How many steps are involved in recovering a rocket?

I said our BeiDou system was built, and you said other people's rockets can be recovered. I said our space station was built, and you said other people's rockets can be recovered. I said we dug soil from the moon, and you said other people's rockets can be recovered. I said we are about to send a man to the moon, and you still said other people's rockets can be recovered. Well, today is the last episode of the Sky Survey Arrow series, today we will talk about rocket recovery.

Early rocket recovery

In fact, as early as the early days of human exploration of space, recovering rockets was a beautiful wish. After the end of World War II, the United States used the V-2 missiles obtained from Germany to conduct a series of flight tests. However, the number of missiles brought back was limited, and each test was one less. They thought, can I add a parachute to it so that it can land slowly instead of crashing to the ground? But ideals are full, and reality is very skinny. At that time, the V-2 fell too fast, and as soon as the parachute was released, it was torn into pieces as expected, which was not mature.

Later, the United States and the Soviet Union launched a star war in the field of space. At this time, both countries were pursuing the first at all costs, the first artificial earth satellite, the first manned spacecraft, the first spacewalk, the first manned moon landing, the first space station, etc. As for how much it cost? No matter! Later, the success of the Apollo program gave the Americans an objective lead in the space race. While they were cheering, they looked at the bill and felt a little painful.

So, they began to try reusable rockets - space shuttles . Does what I said above sound a bit awkward? Is it a rocket or an airplane? In fact, we can think of it as an airplane-shaped rocket. The power used for takeoff is the three liquid hydrogen and liquid oxygen rocket engines in the middle and the two solid rocket engines on the side, so of course it is a rocket. When returning, it glides and lands on the airport runway like an airplane, or more precisely, a glider. The boosters on both sides of it are recovered with parachutes. As for the large tank in the middle, it is the cheapest part of the space shuttle system. It is basically just a tank, so it is not recycled.

In the American imagination, each space shuttle can be reused 100 to 150 times, which means that for each launch, the manufacturing cost will be only 1/100 to 150. No matter how you think about it, you can save money. Taurus will laugh even in their dreams. But after using it, they found that it was over. This is a manned spacecraft, which has much higher requirements than ordinary rockets, and the system is much more complicated. Although it can be recovered, the maintenance cost after recovery is ridiculously high, no less than rebuilding an ordinary rocket. Moreover, the empty weight of the space shuttle is about 70 tons. These 70 tons have to go into space anyway, and in ordinary rockets, they have to be counted as payload. In other words, no matter what else I want to bring into orbit, I have to consider sending these 70 tons up first. It is similar to a heavy truck. If I use it to pull heavy things, such as components of the space station into space, it is cost-effective, but if I only send a few people to the space station, it is like driving a heavy truck to go out to buy vegetables, which is not cost-effective. So in the end, the space shuttle did not save money . No single one was used 100 times. The five shuttles that could go into space were only used 135 times in total. Two of them exploded, killing 14 people. It was a costly and life-threatening thing.

Vertical landing technology

Although the space shuttle did not achieve the expected stingy purpose, the stingy goal of astronauts will not change. At the same time as the space shuttle was retired, the American SpaceX company began to try to let the rocket control itself to fall vertically to the ground, without the need for a runway like an airplane, nor a parachute. Of course, this is not their first technology. As early as 1993, McDonnell Douglas built a rocket called " Triangle Clipper ", which can take off vertically and then fall vertically, proving that the rocket can control itself to stand on the ground. The other problem that needs to be solved is to let the rocket to be recovered separate from the upper part, fly to the landing site by itself, reduce the speed to a small enough level, and then aim at the landing point.

We all know that SpaceX has already completed the above process relatively maturely, and they use the Falcon 9 rocket, which is a two-stage rocket, and only the first stage will be recovered vertically . Its first stage has nine "Merlin" liquid oxygen-kerosene rocket engines. At takeoff, nine engines work at the same time to provide sufficient takeoff thrust. After about two and a half minutes, the nine engines of the first stage will be shut down at the same time, and then the first and second stages will separate, the second stage will ignite, and the first stage will embark on the recovery journey.

Everyone, please note that from this point on, whether the first stage can be successfully recovered has nothing to do with whether the payload it launched can enter orbit safely. Isn't the psychological pressure of recovering the first stage rocket much less? If the recovery is successful, it will be profitable. If it is not successful, it will be the same as other rockets on the market. For example, the space shuttle must be recovered in one piece, and it must be recovered with the people. The recovery process cannot afford to make any mistakes. The pressure is huge. This is also a major reason why SpaceX can recover and fail repeatedly in the past, accumulate a lot of experience and quickly iterate products.

At the beginning of the recovery, the fuel of the Falcon's first-stage rocket has been almost consumed, so it is relatively light and does not require a lot of thrust to achieve its deceleration and landing. In fact, it only retains 6% to 10% of the fuel for landing, so after separation from the first stage, only three of the nine engines need to work to decelerate the rocket. The flight trajectory of the first stage for land recovery and sea recovery is different, but both require the three engines to be turned on and off multiple times to slow down. When it finally lands, the weight of the rocket will be lighter, and only one engine needs to work.

