What shape should be my satellite?

If you are a first-timer in satellite design and looking for information about how to choose and start design of your satellite structure, then you are at the right place!

A satellite structural platform is a subsystem of the satellite which provides a mechanical base to hold its other subsystems. A satellite platform is expected to (a) withstand structural load, stresses and vibration experienced during launch, (b) maintain structural integrity and stability while in orbit, as well as, (c) protect the satellite from the damage due to the harsh spacecraft environment.

Design drivers for Selecting a Structural Platform

The design of the structure involves determining the desired shape and  size of the structure so that it is big enough to house all the components of the satellite. After this, material selection is done and the loads acting on the structure are calculated. The satellite is iteratively redesigned until all components fit and the structural loads are manageable.  The following parameters usually drives the design:

  1. Size and weight of the satellite subsystems
  2. Availability of materials with high strength-to-weight ratio and thermal resistivity
  3. Ease of manufacturing
  4. A ready availability of other raw materials
  5. Radiation protection coating
  6. Cost considerations
  7. Launch adapter integration
  8. Expected space environment
  9. Ease of assembly, reusability and extensibility

These aspects would act as objectives and constraints for selecting the optimal design of the structure.

Material Selection

Aluminium and Aluminium alloys are most widely used in space applications because of their high strength-to-weight ratios. Recent advances in the manufacturing industry has introduced  the use of composite materials integrated with the Aluminium metal. A list of desirable material properties with recommended materials to be used is given below.

Desirable Property Examples
High strength Stainless steel, Al alloy, Composites
Light weight Composites, Al
Easily available Al, steel
Protect against radiation Lead
High thermal resistivity Ceramic, Nickel and cobalt alloys
Low cost Al

Estimation of loads on the satellite

The load acting on the satellite while in orbit is negligible, whereas during launch, the load due to the launch vehicle vibration and dynamic load due to gravity is enormous. Typically, the launch vehicle provider furnishes details about the expected launch loads. The satellite platform is to be designed to withstand these launch load by a suitable margin. The launch service provider also defines an adapter for fitting the satellite onto the launch vehicle, with suitable auto-eject mechanism. For instance, IBL230/298 adapters are used for PSLV of ISRO. Finite Element Models (FEM) analysis of the whole structure including the launch vehicle, is usually carried out to verify that the satellite-adapter system can withstand launch loads. See here for more details.

Platform Shape Selection

Common shapes used for micro and nano satellites are;

  1. Cube
  2. Hexagonal prism
  3. Octagonal prism
  4. Sphere

The selection of the shapes depends on subsystems used for the mission. Also, the surface area of particular shapes plays a major role in heat transfer and radiation when the satellite is in orbit.

Pros and cons of various satellite platform are listed in the table below.

Advantages Disadvantages Examples
Cube Six plates of metal joined together to form a cube therefore easy manufacturing i) Lesser surface area, implantation surface

ii) mounted solar panel may not be sufficient for the mission demanding high power. Deployment of solar panel involves, extra weight and higher risk of failure due to deployment.

AprizeAprize
Less material used which corresponds to lesser weight and lower cost iii) Problems in arranging subsystems in the sharp acute corners. CanX5CanX5
Larger surface area for the same volume compared to other shapes would help in heat dissipation and larger area for solar panel
Less number of joints compared to other shapes
Easy integration of constellation solar arrays.
Sphere Larger volume for lesser material Difficult to manufacture SpinSatSpinSat
Cannot utilise full space available due to the curved surface
Difficult to integrate with  launch vehicle
Hexagonal prism Little larger volume compared to cube for the same quantity of material Difficult to build a perfect structure due to the angular constraints OCO-2OCO-2
Sharp corner problems are lesser as compared to the cube shape Too many joints which need  many bolts and nuts, this slightly increases the weight NanoSat-1NanoSat-1
May affect panel folding
Octagonal Similar to hexagonal prism but it has slightly lower volume than that of cube for the same quantity of material Increased difficulty in building SpartnikSpartnik
Increased number of joints and therefore increased risk of failure due to the joints and increased weight

In general, the space provided in the launch vehicle for a satellite is cuboidal. It is necessary that the designed satellite structure fits within this space. So, naturally cube platform is preferred over other shapes to utilize fully the space provided in the launch vehicle for the satellite.

From our literature survey, we found that about 80% of the satellite platforms are cube shaped. The graph below shows the result of the survey. One rich source of information about satellite missions is the eoPortal, which we have used for this survey.

satellite-shape-histogram

Conclusion

The reason that most of the micro and nano satellites are cube is because of its various advantages over other shapes. It is wise to choose cubic platform unless the mission has specific requirements like high surface area for mounted solar panels where other shapes may be more effective. A compact structure with lower length-to-width ratio is preferred not only to minimize the size of the satellite but also to avoid vibration. Being abundant, light-weight and inexpensive, Aluminium and Aluminium alloys are preferred materials for micro or nano satellite structures.

Study carried out by Indra Muthuvijayan, Intern at Astrome Technologies.