"How SpaceX engineers taught a launch pad to grab the lower stage of their Starship.
What a spectacle it was that unfolded in the first light of dawn on October 13, 2024 in Boca Chica, South Texas. Six minutes after the fifth test launch of SpaceX's heavy-lift rocket Starship, the 71-meter-long lower stage plunged toward the launch site at supersonic speed. The 33 engine nozzles glowed in the heat of air friction, as if a giant had thrown away a lit cigarette. Then 13 of these engines ignited and slowed the falling cylinder to the speed of a car in city traffic. Now the rocket stage maneuvered itself on the jets of the last three engines still in operation between two pincer-like arms attached to the launch tower. The monster, also known as Mechazilla, grabbed hold, boldly, it seemed, but gently.
When the Starship landed on last Tuesday, when the rocket completed its sixth test flight, this photogenic maneuver did not take place, to the disappointment of many spectators. But on October 13, when the recovery was attempted for the very first time, it worked. But how can something like that even work?
With a total height of 121 meters, Starship is the largest and most powerful rocket ever launched. When it takes off, it develops more than twice as much thrust as the legendary Saturn V, which the Apollo astronauts flew to the moon with.
The idea of returning its 232-ton lower stage to the launch tower and having it recover it so that it could then be quickly reused sounds like science fiction and was long considered as such by many.
However, this had also been the case with SpaceX's Falcon 9 before a lower stage of this rocket landed safely on Earth for the first time in December 2015.
It is now routine: as of today, SpaceX can look back on 352 successful landings of these boosters. Some of them have already been used up to 23 times.
But the Falcon 9 stages land on legs that have been unfolded shortly before after being slowed down by their own engines - and usually do so on drone ships far out in the ocean, at a safe distance from the expensive infrastructure at the launch site, to which they then have to be brought back in a complex and time-consuming process. A return directly to the launch site is a high-risk, high-reward approach, but it is a characteristic component of the strategies that SpaceX founder Elon Musk likes to pursue. Musk has set himself the goal of not only building a rocket that is large enough to fly to the moon and, in principle, even to Mars. It should also be the first that is completely reusable. With the Falcon 9, he had to limit himself to the lower stage.
Proof has now also been provided for Starship's lower stage, called Super Heavy. Before that, however, Musk's engineers had to solve a number of problems. Mechazilla's arms, also known as chopsticks, are used not only to catch the lower stage, but also as a lifting device to position the two stages, the Super Heavy and the upper stage - the Starship in the narrower sense - and to assemble them on top of each other. To do this, the chopsticks slide up and down the tower on rails, operated by a steel cable system. To catch the rocket, the arms, each weighing around a hundred tons, have to grip much faster than during assembly maneuvers, which makes precise control of the hydraulics more difficult. There is a risk of vibrations, which must be minimized by clever designs.
The tolerances are tight in other respects too. The 20-meter-long catch rails on the inside of the chopsticks sit on shock absorbers that can be lowered by up to 85 centimeters during the catch process. The inside of the rails are covered with elastically deformable foam cushions in thin metal covers that can press against the wall of the rocket stage without damaging it. However, they are not designed to clamp Super Heavy and hold it in place. This is done by two small attachments on top of both sides of the rocket stage. They each end in 17 centimetre wide cylinders pointing downwards, which rest on the arresting rails when landing. They can be moved at a small angle to their longitudinal axes to ensure smooth contact with the rails even if the booster is caught at a slight angle.
However, if the booster arrives twisted with a roll angle of more than nine to 17 degrees, these attachments would miss the rail. The rocket stage would slip through and would hit the base of the launch tower after 40 metres, causing the remains of the methane fuel to explode and not only destroying the booster, but also severely damaging the entire system. However, such twisting is unlikely, as this roll axis is the easiest to control in rockets.
The risk of incorrect timing is much greater, for example if the chopsticks take hold too slowly or the rocket stage is still too fast, perhaps because not all the engines were working properly beforehand. The access is therefore carried out in precise coordination between the Mechazilla and the telemetry data of the falling rocket stage. And there is a sophisticated abort protocol: Left to itself, the booster automatically steered into the sea east of Boca Chica. It only returns to the launch tower using aerodynamic control via its four grid fins if ground control actively gives it a signal. This is only sent if all the parameters on the booster or the tentacles are exactly right. After the sixth test launch last Tuesday, they were obviously not all correct.
But even if the flight director gives the booster the "go for catch", it could still happen that not all 13 movable engines, which ignite for a few seconds at a height of around one kilometer, are working properly. Its job is to slow the rocket stage down from around 1100 to 60 kilometers per hour after it has fallen through the atmosphere. If this does not work as planned, the booster will automatically steer itself away from critical infrastructure. It will then no longer reach the sea, but will hit a sandy plain east of the tower.
All in all, a lot of things could have gone wrong on October 13th, and probably not even the engineers at SpaceX expected the maneuver to work straight away. However, catching it may have been the easier exercise compared to an undamaged return of the upper stage after an orbital flight, i.e. from an orbit around the Earth. Only this would make Starship the first fully reusable rocket. The upper stage did indeed reach the intended landing site in the Indian Ocean west of Australia exactly during the two most recent test flights. This at least shows that the spacecraft's movable fins did their job despite the heat and mechanical stresses of re-entry. But those were only suborbital flights. On the most recent one, however, several of the heat protection tiles on the stainless steel hull of the Starship had been left out - including where the capture attachments will be located in the future. Because in the end, Mechazilla should also be able to capture the upper stage." [1]
1. Raketenfangen leicht gemacht. Frankfurter Allgemeine Zeitung Von Ulf von Rauchhaupt; Frankfurt. 23 Nov 2024: B2.
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