Grading on a suborbital curve
by Jeff Foust
|That superposition of failure and success comes to mind with last week’s long-awaited first integrated launch of SpaceX’s Starship vehicle. It would, most definitely, be a test flight.|
When it comes to test launches, though, those shades of gray become a kaleidoscope. Even with ground tests and extensive modeling, the only way to fully test a launch vehicle is to launch it, accepting that there is a chance that something will keep the rocket from reaching orbit. That would make the launch a failure, in the sense it could not complete its mission and place any payload into orbit. Yet the data collected might be entirely satisfactory for engineers to revise the vehicle’s design and ensure future vehicles do make it to orbit: a success in the long run.
That superposition of failure and success comes to mind with last week’s long-awaited first integrated launch of SpaceX’s Starship vehicle. The company had flown the Starship upper stage several times on low-altitude test flights—most ending with crashes and/or explosions—but this would be the first time that it would lift off atop its Super Heavy booster with 33 Raptor engines. It would, most definitely, be a test flight.
It would not even be an orbital test flight. While SpaceX had referred to the launch as an orbital flight test for some time, it switched to calling it an integrated flight test in the weeks closer to launch, reflecting the true nature of the flight. In all went according to plan, Starship would complete less than one orbit of the Earth, splashing down off the Hawaii coast 90 minutes after liftoff without performing any orbit insertion or deorbit burns. An FAA official, talking to reporters in a background briefing in the days before launch, called the flight a “nearly orbital” trajectory that would reach a peak altitude of about 235 kilometers.
Of course, the launch did not go according to plan. The first launch attempt was scrubbed less than ten minutes before the scheduled liftoff April 17 at 8:20 am local time from Boca Chica, Texas, when a valve that is part of the system used to pressurize tanks in the first stage became stuck. The company kept the countdown going to the T-40 second mark, using the remaining minutes as a wet dress rehearsal for the vehicle. SpaceX later said moisture in the valve froze.
SpaceX initially said they would need 48 hours before making another launch attempt in order to replenish supplies of liquid oxygen and methane propellants, then pushed it back another day to April 20. Crowds returned to the tip of South Padre Island, the closest public viewing site for the launch, and this time the valves behaved. At 8:33 am CDT, the Raptor engines fired up and, at T+6 seconds, the vehicle started to ascend.
It was clear almost immediately, though, that the launch was not going precisely as planned. SpaceX, to its credit, displayed graphics on its webcast that showed the vehicle’s speed, altitude, propellant levels, and the status of its engines. When that graphic first appeared at T+15 seconds, three of the Raptor engines were off: two next to each other in the outer ring and one engine in the center that, unlike those in the outer ring, can gimble for attitude control.
|“We should have had separation by now,” SpaceX’s Insprucker said at T+3:45. “Obviously, this does not appear to be a nominal situation.”|
As the flight continued, more Raptor engines shut down, with at least six out of service at some phases of flight. There was also debris that came off near the base of the booster at about T+30 seconds, with some flaring visible in the plume. Yet the rocket continued to accelerate and ascend, powered by its remaining Raptor engines and, seemingly, the ever-louder cheers of SpaceX employees at the company’s Hawthorne, California, headquarters that drowned out the commentary on the launch webcast.
The vehicle powered through max-Q, or maximum dynamic pressure, and continued to fly, but by about two and a half minutes into the flight the vehicle began a slow tumble. The webcast commentators at first suggested this was part of stage separation activities, with Starship set to separate from Super Heavy at T+2:52, according to the timeline SpaceX provided before launch.
That time came and went, though, without a shutdown of the remaining Super Heavy engines or stage separation. The slow tumble continued, but the vehicle remained intact. The cheers in Hawthorne faded. “We should have had separation by now,” acknowledged SpaceX’s John Insprucker on the webcast at T+3:45. “Obviously, this does not appear to be a nominal situation.”
Seconds later, just before the T+4 minute mark, the vehicle burst into a white cloud with only a faint hint of a fireball. (SpaceX later said it triggered the flight termination systems on both Super Heavy and Starship.) The first integrated flight of Starship and Super Heavy was over, far short of that planned splashdown near Hawaii. Yet, counterintuitively, cheers erupted again from Hawthorne. “Everyone here is absolutely pumped to clear the pad and make it this far into the test flight,” Siva Bharadvaj, another host of the SpaceX webcast, explained.
Starship/Super Heavy in flight. The image shows that several Raptor engines are not working at this phase of flight. (credit: SpaceX)
Were the SpaceX employees, not to mention their fans gathered in person on South Padre Island or virtually online, right to cheer a launch that ended explosively four minutes into a 90-minute flight? SpaceX CEO and founder Elon Musk thought so. “Congrats @SpaceX team on an exciting test launch of Starship!” he tweeted shortly after the launch. “Learned a lot for next test launch in a few months.”
Congratulations came from outside the company as well. “Congrats to SpaceX on Starship’s first integrated flight test!” said NASA administrator Bill Nelson in a tweet shortly after the flight. “Every great achievement throughout history has demanded some level of calculated risk, because with great risk comes great reward. Looking forward to all that SpaceX learns, to the next flight test—and beyond.”
|“Everything is proceeding per the plan. We need those guys to get off the ground. We need them to get uncrewed demos done,” NASA’s Kshatriya said.|
NASA, of course, has a very strong interest in Starship, having selected that vehicle to serve as the lunar lander for at least the first two crewed Artemis landings, Artemis 3 and 4, under contracts with a combined value of more than $4 billion. The test flight was not an official milestone in those contracts but one that the agency was nonetheless watching closely.
