With the frequent launch of multiple satellites with one rocket, what technical challenges need to be overcome?

Recently, rocket launches by various countries have frequently seen multiple satellites launched with one rocket. So what are the benefits of launching multiple satellites with one rocket? What technical challenges need to be overcome? In the future, what new technologies will be used to improve and enhance multiple satellites with one rocket?

Reduce costs and speed up networking

The so-called "multiple satellites in one rocket" is to use a carrier rocket to launch more than two satellites into the predetermined orbit. This is actually the result of "military-to-civilian conversion", which originated from the multiple warhead technology of ballistic missiles. It was first proposed by the United States in the late 1950s to break through the Soviet anti-missile defense system and improve the penetration capability of ballistic missiles.

In 1960, the United States launched two satellites with one rocket for the first time, and successfully launched three satellites with one rocket the following year. Subsequently, major space countries and organizations in the world have made breakthroughs in this field. Currently, the United States, Russia, ESA, China, India and Japan all have the ability to launch multiple satellites with one rocket.

In recent years, the demand for launching a large number of small and medium-sized satellites and establishing global communication and navigation network systems has continued to increase, and the scale of constellation networking has continued to expand, making the launch of multiple satellites with one rocket more and more frequent, and the number of single satellite launches has continued to set new records. In 2021, the US SpaceX company successfully launched "143 satellites in one rocket" using the Falcon 9 rocket, setting a new world record.

Schematic diagram of stacked flat-panel satellite separation and deployment

So what are the benefits of launching multiple satellites with one rocket?

On the one hand, it can give full play to the rocket's carrying capacity and reduce the cost of satellite launches. Traditional rockets use a one-satellite-per-rocket launch method. Since the satellite design quality cannot be completely consistent with the rocket's carrying capacity, it will inevitably cause a waste of rocket carrying capacity.

The multi-satellite launch model can reasonably select the payload according to the rocket's carrying capacity, increase the chances of satellite launches, and reduce the average launch cost of a single satellite. Especially for micro-satellites and micro-nano-satellites, which are usually impossible to be launched as the main payload on a single launch, they can only be put into orbit as the payload of other launch missions. The multi-satellite launch model obviously provides many small satellites with the opportunity to "carpool in space".

On the other hand, launching multiple satellites with one rocket can speed up the construction of constellations. For constellations composed of multiple satellites, if the conventional one-satellite-per-rocket launch mode is adopted, it may take a long time to complete the overall networking, and it is difficult to fully exert its effectiveness in time. In particular, in recent years, many parties have proposed plans to build giant constellations with tens of thousands of satellites, and they attach more importance to networking time and comprehensive benefits. With the one-rocket-multiple-satellite launch method, small constellations can complete the overall networking through one or several launches, and the benefits are significantly improved. Giant constellations can greatly shorten the networking time, and make it possible to build ultra-large-scale constellations with tens of thousands of satellites.

Multi-satellite deployment is difficult

Depending on the deployment method, the launch of multiple satellites with one rocket can be divided into two categories: one is to launch all payloads into the same orbit; the other is to launch different satellites into their respective orbits or at different phases of the same orbital plane.

Obviously, the first type of mission does not require high accuracy of orbital altitude and orbital phase. The last stage of the rocket can release satellites once or one by one on a specific orbit. The key is to avoid collisions after the release of each payload. This requires the design of a reasonable separation strategy to make the separation time and separation speed of each satellite different, so that the satellites can gradually and safely "drift away". In 2017, an Indian rocket achieved the launch of "104 satellites in one shot", which is a case in point.

The second type of mission has higher requirements for orbital altitude or orbital phase. In this case, to carry out a multi-satellite launch mission, the carrier rocket must be equipped with an advanced upper stage, which can ignite and start the upper stage multiple times, maneuver to different orbital altitudes or different phases of the same orbital plane, complete satellite separation and orbital deployment, and meet the altitude or phase requirements of different satellites entering orbit.

Generally speaking, using the upper stage for multi-satellite launch and orbital deployment can make full use of the upper stage's strong mobility, reduce the multi-satellite deployment time, and reduce the requirements for the satellite's own mobility. However, compared with the traditional rocket's last stage, the upper stage needs to have the ability to start multiple times and stay in orbit for a long time, and the development team needs to master certain technical capabilities.

In addition, the satellite distributor is a key factor in the successful launch and deployment of multiple satellites with one rocket. The so-called "satellite distributor" is mainly used to provide the installation layout position of the satellite in the rocket fairing, figuratively speaking, it provides a "seat" for the satellite.

When multiple satellites of comparable size are launched, such as navigation satellites, communication satellites, etc., the mass and size of each satellite are basically the same. Multiple satellites can be hung on the circumferential side walls of a large cylinder. This is called a central load-bearing cylinder type satellite distributor. They can also be placed on a large disk, which is a disk type satellite distributor.

When multiple satellites with different masses and sizes are launched, there is usually one main mission satellite and the rest are small satellites. The main mission satellite is usually located at the top of the satellite distributor, and the small satellites are located on the side wall or

All around below.

The development team needs to design a reasonable separation plan based on the layout of multiple satellites, and unlock them on time according to the predetermined separation sequence, so that the satellites can be pushed away from the rocket's last stage or upper stage one by one by the separation springs, achieving safe "getoff".

In addition, there are other ways to achieve multiple satellite launches with one rocket, such as using spacecraft to transport microsatellites to the space station and releasing and deploying multiple satellites on the space station. The International Space Station is equipped with a special small satellite release device that can "pop out" the satellite in the opposite direction of the International Space Station's operation for release. Astronauts can also release micro-nano satellites by hand during spacewalks.

Stacked launch future direction

In recent years, in order to deploy the Starlink constellation consisting of tens of thousands of satellites, the US company SpaceX has innovatively designed a new multi-satellite launch mode with one rocket - stacked satellite launch.

Since Starlink satellites use a unified standardized flat-plate structure, they can be installed and connected in a stacked manner using load-bearing support interfaces, and many satellites can be stacked tightly together like potato chips.

In addition, Starlink satellites use an unusual separation method, that is, by slowly rotating the rocket's final stage, a whole pile of "packaged" Starlink satellites gradually detach from the rocket's final stage at a certain rotational angular velocity and are thrown into the space orbit. Because these satellites are originally stacked, they have the same angular velocity before release, but different rotation radii, resulting in a slight speed difference between each satellite when separated and released. The initial speed of the upper satellite is fast, and the initial speed of the lower satellite is slow. Over time, these satellites will pull away from each other, like spreading a deck of playing cards on a table.

This stacked satellite launch method is a major breakthrough in the development of multiple satellite launch technology. It does not require a satellite distributor for each satellite, thus saving payload mass and making better use of the internal space of the launch vehicle fairing, thus improving launch efficiency. With the growing demand for low-orbit communications and remote sensing constellation missions, it is believed that stacked satellite launch will be an important development direction for multiple satellite launch technology in the future. (Author: Zhang Liu, review expert: Jiang Fan, deputy director of the Science and Technology Committee of China Aerospace Science and Technology Corporation)