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SpaceShipOne Rocket-Engine Design
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pacecraft have used both solid and liquid rockets, and in some cases both, to blast out of the atmosphere, into orbit, to the Moon, and out of the Solar System. The Space Shuttle, for example, uses two solid rocket boosters (SRB) mounted to the external tank (ET) and its three liquid-fueled main engines to reach orbit.
SpaceShipOne had a much different set of challenges to face, so its rocket engine had to be equally unique. There was no off-the-shelf rocket engine that Scaled Composites could simply install. Rutan had to design the rocket engine from scratch. It would be the first that Scaled Composites would have to build. Once the design was complete, Sealed Composites enlisted four subcontractors to provide the rocket-engine components that were not built in-house.
SpaceShipOne would be the first manned spacecraft to use a hybrid rocket engine. Figure 5.1 shows an external view of SpaceShipOne’s hybrid rocket engine.
The Rocket Engine
In 1999, Scaled Composites began researching rocket-engine technology. By January of 2000, it had not only identified the type of rocket engine and selected the propellants, but it had developed a new concept for its configuration.
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Fig. 5.1. A hybrid rocket engine offers advantages of both liquid-fueled and solid-fueled rocket engines. The rocket engine can be shut off at any time during the burn and can be constructed without complicated plumbing and pumps. The disadvantage, though, is that it has lower performance than the other two types. Mojave Aerospace Ventures LLC, photograph by David M. Moore ______________________________________________________________________________________________________
Rutan believed that the highest risk of the program from the technical stance was the operation of the rocket engine. Reentry was dangerous, of course, but the “carefree” approach using the feather dramatically minimized this danger.
“I ruled out solids because I couldn’t do flight tests with them,” Rutan said. “I couldn’t do flight-test envelope expansion. I couldn’t do partial burns. Also, I knew that likely during a burn, I might be accelerating into a Mach number that I’d never been to. And I may not like it. I wanted to be able at any time to shut the motor off just like that.
“I ruled out liquids because they had a large number of failure points that were difficult to improve safely by making them all redundant. If you did, you ended up with a complex system, which historically has been shown to be less safe than not having the redundancy.”
A hybrid rocket engine fit Rutan’s requirements. It was very safe and very simple and very robust. Just as the name suggests, a hybrid
rocket engine is part liquid rocket engine (like the Space Shuttle’s main engines) and part solid rocket engine (like the Space Shuttle’s solid rocket boosters). Figure 5.2 shows the basic designs of liquid, solid, and hybrid rocket engines.
Essentially, a hybrid rocket engine is a tank that contains the liquid part and a motor that contains the solid part. Upon ignition, the liquid flows into the motor and out come the flames. It can be stopped instantly, unlike a solid, and its propellants are room temperature as opposed to cryogenic. However, there is a tradeoff. Hybrid rocket engines are typically less efficient than liquid or solid rocket engines. This means that for equal amounts of propellant by mass, hybrids deliver less thrust. But in the case of SpaceShipOne, the lower performance was acceptable.
“Would I use a hybrid motor to go to orbit? Probably not unless we could develop one that was close to the efficiency of the liquids,” Rutan said.
Liquid Rocket Engine
Fuel Pumps Throat
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Oxidizer
Solid Rocket Engine
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