QUESTION:
While the Shuttle is in space flight, is the fuel that is used burned or just jettisoned for thrust? If burned, why do the main engines at launch need to be ignited externally rather than internally?

ANSWER from Mike Wilhoit on December 3, 1997:
The fuel used to maneuver the Shuttle while on orbit is indeed burned and is different stuff than what is used in the main engines during the eight and a half minutes of launch. The maneuvering fuel and oxidizer are hypergolic, which means that they will ignite merely by contacting each other, without an extra ignition source. The main engines use liquid hydrogen and liquid oxygen and require a electronically produced spark to catch fire. There are igniters, which are like fancy spark plugs, in several locations on each engine because the propellants are "pre-burned" to help drive the liquid pumps, then remixed and burned again in the main combustion chamber. What you see coming out of these engines during launch is a slightly fuel-rich steam, mostly water. During the first seconds of the ignition phase there are sparks going off all over the place inside the engines, but the flying sparks you see just before main engine start are another matter. These visible sparks are strictly for helping to detonate any free hydrogen in the area of the engine nozzles before ignition - they are a safety device and they don't contribute to the start-up ignition at all.