I. Literature Review




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Finalized Approach


It was determined that the prototype deorbiting system was to be an inflatable device. Using rigid structures to increase the surface area would be impractical due to the added mass of the rigid structure, actuators, gears, motors etc. In addition, inflation can occur as a passive action. Once the valve to the pressurant gas is opened no further electrical or mechanical input is necessary. With the rigid structures enough battery reserve must be maintained to run the electric motors/actuators to drive the gears and tracks. Electrically intensive experiments may not provide enough end-of-life power margins to accommodate that. There are also fewer moving parts with the inflatable. For example launch loads may accidentally misalign gears and tracks or electric motors may seize with the rigid structure whereas the lid and valve are the only moving inflatable parts and are inherently simple.
  1. Materials


As mentioned previously, the primary materials required for the successful demonstration of the deorbiting technology include: aluminum 6061-T6, Elastosil S36 adhesive, Suva 236fa refrigerant, and Kapton polyimide film. During the course of designing and fabricating the prototype, as with all projects, substitutions and compromises had to be made. These compromises were approached from the standpoint that the purpose of the project was to demonstrate our ability to communicate with the CubeSat bus and trigger the deployment of an accurately sized and operational deorbiting device.

While NASA regulations require that structural components for CubeSats be manufactured from certain alloys of aluminum, the current prototype would not actually be utilized on a launch vehicle. With that being said the decision was made to work with the machine shop and utilize the materials they had on hand. The design constraint was that the deorbit device enclosure must still fit within the size restrictions as originally outlined. The added benefit of this decision was that the container was fabricated from stock, effectively eliminating the cost of procuring aluminum which was originally estimated to be $15-$20. Costs for aluminum were subsequently verified upon contact with several suppliers throughout the greater Hampton Roads area as well as nationwide. Prices ranged from as low as $4 per sheet for a couple square feet to as much as $146 for 48 ft2. Substituting stock material for aluminum 6061-T6 was deemed acceptable as it will not impact the overall success or failure of the prototype demonstration.

For demonstration purposes, the inflatable device must be activated similarly to how it would be when actually deployed on the satellite. Developing the prototype as an integrated whole and also incrementally in stages requires many test inflations. Utilizing Suva 236fa for each test firing was determined to be impractical; each time a test was to be performed the small cartridge would have to be charged with new refrigerant and sealed. The lab in which testing and integration is carried out provided an option to eliminate the cylinder recharging from the routine. A metered air supply from a large reservoir is fed to the lab via air line, which has a built in pressure gauge. Air is available in pressures ranging from just barely above atmospheric to as much as 100 psi. This pressure range is more than viable for the inflatable. On orbit, the Suva would be providing a pressure of 84.7 psi for inflation into a vacuum. To compensate for sea level the pressure on the air supply can be metered to 99.4 psi mimicking the Suva and also compensating for ambient conditions. Using the lab air supply was a conclusion based on both necessity and practicality: necessity because attempts to contact a Suva 236fa supplier were unsuccessful, and practical for the reasons outlined above. Success of the prototype demonstration will not be impacted by this substitution and the results will actually be improved. Improvement will come from the fact that the pressure supplied to the inflatable can be more readily controlled with the air supply as opposed to a small gas cylinder whose pressure is sensitive to temperature fluctuations. Also, the effects of under pressurization and over pressurization can be easily studied by simply increasing the flow from the air supply. A benefit in project cost is seen in the elimination of purchasing refrigerant, obtaining a suitable gas cylinder, and EPA regulations.

A further challenge was encountered due to the specification of the Elastosil S36 adhesive. Wacker Chemie AG was contacted in an attempt to obtain a price quote for small quantities of this specific product. It was discovered that they had no knowledge of Elastosil S36, and were certain they did not produce it. Going back to the drawing board the base specifications for an adhesive applicable to the CubeSat environment were determined as tabulated in Figure 6 below.

Temperature Range

-110oC to +250oC

Outgassing

Low

Radiation Resistance

High

Tensile Strength

≥ 4.5 N/mm2

Figure : Specifications for CubeSat adhesive

Upon returning to Wacker Chemie with these specifications they suggested another one of their products, Elastosil S691. The 691 series is a two part RTV silicone rubber approved for space applications in terms of being both low outgassing and radiation resistant. It has a slightly narrower temperature range however, from -110 oC to +148 oC. Another area for concern was that the low range of the tensile strength of S691 is 0.5 N/mm2 below the NASA mandated minimum tensile strength of 4.5 N/mm2. This is not a terribly large difference, but it should be kept in mind as a non-ideal curing or mixing environment may cause the cured adhesive to lean towards the low end of the strength range. In light of this development and due to the fact that Wacker Chemie can only quote bulk quantities of adhesive, it was concluded that alternative adhesives would be explored for the deorbiting prototype.

