Northrop Adopts Twin-Booster for SLI

Frank Morring, Jr. / Washington

Northrop Grumman has settled on an all-cryogenic winged core booster with twin kerosene-fueled fly- back boosters as its proposal for NASA's Space Launch Initiative reusable launch vehicle technology program.

The twin boosters would have folding wings that would open once the boosters separate from the core vehicle. Instead of gliding back to the launch site, the boosters would use air-breathing jet engines for greater range on the flyback. 

"The boosters fly back, which allows the vehicle to stage at a higher Mach number and a lower dynamic pressure," said Doug Young, director of space programs at Northrop Grumman Integrated Systems in El Segundo, Calif. "We believe it's a less technically risky approach than staging earlier so that the boosters are closer to the launch site. With the air-breathing engines you can fly the booster back from farther downrange and thereby reduce the risk of a separation event."

The winged, hydrogen-fueled upper stage would ignite at the same time as the two boosters and continue all the way to orbit. Its payload would either be the crew vehicle under development by Northrop Grumman's SLI teammate, Orbital Sciences, or cargo carried internally in a cargo bay. Company engineers continue to run trade studies on whether to mount the crew vehicle on the nose or the back of the cryogenic core vehicle, Young said.

The initial round of SLI studies led to the basic configuration Northrop Grumman has proposed, with a relatively small first stage using hydrocarbon fuel to power both its rocket engines and the jet engines that power it back to the landing site, and a cryogenic upper stage (AW&ST Apr. 1, p. 28). Young said the twin-booster configuration would provide greater flexibility to meet commercial and NASA requirements of lifting as much as 45,000 lb. to orbit, and the smaller military demand for as much as 20,000 lb.

“One of the things we've built into our architecture, and is something NASA’s looking for in all the architectures, is evolvability and flexibility," Young said. "We feel a fair degree of that is afforded by the approach we've taken." Northrop Grumman would adjust the size and combination of flyback boosters.

Young noted that NASA’s need could change after we system requirements review at year-end, "and that will steer where the architecture's going." NASA and military space users are expected to reach agreement early next month on joint efforts to meet their space transportation needs (AW&ST Sept. 9, p. 29).

The twin-booster configuration would also enhance safety, Young said, because it would provide more stability in meeting an engine-out emergency during ascent. The architecture would produce a vehicle with a gross weight at liftoff of about 4 million lb. If the total stack height worked out to be 200 ft., the boosters would rise 80-100 ft. from the pad.