Stacking satellites layer by layer? "One rocket launches a hundred satellites" is not a dream!

Since the birth of the first artificial satellite, the shape of satellites has been evolving from simple to complex, from spherical, cylindrical to prismatic and polyhedral. In recent years, stackable flat-panel satellites have given people a sense of simplicity. What is a stackable satellite platform and why do we use this design? Let's take a closer look.

▲ Stacked "Starlink" satellites

What is a stackable flat panel satellite platform?

As the name implies, the stackable flat-panel satellite platform is a satellite designed in a flat, open configuration. Traditional satellite platforms are usually three-dimensional shapes such as cuboids and cylinders. If the traditional satellite platform is compared to a desktop computer, the stackable flat-panel satellite platform is a notebook or tablet computer. It has a simple form and a compact layout, which is convenient for "assembly line"-style batch manufacturing. It is a revolutionary innovation of the traditional satellite design concept, an epoch-making and transformative technology, and is gradually becoming an important driving force for the development of my country's aerospace new quality productivity.

▲ Flat-panel satellite

The Shanghai Satellite Engineering Research Institute is one of the earliest domestic research and development units to develop stackable flat-panel satellite platforms. It has made breakthroughs in the flattened stacked satellite bearing structure technology, decentralized stacked satellite unlocking and separation technology, and stacked satellite cluster collision-free dispersion technology. Currently, two types of satellite constellations have entered the prototype development stage using this platform. With the continuous increase in research and development tasks, the Shanghai Satellite Engineering Research Institute will further shorten the development cycle, improve production efficiency, and quickly respond to emergency tasks or sudden demands through modular design and mass production.

Why do satellites use a stackable flat-panel design?

Satellites are launched into space by carrier rockets. The space in the fairing of carrier rockets is small and the number of satellites that can be accommodated is limited. Therefore, for traditional three-dimensional satellite platforms, usually only one or a few satellites can be launched at a time. Stackable flat-panel satellites can be compactly stacked in the fairing of carrier rockets like flat plates. Through reasonable space layout design, a rocket can accommodate dozens or even hundreds of satellites, making the "one rocket, one hundred satellites" launch mode possible. Both my country and the United States have proposed plans to build a satellite constellation consisting of tens of thousands of satellites. The emergence of stackable flat-panel satellite platforms has brought a qualitative leap in satellite launch efficiency and will strongly promote my country's satellite networking construction plan.

▲ "Starlink" satellite launches 60 satellites

The stackable flat-panel satellite platform has also brought about changes in the cost of satellite development. The platform's design fully considers the convenience of automated manufacturing and assembly, gives priority to the use of mature domestic components, invites a number of private enterprises to join the product supply chain, and encourages suppliers to learn Toyota's zero-inventory production model. Through mass manufacturing and assembly-line assembly, the development cost of a single satellite will be reduced to the level of millions to tens of millions of yuan, which will enable more companies to participate in satellite development and jointly promote the large-scale networking of satellites.

How do stacked satellites connect to the launch vehicle?

Traditionally, satellites for multiple launches are mostly installed in the carrier fairing through a central bearing tube, that is, multiple satellites are installed on the central bearing tube provided by the carrier. This satellite-rocket connection method has two obvious shortcomings. First, the existence of the central bearing tube occupies the fairing space, reducing the number of satellites that can be accommodated in one launch, affecting the efficiency of satellite constellation networking; second, the installation method of the central bearing tube raises the center of mass height of the satellite group, causing the satellite to be subjected to more stringent mechanical conditions during the launch phase.

▲ The star-rocket connection method uses a central load-bearing cylinder

▲ The star-rocket connection method without the center-bearing cylinder

In order to improve the space utilization rate of the launch fairing and reduce the center of mass height of the satellite cluster, the stackable flat-panel satellites are directly connected through on-board connecting sleeves and tightened with pull rods to form a multi-layer layout with two satellites on each layer. The satellite on the bottom layer is installed on the satellite-rocket connecting ring through the on-board connecting sleeve to achieve connection with the launch vehicle.

▲ Schematic diagram of stacked satellite clusters

How does a flattened configuration improve mechanical properties?

The connection method of stacked satellites brings difficulties to the strength and stiffness design of the structure. The stacked satellite cluster is supported by only four connection sleeves at the bottom. The span between the support points is close to 3m. The weight of a single satellite is over 100 kilograms, resulting in a low natural frequency and a less than ideal mechanical environment for the satellite payload. In addition, there is less connection area between the connection sleeve and the satellite structure, and the stiffness and strength of the connection are also one of the main difficulties in structural design.

▲ Schematic diagram of stacked satellite clusters

In response to these difficulties, the satellite structure uses topology optimization methods to assist in planning the location and quantity of the cabin panels, and uses lightweight honeycomb sandwich panels or high-specific modulus and high-specific strength grid metal structures to improve the overall stiffness and strength of the structure. For local structures, refined design and simulation are carried out, and high-strength materials and damping materials are used to improve strength and absorb vibration energy.

How are stacked satellites separated after launch?

The stacked satellites are pressed together by the pull rod and the locking device during launch, and are installed in the launch vehicle by the adapter at the bottom. After launch into orbit, when the satellite cluster rotates to a stable position in orbit, the locking device at the top of the satellite cluster is unlocked and continuously ejects air, causing the pull rod and the locking device to swing out quickly, thereby releasing the restraint connection between the satellite cluster and the vehicle.

▲ Schematic diagram of the stackable flat-panel satellite cluster being unlocked in orbit

When the satellite cluster separates from the launch vehicle, the constraints between the satellites are also released, and the separation between satellites is achieved by spin unlocking. The center of mass of each satellite should perform inertial motion along the speed at the unlocking moment, while the satellite body rotates at a uniform angular velocity at the unlocking moment. Taking two satellites in a layer as an example, the on-orbit separation process between satellites is shown below. The relative position relationship and speed of the satellites under the maximum collision risk state can be calculated through simulation analysis, and the satellite configuration size and center of mass position can be optimized accordingly, effectively reducing the collision risk during the separation of the satellite cluster.

▲ Schematic diagram of the in-orbit separation of stackable flat-panel satellite clusters

Stackable flat-panel satellite platforms have shown great development potential in various fields such as communications, observation, and scientific research, but they are still in the development and verification stage in China, and the maturity and reliability of the products will continue to improve. Driven by national planning and demand, stackable flat-panel satellite platforms will quickly come onto the stage in the future, making satellite "assembly line" manufacturing factories a reality and serving the society, injecting innovation and vitality into the development of the global aerospace industry.