The promise of methane

Both SpaceX and Blue Origin are developing new engines that use liquid methane for propellant. The RS-25, on the other hand, uses liquid hydrogen, while many other first-stage engines use RP-1, a blend of refined kerosene. All of these fuels must be combined with liquid oxygen to burn, similar to the way an automobile engine needs both gasoline and air.

What's the difference?

Hydrogen is the first element on the periodic table, meaning it's the lightest. It releases the most energy when burned, and it burns clean, which, in the space shuttle's case, made it good for reusability.

The Atlas V RD-180 uses RP-1. So does SpaceX's Falcon 9 Merlin. RP-1 is less efficient and heavier, but on the other hand, it's denser, so your fuel tank can be smaller. And at sea level, it actually produces more thrust. It's also cheaper and easier to deal with, since it doesn't need to be supercooled like liquid hydrogen.

So which is better? "It all depends on what you need out of a stage," said Steve Wofford. "Liquid hydrogen was the best propellant for the SLS stage and what we needed it to do, in combination with the solids. For a single stage to orbit, hydrogen is the best option."

If, on the other hand, your rocket uses multiple stages during its climb to orbit, RP-1 might be a better bet for the core stage. The Atlas V and Falcon 9 are in good company with this approach—the mighty Saturn V used RP-1 at liftoff.

The efficiency of a rocket engine—essentially, how much change in velocity it delivers per quantity of propellant consumed—is measured in seconds of specific impulse, a metric known as Isp. The RD-180, which runs on RP-1, has a sea-level Isp of 311 seconds. The RS-25, burning hydrogen, is more efficient, at 366 seconds.

An in-between option is liquid methane. Glenn Case told me that in theory, methane offers about ten more seconds of Isp over RP-1. "Engine designers will kill for a few extra seconds of Isp," he said.

Methane doesn't need to be as cold as hydrogen, which makes it easier to work with. It also burns cleaner than RP-1, making it attractive for reusable engines. SpaceX is using methane for their new Raptor engine, while Blue Origin chose it for their upcoming BE-4. Both of those engines will also employ closed-cycle designs, giving them increased performance.

Cost

Depending on whom you ask, NASA paid about $60 million each for its current fleet of RS-25 engines. Coincidentally, that's about the same price SpaceX advertises for an entire Falcon 9 flight.

SLS and the Falcon 9 are two very different launch vehicles bound by separate programmatic and technical constraints. But there's no way around the glaring truth that SLS is incredibly expensive.

Aerojet says they plan to reduce the cost of the RS-25 by 33 percent. The savings will come from modernizing production lines to increase efficiency, as well as advances in materials science. Steve Wofford said engineers can now forge and cast large engine parts more precisely, rather than casting rough shapes and spending thousands of hours machining them to their final forms. "We're taking advantage of 40 years' worth of manufacturing lessons learned in building this engine," he said.

Another cost-saving technique is the utilization of additive manufacturing, or 3-D printing.

"My office is really leading the charge across NASA—and in many respects, across industry—in additive manufacturing technology as applied to rocket engines," Steve Wofford said.

So is a 33 percent cost reduction realistic? "I certainly think it would be doable," Glenn Case said, while noting that every manufacturing change is a potential trade-off in terms of demonstrated flight reliability, since new components must be shown to be as sound as the ones they replace. "In the rocket business, we are a slow folk to change," he said.