ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
493   Figure 6. Monitored intensity during burning thermo-vacuum test  Change of direction. New Burning Voltage  Once the resistance was dimensioned to burn at 3.3 V, at a specific value of 5.6 Ω, changes were made  to the project from outside our design process. The voltage supply was changed from being a set figure to  one somewhere in the interval between 4 and 5 V. This not only caused a delay and meant the work done so far could not be applied in the Xatcobeo project, but it also posed a new problem for the task of guaranteeing correct resistance operation, which did not now have a fixed voltage value but instead had a rather wide range of values. 

The challenge set by the new resistance values lay in guaranteeing that the selected resistance would burn the wire at the coldest temperature and with the lowest voltage (now 4 V), while at the same time ensuring that the resistance would not be destroyed at the highest voltage (5 V) and at the highest temperature, i.e., room temperature. Furthermore, the limitations still existed of using commercial values and consuming the least power possible. 

One possible consideration was to select a resistance of 0.25 W, as it would then be possible to dissipate more power and, in theory, have more room for manoeuvre. Thus, two sub-chassis were designed independently, each one adapted to each resistance type. However, when the burning tests were carried out, it was observed that, the 0.25-W resistance was much more easily destroyed than the 0.125 W one contrary to what we had first thought. This meant this options had to be totally discarded and the tests became more centered on finding a suitable resistance at 0.125 W.   Figure 7. Burning thermo-vacuum test Figure 8. Destroying a 0.125W resistance  Thermo-vacuum tests were carried out at -100ºC using several resistance values to determine the wire burning. The burning tests consisted of a three-minute continual burn in order to check both that no damage was caused to the resistance and also the burning time at 4 V and 5 V. Observation was also made of the current variation produced by the deterioration of the resistance. In this way all possible effects were checked and operation was guaranteed in the face of much higher time values than the real
495   Figure 9. Independent fixation  We finally chose the last option which included a shift of the attachment point from antenna S to E. This  led to improved antenna E retention and to the sub-chassis' center of gravity moving towards the geometric center of the piece because a practically perfect anti-symmetric piece was attained by this change.  Retention wire and mechanical interface with the Switch  In the mechanism presented in 2010, the detection switch was used to know the state of the antennas after each period of supply to the burning resistance. This meant that the retention wire pressed the switch tab and thus indicated that the antennas were folded. The loss of tension allowed the switch tab to lift and indicate that the antennas were open when the wire was cut. 

Mechanically, the wire had to go through (in order, from the outside of the sub-chassis ring towards its inside) the enveloping film, and the antennas, the sub-chassis' outer wall, and pass over the resistance, close the switch, pass through the inner wall of the receptacle and be tied in a hole located a few millimetres away from the line of the wire. 

A problem arose in the hole through which the wire had to pass in the internal wall of the burning receptacle. In some of the tests the tab was not able to open as a result of the trajectory curve that the retention wire had to follow. In order to solve this, the hole had to be designed above its previous position, although this compromised both the wall's integrity and the ease of tying as it made it difficult to achieve the tension needed to keep the antennas tied up.  This was achieved by introducing a second hole and tie attachment in such a way that the wire was tied between the two holes with no possibility for it to turn either before or after the burning. This detail increased the tension attained when tying and security, and also improved the wire's behavior in our search for prefect switch operation.
496 Figure 10. Burning receptacle Figure 11. Detail of the interface between  resistance support, sub-chassis and shielding  Resistance support piece  After many trials burning the retention wire, and bearing in mind that the order of events made it possible  to produce successive burns in the case that the switch did not open after the burn, it was decided to test successive burns carried out on the same resistance, the same sub-chassis, and with the same assembly.  The result was that the resistance withstood all the successive burns perfectly well, without too much deterioration and without putting the retention wire's burning capability at risk. However, the heating up of the conductors in the resistance itself, together with the force applied by the tightened wire in contact with the resistance, produced a process similar to hot wire cutting on the part of the sub-chassis used for holding the resistance in the correct position.  This was a risk when several successive burns were needed to burn the retention wire. That is why a part was designed to substitute that part of the sub-chassis. This part holds the resistance in an unchanging position regardless of how many burns are used.  A material was sought that could withstand high temperatures of around 300ºC, would comply with values for outgassing, and would, in addition, present no ESD problems, i.e., one that was electro-dissipative without being conductive. The resistance could then be supported directly by it. The solution is a technical polymer made of PEEK, graphite and carbon fiber, which is also machinable and can thus be easily manufactured.  Finally the sub-chassis was redesigned to make this piece fit the lower part, where it is inserted, in such a way that the sub-chassis attachment itself was used for this piece. In this way we avoided adhesives or new mechanical joints and kept the assembly light. It also avoided the inclusion of new elements in the burning receptacle area, which was already congested. 

