Less than three minutes after take-off, the Super Heavy booster separated from the Starship spacecraft and started its return. Approaching its maximum altitude of 96km, it decelerated to 1,200km/h before speeding up again as it pointed its base back towards Earth.
Just three minutes later, it broke through the plume it had traced through the sky mere moments ago. 1km above the surface, it ignited 13 Raptor engines, quickly slowing down from more than 1,000km/h. 10 then switched off, leaving just the three in the centre of the base.
The engines’ exhausts swivelled from side to side as the rocket negotiated the last few hundred metres. Descending at a slight angle, it finally aligned with the tower at the Starbase in Texas. As it slowed to a hover, the ‘chopstick’ arms of the tower gently closed around it, catching the giant structure out of the air seven minutes after it had launched.
“This is a day for the engineering history books,” said SpaceX senior manager and webcast host Katie Tice. Aimed at demonstrating Starship and Super Heavy’s ‘fully and rapidly reusable design’, it is hard to overstate the challenges that were overcome to achieve the feat, the equivalent of catching a 22-storey building.
Thousands of distinct vehicle and pad criteria had to be met prior to the catch attempt on Sunday (13 October). Together with Starship’s successful ascent to outer space and splashdown in the Indian Ocean, SpaceX said “the world witnessed what the future will look like when Starship starts carrying crew and cargo to destinations on Earth, the Moon, Mars and beyond.”
Here are some of the engineering innovations, approaches and techniques that enabled the historic achievement.
The catch
Known as ‘Mechazilla’, the Starbase tower caught the Super Heavy using its two chopstick arms. Left wide open as the rocket approached, the chopsticks only closed in the last few seconds of the descent, gently catching the rocket as it came to a rest on catch fittings near its top.
Making the arms sufficiently strong “isn’t that hard”, said Dr Alistair John, programme lead for aerospace engineering at the University of Sheffield, to Professional Engineering. “But getting them to move in the right way, to have the right damping, to be able to softly catch the booster, it can swing in and touch it without smashing into it, scraping or breaking everything… it’s obviously a huge amount of work by a huge team of people.”
Catching the rocket meant that it did not need a landing gear, said Dr Peter Shaw, senior lecturer in astronautics at Kingston University, cutting the overall mass of the rocket. “If you can put more mass into space, then you can start developing it in different ways – space stations, mining,” he said to Professional Engineering.
“When you do the trade-offs on it, it makes more sense to capture the Super Heavy than it is to put these massive landing legs on it.”
On smaller boosters, the mass of landing legs is roughly two- to three-times that of the air brake system, Dr Shaw said. The Super Heavy has four three-tonne aerofins, meaning potential savings of 24-36 tonnes.
The system could help enable rapid reusability in future, he added, if qualified and tested rockets can do multiple flights before reinspection. Captured boosters could be brought back onto a clamp release system, have the next spacecraft fitted on top, and prepare for the next launch.
“Looking at it from a complete systems point of view – to do this kind of chopstick manoeuvre – the fact that they managed to do it is absolutely amazing,” he said. “To a lot of people, those two boosters landing a couple of years ago, this one where the Super Heavy was caught – they are our generation’s ‘man on the Moon’ moment.”
Rapid reusability will depend on solving some further challenges, however. Launching several times before a full service will repeatedly expose components to extreme heat, cold, and massive vibrations, Dr Shaw said, posing a significant regulatory challenge. Frequent relaunch will also mean new manufacturing challenges as the production line expands from a more bespoke, custom-built approach without reaching true factory-line production.
Flight control
The Raptor engine is a key component of SpaceX’s packed launch calendar – and now it has an important role in the chopstick manoeuvre. The Super Heavy has 33 of the reusable methane-oxygen staged-combustion engines, 13 of which have gimbal actuators for precise directional control.
The engines use full-flow staged combustion, which could allow them to be reused up to a thousand times. The process is “an advanced and very efficient rocket engine cycle, where the output from the turbo pumps goes straight into the main engine,” said Dr John. “That means you have to have everything operate in a very high pressure. You have to have materials that can operate at high pressures and temperatures in an oxygen rich environment.”
Thrust vector control is key to precise landing, provided by the gimbals. “That gives fine control of the system,” said Dr John. “Anyone who has ever tried to design a control system like this, you don't want to overshoot, you don’t want it to undershoot. And you saw how precise the control was.”
Modelling helps predict how the whole system will behave, Dr John said, including how the rocket will tilt forwards and back under different inputs from the engines. “They must have had a huge amount of simulations done in the background, and probably based on the last flights they did,” he said. “They would have got data from that on the inertia of the booster, how it moves, how a small angle change of an engine will cause that to defect.”
Rapid iteration
Falcon 9 rockets were the first to land, but it took many attempts – and many failures – before the first success in December 2015. “There's different things that have failed. Valves fail, running out of propellant, running out of the cold gas, reaction control systems, landing legs failing, etcetera – they learn a load through that,” said Dr John.
“They had lots and lots and lots of Falcon 9s that they tested. They didn't just try and build one of these and get it right first time. They were trying again and again and learning every single time from the flight test data, from the results. So that's one of the key things, really – that willingness to attempt it and willingness to fail.”
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.