alb@alice.UUCP (Adam L. Buchsbaum) (08/03/85)
The sparks you see are to ignite any hydrogen that may have leaked before ignition. The engines each have their own ignition system; they ignite separately, 120 milliseconds apart, in the computer controlled sequence. They do not ignite again after launch because (1) there is no more fuel left once in orbit and (2) the main engines are not designed to be reignited. The solids, too, have their own ignition devices. The solid fuel is basically an aluminum powder.