Outgassing and ESD 

All the compounds making up the device must be selected so that the outgassing values do not exceed 1% in the TML and 0.1% in the CVCM. These values are demanded by the ECSS standard.
497 Figure 12. Sub-chassis Figure 13. Resistance support 

This is not a problem when working with metals, as no conventional metals have appreciable outgassing values, which means that there is no problem when working with aluminum, steel or titanium. However, when polymers are used it becomes more complicated to comply with the outgassing values demanded by the standard. It is often impossible in practice for the suppliers of finished parts to give outgassing values as they are parameters for very specific applications.  As this is considered to be a low-cost research project, we do not work with suppliers from the aerospace industry as they are far too expensive. Which means that obtaining the outgassing values for the polymers we work with has been complicated. We have already analysed the case of the polyamide, which, due to it containing fiberglass and being manufactured using laser sintering, has values below the limits according to the ESTEC/544 (QM/3958) reference test from 2004.  PEEK with Graphite and Carbon Fiber is an extremely expensive product, which in our case does not have major repercussions on the project given that the piece we use is very small. Here are the outgassing values for this compound:   Total Mass Loss (TML) [%] 0.16 ESA, ESA PSS-01-702  Collected Volatile Condensable Material (CVCM) [%] 0.00 ESA, ESA PSS-01-702   As for the dissipative film, it can be seen to be a conflictive element for several reasons, but above all because of the ESD risk. Regardless of this, it also had to comply with the outgassing parameters. Below are the values.   Total Mass Loss (TML) [%] 0.32 ASTM E-595  Collected Volatile Condensable Material (CVCM) [%] 0.00 ASTM E-595   The other elements on the device are metals such as Ti6Al4V for the mechanical attachment, or aluminum for the shielding, which do not cause problems with regards outgassing.  ESD  Much has been said about the way in which the device retains the folded antennas by using a polymeric enveloping film, and also about the sub-chassis structure that acts as the support for all the device's elements. Both share the same feature of being made out of polymeric materials, which not only reduces weight but also acts as electrical insulation at the same time.
498 Initially, this feature seems to be nothing but advantageous, but when the space environment is analysed  it can be seen that there are several serious drawbacks, some of which we have already mentioned. These include steep temperature gradients, expansion forces, heat build-ups due to the lack of convection in heat transmission equations, and, in addition to others not mentioned here, the risk of electrostatic discharges (ESDs) appearing due to the high doses of ionized plasma that exist in the space environment.  After analysing the needs and requirements set out in the ECSS-E-ST-20-06C standard, we selected a commercially available electro-dissipative film. This is Polyethylene terephthalate (PET) sheeting with an aluminized face with anti-corrosive primer. 

We decided to check whether this material was apt for our specific application. ESA experts in ESD were consulted and by combining their advice with the standards we made a series of calculations that show the material to be completely valid for the working environment it will have in space, even in the worst case.  The first hurdle is the unit in which surface resistivity is and the calculation of the electro-dissipative film 's total resistance. The characteristics film gives a value of 6.81 e10 Ω/Square, which is the unit used to  differentiate resistances of surface resistivities. 

First, we had to calculate the film 's total resistance:   R=ρ·l w·t   Where ρ is the surface resistivity, l the length of the strip, w its width and t its thickness. 

R=6.81	e10·500 4,5·0,0508 

R = 1,4895	e14	Ω    As mentioned, the standard gives a current density limit of J =10 nA/cm 2 and, in order to avoid  electrostatic discharges in inverse voltage gradients (e.g. between a dielectric and a conductor at a greater negative potential), a maximum potential difference (V) is established between dielectric and adjacent conductor exposed to the vacuum of +100 V. Furthermore, when A is the area exposed in cm 2,  we know that: 

R=V J·A 

R=100 1	e−8·50·4,5= 4,44	e+08	Ω	    That is, for our case: 

R ≥ 4,44	e+08	Ω    If we reverse this reasoning and take R as data to calculate the current density resulting from using the resistance:   J=V R·A =100 1,4895	e+14·50·4,5= 2,98385	e−14	A/cmଶ≪ 	1	e−8	A/cmଶ
501 temperatures, not from the resistance but from the conductors located on both sides of it.  Further details will be given in the section dealing with the burning resistance. 

