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What shape should be my satellite?

  • August 18, 2015June 21, 2017
  • by admin

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.

Blogs

How do Satellites survive Hot and Cold Orbit Environments?

  • July 22, 2015July 21, 2017
  • by admin

Satellites orbiting within few hundred kilometers from the Earth’s surface are multiplying every day due to the rise in demand for communication, navigation and monitoring technologies. Typical applications of these so called Low – Earth Orbit(LEO) satellites include, air , sea and road traffic monitoring, remote sensing, communication services, atmospheric research and weather forecasting. In space, these satellites are exposed to a harsh space environment that varies widely in temperature as the satellite orbits around the Earth. These extreme temperatures pose a major threat to the electronics housed inside  the satellite. Generally, the electronic boards are designed to operate optimally within a certain range of temperatures defined by the manufacturer. In addition to the thermal loads from the environment, the electronic components themselves generate heat which has to be managed. Hence, it is critical that the satellites maintain operational temperatures to avoid any subsystem failures.

The responsibility is on the Thermal System Design Engineer to solve these challenges with an efficient and affordable system. This article will present important factors that govern the design of the Low Earth Orbiting satellite from a thermal point of view.

Orbits and their Environments

A sketch illustrating different orbits. Source: SpaceSIM

Before we get into the details of the thermal system design, it is wise to find out about the different types of orbits and the nature of space environment experienced by a satellite in each of these orbits. Most of thermal design choices are heavily dependent on the type of orbit chosen for a particular satellite. Typical LEOs are: Sun – Synchronous Orbits (SSO), Polar Orbits, Inclined Orbits or Elliptical Orbits. Sun – Synchronous Orbits are of two types: Dawn – Dusk SSO and Noon – Midnight SSO. The two orbits are completely different from the thermal perspective as will be explained shortly.

Temperature vs distance from sun. Source:  (1) Spacecraft Thermal Control, Lars Bylander, and (2) Spacecraft Thermal Control Handbook, David G. Gilmore

Space thermal environment experienced by a LEO satellite is, for all practical purposes, defined by three parameters: Solar Flux (S), Albedo and Earth’s Infrared radiation. The latter two parameters are a function of altitude of the orbit while the first one is that of distance from sun. A typical orbit of a satellite around earth can be divided into two phases – (1) Sun-lit phase, and (2) Eclipse phase. During the sun  – lit phase of the orbit, the satellite heats up from all of these three effects. As a result, the temperature of the satellite goes to a maximum. When in the eclipse phase, the Solar Flux and Albedo effects are not encountered and the satellite is exposed to temperatures as low as the Earth’s Average Infrared temperature of -18 ˚C. Dawn – Dusk SSO doesn’t have an eclipse phase and hence it experiences a high temperature environment for the whole time but Noon – Midnight SSO and all other orbits experiences both hot and cold environments as they have both sun – lit and eclipse phases. The duration of these phases though, depends on the type of orbit. Thus, based on altitude and duration of the two phases, satellite in each type of orbit will experience a different hot and cold environment. However, the typical range of temperatures was found to be from  -170 ˚C to 123 ˚C for LEO satellites while -250 ˚C to 300 ˚C could be experienced in other orbits.  For better understanding, an example providing different thermal loads and the temperature of the satellite orbiting at an altitude of 1280 km is presented below.

Typical spacecraft component’s temperature limits. Source: Fundamentals of Space Systems, Vincent L. Pisacane

Thermal System Design Considerations

Given the extremely cold and hot ambient temperatures that a satellite is exposed to, it is impossible to design a satellite and sustain its operation without a thermal control system (TCS). Once a specific orbit is selected for the mission, a thorough understanding of the space environment for that orbit coupled with the mission requirements, provide the ideal thermal system to be implemented. Representative operational temperature limits of the typical electronic components used in a satellite is given here.

Thermal Control System (TCS)

Passive and active thermal system components. Source: Satellite Thermal Control Engineering(prepared for “SME 2004”), Philippe Poinas, European Space Agency, ESTEC, Thermal and Structure Division.

Having learnt the “why” and “how” a thermal control system is selected, we can further explore the different thermal system design components available. In general, there are three categories of thermal control systems used in satellites: 1) Passive Thermal Control System, 2) Active Thermal Control System, and 3) Partially – Active Thermal Control System. They differ in the way they function and maintain the temperature of a section or the whole satellite. Passive TCS requires no mechanical moving parts or moving fluids and no power consumption. It is simple to design, implement and test. It has low mass and cost and is highly reliable. However, it has limited temperature control capability. Active TCS requires mechanical moving parts or moving fluids or electrical power. It has complex design and generates constraints on spacecraft design and test configurations. It has a high mass and cost and it is less reliable than Passive TCS. Partially – Active TCS is a hybrid system that uses both Passive TCS and Active TCS components. It uses less power and is of lower cost than the Active TCS. It is comparatively low in mass and offers better reliability than active TCS. It also provides better temperature control as compared to the Passive TCS. The figure on the right highlights all the regularly used passive and active thermal system design components.

Orbits and suggested TCS

MLI and Radiators. Source: Satellite Thermal Control Engineering(prepared for “SME 2004”), Philippe Poinas, European Space Agency, ESTEC, Thermal and Structure Division.

Any one or a combination of the thermal system components mentioned earlier can be used to establish the required thermal equilibrium in the satellite. Whether to choose passive or active or both depends on the type of selected orbit. Satellites in Dawn – Dusk Sun – Synchronous orbit will not require heater power to increase the temperature of the spacecraft as it is sun – lit throughout the mission. But, to avoid the temperature from rising above the maximum allowable operating temperature, a cooling down mechanism is required. Heat can be distributed along the structure of the satellite by suitable construction material (Eg. Aluminum) or through heat pipes or fillers. Multi – Layer Insulation (MLI) blankets and paint on the surface with suitable coating material are also used. These techniques come under the Passive thermal control system.

Heaters used in battery compartment. Source: Satellite Thermal Control Engineering(prepared for “SME 2004”), Philippe Poinas, European Space Agency, ESTEC, Thermal and Structure Division.

Satellites in Noon – Midnight Sun – Synchronous, Polar, Inclined and Elliptical orbits will require heater power to increase the temperature of the spacecraft during cold eclipse phases. During the rest of the orbit when the satellite is sun – lit, to avoid temperatures from rising above the allowable operating temperature limit, similar thermal control methods as used in the above case can be used. Since, this thermal control system uses both passive components and active electric heater system, the system is Partially-active.

Satellites in any orbit will require a surface coating with a specific surface property (Absorptivity and Emissivity controls the heat load input and output) as required to manage the thermal loads in that orbit. Secondary surface mirrors (SSM) or Optical solar reflectors (OSR) sometimes replace surface coating but add an extra expense to the cost of the satellite. Thermal radiators are used in satellites to manage internal heat generated by electronics. Thermal doublers are usually used in the radiators of large satellites like RADARSAT-2 which generate enormous amount of heat. Louvers are usually not used unless there is a stringent condition to maintain the temperature of the spacecraft as a function of time as in the ROSETTA mission.

Conclusion: Options are too many. A precise thermal control can be achieved using expensive components which in turn affects the satellite cost and mass budget, while reasonable temperature control can be achieved using partially active or passive components that are low cost and more reliable. It is upon the thermal system design engineer to choose the optimal design. He or she will have to explore all the available options to find the most efficient and affordable thermal control system such that the temperature limit constraints are met with good tolerance and the costs are kept within the budget.

Study carried out by Miracle Israel, Intern at Astrome Technologies.

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