Key technologies for vertical recycling

In the above processes, there are some key technologies, such as how to make the rocket fly to the predetermined landing area. In addition to the control of the rocket engine and the side jet control, the Falcon 9 also uses the grid rudder we talked about earlier. This thing allows the rocket to obtain great control when flying within the atmosphere and effectively slow down . Because the rocket falls downward when it is recovered, and the flight direction is exactly the opposite of that when it was launched, the grid rudder is folded up and does not work during launch, and it is unfolded when it is recovered. Although China has not yet achieved vertical recovery of rockets, in 2019, this grid rudder has been used in advance to conduct precise control experiments on the landing area of ​​the first stage of the rocket, which is actually the part that allows the rocket to fly to the designated landing area.

The next problem to be solved is to let the engine slow down the rocket. As you can see from the description of the launch and recovery process of Falcon 9, in the mission of this rocket, the first-stage engine has to be turned on and off several times. This is called multiple ignition technology . During the falling process, the fuel in the fuel tank may sway and not sink to the bottom, resulting in insufficient pressure or even interruption of the fuel supply. Stable and reliable flames are also needed for multiple ignitions. These are the key and difficult problems in multiple ignition technology.

After sufficient deceleration, if you want the rocket to land on the ground at a very low speed, the thrust of the rocket engine needs to be adjusted very accurately over a large range . For example, the thrust of the Merlin engine of the Falcon 9 can be adjusted between 60% and 100%. In the episode about rocket engines, we said that the thrust adjustment of liquid rocket engines is relatively simple. If you give more fuel, the thrust will be greater, and if you give less fuel, the thrust will be smaller. But this is only relative to solid rocket engines. If you want to achieve a large range of precise adjustments, liquid rockets have to solve many problems. For example, the required thrust is different, the fuel flow rate sent to the combustion chamber is different, and the pressure in the combustion chamber is also different. At this time, the nozzle that sprays the fuel into the combustion chamber must be coordinated with the fuel flow and the first-level combustion chamber pressure, otherwise combustion instability may occur. In addition, the coordination of these mechanisms such as the turbopump and the thrust regulator also needs to be precise, and the design of the thrust chamber and the nozzle must also be able to adapt to the combustion state under different thrusts. These are the technical difficulties of thrust adjustment.

In addition, the landing mechanism is also a key technology. For the Falcon 9, its landing mechanism is the three foldable legs. When landing with such legs, the attitude accuracy must be relatively high, and the landing must be as gentle as possible. It is also necessary to prevent such folding mechanisms from being damaged by aerodynamics, high temperature, and vibration during the landing process. This is also a challenge. Therefore, in addition to standing on the ground with legs, engineers are now beginning to use other solutions. For example, SpaceX's Starship system is expected to use two recovery arms to support the rocket's grid rudder, just like holding a sausage with chopsticks. In our country's reusable rocket plan, it is also considered to use four ropes to support the rocket for recovery. Such a solution can reduce the number of moving parts on the rocket and increase reliability. In addition, the thrust control requirements during landing will also be reduced because the recovery arresting mechanism will buffer and slow down the rocket.

Makes using rockets more convenient

At present, it seems that SpaceX's Falcon rocket has indeed greatly reduced the cost of aerospace, but their goal is far more than that. They want to make the use of rockets as convenient as using airplanes , landing, refueling, and then continuing to fly. The current Falcon rocket still needs to enter the factory for renovation and cleaning after recovery. One of the bigger reasons is that the liquid oxygen-kerosene engine it uses will produce coking , which is similar to the carbon deposits when we drive, so it needs to be cleaned after each launch. And just to facilitate cleaning and make the engine structure simpler, the engine cycle it uses is also relatively primitive and inefficient open cycle.

In order to prevent the rocket from having to be disassembled, cleaned, and repaired in the factory between launches, people have focused on liquid oxygen-methane engines . This type of engine does not coke and can be used almost immediately after being recovered. Therefore, this type of rocket engine is used on SpaceX's starship and its boosters, and its efficiency is improved through a closed cycle. In addition, China's Blue Arrow Aerospace and other companies are also researching liquid oxygen-methane engines. Blue Arrow is also the first team in the world to use this engine to put a payload into low-Earth orbit. It can be seen that this engine may be one of the main tracks for future astronauts.

Looking back at the history of human aviation, from stumbling takeoff to passenger planes serving ordinary people all over the world, it only took a few decades. In the future, aerospace technology will surely enter and change the lives of more ordinary people due to the maturity of technology and the reduction of costs. From killer demons to interstellar trains, human advanced technology often starts with killing, but it will only have real meaning if it can benefit more people.

Having said that, our Arrow of the Sky series has come to an end. I hope that through these six episodes, you will be able to master the basic principles of exploring the starry sea. I am Gou Sheng, and I will bring you hardcore and interesting aerospace knowledge. Just like and follow me and you can end the get out of class.

This article is a work supported by Science Popularization China Starry Sky Project

Author | Liang Yichen (Teacher Gou Sheng) Lecturer at Xi'an Aviation University

Review | Zhou Binghong, Researcher at the National Space Science Center of the Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.