How do we achieve rocket sharing when launching multiple satellites into space?

In a blink of an eye, 2023 is drawing to a close, but China's space industry is still working hard to maintain a high-density launch, making continuous breakthroughs and delivering impressive results time and time again. On the 26th, China successfully launched the 57th and 58th Beidou navigation satellites at the Xichang Satellite Launch Center using the Long March 3B carrier rocket and the Yuanzheng 1 upper stage. But launching two satellites in one rocket is just a "small scene". As early as June this year, China successfully launched 41 satellites of the Jilin-1 high-resolution 06A magnitude using the Long March 2D carrier rocket at the Taiyuan Satellite Launch Center, breaking the record for the largest number of satellites launched in one go in China , which is of extraordinary significance.

Image source: Xinhua News Agency (Photo by Zheng Bin)

Launching multiple satellites with one rocket refers to the technology of sending multiple satellites into the predetermined orbit simultaneously or successively with one carrier rocket . How do the satellites "sit in a row" on the rocket and how can they "get off" in an orderly manner after arriving at the station? Today, let's talk about the story behind "combining rockets" to go to the sky.

01

High-end " ingredients " are often cooked in simple and superior " cooking methods "

Compared with the traditional single-satellite launch, "multiple satellites in one rocket" can make better use of the rocket's carrying capacity and improve launch efficiency. It seems to be a "high-end operation" of major aerospace companies, but in fact it is a conventional choice to save the launch fee of carrier rockets and reduce launch costs in response to the increasingly competitive commercial space launch market .

The first country to achieve multiple satellites with one rocket was the United States. In 1960, the United States launched two satellites with one rocket for the first time, and in 1961, it launched three satellites with one rocket. Then, the Soviet Union launched eight satellites with one rocket many times. Later, the European Space Agency also mastered this launch technology. In September 1981, China successfully used a "Storm 1" carrier rocket to send a group of three "Practice 2" satellites into the Earth orbit, becoming the fourth country to independently master the technology of launching multiple satellites with one rocket. India and Japan achieved multiple satellite launches with one rocket in 2008 and 2009 respectively.

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These early multi-satellite launch missions were basically special payloads serving the government. Their basic feature was that at the beginning of development, the satellite payloads had detailed communication and coordination with the launch vehicle. Both parties carried out corresponding adaptive designs for the complete success of the mission, including additional brackets, load-bearing structures, etc. However, these structures are generally not universal and will be of no use after the mission is completed.

With the rise of communication satellite constellations, the aerospace industry has a need to repeatedly launch multiple satellites with one rocket, such as Iridium and Globalstar. Due to the large volume, in order to serve these large customers, launch vehicles generally customize special satellite brackets and even upper stages for them. For example, in order to launch Iridium, China's Long March 2C launch vehicle developed and added an upper stage parallel dual-star intelligent distributor .

Later, micro-satellites appeared. These satellites are light, small, and require little investment. Most of them are used for technical research, and they hope to enter space at a low price. Therefore, the launch vehicle often leaves an additional position on the satellite bracket of the main payload or the last stage of the rocket to "take" it into space. This method is called "piggybacking". The piggyback satellite is completely subject to the main payload in terms of development progress, launch orbit, and environmental requirements, and has no say. It can be said that in order to enter space, they have become "humble little transparent."

In recent years, with the vigorous development of the small satellite market, the demand for small satellite launches has increased rapidly, and the number of small satellite launches accounts for the vast majority of the total number of satellite launches each year. According to a forecast made by a European consulting firm in 2021, the world is expected to launch a total of 13,912 microsatellites from 2021 to 2030, with an average launch mass of only 180 kilograms per satellite. Under this situation, rocket group orders have become popular, and the scale of multiple satellites in one rocket has become larger and larger. India launched 104 satellites in one rocket in 2017, and the US SpaceX company set a record of 143 satellites in one rocket in 2021.

The difficulty in organizing the launch of a large number of small satellites has spawned a number of intermediary companies specializing in "group ordering" . They need to carry out a lot of customer coordination work to solve the impact of some customers' delays and changes on the overall progress. Among them, the famous launch brokerage companies include Exolaunch in Berlin, Germany, D-Orbit in Italy, and Spaceflight in Seattle, USA.

On December 23rd local time, SpaceX successfully launched and recovered 23 V2mini Starlink satellites (G-32). This was the first time they used reusable rockets, breaking the previous record.

