I. Literature Review

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НазваниеI. Literature Review
Дата конвертации14.02.2013
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II. Rationale

The current topic of development is CubeSat deorbiting. Designs for deorbit mechanisms can range from long tethers to inflatable assemblies. The current level of deorbit mechanism research provides many new areas to explore. As electronics size and power requirements have decreased, small electromechanical devices have also miniaturized.

The method chosen to best suit a CubeSat deorbit device is one that increases surface area, producing drag. This design relies on creating a drag force to slow the orbital velocity of the spacecraft (R. Janovsky). It can be shown that as the orbital velocity decreases, the orbital altitude must also decrease. The drag force is a function of the density at the respective altitude. Atmospheric density increases exponentially towards the earth’s surface; therefore drag is higher at lower altitudes.

In spacecraft design a single term, ballistic coefficient, can be used to describe the drag efficiency. Ballistic coefficient is defined as where M is the mass, CD is the drag coefficient, and A is the frontal area (D.C. Maessen). In this case, a lower ballistic coefficient will have a larger surface area and be a better deorbit mechanism. A 1U CubeSat, with a mass of 1 kg, cross sectional area of 0.01 m2, and a CD of 2, will have a ballistic coefficient of 50.

The method for increasing the surface area of the spacecraft requires some sort of deployable mechanism. The two main ways to increase the surface area of a spacecraft are to deploy a rigid structure/array or an inflation device. Both of these methods require some sort of activation technique and movement.

The deployable array technique would be similar to deploying additional solar panels. The sides of the spacecraft would fold outwards to increase the total surface area. This technique requires a significant level of sophistication and will not be discussed further.

The second method for increasing surface area is to inflate a generic volume of some sort. From a drag perspective, the shape is irrelevant as long as the critical surface area for producing the required lifetime is achieved. An inflatable must use some sort of gas to inflate a closed thin walled volume. The volume must be able to be packed sufficiently small and the gas must also be stored compactly. The material for the thin walled volume is also of importance since it must remain in space for extended periods of time. Puncture resistance is also a consideration, whether micrometeoroid impacts are a concern or not (D.C. Maessen).

If an inflatable device is used, one possible modification is the addition of a conductive ring inside the material. The ring would allow a current to flow, which would interact with the Earth’s magnetic field according to the equation F = I*L x B. This interaction could be beneficial in various ways. First, if the ring is perpendicular to the magnetic field lines, then the ring experiences a torque, which could be used to orient the CubeSat. Second, if the ring is parallel to the magnetic field, by applying the correct direction of current (clockwise versus counterclockwise), the interaction causes the ring to expand. This expansion could be useful. For example, if the inflatable device experienced a leak, the expansion would help keep the material from collapsing in on itself, making it easier for the gas to inflate the structure.

Although a conductive ring would be useful, it adds complexities to the deorbiting device. One would need to ensure that dissipated heat from the ring does not degrade the adhesive in the inflatable structure. The folding and packaging of the material would need to be slightly altered. Also, the CubeSat would need to store more power, in order to supply the current of the ring. Lastly, and most importantly, a detailed computer program would be necessary to predict the magnetic field of the Earth at the exact position of the CubeSat. The purpose of the computer program is to ensure that the magnetic forces are pointed in the correct direction, according to the right-hand rule. Due to these complexities, a conductive ring will only be considered if a standard inflatable structure does not suffice.
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