Potential replacements for the Space Shuttle are taking shape as NASA struggles to finalise the requirements for a second-generation reusable launch vehicle

Shifting requirements could force NASA to postpone the next milestone in its ambitious programme to develop a second-generation reusable launch vehicle (RLV) to replace the Space Shuttle. Industry believes the system requirements review scheduled for next month could be pushed well into next year as NASA wrestles with issues of supporting the International Space Station (ISS) and accommodating possible US Air Force involvement.

NASA says the five-year, $4.8 billion Space Launch Initiative (SLI) programme is undergoing a headquarters review ordered by new associate administrator for aerospace, Jerry Creedon. Wrapped up in this review is a decision on the crew rescue/transfer vehicle that is required if the ISS is to realise its full potential. This decision could determine the path NASA takes towards replacing the Space Shuttle.

Design selection

Over the past two years, Boeing, Lockheed Martin and Northrop Grumman have studied thousands of potential designs for a second-generation RLV, with each company narrowing down to one preferred architecture in preparation for the system requirements review. Trade studies continue as NASA and industry try to understand the cost implications of emerging requirements.

Boeing has made a "soft downselect" to a two-stage-to-orbit vehicle carrying a dedicated orbiter, says Kevin Neifert, director advanced space and launch systems. The first and second stages are hydrogen-fuelled and powered by "fundamentally common" engines. "There is one engine design, modified for different thrust levels," he says. Each stage has multiple engines, allowing the vehicle to continue to orbit if one fails.

The first stage is a fly-back booster, with jet engines to power the vehicle back to a runway landing at the launch site. The second stage goes into orbit, releases the orbiter, re-enters and glides back. The two stages have different designs. "We had configurations where they were the same, but they were not as attractive," says Neifert. "There is commonality between the stages, but not at the visible vehicle level." The stages are processed horizontally, stacked, and the vehicle lifted to the vertical position for launch.

The orbiter is smaller than the Shuttle, with a 4.6m diameter, 13.7m-long (15 x 45ft) payload bay, and is capable of carrying either cargo or crew. The design is scaled up from Boeing's X-37 reusable spaceplane. "The orbiter has the flexibility to do several different operations. It can go to the ISS, do a spacecraft servicing mission or take satellites to geostationary transfer orbit," says Neifert. "It puts the variation in the on-orbit element and keeps the variability down in the first and second stages. We can do a lot of missions with the same first and second stages."

Because it is a reference configuration to be used in validating the requirements and not the final design, Boeing is continuing trade studies into options including kerosene-fuelled and glide-back first stages. "We could change to a hydrocarbon first stage," says Neifert. "The risk would be the need to develop another engine. The benefit would be the capability to fly using jet fuel, which is everywhere."

Boeing is "looking very hard" at a glide-back first stage. This would eliminate the need to carry jet engines and their fuel. "Fly-back gets us back to the launch site. Glide-back would go someplace else," says Neifert. "We've looked at the requirements and think we could land in Bermuda." The first stage would be transported back by ship. Boeing already transports Delta IV core stages by ship, the Delta Mariner, and could use the vessel to recover the RLV first stage. "Sizewise they are about the same," he says.

Northrop Grumman has narrowed its focus to a two-stage-to-orbit vehicle with a winged core stage and two small fly-back boosters. The core stage is hydrogen-fuelled, but the two identical boosters are kerosene-fuelled and fly back under jet power to a runway landing at the launch site.

The cryogenic core stage goes into orbit, carrying cargo in an internal bay or a crew transfer vehicle on the nose, releases its payload, re-enters and glides back to the launch site. Teammate Orbital Sciences is designing the crew vehicle. "We are still in a very conceptual stage," says Doug Young, director of space programmes. "We are doing trades on whether the crew transfer vehicle should be on the nose or the back of the core stage."

Fly-back benefits

The vehicle is processed horizontally and launched vertically. The core stage and 25-30m-tall boosters each have multiple engines. "We are doing trades on the booster size and the number of engines," Young says. The boosters unfold their wings after separation and fly back under the power of "heavily modified" commercial turbofans. Fly-back boosters allow the vehicle to stage further downrange, at a higher Mach number and lower dynamic pressure, to provide a more benign separation environment, he says.

Young says Northrop Grumman has selected the safest, "most evolvable and most operable" architecture. "We can lose an engine on the pad, or in the early stages of the launch, and continue and complete the mission," he says. Operability is a result of "prudent system engineering". Young says the design could be evolved by using larger or smaller boosters or by developing and flying the booster before the core stage. "It gives us development options." Rather than trying to reproduce the Shuttle, "we can take a lower-risk approach and evolve the vehicle", he says.

Lockheed Martin's preferred architecture is also a two-stage-to-orbit vehicle, combining a kerosene-fuelled first stage with a hydrogen-fuelled second stage. The fly-back first stage returns under jet power to the launch site, while the second stage continues into orbit. Cargo is carried in a fairing on the back of the second stage, which opens to release the payload. The vehicle then re-enters and glides back to a runway landing.

Alternatively, an orbital spaceplane (OSP) can be mounted on the second stage in place of the cargo fairing. The spaceplane would detach itself, operate in orbit, then re-enter and glide back. "The OSP is manned and we are configuring it for manual control," says Bob Ford, SLI programme director. "We have a fairly detailed conceptual design for the OSP, including its capabilities, weights, costs and operations."

In common with Northrop Grumman, Lockheed Martin has opted for a hydrocarbon-fuelled first stage. "Kerosene works quite well as a first-stage fuel," says Ford. "There is not a great [performance] benefit for hydrogen over kerosene, and we get a lot of operability benefits. Hydrogen is a difficult fuel to handle. Kerosene is a piece of cake," he says.

All three companies underline the importance of the engines to meeting the design goals for the second-generation RLV. "We are looking at clean-sheet engines," says Neifert. "We have got to have truly reusable engines. They are the key enablers." The goal is an engine life of 100 missions, with 50 flights between major maintenance. Shuttle main engines must be refurbished after every flight.

Engine development

"The success of our architecture depends on the success of NASA's engine development programme," says Young. The space agency is funding work on four main engine candidates, two hydrogen-fuelled and two kerosene-fuelled. Pratt & Whitney and Aerojet are developing the Cobra, a 600,000lb-thrust (2,670kN) hydrogen-fuelled, staged-combustion, first- and second-stage engine, while Boeing's Rocketdyne division is working on the 650,000lb thrust-class RS-83. Rocketdyne is also pursuing the RS-84, a kerosene-fuelled, staged-combustion, first-stage engine generating 1,100,000lb thrust, while TRW is developing the 1,000,000lb thrust-class TR107.

The plan is to test two prototype engines at a cost of $1.3 billion. "NASA will go for prototype engines that bracket the requirements of the three contractors," says Ford. He suggests the emphasis has shifted towards the kerosene-fuelled engines. "NASA wants to address kerosene first to reduce risk," he says. The USA has little experience with kerosene-burning rocket motors, having focused for decades on cryogenic engines.

Despite the prospect for postponement of the system requirements review, and a knock-on delay in the planned downselect to two competing architectures, all three companies believe NASA remains on the right track to make a decision in 2006 on whether to begin full-scale development of a Shuttle replacement. "NASA is doing its homework, getting to know the cost of the requirements," says Young. "That takes a little time to do, but once it's done they will be on the right path."

Source: Flight International