Integration of the Remove Before Flight   For the Remove Before Flight assembly, we had to bear in mind the great precision that had to be attained in the trigger position and, therefore, in the way this was guided towards the interior switch. It was decided that the best way to guide the trigger was by means of a threaded hole, and for the trigger to be a 16-mm long M2.5 bolt, as it had to close the switch at this distance until the RBF was removed.  In order to guide the bolt conveniently, one option was to use a single nut because the shielding is too thin for a hole to be threaded. However, the option to attach a nut using some kind of adhesive posed the problem of not being able to achieve suitable precision and of the bolt being likely to deviate with respect to the switch.  To solve this problem we resorted to a type of self-clinching nut called a PEM nut, which consists of a mechanical joint that uses pressure to embed itself thanks to the geometry of the nut itself. We designed an installation method which meant there could be no over pressure, and therefore no deformation of the shielding, using a simple installation tool for the nut and a small manual hydraulic press.  To check whether the nut would hold up after installation, several tests were carried out on a shielding that was identical to that for the flight. These consisted of severe mistreatment by means of heavy blows that destroyed the shielding. However, the PEM nut remained unmoved by all the knocks and accelerations that were produced. Finally, it is worth pointing out that in the vibration test (which was more professional and better monitored) that formed part of the Qualification tests, the PEM nut behaved perfectly and as expected.   Integration and Qualification   During the assembly tests for the mechanism, there were never any problems arising regarding the integration of the mechanism; nor were there any problems stemming from the mechanical interfaces. These tests had been carried out with a shielding that was identical to the one for flight, and the coaxial meshes of the connection cables for the antennas had been frayed to improve the coupling between cable, sub-chassis and shielding. However, the flight connection cables had been made in such a way that the coaxial mesh was compact, cylindrical and soldered. This led to the coupling between these three elements being highly unsatisfactory.   It must be said that the design had a margin of 0.2 mm with regards the limit for the envelope given in the Cubesat standard, set at 6.5mm. However, the pressure generated when the sub-chassis attachment bolts were tightened on the rolled up coaxial meshes produced a bending moment located at each of the four attachment points. This deformed the piece elastically to the point where the corners of the sub-chassis exceeded the available envelope dimensions by several millimetres.        Figure 14. PEM nut for guiding the RBF
502 Figure 15. Integration problem with the coax Figure 16. Coated Sub-chassis during the  integration process  To solve this integration problem two measures were taken to avoid protrusion of the envelope. 

 It was decided to look at the way in which the coaxial meshes were integrate, that is, we undid  the rolling and the soldering and frayed them all. The aim was to increase the contact surface (and therefore reduce local pressure) and reduce the thickness of the mesh, as both factors were producing the excess in dimensions.   The sub-chassis had been designed at the limits of its possibilities, but thanks to the results of the  material chosen and the manufacturing method turning out to be better than expected, we decided to reduce the thickness of the upper tab on the sub-chassis by 0.15 mm and the lower one by 0.05 mm, which gave us 0. 2mm of additional margin.  Both these actions combined resulted in the mechanism comfortable complying with the standard set for the film and all the measurement taken showed that the mechanism entered with a margin of at least  0.1 mm at any point on the design inside the envelope dimensions laid down.  Qualification Tests  The final step for the project was to carry out qualification tests on the satellite. We will focus here on those to do with the antenna deployment mechanism. These are the thermal-vacuum test and the vibration test.  Both tests were undertaken at the INTA facilities (Madrid) and had support from many members of the Xatcobeo project and INTA staff. The first was a complete simulation of the heat process during launch and orbit initiation, and a real-time simulation of orbital periods. During this test there was deployment of a duplicate of the antenna deployment mechanism, given that the main one was assembled on Xatcobeo. The results from the deployment were more than satisfactory.  With regards the vibration test, this was carried out at room temperature with a TEST-POD MkII, following a random profile as given in the reference document used as a guide in the tests. This reached up to 28g of acceleration. The test results were satisfactory as all the components in the deployment mechanism withstood the test with no shifts in their positions, no breaks, and no need to fine tune any part of the design. In short, there were no setbacks from the vibration test, which was the most ambitious one from the outset.
503 Conclusions 

After almost three years of development, we have achieved a design that fulfils all the standards involved  in the project. At the same time we have attained extreme lightness and great simplicity in operation and assembly. A great deal of the design's success lies in the use of polymers and the way their characteristics have been applied correctly after a process of testing, modification and verification.  Many of the problems and drawback found in the project have also stemmed from the use of these materials, and their use is an engineering risk when the aim is the search for extreme lightness. But thanks to this risk, a mechanism weighing in at only 13g has been completely validated even when faced with the most demanding tests.   Figure 17. Rear side with the antennas deployed Figure 18. Front side with the antennas folded   We believe that the design process has sought validity within the characteristics decided upon for this mechanism in the face of all the important factors. Also the testing process has been exhaustive. The results have shown that the durability and reliability of not only the retention system but also the burning and detection system are all proven and guaranteed. The definitive test will soon take place when Xatcobeo is placed in orbit in the VEGA inaugural flight. However we are confident of the results shown by the mechanism in testing and of the reliability that the design has shown on the ground.    Given all that has been described in this and the previous article, we believe that using a combination of polymeric materials as a base and fast prototyping as a manufacturing method provides major advantages. These include the speed of the design process, the lightness of the assembly and the reduced cost of the project, and makes it possible to obtain feedback from flight parts at a reduced cost. It is also possible to overcome drawbacks stemming principally from the need for a direct correlation between the computer model and the manufactured item.   The final conclusion, as stated above, will be seen on launch. However, the results obtained so far show future possibilities for this combination of material and manufacture, for the response has been highly satisfactory.  

References 

1. Encinas Plaza, J.M., