How would a hybrid rocket work?

Question

How does a solid fuel liquid oxidizer hybrid rocket work? How do they throttle them up and down?

Virgin Galactic plans to use a hybrid engine, and I was also wondering how they were planning to store them together.

Answer 1

The typical hybrid rocket arrangement consists of a hollow cylinder of solid fuel, with liquid or gaseous oxidizer in a separate pressurized tank, introduced at the top end of the cylinder. The oxidizer flows down the hollow, combusting with the fuel all along the inner surface of the cylinder.

Rocket engines used for ascent often don't need to be throttled at all, but a hybrid can be throttled by reducing the oxidizer flow. This changes the oxidizer/fuel mixture ratio, obviously, so specific impulse (fuel efficiency) decreases as well as thrust.