Brian J. Cantwell, a professor in Stanford University's department of
aeronautics and astronautics, has an article at the Institute of
Electrical and Electronic Engineers (IEEE) Spectrum Magazine web site
about he and his colleagues efforts to design safer rocket engines using
a liquid/solid hybrid approach where they are using paraffin wax as
the solid component.
Quote:
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Originally Posted by Brian Cantwell, IEEE Spectrum
The overall design of a hybrid rocket is quite simple. The solid fuel is built into the combustion chamber in the form of a cylinder with one or more channels hollowed out along its axis. After the liquid oxidizer is released from its storage tank, it becomes a gas and flows through these openings. After ignition, combustion takes place between the oxidizing gas and material evaporating from the surface of the fuel that is exposed inside the channels. The hot, high-pressure gases that result flow out of a suitably designed nozzle, generating thrust.
The problem is that fuel. The heat inside the combustion chamber causes some fuel to vaporize, at which point it readily burns with the oxidizer. But despite the intense heat in the chamber, the polymeric fuels normally used in hybrid rockets vaporize slowly, making it very difficult to produce the large amount of thrust needed for a launch into space.
To compensate, designers often try to expand the surface area of the fuel that’s exposed to combustion. That’s easy enough—just increase the number of channels. This strategy works to some extent, but if taken too far it can make the mass of fuel look like a block of Swiss cheese and may cause the fuel to fracture as it burns away.
The synthetic-rubber fuel used in the motor for SpaceShipOne required four channels for combustion to produce the needed thrust, and that caused some difficulties. As the fuel was burned, these openings became larger and larger, and the walls of solid fuel between them shrank. Toward the end of a burn, the expanding holes made the solid fuel prone to breakage. Indeed, on at least one flight of SpaceShipOne, large chunks of solid fuel went flying out of the nozzle, causing intense shaking and reducing the rocket’s performance—and making the pilot think at one point that the tail of his craft had blown off.
At Stanford, we have focused our research on ways to increase the thrust of hybrid rockets without adding these problematic extra channels.
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Full article here :-
http://spectrum.ieee.org/aerospace/s...ets-more-oomph
Gary Kopff
IEEE Member 37 years
Hybrid rocket engines using paraffin wax - IEEE Spectrum article
