Each of the four RS-25 engines currently positioned at the base of NASA’s Space Launch System has gone to space many times before, and each has an interesting story to tell. One first flew in 1998, boosting astronaut John Glenn to orbit. Soon, if all goes well, these veteran boosters will propel NASA into the Artemis era.
NASA’s Space Launch System is the most powerful rocket ever built, capable of lifting more than 57,320 pounds (26 metric tons) of cargo and crew to the Moon. Future configurations could see the rocket lift as much as 99,208 pounds (45 metric tons). It’s an engineering marvel—at least we hope so—with its maiden voyage scheduled for this Saturday at 2:17 p.m. ET. But as NASA takes a bold leap into the Artemis era and a steady succession of increasingly sophisticated missions to the lunar environment, it’s important to remember that SLS is a new rocket that’s made from a bunch of old parts.
The fully integrated heavy-lift launch vehicle is a tidy amalgam of previous NASA launch systems, most especially the Space Shuttle, which the U.S. retired in 2011. In fact, components from 83 Space Shuttle missions have been cobbled together to build SLS and the Orion crew capsule. It uses the Space Shuttle Main Engine, now known as the RS-25 engine, built by Aerojet Rocketdyne, while its two extended solid rocket boosters were likewise borrowed from Shuttle. An engine that previously maneuvered the Shuttle also found its way into Orion.
As Congress told NASA back in 2010, the new rocket and crew capsule was to be built with “space shuttle-derived components...that use existing United States propulsion systems, including liquid fuel engines, external tank or tank-related capability and solid rocket motor engines.” With this in mind, and not wanting to waste good, flight-proven hardware, NASA stripped the retired shuttles of their main engines and stored them for safekeeping. This was in keeping with the directive to make good use of heritage hardware when building SLS, “to save cost and expedite the schedule,” according to the space agency.
That the RS-25 is both powerful and reliable is not in doubt. The engines were upgraded five times over the course of the Shuttle program, during which time they participated in 135 missions, ignited across more than 3,000 starts, and remained powered over the course of 1 million seconds during both ground tests and flight operations. In total, NASA accumulated an inventory of 16 RS-25D engines from the Shuttle program to support the first four SLS missions. Of these 16 engines, only two have never gone to space.
The Space Shuttle was equipped with three RS-25 engines, whereas SLS has four. Fueled by liquid hydrogen and liquid oxygen, the four engines are arranged roughly in a square to ensure stability and equal distribution of force during liftoff. Each RS-25 engine can produce 2 million pounds of thrust, which, combined with the two five-segment solid rocket boosters, will offer 8.8 million pounds of thrust at launch. During the Shuttle era, the RS-25s operated at 104.5% of rated thrust (491,000 pounds vacuum thrust), but for SLS, these engines have been modified such that they’ll operate at 109% of rated thrust (512,000 pounds vacuum thrust), NASA says.
“For SLS, the engines will experience increased propellant inlet pressures and temperatures,” according to NASA. “In addition, the existing inventory is receiving new engine controllers with contemporary avionics, and new exhaust nozzle insulation for the higher heating environment.”
The current SLS configuration is known as Block 1, and it includes four very experienced RS-25 engines. For the inaugural flight of SLS, NASA will use engines E2045, E2056, E2058, and E2060. In total, these four engines participated in 21 Shuttle flights across three decades.
The first engine, E2045, is the most experienced of the bunch, having flown on 12 Shuttle missions. It first flew in January 1998 during the STS-89 mission, while its final Shuttle flight occurred in July 2011 during the STS-135 mission. Astronaut John Glenn experienced the power of E2045 first hand in 1998 when he flew as part of the STS-95 mission.
The second engine, E2056, is the veteran of four Shuttle flights (including STS-114—the first mission after the Columbia disaster), while the third engine, E2058, participated in six flights. The fourth engine, E2060, is the least experienced of the bunch, having flown on three missions, including STS-135—the final Shuttle mission.
It’s that third engine that engineers blamed for the launch scrub on Monday, August 29, when it failed to reach the ultra-cold temperatures required for launch, but the team later traced the problem to a faulty sensor. As John Blevins, SLS chief engineer, told reporters yesterday, there’s nothing wrong with E2058, as engineers were able to confirm the “good flow” of cooling propellant through engine #3.
For each of the 16 RS-25 engines left over from the Shuttle era, their next flight will be their last. SLS is an expendable rocket, with the core stage expected to splash down in the Pacific Ocean (the side boosters will crash into the Atlantic). Once NASA exhausts its inventory of RS-25D engines, the space agency will switch to RS-25E engines currently being built by Aerojet Rocketdyne. The new engines will cost around 30% less than the previous engines and provide 111% of rated thrust (521,000 pounds vacuum thrust).
NASA needs SLS for the upcoming Artemis missions to the Moon. The heavy-lift system will play a key role during Artemis 2, in which a crewed Orion capsule will venture to the Moon and back in late 2024, and also Artemis 3, the first crewed Moon landing since the Apollo era. The Artemis missions are also meant to prepare NASA and its partners for the first human journey to Mars, for which SLS is expected to play a key role.
As exciting as all this sounds, the price tag may simply be too high. Since 2011, NASA has spent more than $50 billion in development costs for SLS and Orion, according to the Planetary Society. But to operate SLS, NASA’s inspector general estimates that it’ll cost NASA upwards of $4.1 billion per launch for each of the first four Artemis missions—a price that inspector general Paul Martin has described as “unsustainable.”
Through its Artemis program, NASA is seeking a permanent and sustainable return to the Moon. If this is to happen, however, NASA will need to rein in the soaring costs.