As mentioned in the discussion of the aluminum and Suva this prototype would not see actual flight and so some liberty could be taken in terms of materials when practicality dictated otherwise. Since only one prototype was to be constructed and the ultimate goal was to demonstrate the ability to initiate and perform the inflation commonly available adhesives were chosen to seal the edges of the inflatable, which also further reduced cost. A consumer contact cement was chosen as an acceptable adhesive for the thin film media in use.

One aspect of the material selection that remained unchanged was the Kapton polyimide film. Kapton had been accrued in the previous semester in sufficient quantity so that no further product need be purchased. However, sixty feet of 2 mil Mylar film was procured for testing purposes. This will provide a media similar to the Kapton so that more testing can be conducted.

  1. Testing

First round testing was conducted with the intent of determining the feasibility of the current inflatable folding scheme. This is a critical aspect of the inflatable deorbiting device. How the bag is stowed prior to deployment helps determine the volume that must be allotted to contain it. The less efficient the folding technique the more mass and space will be required to stow it.

The test utilized the air line in the lab for pressurizing purposes. Pressure was read from the pressure gauge supplied with the line. For this first test the inflation pressure was regulated manually to 14.7 psig. This provided a slow enough deployment to ensure that all the seams of the test inflatable were securely fastened and there were no leaks prior to higher pressure testing. The dummy inflatable was constructed from waste bin liner, as this material is both elastic and flexible, much like the Kapton material. Two dummy inflatables were fabricated to test two different adhesives. On one bag the seam was sealed with Loctite brand adhesive while Mod Podge brand adhesive was utilized on the other. To initiate inflation a slit was cut in one corner of the bag where the air line was inserted and then sealed. Figure 7 below is a diagram of the test set up.

After curing, both bags were folded using a simple folding technique similar to that outlined in the base research, detailed in Figure 7 below (Lokcu).

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Figure : Corner Folding Method (Lokcu)

Both bags were initially tested at 14.7 psig. It was found that the Mod Podge adhesive had not adequately cured, rupturing the seal. The Loctite sealed bag was then admitted to round two of testing in which the inflation pressure was increased to 60 psig with the same simple folding. The importance of folding technique was demonstrated by this test, as even the elevated pressure was not enough to completely expand the inflatable. The problem is believed to be that one seam of folding was creased more tightly than it had been in the previous test and the pressure from the adjacent expanding area actually acted to compress the fold further. To ensure the sealing properties the inflatable was manually unfolded and the inlet pressure increased until the seam burst at approximately 115 psig. As a result of this testing two conclusions were drawn: 1) the Loctite adhesive was adequate for testing purposes and 2) the folding technique previously prescribed must be further analyzed and optimized. A photo of the test is shown in Figure 8 below.



Figure : Test Photo

  1. Valve

The choice of valve was a critical step for the implementation of the deorbit device. The sourcing for a space ready solenoid valve that meets the volume requirements proved to be quite difficult. It has been determined that a custom valve will need to be designed for the purposes of the space qualified solenoid valve.

Continuing the practice of using readily available suppliers for the prototype, a small solenoid valve was purchased. The Clippard Company manufactures a number of small pneumatic and hydraulic components. It was determined that an EM series 2 – 12 solenoid valve will meet the requirements of testing. At this point several local suppliers were contacted. Quotes were obtained from Hydraulic Pneumatic Systems, Inc. and Carolina Fluid Components, LLC. The prices were compared with the Clippard factory supplier and a purchase from the factory was the most economical.

The physical properties of the EM-2-12 solenoid valve will permit the use of the lab supplied air supply. The valve operates on 12 VDC and has a temperature range of 0 to 180 degrees Fahrenheit. The physical properties of the valve are quite compact, roughly 1 inch tall and 0.75 inches in diameter. The small size of this valve limits the flow rate of the system. The maximum flow rate at the pressure we will be using is 0.6 scfm (Clippard Instrument Laboratory).

The valve is connected to the lab air supply via a machines steel manifold. This was selected from the available list from Clippard. The chosen manifold was the 15491-2 inline manifold. This extended the total length of the apparatus by 0.8 inches. The manifold has 1/8-27 NPT and 10-32 threaded connections for connection to the air supply (Clippard Instrument Laboratory). The assembled valve and manifold can be seen below in Figure 9.

