Spaceflight Insider

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

SLS launch

Artist’s rendition of NASA’s Space Launch System (SLS) in flight. Image Credit: Nathan Koga / SpaceFlight Insider

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.

An artist’s rendering of the SLS being assembled inside the massive Vehicle Assembly Building at Kennedy Space Center, Florida. Image Credit: NASA

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.

This artist's rendering shows a concept for a future NASA mission to Europa in which a spacecraft would make multiple close flybys of the icy Jovian moon, thought to contain a global subsurface ocean. Image Credit: NASA/JPL-Caltech

This artist’s rendering shows a concept for a future NASA mission to Europa in which the spacecraft would make multiple close flybys of the icy Jovian moon, thought to contain a global subsurface ocean. At present, the mission is slated to be launched atop the agency’s new Space Launch System rocket. Image Credit: NASA/JPL-Caltech





Curt Godwin has been a fan of space exploration for as long as he can remember, keeping his eyes to the skies from an early age. Initially majoring in Nuclear Engineering, Curt later decided that computers would be a more interesting - and safer - career field. He's worked in education technology for more than 20 years, and has been published in industry and peer journals, and is a respected authority on wireless network engineering. Throughout this period of his life, he maintained his love for all things space and has written about his experiences at a variety of NASA events, both on his personal blog and as a freelance media representative.

Reader Comments

How much you wanna bet that the Spacex BFR beats the SLS to launch?

How much wanna bet that the SpaceX fans cry bitter tears when it exceeds all expectations?

I wonder how many BFRs or New Glenns you could get for the cost of a single SLS.

Every time I see the SLS I think, another year and another 2 billion (or is it 3 billion with Orion?) that could have helped humanity acquire advanced circular systems which could solve sustainability issues here on Earth as well as in space. I understand that the BFR is not yet constructed and that many jobs are on the line but either way this will mean a decade delay in solving these issues, it’s not like there’s unending time for these solutions to arrive with or without belief in climate change. Are we to expect Elon Musk and his companies to develop every single piece of essential 21st century technology before the rest hop on the bandwagon? Right now, that’s what it feels like.

SLS. Might see one flight and then goodbye. It’s obsolete already.

This is the worst kept secret of the SLS program. NASA grossly oversized the SLS core stage relative to the requirements of the NASA Authorization Act of 2010 Section 302. That’s why the project is 5 years behind schedule and $10 billion over budget.

The worst part is that the extra performance to LEO is almost useless because iCPS is so small. Dragging that massive core stage higher so that iCPS can do TLI is grossly inefficient. EUS would be better but won’t be available until 2024 at the earliest.

Block 1 performance to TLI, Mars, or Jupiter is only about 20% better than FH. But FH is available now, and is 90% cheaper.

Gary, you know they’ll just hate it all the more if it succeeds.

Nathan, depends what you mean by success. Will it fly, I expect that it will. Is it of much use in the 21st century, that is questionable at best. I can’t say I hate the SLS, it’s more like profound disappointment at this point.

“Success” for the SLS is measured by the vast amounts of Taxpayer dollars it consumes and lobbyists it keeps employed.

James Lunar Miner

Launch on need fast backup crew rescue capabilities in LEO and across all of CisLunar Space is one of many important uses for the SLS and International Orion.

The SLS will probably be an early user of highly efficient 900+ Isp nuclear thermal powered and super efficient large 4000+ Isp electric propulsion upper stages that can significantly improve the cargo and other payload hauling capability of the SLS to MEO, GEO, the Moon, asteroids, and Mars.

If or when metallic hydrogen becomes available, the evolvable SLS will probably make full use of that future propellant that could offer 900 Isp to 1,700 Isp performance options and a massive increase for the SLS in putting payloads into LEO and TLI.

The SLS enjoys broad national and international political support and is an open architectural design that is expected to evolve in different ways, including ways that could significantly increase its LEO payload performance while also diminishing its environmental impact.

Full reusability reduces by about 50% a launcher’s LEO payload when compared to a similar but non-reusable launcher. Reusable launchers do about twice the direct damage to the Ozone Layer when compared to similar but non-reusable launchers.

If Congress wants a reusable vertical landing SLS core, NASA might add a centrally located and relatively cheap, simple, and easy to start hydrolox expander bleed cycle 331,000 lb thrust LE-9 rocket engine to the base of the core of the SLS. Other options for SLS reusability should also be doable.

