Starship test : official explanations

In a wide-ranging talk on Saturday night, SpaceX founder Elon Musk reviewed the debut launch of the Starship rocket on April 20. The bottom line, he said, is that the vehicle’s flight slightly exceeded his expectations and that damage to the launch site was not all that extensive. He expects Starship to fly again in as few as two or three months. “Basically the outcome was roughly sort of what I expected and maybe slightly exceeded my expectations,” he said. “And I’m glad to report that the pad damage is actually quite small, and it looks like it can be repaired quite quickly. It was actually just good to get this vehicle off the ground because we’ve made so many improvements in Booster 9 and beyond.” Musk spoke for about an hour during a Twitter spaces event, responding to questions from several journalists and spaceflight enthusiasts. For those unable to listen, what follows is a summary of what Musk said.

On the flight

When the rocket lifted off, there were three engines whose ignition was terminated because the flight software did not deem them “healthy enough” to bring to full thrust. That left 30 of the Super Heavy first stage’s 33 engines in good condition, which is the minimum allowable number for liftoff. Musk said he did not believe these three engines were damaged by the gravel and concrete kicked up by the immense thrust created by the rocket as it slowly lifted off from the pad. “Weirdly, we did not see evidence of the rock tornado actually damaging engines or heat shields in a material way,” he said. “It may have been, but we have not yet seen evidence of that.” At 27 seconds into the flight, engine 19 lost communications concurrent with some kind of “energetic event,” Musk said. This also liberated the outer heat shield from four nearby engines. SpaceX engineers are still assessing exactly what this energetic event was. “So something bad happened at T-plus 27 seconds because the engine 19 lost all communications, and some kind of explosion happened to knock out the heat shields of engines 17, 18, 19, and 20,” Musk said. “There were visible fires seen from the aft end of the vehicle for the remainder of the flight, but the rocket kept going. At T-plus 62 seconds, we see additional aft heat shield damage near engine 30; however, the engine continues to run. And then T-plus 85 seconds is where things really hit the fan.” At that point, the rocket started to lose its thrust vector control, or the ability to steer itself. This led to the initiation of the flight termination system.

Flight termination system

Just before a minute and a half into its mission, the rocket’s flight termination system was initiated to break up the vehicle before it veered too far off course. Essentially, the ordnance on board the rocket detonates to rupture its fuel tanks, leading to a breakup. However, in this case, there was about a 40-second delay in the initiation of the system and the rocket breaking apart. This time lag posed no safety issues with the rocket safely offshore, but it is an unacceptable lag for a system that is supposed to terminate flight almost immediately. Musk said the problem could be solved with a “longer detonation cord” to make sure the propellant tanks are fully unzipped rapidly. However, he acknowledged that working through this issue with the Federal Aviation Administration may take some time. “The longest lead item is probably requalification of the flight termination system,” Musk said. “That’s obviously something that we want to make sure of before proceeding with the next flight.”

Musk also addressed the damage observed at the launch site, including a large hole dug by the rocket’s thrust. The damage from what he described as the “world’s biggest cutting torch” ripped through a material called Fondag, which is one of the most heavy-duty concretes in the world. “We’re going to be putting down a lot of steel,” Musk said of the area at the rocket’s base, which he characterized as a ‘mega-steel pancake.’ This would provide both strength below the rocket and a regenerative cooling system by pumping water upward to dampen exhaust from the rocket’s 33 engines. “That is basically a water-jacketed sandwich that’s two layers of plate steel that are also perforated on the upper side,” Musk said. “So that is basically a massive, super-strong steel shower head pointing upward.” This approach should reduce damage to the launch site and eliminate the propagation of concrete bits and dust that were observed during the initial test flight last month. “The debris is really just basically sand and rock, so it’s not toxic at all or anything,” he said. “It’s just like a sandstorm, essentially. Basically a human-made sandstorm. But we don’t want to do that again.”

Thrust vector control

Had the Super Heavy booster not lost thrust vector control, the vehicle may have made it to stage separation, Musk said. Ensuring the ability of the rocket to continue to steer itself, even with multiple engine failures, is key for the next flight attempt with Booster 9, the company’s next-in-line rocket. “Booster 9 is a lot easier because we use electric motors to steer the engines as opposed to hydraulic actuators, where you’ve got a common manifold between the hydraulic actuators,” Musk said. “The electric actuated engines will be much more isolated.” It will be key to ensure that any single engine failures are isolated, and the company has made the rocket more robust for this purpose, he said. “If you have extremely good engine isolation and an engine fails, it does not cause a failure of neighboring engine or the stage itself,” Musk said. “Because then if you lose one of 33 engines, that’s a 3 percent thrust loss. It’s not a big deal. If you do not have good engine isolation, then an engine failure can domino to other engines or to parts of the stage, then you have an extremely unreliable design.”

Expectations for the next flight

Because of the rocket and launch pad upgrades, Musk said he anticipates SpaceX being ready for a second Starship launch attempt in six to eight weeks. However, he acknowledged that closing out work with the Federal Aviation Administration on the flight termination system and taking other measures necessary for a launch license may take longer. He is cautiously optimistic about the next launch attempt, which will repeat the same mission profile—Super Heavy launches and lands in the Gulf of Mexico; Starship separates, nearly reaches orbital velocity, and then returns into the Pacific Ocean north of Hawaii. “I think this time, we’ve got a better than 50 percent chance of reaching orbit,” he said. “I’m hopeful we can get four flights out this year, or maybe five.” The goal of these initial flights is to continue to gather information about the performance of Super Heavy and Starship. After the launch system can reliably reach orbit, the next phase of the program will involve demonstrating in-space fuel transfer and beginning to land and reuse both the booster and upper stages. “It’ll probably take us a few more years to achieve reusability on a regular basis, where we bring the booster back and bring the ship back,” he said. “It’ll take a few years to get to where Falcon 9 is today, where it is quite normal for the rocket to land.” Musk estimated that SpaceX will spend about $2 billion on the Starship program in 2023 but that he does not anticipate needing to raise additional capital this year.