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Figure : Valve and Manifold Assembly

  1. Software



  1. Satellite Tool Kit

For this project it was absolutely essential to have a reliable astrodynamics program capable of performing the complex calculations necessary to monitor the decay of an on-orbit satellite. The ideal program would have to be versatile enough to allow for all orbital parameters to be variable, would have to take into account solar cycles for solar heating and atmospheric density variation, and also industry standard so that the results would be able to be verified. All of these characteristics and more are represented in Satellite Tool Kit (STK) developed by Analytical Graphics, Inc.

STK allows for the simulation of all manner of vehicle: satellites, surface ships, submarines, and land vehicles. STK is able to link the time dependent dynamic positions of all of these various vehicles along with over 40,000 ground stations worldwide. With this information mission planners can determine when a certain satellite in a specific orbit will be within communications view of specific targets. In addition to this another powerful utility of STK is that it can solve complex orbital equations. For problems that deal with real orbits the assumption of a single point mass orbiting a much larger point mass is not enough. This is referred to as the classical two-body problem and is extensively covered in the literature (Bate, Mueller and White). In actual orbits there a several sources of orbital perturbation. Perturbation is defined as a deviation from the norm, or in this case, a force that causes deviation from a prescribed orbit. The fact is that the Earth is not perfectly round. Earth's ellipsoidal character is known as oblateness and is one of the primary sources of orbital perturbation. Another source of orbital perturbation is the presence of the Moon. In fact every celestial body exerts a force on each other; this includes the solar system and the galaxy. A precise orbit determination is dependent upon all of these forces. Each applicable force is rolled up into a single equation that gives the acceleration of the orbiting body of interest in relation to the coordinate system. This equation is





The above equation is a second order, nonlinear, vector differential equation that has yet to be solved analytically (Bate, Mueller and White). As such, numerical solutions must be found to accommodate the complexities just in the summation of the forces, which include perturbations, gravitational forces, and of course drag forces, which are all functions of velocity, position, and time. This also happens to be the equation governing the present problem of orbital decay. STK provides such numerical computing power.

Specific geometries of the deorbiting system can be supplied to STK and the program can calculate the time dependent orbital parameters. NASA and international regulations requiring that orbital objects have a life of 25 years can be satisfied by the present inflatable deorbiting device, and this can be verified using industry standard STK software. Licenses were obtained both for this project and for the Mechanical and Aerospace Engineering department at Old Dominion University.



  1. CubeSat Plug and Play Development Kit



The Air Force Research Lab provided the plug and play development kit for prototyping purposes. Most of the architecture is done for handling the base health functions of the bus. However the integration of sensors is done by coding in C language. The challenges are compounded by the compiling which is done off site by the kit developers. In this manner, the software development is a two part operation. One part is done on site at Old Dominion University, and a second part is done automatically on offsite servers.



  1. LabVIEW



LabVIEW is being used to test and operate the prototype setup in the laboratory. LabVIEW has the interactive GUI setup that can be used to actuate the relay and valve for inflation. The user must also have LabVIEW’s multifunctional Data Acquisition (DAQ) device. The DAQ device contains analog and digital inputs/outputs, allowing one to interface with different systems. In order to initiate inflation, the user must first create a Boolean program. This allows one to control the program through a “true” or “false” statement. When inflation is desired, the user simply switches the Boolean statement. The change in statement commands the program to send a small signal (1 to 5 Volts) to the DAQ device. The device outputs the signal to a relay, where the small voltage is used to close the relay contacts. The relay is used to switch the 12 V power supply for the solenoid valve. Inflation can thus be actuated with one click on LabVIEW’s user interface.



  1. Machine Shop Work

The Old Dominion University machine shop was used to construct portions of this project. The machine shop built an actual 1 unit CubeSat shape for volume purposes Figure 12. The most important work done by the machine shop was a replica of the volume allowed for the entire deorbit device. The sketch passed to the machine shop is shown in Figure 10. The finished piece is made of aluminum and shown in Figure 11.

These parts were necessary to build the prototype to the design constraints. The physical limitations of the part in Figures 10 and 11 have proven very useful. The cost of these parts was free of charge. This was due to the use of scrap materials from the machine shop.



Figure : Machine Shop Drawing


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Figure : 1U CubeSat Shape Figure : Deorbit Volume Requirement
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