Rocket Lab Flatellite Satellite Wins $397M Contract

If you want to keep a continuous watch on moving targets from orbit, you have to cope with a basic problem in orbital mechanics: a single satellite in low Earth orbit only remains over any given point on the ground for a short period of time - typically just a few minutes - because it must travel at around 7.8 kilometres per second in order to remain in a stable orbit against the attraction of Earth’s gravity, a fact which is explained in A-level physics under the section on circular motion and gravitational fields.

To make sure that an area is constantly covered, it is necessary to have a sufficient number of satellites in orbits suitable for the task so that when one satellite goes out of sight another takes over and assumes its place. It is this requirement which leads programmes such as SB-AMTI to choose large constellations consisting of a great many small and relatively simple satellites rather than a small number of large and extremely capable ones, since the degree of coverage depends just as much on the number of satellites and the way in which their orbits are arranged as it does on the quality of each individual satellite’s sensor.

The engineering required when designing a satellite for mass production is entirely different from that needed when designing one with the aim of achieving the highest possible individual performance.

Traditional satellites are generally built on a central bus, with the solar panels, antennas, and instruments added as separate parts, each of which needs its own mounting fittings, wiring harnesses, and a separate structural analysis to make sure it can endure the vibrations during launch. With a flatellite, on the other hand, the propulsion system, the avionics, the reaction wheels, the star trackers, the radios, and the fuel tanks are all built directly into one single flat structural frame, which in turn reduces the number of separate components, connections, and structural interfaces that have to be designed, tested, and assembled individually. Since there are fewer different parts and the assembly process is simpler, production can be carried out more quickly and at a lower cost, a factor that is particularly important when the objective is to build dozens or hundreds of satellites rather than just one or two.

The fact that the satellite has a flat shape also deals with a specific problem in the design of rocket payloads: fairing volume. The amount of space available in the nose cone of a rocket is fixed, and conventional satellites, with their solar panels, antennas and other projecting components, generally make inefficient use of that space, which means that there are always some unused gaps on each launch.

Since flat satellites can be stacked with only small gaps between them, just as flat-pack furniture is more efficiently shipped than assembled furniture, a single Neutron launch could carry a significantly larger number of flatellites than the same fairing volume would be able to hold if it contained traditionally shaped satellites. For a project of the scale of a constellation, this packing efficiency directly cuts down the number of launches needed to obtain full operational coverage, which is one of the principal cost factors involved in setting up and keeping any large satellite network.

You won’t get any advantages from this setup unless you make certain concessions. A flat and thin design is less naturally rigid than a more boxy one, and as a result of so many subsystems being built into a single common frame, thermal management becomes more difficult because flat satellites lack the natural separation that enables the heat-generating components on a conventional bus to radiate into space independently of each other. The fact that the reaction wheels, the radios, and the avionics all give off waste heat means that, since they are all placed in this shared flat structure, the engineers have to carefully examine where the heat will collect and guide it to the radiating surfaces, so that it doesn’t build up close to the sensitive electronics.

Rocket Lab’s approach appears to view these problems as constraints which can be dealt with so as to benefit from the savings in both launch and manufacturing that are obtained from the flat design, choosing the cost and schedule benefits of mass production rather than carrying out the additional thermal and structural engineering work needed to ensure that the flat satellite operates reliably in orbit.

For anyone with an interest in aerospace engineering, the flatellite serves as a useful example of the types of systems-level trade-offs that are rarely found in textbook physics problems; instead of merely or even mainly focusing on maximising the performance of any one subsystem, the decision regarding a satellite’s shape involves optimising the entire process from the factory floor to orbital coverage.

The reason why a constellation has to comprise many satellites is explained by circular motion and orbital mechanics, but it is manufacturing engineering and payload fairing geometry which have caused a flat design - rather than a more usual satellite bus - to win a defence contract amounting to almost $400 million. This shows that engineering decisions in the aerospace industry are just as affected by production lines and rocket geometry as they are by orbital physics.