Image source: SpaceX official account

In order to seize the small satellite market, SpaceX has also used the trick of "flight-based", that is, like a high-speed train, regardless of whether all the customers who have bought tickets are gathered, the train will depart on time to increase the certainty of the launch. In addition, the launch frequency is increased, and customers who are late do not need to panic. They can pay a little more and catch the next train. In this way, SpaceX has become the first choice for most small satellite customers, establishing its own dominant position. A large number of customers also makes flight-based a practical guarantee, which in turn further strengthens this dominant position. Just imagine, would you choose a punctual bus or a "black car" that has to wait until it is full before leaving?

02

Satellites must not only accommodate passengers, but also be comfortable to sit on

To achieve multiple satellites with one rocket, the number of satellites is not the key. What matters is how to ensure that all satellite "passengers" have comfortable seats, avoid bumps and squeezing during launch, and reach space safely. This is what highlights the technical content.

To safely and smoothly send so many satellites into orbit at one time, the first thing to solve is the layout of the satellites in the fairing, which must not only ensure that there is enough space for them to sit, but also that they sit comfortably. To this end, it is necessary to optimize the "seat" layout based on the satellite structure and mission requirements.

In order to provide solid support for the satellite and transmit the rocket thrust to the satellite firmly, the rocket is generally equipped with a conical satellite bracket. The end with a larger diameter at the bottom is connected to the cylindrical outer shell of the rocket's final stage, and the end with a smaller diameter at the top is connected to the satellite's rocket-star docking frame.

When enjoying a rocket launch mission alone, a satellite can exclusively use the rocket's satellite bracket, straps and other unlocking and separation devices. However, for multiple satellites launched with one rocket, "there are too many people and too little porridge", and one docking frame is far from enough. These satellites vary in size, weight and shape, and the space in the rocket fairing needs to be effectively constrained, allocated and utilized to allow the satellites to be successfully loaded and separated.

This requires a conversion device to convert multiple different satellite interfaces for all satellites to dock and install. This device is generally called a "multi-satellite distributor". Distributors come in many forms. If satellites of similar types are launched, a central load-bearing cylinder distributor or a disk distributor can be used, or a cone distributor can be used to facilitate the release of satellites .

On the multi-satellite distributor, satellites are arranged according to their respective characteristics. Some sit upright on the top, some are hung on the wall sideways, and some are packaged in release devices, ensuring that everyone is in their proper place and can withstand the flight environment during rocket launch and enter space safely.

03

The moment of separation : " Unbuckle your seat belt and get out of the car safely ! "

After the rocket safely sends the satellite into space, it still needs to complete the last critical process, which is to "help the satellite down" and send it to its promised land. This requires mastering the stable and reliable "satellite-rocket separation" technology. The satellites must be separated from the multi-satellite distributor without colliding with each other. It is also necessary to choose the best flight route and determine the best separation time so that so many satellites can operate in their respective orbits . In addition, the impact of changes in the rocket structure and center of gravity when the rocket gradually separates the satellites must also be considered to avoid the rocket's own attitude getting out of control and colliding with the satellite.

The more satellites there are, the greater the risk of collision in orbit after the satellites are separated from the rocket. In order to avoid crowded collisions, the satellites on the multi-satellite distributor are generally grouped by layer, and the satellites are separated in groups one by one. After each group of satellites is separated, the rocket opens the reverse thrust rocket to increase the distance between itself and the satellites just released, and then starts the separation of the next group of satellites until the end . In this way, the safety of separation between each other is guaranteed.

After the Long March 2D carrier rocket flew for about 840 seconds, 41 satellites began to separate from the rocket one after another.

Image source: CCTV News screenshot

The choice of multiple satellite separation method has a great impact on the accuracy and safety of satellite separation, and many factors need to be considered, such as the order, time, angle and speed of separation of different satellites. At the same time, since the separation process will be disturbed by multiple factors such as attitude stability, earth gravity and air resistance, a large number of simulation calculations are required to realize the separation process, and various actions are precisely controlled in the actual process to ensure the accuracy and safety of the spatial position and orbital parameters of the satellite after separation.

As satellite technology continues to develop in countries around the world, new commercial vitality of "satellite express" is activated, and the picture of more humans entering space in the future is also being outlined.

Author: Yu Yuanhang, Senior Engineer, Beijing Institute of Aerospace Systems Engineering

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.