gasify the pentane than was needed for conventional solid fuels. Cantwell and Karabeyoglu felt that this explanation was inadequate in light of the "blocking effect," which limits the amount by which fuel evaporation can be increased simply by increasing the rate of heating.
The effect occurs because the increasing evaporation pushes the flame away from the surface and blocks heat transfer even as the heating rate increases. Karabeyoglu proposed an alternate mechanism. He suggested that as oxidizer flows over it, the surface of the pentane melts to form a low-viscosity liquid layer that becomes unstable and forms waves that are easily pulled off the liquid surface as a spray of droplets that evaporate, mix and burn to produce thrust. Karabeyoglu then set out to find materials that have the same physical properties as frozen pentane but that are naturally solids at room temperature. The paraffin waxes were the perfect candidates.
The Stanford team first tested paraffin in a laboratory-scale rocket motor in November 1998 and found that like solid pentane, it burned three to four times faster than conventional solid fuels. To date, they have conducted more than 250 laboratory and field tests in collaboration with engineers at NASA Ames Research Center.
They have tested rocket motors with 2,500 pounds of thrust, the amount that might be needed for a third stage rocket in a launch system. "Further scale-up tests are needed before paraffin-fueled rockets can be utilized in lower stage rockets requiring thrust levels of 200,000 pounds or more," Cantwell said. Cantwell projects that commercial application of paraffin fuels could become a reality in as few as three years.
Stanford has secured a patent on the use of paraffin in rocket fuel applications, and Cantwell and Karabeyoglu have started Space Propulsion Group Inc., a company geared toward commercializing the technology. David Altman, a consulting professor of engineering who is a co-founder and co-inventor of the technology, heads the company. Cantwell has high hopes for paraffin-fueled motors. "Solid rocket boosters remain among the most dangerous part of any space shuttle mission," he said. "I think [the paraffin-based hybrid] would be a good candidate for replacing the shuttle solid rocket boosters."
Hybrid Gels
The major weakness of the solid-fuel rocket is the fact that, once lit, it burns to completion, and the only thing that can be done is to divert the thrust when it is no longer needed. The lack of burn control for solid-fuel rockets has led to the development of "hybrid" rockets that use a solid-fuel core along with a liquid oxidizer. The solid fuel component in a hybrid rocket is not impregnated with large quantities of an oxidizer material, which makes the rocket much safer to handle and store since it cannot burn efficiently on its own. Lockheed Martin has static-tested a hybrid motor with a butadiene-type solid fuel and liquid oxygen oxidizer. Lockheed Martin has also investigated the use of paraffins as propellants; "paraffins" in this case of course refers to the American usage of the term, meaning candle waxes and related solid hydrocarbons, and not the British usage of the term, which is what Americans call kerosene.Burt Rutan's famous commercial suborbital manned spacecraft, "SpaceshipOne", uses a hybrid propulsion system, with a butadiene-type solid fuel and nitrous oxide oxidizer. In this case, the propulsion system is designed for low cost and ease of handling instead of optimal thrust levels. SpaceshipOne is probably the first thing resembling a operational space vehicle to use hybrid propulsion, and after many years of tinkering the technology seems to be coming of age.
Experiments have also been performed on another approach to the same problem, in the form of "propellant gels". The idea is to take storable propellants and turn them into gels: hydrazine can be gelled by adding cellulose, and nitric acid can be gelled by adding silicon dioxide (sand, more or less). The results have the consistency of toothpaste. Aluminum powder can be added to provide more "kick". Since hydrazine and nitric acid are hypergolic, if the two gels come in contact with each other they burn spontaneously -- but not for long, since a crust builds up between them that inhibits further combustion. This makes them much safer to handle than their liquid forms.
To get them to burn in a combustion chamber, they are fed under pressure through an orifice that turns them into an aerosol, allowing them to mix properly. The potential advantages of this approach are high energy density, throttleable operation, and relative safety in handling. Experiments have been performed in determining the suitability of gelled propellants for military missiles. The status of research into gelled fuels remains unclear.