NASA has said that one of the strengths of its Space Launch System rocket is that the massive booster, in part, uses legacy hardware. These proven technologies, such as the space shuttle's main engines and the side-mounted rocket boosters, provide the agency with confidence that when it finally flies, the SLS will be reliable.

However, one problem with legacy hardware, built by traditional contractors such as Orbital ATK and Aerojet Rocketdyne, is that it's expensive. And while NASA has not released per-flight estimates of the expendable SLS rocket's cost, conservative estimates peg it at $1.5 to $2.5 billion per launch. The cost is so high that it effectively precludes more than one to two SLS launches per year.

The space agency recognizes this problem with its rocket, and in the past it has solicited ideas on how best to cut the production and operations costs for its SLS rocket. Now, the agency appears to be actively considering alternative hardware, including the use of potentially lower-cost engines from a new space rocket company, Blue Origin.

Exploration Upper Stage

The SLS rocket consists of side boosters to give it a kick off the launch pad and a core stage to power the system into orbit. It will also have an upper stage to shove the rocket's payload out of low-Earth orbit and on to its ultimate destination, whether it is the Moon, Mars, or one of Jupiter's moons, Europa.

For its maiden launch, likely in 2020, the SLS rocket will use a makeshift upper stage. However, NASA is developing an "Exploration Upper Stage" to give future variants of the SLS more lift capacity, and this new upper stage will debut on the rocket's second flight, which may occur in 2023 or 2024. Initially, this new stage will use four RL-10 engines, which have a long, proud heritage since their first flight in 1961.

But these engines, manufactured by Aerojet Rocketdyne, are also costly. (Ars understands that NASA paid an average of $17 million for each RL-10 engine for the maiden Exploration Upper Stage vehicle). So in October, to power the EUS, the space agency issued a request for information to the aerospace community for "a low cost drop-in replacement engine to minimize program cost." According to the document, the initial set of four engines would be needed in mid-2023 to prepare for the third flight of the SLS rocket, known as Exploration Mission-3.

Then, after an extension of the deadline for responses beyond mid-November, NASA revised the RFI on December 1. The revised document no longer seeks a "drop-in replacement" for the RL-10 engine, rather it asks for a "low-cost replacement engine." Although this seems like a subtle change, sources within the aerospace industry indicated to Ars that it is significant. According to NASA, it was done to increase the number of responses.

"The original RFI was issued as part of an effort to reduce costs and improve affordability for the block 1B version of the Space Launch System rocket and focused on the EUS engine itself," an agency spokeswoman, Kathryn Hambleton, told Ars. "The revision takes a more system-level performance view to open up the field of possible responses while still ensuring compatibility with EUS design."

Opens door to Blue Origin

The original wording of the RFI could be seen as a message of sorts for Aerojet, the engine manufacturer, that NASA expected to get a discount on future RL-10 engines. However, by striking "drop-in replacement" for a more generic replacement, NASA has signaled it's now open to an architecture that uses fewer than four engines.

That would probably include Blue Origin's BE-3U engine, which the company plans to use for its upper stage on the New Glenn heavy lift rocket. This is a modified version of the BE-3 engine that powers the New Shepard rocket, which has now flown successfully seven times. Blue Origin has previously marketed the BE-3U to Orbital ATK for its Next Generation Launch System, which is looking for an upper stage engine. A single BE-3U provides about 120,000 pounds of thrust, which exceeds the 100,000 pounds of thrust provided by four RL-10 engines.

It is not clear which companies responded to NASA's RFI, which had a deadline of Friday, December 15. However, this seems to be an effort to provide the agency with options to enhance the affordability and operations of SLS and to open the door to new technologies and new space companies for the massive rocket.