As SLS’s Block 1 design matures, its capabilities come into clearer focus

Curt Godwin

NASA has been touting its Space Launch System (SLS) as the “most powerful rocket ever built” since its inception, and initially listed its payload capability to low-Earth orbit (LEO) as 154,324 pounds (70 metric tons). Though less than what the Saturn V could launch when it was active, it was a significant increase over the Space Transportation System’s – better known as the Space Shuttle – 60,600 pounds (27.5 metric tons) offered.

Indeed, when SLS began its development, United Launch Alliance’s (ULA) Delta IV Heavy was the most capable launcher in the U.S. fleet, with a 63,470 pound (28.79 metric ton) capability to LEO. Therefore, with an ability to loft more than twice the payload of its closest stablemate, SLS would clearly meet the agency’s need for a vehicle capable of supporting deep-space human exploration.

However, with the rapid growth of NewSpace companies like SpaceX and Blue Origin, many pundits began to wonder if there was a need for a rocket like SLS. In fact, when SpaceX launched its Falcon Heavy – with a LEO capability listed as 140,700 pounds (63.8 metric tons) in its fully disposable configuration – those voices became even louder.

NASA continued to position its super-heavy-lift rocket as a deep-space vehicle suited to the agency’s goal of sending crews to the Moon and Mars. NASA even changed SLS’s stated capabilities from LEO to an exploration-focused Translunar Injection (TLI) rating.

With the new metrics in effect. SLS in its Block 1 configuration was advertised of being able to deliver more than 57,320 pounds (26 metric tons) to TLI, replacing the original LEO numbers on the agency’s infographics. Since SLS is designed to deliver crew and cargo to destinations beyond LEO, this shift made sense.

Nevertheless, SLS was thought to fly only once in its initial Block 1 configuration. Later missions, including crewed flights to the Moon and the launch of the Europa Clipper to the Jovian system, could use the upgraded Block 1B model. This upgrade could boost the vehicle’s capabilities by more than 17,637 pounds (8 metric tons) to TLI, and if everything goes as it is currently planned – would serve as the workhorse of the SLS fleet.

However, when it became apparent that the Block 1 design might make more than a single flight — including the mission to Europa — some began to voice concern that the Block 1 design would not be capable of delivering the 13,277-pound (6 metric ton) spacecraft to its destination, negating the push to use SLS in the first place.

Spaceflight Insider (SFI) reached out to public affairs personnel involved with the SLS program at NASA’s Marshall Space Flight Center (MSFC) to find out if these concerns were warranted. Could SLS, in its Block 1 design, still deliver the spacecraft directly to Europa without the need for time-consuming gravity assist maneuvers?

According to the agency: “NASA’s early analyses of launch windows for Europa Clipper in 2022, 2023, 2024, or 2025 indicate that direct trajectories are feasible for SLS Block 1. Further analyses are expected to confirm these early findings with the possibility of some minor configuration modifications. June 2022 is the earliest launch window. Each launch window is about 30 days, each window can get the spacecraft to Europa in 2.5 to 2.7 years, and each window can support a mass for Europa Clipper of at least 6 metric tons.”

It would appear, then, that SLS’s Block 1 would be capable of shortening the cruise duration by more than half when compared to missions like Galileo. That spacecraft, launched from the payload bay of Space Shuttle Atlantis and took nearly six years to reach Jupiter.

While a comparison between NASA’s SLS Block 1 and SpaceX’s Falcon Heavy is often made, the gulf between the two has actually widened. As the Block 1 design has matured, the agency has refined the vehicle’s capabilities by a significant amount. Though NASA prefers to position SLS as a deep-space rocket, SFI sought a clarification of the vehicle’s capabilities to a more common destination for rockets: low-Earth-orbit (LEO).

NASA replied: “Now that the SLS design has matured and the program has more data as a result of progress with hardware manufacturing and testing, our current analysis shows the Block 1 configuration of SLS can deliver an estimated mass of 95 metric tons (209,439 pounds) to low-Earth orbit based on a 200 by 200-kilometer orbit with a 28.5 degree inclination, which is a commonly used orbit in the industry for estimating performance.”

This represents nearly 50 percent more payload to LEO than the next-closest launch vehicle, SpaceX’s Falcon Heavy. While SLS might not be aiming for LEO as its destination, it further highlights the difference between the two vehicles.

The agency continues to ready SLS for its debut flight, which is currently scheduled to take place in 2020, with the massive liquid hydrogen test stand at MSFC gearing up for element testing in December of this year (2018). Stennis Space Center in Mississippi, meanwhile, eyes 2019 for the core stage’s green run test.

Regardless of when SLS evolves to its Block 1B and Block 2 configurations, it would appear that the progenitor of NASA’s next series of deep space rockets should be more than capable of the roles laid out for it to complete.