The "Starship" was successfully recovered for the first time! What is the difficulty of "holding a rocket with chopsticks"?

At about 8:30 a.m. Eastern Time on the 13th (about 8:30 p.m. Beijing Time on the 13th), the new generation of heavy-lift rocket "Starship" of SpaceX conducted its fifth test launch. The company said that the launch tower's robotic arms (these two metal arms are called "chopsticks" by the outside world) successfully clamped the Super Heavy rocket (Super Heavy) returning to the ground. "Chopsticks clamping rockets" is regarded as a breakthrough in aerospace engineering technology.

What is "chopstick rocket"?

On that day, the "Starship" was launched into space. About two minutes later, the first-stage "Super Heavy" rocket separated from the Starship and began to return. More than six minutes after the launch, the booster was successfully suspended between two huge metal arms. This new rocket recovery mode became the biggest highlight of this test flight.

It is reported that there are three robotic arms on the huge "Starship" launch tower named "Mechazilla". One is the "quick disconnect arm", which is mainly used to refuel the "Starship" and keep the rocket stable before launch. The other two 36-meter-long robotic arms are used to capture rockets and are likened to a pair of "chopsticks". They are installed on super-heavy winches, which are driven by giant motors and can move up and down the launch tower.

According to the records of this test flight, in the final stage of landing, when the "Starship" booster was about 2,000 meters from the ground, 13 engines were simultaneously started to brake and slow down. These engines have "universal thrust" capability, that is, the nozzles can rotate and adjust the direction to control the booster's attitude and flight trajectory.

After landing at a certain height, the outer engines shut down, and the three engines in the center continue to work to further slow down the booster and land almost vertically between the two "chopsticks". At a height of about tens of meters from the ground, the booster hovers vertically in front of the launch tower. Within a few seconds, the "chopsticks" move slightly and firmly grasp the pins under the booster grid wing, and the engine shuts down.

In fact, rather than calling this process "chopsticks clamping a rocket", it is better to call it "parking the rocket on chopsticks". The technical difficulty does not lie in the dexterity or strength of the robotic arm, but in how to accurately hover the huge booster between the two "chopsticks". Any slight deviation will cause it to hit the "chopsticks".

'Chopstick rocket' designed to be quickly reused

At the beginning of the development of the "Starship" rocket, SpaceX proposed the concept of "rapid reuse", that is, to quickly recover, refuel and re-launch the booster. The company once proposed the goal of launching three times with the same booster in one day.

To achieve this goal, the only option is to land the booster on the launch tower for on-site maintenance, refueling and relaunching. Even if it lands on a landing pad near the launch tower, such a huge rocket booster will inevitably require even larger cranes, transporters and other equipment to lift and transport it to the launch tower to wait for launch. This process may take hours. The salvage, cleaning and transportation of the booster that lands in the sea is even more complicated.

It is also critical that SpaceX's existing technology, the landing legs on the Falcon 9 booster, are "disposable" and must be inspected and refurbished after each landing. For the Falcon 9, since it uses liquid oxygen and kerosene as fuel, the engine needs to be thoroughly cleaned between launches, and replacing a landing leg does not consume additional time. The Starship uses liquid oxygen and methane as fuel, and simple maintenance is required before launching again, so replacing the legs will waste time.

How difficult is it to pick up a rocket with chopsticks?

The tower used to hold the "starship" is called Mechazilla, which literally means "Mechanical Godzilla". Both mechanical arms are 36 meters long and 18 meters high, and can bear more than 700 tons.

Unlike the Falcon 9 rocket, which uses four folding legs to land directly, SpaceX did not design landing legs for the Super Heavy booster and Starship spacecraft from the beginning.

According to Professor Wang Huiquan of the School of Aeronautics and Astronautics of Zhejiang University, “chopstick recycling” has two main benefits:

First, when recovering, the rocket is hung on the robotic arm in the air, and then the first and second stage rockets are stacked together. They can be refueled and repaired immediately, and then launched again directly, which greatly speeds up the reuse speed.

The Falcon 9 rocket is much more troublesome. After landing, it must be transported back to the factory for refurbishment, replacement, and inspection of many parts. In addition, the methane engine of the "Starship" has almost no residue after combustion, while the kerosene engine of the Falcon 9 has a lot of coking and carbon deposits that need to be cleaned and must be returned to the factory.

Secondly, by eliminating the landing legs, the rocket's payload is greatly increased, allowing it to carry more people, cargo, and fuel.

"However, there are still three major obstacles to achieving tower grabbing and recovery," Wang Huiquan reminded.

First, accuracy. Tower recovery requires extremely high precision. From the video, even the load-bearing points on the rocket body must be aligned with the robotic arm. A slight mistake may lead to failure or even collision and explosion.

Second, it must be strong. The load-bearing points of the rocket body must be strong enough. If the "chopsticks" break the rocket, it will not be reusable.

Third, stability. The two "chopsticks" must have strong shock-absorbing capabilities, be able to gently catch the arrow body, and minimize the impact force.

Review of the first four test flights

The Starship rocket is about 120 meters long and 9 meters in diameter. It consists of two parts: the first stage is a 70-meter-long "super heavy" booster, and the second stage is the Starship spacecraft. Both stages are reusable. The rocket is designed to send people and cargo to Earth orbit, the moon, and even Mars.

Starship has now conducted four orbital test flights.

During its first test flight in April 2023, the rocket exploded before the separation of the first and second stages.

During the second test flight in November 2023, the first and second stages of the rocket separated successfully, but then the booster and spacecraft exploded one after another.

During the third test flight in March 2024, the first and second stages of the rocket separated successfully, but the booster unexpectedly disintegrated after attempting to land and ignite, and the spacecraft lost contact when it re-entered the atmosphere.

During the fourth test flight on June 6, 2024, "Starship" basically completed the testing of all preset projects: the "Super Heavy" rocket successfully achieved a controlled soft splashdown. Although the Starship spacecraft's flaps had damage to the heat-absorbing tiles during its return to Earth, it was considered a successful splashdown in the Indian Ocean.

Source: Huanqiu.com, Science and Technology Daily, China Business News, China Economic Net