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Foust Forward | A tale of two launch vehicle programs

SN8 upon landing

There was no clearer set of contrasts between how SpaceX and NASA approach launch vehicle development than the dueling tests the two performed in early December of Starship and Space Launch System, respectively.

It was hard to miss the Dec. 9 test flight of SpaceX’s Starship SN8 prototype, the first time the vehicle flew more than a couple hundred meters off the pad. It soared into the Texas sky propelled by its three Raptor engines, then descended back to the pad — only to explode after coming in a little too fast. The flight won praise from its fans, but that explosive end prompted a backlash from critics.

Two days earlier, NASA started a test of the SLS core stage at the Stennis Space Center. There would be no launch that day, or even an ignition of the stage’s four RS-25 engines, just loading the tanks of the giant stage with liquid hydrogen and liquid oxygen. But even that modest step did not go as planned: engineers stopped the test when the liquid oxygen flowing into the tank was a few degrees warmer than expected.

The two events clearly illustrate the differences in development philosophies between the two organizations. SpaceX’s approach is the literal manifestation of the Silicon Valley philosophy to “move fast and break things,” in this case Starship prototypes. The SN8 prototype that met its demise was the latest in a long line of test articles, and hardly the first to have exploded, burst or crumpled.

NASA’s approach to testing SLS has been far more cautious. While SN8 was simply a prototype never intended to fly to space, the core stage at Stennis is flight hardware that NASA will use for the Artemis 1 mission in late 2021. In a call with reporters, NASA managers emphasized they were being deliberate and cautious during the ongoing Green Run test campaign, not wanting to take any risks with the hardware.

It’s tempting to look at that slow pace and ask, “Why can’t NASA be more like SpaceX?” That’s certainly what many fans of SpaceX have wondered, looking at the relatively rapid development of Starship and comparing it to the snail’s pace of SLS. In their view, the SpaceX approach is clearly better than how NASA is developing SLS.

But it’s hard to see NASA adopting a Starshiplike approach to SLS development. The tolerance for risk is very different for a government program like SLS, with a budget of more than $2 billion a year and a large number of stakeholders in government and industry, than for a private program that answers ultimately to just one person, Elon Musk. SLS stages exploding on test stands would invite not just bad press but also congressional scrutiny.

Yet the two efforts are not nearly as different as they might appear. While NASA is being very careful with the core stage on the stand at Stennis, the agency tested other SLS hardware to destruction to confirm its structural strength matched models. (One NASA release about such a test emphasized a propellant tank was burst “on purpose,” possibly to preempt any criticism about the test being a failure.)

SpaceX, meanwhile, is not as reckless as all the dramatic explosions during testing would lead some to assume. People who closely track all the activities at Boca Chica have documented many test attempts, like static firings of Starship prototypes, that were scrubbed for one reason or another. It’s a sign of measured risk taking by SpaceX even as the Starship effort has fallen behind Musk’s aggressive schedule. At an event at Boca Chica last fall, he expected the first high-altitude test flight to take place in one or two months, but Starship SN8 finally flew 14 months later.

Expecting SLS development to move faster is probably just as unlikely as expecting that Starship development will have fewer explosions. Each is optimized for the risk tolerance of their organizations and those funding them. Whether either approach is optimized for long-term success, though, remains to be seen.



Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. His Foust Forward column appears in every issue of the magazine. This column ran in the Dec. 14, 2020 issue.


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Our Space Launch System Rocket’s “Green Run” Engine Testing By the Numbers


We continue to make progress toward the first launch of our Space Launch System (SLS) rocket for the Artemis I mission around the Moon. Engineers at NASA’s Stennis Space Center near Bay St. Louis, Mississippi are preparing for the last two tests of the eight-part SLS core stage Green Run test series.

The test campaign is one of the final milestones before our SLS rocket launches America’s Orion spacecraft to the Moon with the Artemis program. The SLS Green Run test campaign is a series of eight different tests designed to bring the  entire rocket stage to life for the first time.

As our engineers and technicians prepare for the wet dress rehearsal and the SLS Green Run hot fire, here are some numbers to keep in mind:


212 Feet

The SLS rocket’s core stage is the largest rocket stage we have ever produced. From top to bottom of its four RS-25 engines, the rocket stage measures 212 feet.


35 Stories

For each of the Green Run tests, the SLS core stage is installed in the historic B-2 Test Stand at Stennis. The test stand was updated to accommodate the SLS rocket stage and is 35 stories tall – or almost 350 feet!


4 RS-25 Engines

All four RS-25 engines will operate simultaneously during the final Green Run Hot Fire. Fueled by the two propellant tanks, the cluster of engines will gimbal, or pivot, and fire for up to eight minutes just as if it were an actual Artemis launch to the Moon.


18 Miles

Our brawny SLS core stage is outfitted with three flight computers and special avionics systems that act as the “brains” of the rocket. It has 18 miles of cabling and more than 500 sensors and systems to help feed fuel and direct the four RS-25 engines.


773,000 Gallons

The stage has two huge propellant tanks that collectively hold 733,000 gallons of super-cooled liquid hydrogen and liquid oxygen. The stage weighs more than 2.3 million pounds when its fully fueled.


114 Tanker Trucks

It’ll take 114 trucks – 54 trucks carrying liquid hydrogen and 60 trucks carrying liquid oxygen – to provide fuel to the SLS core stage.


6 Propellant Barges

A series of barges will deliver the propellant from the trucks to the rocket stage installed in the test stand. Altogether, six propellant barges will send fuel through a special feed system and lines. The propellant initially will be used to chill the feed system and lines to the correct cryogenic temperature. The propellant then will flow from the barges to the B-2 Test Stand and on into the stage’s tanks.


100 Terabytes

All eight of the Green Run tests and check outs will produce more than 100 terabytes of collected data that engineers will use to certify the core stage design and help verify the stage is ready for launch.

For comparison, just one terabyte is the equivalent to 500 hours of movies, 200,000 five-minute songs, or 310,000 pictures!


32,500 holes

The B-2 Test Stand has a flame deflector that will direct the fire produced from the rocket’s engines away from the stage. Nearly 33,000 tiny, handmade holes dot the flame deflector. Why? All those minuscule holes play a huge role by directing constant streams of pressurized water to cool the hot engine exhaust.


One Epic First

When NASA conducts the SLS Green Run Hot Fire test at Stennis, it’ll be the first time that the SLS core stage operates just as it would on the launch pad. This test is just a preview of what’s to come for Artemis I!

The Space Launch System is the only rocket that can send NASA astronauts aboard NASA’s Orion spacecraft and supplies to the Moon in a single mission. The SLS core stage is a key part of the rocket that will send the first woman and the next man to the Moon through NASA’s Artemis program.


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