Tight budgets and a tight schedule mean NASA will use an existing upper stage for the first two tests of its Space Launch System (SLS) human-exploration rocket, instead of flying the Orion multi-purpose crew vehicle (MPCV) with the human-rated J-2X engine already in full-scale ground test.

Engineers developing the heavy-lift SLS are polling the worldwide launch industry in search of an upper stage they can use to kick the Orion MPCV on two test flights around the Moon. Under pressure from Congress to build and fly the big new rocket quickly, the U.S. space agency has scheduled an unmanned lunar flyby late in 2017, followed by a lunar-orbital mission with a four-person crew in 2021.

Because of the relatively low performance requirements for those missions, an upper stage already in use probably can fulfill them, says Garry Lyles, NASA's SLS chief engineer.

“We're on what we're calling a capabilities-driven exploration,” he says. “So with the budget we have, we've laid out an evolution plan for the launch vehicle that's consistent with the need, and so that budget allows us basically to develop a capability that, with a small stage, a small delta-V with the launch vehicle, we can actually go beyond low Earth orbit with our initial launch vehicle capability.”

That 70-metric-ton initial SLS lift capability is to be provided with a core stage powered by surplus RD-25D space shuttle main engines and two copies of the five-segment version of the four-segment shuttle solid-fuel booster originally developed as the first stage for the Ares I crew launch vehicle (AW&ST Nov. 14, 2011, p. 39).

Specifications for an “Interim Cryogenic Propulsion Stage” (ICPS) published in a Jan. 9 solicitation via the NASA Acquisition Internet Service call for “three engine ignitions to achieve greater than 3,050 [meters per second] delta-V.” By comparison, Lyles says, the SLS upper stage to be developed by Boeing will be able to deliver at least twice that velocity change, even after an ascent burn to help get a full-up Orion to low Earth orbit on missions destined to go beyond the Moon.

“This really is just an interim solution that allows us to actually go beyond low Earth orbit with what I would call a minimum launch vehicle capability,” says Lyles. “What we actually derived is a set of test flights to demonstrate the spacecraft, the Orion MPCV capability, and we needed a rather small delta-V to do that test-flight demonstration.”

For the first two flight tests, NASA wants an upper stage that can push an Orion weighing 24,224 kg (53,404 lb.) to an initial elliptical orbit with an apogee of 900-1,000 nm. It must be able to fire again to circularize that initial orbit, and then send the capsule on its way to the Moon with a “big burn” for trans-lunar injection, according to Lyles.

The payload requirement includes the Orion service module and, on the second mission only, a four-person crew. The first mission would simply swing the unoccupied vehicle around the Moon and return it to Earth for an 11-km/sec. (6.8-mi./sec.) reentry velocity to test its heat shield. The second would use the service module propulsion system to place the Orion in a high lunar orbit while its systems are checked out, and then use service module propulsion for the return to Earth. That propulsion will be supplied by a modified version of the shuttle's Orbital Maneuvering System engine (AW&ST Jan. 9, p. 40).

While the solicitation is open officially to any upper stage operating today, including solid-fuel and storable-propellant systems, Lyles says it is possible that only cryogenic stages would meet the requirement. That could include an upper stage using hydrocarbon fuel instead of liquid hydrogen, as on the SpaceX Falcon 9.

“Even the kerosene stages use LOX [liquid oxygen], so you can say that's a cryo stage,” says Lyles. “We'll take a look at it. It doesn't necessarily have to be a hydrogen stage.”

Possible candidates include the upper stages for the Delta IV and Atlas V, Europe's Ariane V, Russian Proton and Zenit vehicles, and perhaps others. Non-U.S. providers might need to team with a U.S. company to meet “Buy American” and other congressional restrictions.

“For this response, I would expect that we have the potential to have some international responses, and we would have to look at how those responses fit in, not only to our acquisition strategy, but we have to talk to the legal folks to make sure we're dotting every i and crossing every t,” says David Beaman, manager of spacecraft and payload integration for SLS at Marshall Space Flight Center.

Beaman says NASA will provide the interfaces between the ICPS, the SLS core stage below it and the Orion service module above. The agency wants to see 10-page responses from industry by Feb. 7 “solely for the purpose of determining whether or not hardware is currently available and if the procurement may be conducted on a competitive or noncompetitive basis.”

Other criteria that must be met include an overall stage weight less than 71,400 lb., the ability to be human-rated for the second flight, and the ability to maintain three-axis control in space before it is separated from the Orion.

Meanwhile, NASA plans to continue developing the J-2X that will power the final SLS upper stage. Ground testing of the engine, which is based on the Saturn V upper stage engines, is under way at Stennis Space Center. However, work on the SLS upper stage will await actual requirements for it owing to NASA's “metered” development schedule designed to stay within a flat budget.

“We haven't gotten to the point for laying out how the upper stage itself is going to track,” says Lyles. “That will be going through as we go through the negotiations with Boeing.”