Then the spent boosters are jettisoned, and the liquid-fuel engines supply the final thrust toward orbit.

Cost Estimate of $1.25 Billion In its simplest form, engineers said, a heavy-lift booster based on current systems would be some combination of the solid-fuel rockets and liquid-fuel engines of the shuttle but in a configuration with a large payload canister in place of the manned orbiter.

For example, the shuttle's main liquid-fuel engines, for their size the most powerful on earth, undergo repeated trial runs on the ground, and several have exploded in tests that mimic the strains of typical missions.

Rockwell's Rocketdyne division is the nation's largest manufacturer of liquid-fuel engines of the type used on large rockets.

Several modifications were made to the liquid-fuel engines while the shuttles were grounded after the Challenger accident.

If Admirial Truly and James C. Fletcher, Administrator of the National Aeronautics and Space Administration, agree, the Discovery will undergo a practice countdown ending in a 20-second firing of the three main liquid-fuel engines while bolted to the launching pad.

Dynetics' entry uses a liquid-fuel engine, the F-1B, based on the Rocketdyne F-1 which powered the first stage of the Saturn V rocket.

The project to develop the rocket was proceeding smoothly until last year, when one of the liquid-fuel engines blew up.

Marshall had been in charge of developing the liquid-fuel engines that put Apollo astronauts on the moon in 1969.

Meanwhile, the succeeding stage may have ullage rockets, both to aid separation and ensure good starting of liquid-fuel engines.