“They’re doing a lot of great work down in Boca right now,” Amit Kshatriya, director of NASA’s new Moon to Mars Program Office, said at a briefing during the Space Symposium in Colorado Springs hours after the April 17 scrub. He added that NASA “certainly can empathize” with SpaceX given the setbacks the agency suffered getting the first Space Launch System vehicle off the pad last year.
“Everything is proceeding per the plan. We need those guys to get off the ground. We need them to get uncrewed demos done,” he added.
What effect this test will have on that plan, and the overall Artemis schedule, is unclear. A NASA manifest released in March along with its fiscal year 2024 budget request projected Artemis 3 launching in December 2025, giving SpaceX only about two and a half years to get Starship ready to carry astronauts to the lunar surface. That will likely require dozens of test flights to demonstrate the performance of the overall launch system as well as cryogenic refueling of the lunar lander Starship.
Of course, that assume the launch isn’t delayed for other reasons: NASA currently has Artemis 2 scheduled for launch in November 2024, which means it thinks it can turn around ground systems and other elements of Artemis in about a year after projecting two years between Artemis 1 and 2. Nelson, testifying before House appropriators the day before the launch, hinted that Artemis 3 could slip into 2026.
|“This is a very risky flight. This is not something that is a sure thing at all,” Musk said beofre the first launch attempt.|
Others outside the agency also weighed in, supporting SpaceX. “This flight is an important milestone and much will be learned from the engineering data,” Dan Dumbacher, executive director of the American Institute of Aeronautics and Astronautics (AIAA) and a former NASA official, said in a statement. “With Starship, SpaceX is taking bold steps that are helping us accelerate the future of humans living and working off our planet. Flight tests and taking risks will lead to this future.”
Musk himself set expectations low going into the flight. “This is a very risky flight. This is not something that is a sure thing at all,” he said in a discussion for paying subscribers of Twitter, the social media company he acquired last year, the day before the first launch attempt.
Getting off the pad would be good enough, he said. “It would probably take us several months to rebuild the launch pad,” he said of the scenario where the rocket immediately blew up. “So, my top hope is please, may fate smile upon us and we clear the launch pad before anything goes wrong.”
In retrospect, he may not have achieved that. As the Raptor engines ignited, they kicked up huge plumes of dust and sand from around the pad on the shores of the Gulf of Mexico. They may have also picked up chunks of the pad itself. One video released by SpaceX showed material falling into the waters of the Gulf as the rocket ascended, presumably large pieces of pad material, rocks, or other debris liberated by the rocket’s plume.
When photographers who set up remote cameras for the launch were able to return two days later to retrieve their cameras, they also documented extensive damage to the launch mount and nearby tank farm. (Many of the cameras were also damaged or destroyed by the force of the launch.) Unlike other launch complexes, there was no flame trench or other system to divert the exhaust of the rocket at liftoff.
Musk shrugged off the damage. “3 months ago, we started building a massive water-cooled, steel plate to go under the launch mount,” he tweeted the day after the launch. It wasn’t ready in time but SpaceX went ahead with the launch, thinking that the concrete would be eroded instead of shattering. (The problem wasn’t seen in a static-fire test in February, he added, because the engines ran at only half their rated power.) “Looks like we can be ready to launch again in 1 to 2 months.”
Musk, though, is known for his aspirational timelines. In addition to the time to rebuild parts of the launch pad and install the plate or other equipment to mitigate the force of the launch, SpaceX still needs to investigate what happened during the flight. Were the failed Raptor engines, for example, damaged by pad debris, or did they suffer other problems?
|“Maybe the second one will be or maybe the third one will be, but tomorrow probably will not be successful, if by success one means reaching orbit,” Musk said.|
SpaceX may also need to address the environmental effects of the launch. Residents of the town of Port Isabel, near South Padre Island, reported fine-grained debris—presumably sand—falling on the town, kicked up from the launch plume. There were also isolated reports of broken windows. The FAA, which licensed the launch, will look into those issues as part of its oversight into the launch mishap investigation.
Going into the launch, SpaceX anticipated repeating this first flight. In an environmental document filed the same day that the FAA issued the launch license for the first Starship integrated launch, the agency mentioned establishing an additional zone in the Pacific uprange from Hawaii “to account for the potential Starship debris field for the second and third launches of Starship that are not configured to survive atmospheric reentry.” That suggests SpaceX is planning multiple similar launches until it gets it right.
“Maybe the second one will be or maybe the third one will be, but tomorrow probably will not be successful, if by success one means reaching orbit,” Musk said in his Twitter conversation before the first launch attempt. “If we get any information that allows us to improve the design of upcoming builds of Starship, then it is a success.”
Success, though, won’t ultimately be clear until Starship is flying regularly, carrying satellites to orbit and landing astronauts on the Moon. SpaceX has a record of trying and failing repeatedly until it achieves success: the first flights of the Falcon 1, landing Falcon 9 boosters, and early Starship flights. The same may be true here, but it may be years before we know whether to count this abbreviated test flight as a success or not.
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