Engineers planning NASA’s next effort to build a vehicle to replace the retiring space shuttle fleet will start with an amalgam of the shuttle and its moribund Ares I and Orion follow-ons. As before, money remains a problem.

The U.S. space agency’s “reference vehicle design” for a heavy-lift space launch system (SLS) incorporates five space shuttle main engines (SSMEs); a core stage based on the 27.5-ft.-dia. shuttle external tank; two five-segment versions of the four-segment solid-fuel shuttle booster rockets and an upper stage powered by one or two J-2X engines in development for the Ares I crew launch vehicle.

For the new multipurpose crew vehicle (MPCV), crews ranging in size from two to four would ride in the deep-space version of the Orion crew exploration vehicle, adapted to provide backup flight capability to the International Space Station in case commercial crew vehicles being developed separately do not materialize.

NASA officials stress that while the designs meet congressional requirements that the government’s shuttle follow-ons use existing hardware, contracts and workforce as much as possible, the reference vehicles are not necessarily the final choice. Internal NASA teams also are studying a kerosene-fueled alternative to the liquid-hydrogen-fueled SSME, and a modular approach that would array three smaller-diameter stages in parallel—comparable to the approach followed by the Delta IV Heavy.

In addition, 13 companies are conducting six-month studies of technologies that might help NASA refine its heavy lifter. Administrator Charles Bolden has insisted that any future vehicles be “affordable, sustainable and realistic,” specifications the reference launch vehicle does not meet given the $6.5 billion authorization for its development and 2016 deadline for its first flight.

“To date, trade studies performed by the agency have yet to identify heavy-lift and capsule architectures that would both meet all SLS requirements and these goals,” states the 22-page “preliminary” report sent to Congress under the authorization act President Barack Obama signed last year.

“For example, a 2016 first flight of the SLS does not appear to be possible within projected FY 2011 and out-year funding levels.”

Cris Guidi, an engineer who focuses on launch vehicles as deputy director of the Constellation Systems Div. at NASA headquarters, said Jan. 12 the results of the company and internal-NASA studies will drive the final vehicle design.

The reference design was picked because it made maximum use of shuttle hardware and work already done under the Constellation Program that was building the Ares I and Orion until Obama’s Fiscal 2011 budget request and the subsequent authorization act killed it.

“Administrator Bolden is very, very strongly emphasizing that we’re going to develop a credible plan,” she says. “We’re not going to break any laws, but the key thing that NASA needs to go off and do is build a credible plan for the SLS development; make sure it’s affordable, sustainable and realistic or practical.”

The final vehicle also will be shaped by final Fiscal 2011 NASA funding that emerges from the newly convened Congress, and by the Fiscal 2012 budget request due out next month.

The agency is currently funded under a continuing resolution that expires March 4, which contains money for work on Constellation programs like the J-2X and the five-segment solid-fuel booster originally intended as the first stage for the Ares I.