James Lunar Miner

The super high 5,000 Isp of efficient solar or nuclear VASIMR electric propulsion system powered upper stages and OTVs (Orbital Transfer Vehicles) could be used for efficient cargo hauling missions to greatly increase the MEO, GEO, and Low Lunar Orbit payload capabilities of the SLS, New Glenn, OmegA, Vulcan, Falcon Heavy, China’s Long March 9, Japan’s H3, Russia’s Soyuz-5 Super Heavy, Europe’s Ariane 6, and other launchers when compared to energy weak and much lower Isp chemical rocket powered upper stages and OTVs with Isps in the 350 to 465 range.

“NASA envisions delivering about 34 metric tons of useful cargo to LLO in a single flight with a chemically propelled vehicle. To make that trip, about 60 metric tons of LOX-LH2 propellant would be expended. A comparable OTV would employ 5 VF-200 engines powered by a 1 MW solar array. To do the same job, a VASIMR-powered OTV would need to expend only about 8 metric tons of argon propellant. The total mass of such an electric OTV would be in the range of 49 t (outbound & return fuel: 9 t, hardware: 6 t, cargo 34 t).”

From: ‘Variable Specific Impulse Magnetoplasma Rocket’ Wikipedia

Is any business or nation likely to plan on always using “60 metric tons of LOX-LH2 propellant” to transfer 34 metric tons of Moon Village or mining equipment cargo to a Low Lunar Orbit (LLO) when the other and much more efficient, cheaper, and environmentally lower impact option is to use only “about 8 tons of argon propellant” to do the same task?

James Lunar Miner

SLS Nuclear thermal rocket engine notes:

“Reuter noted that in addition to the Restore-L increase, the report accompanying the bill directed NASA to spend $75 million on nuclear thermal propulsion technology, versus $35 million the agency planned to spend.”

From: ‘NASA weighs changes to space technology programs and organization’ By Jeff Foust 3/27/2018

“An additional beyond-LEO engine for interplanetary travel from Earth orbit to Mars orbit, and back, is being studied as of 2013 at Marshall Space Flight Center with a focus on nuclear thermal rocket (NTR) engines.[73] In historical ground testing, NTRs proved to be at least twice as efficient as the most advanced chemical engines, which would allow quicker transfer time and increased cargo capacity. The shorter flight duration, estimated at 3–4 months with NTR engines,[74] compared to 6–9 months using chemical engines,[75] would reduce crew exposure to potentially harmful and difficult to shield cosmic rays.[76][77][78][79] NTR engines, such as the Pewee of Project Rover, were selected in the Mars Design Reference Architecture (DRA).”

From: Space Launch System’ Wikipedia

Posts about a launcher’s beyond LEO payloads and Lunar mission capabilities should include the context of the efficient payload increasing options for using Nuclear Thermal Rocket powered upper stages, OTVs, and Landers.

Various nuclear power based propulsion systems are critical for efficiently building and quickly expanding human and robot colonies on the Moon and across our Solar System.

James Lunar Miner

What important and tangible benefits will justify and sustain the large business investments and national and international expenditures implied in regular SLS launches of the Orion and many other launchers and spacecraft that will be needed to send humans, robots, and cargo to contribute to building the international Moon Village and finding and mining Lunar resources?

The clearest answer is found from gaining and maintaining new systems to help us effectively deal with two unfortunate realities that are far more critical than any human Mars missions.

“More than 90 percent of the global population is breathing in high levels of pollutants, the World Health Organization said Wednesday, blaming poor air quality for some seven million deaths annually.”
From: ‘Nine out of 10 people breathing polluted air: WHO’
By Nina Larson May 2, 2018

“The research indicates we need a complete energy overhaul by 2050 to avoid catastrophic climate change, but at our current rate of progress it’s not going to take another 30 years – it’s going to take another 363 years.”

From: ‘We’ll Need Another 363 Years to Completely Revolutionize Earth’s Energy Systems’
By David Nield 3/19/2018

Many scientists and engineers have long understood that using Lunar resources and industrial capabilities to build large Space-based Solar Power systems (SBSP) in LEO, GEO, and high Earth orbit could help us to efficiently provide clean energy to everyone on the Home Planet.

The SLS and many other launchers will be essential in this massive and long-term effort to use the Moon’s resources to help us decarbonize our Earth’s energy systems.

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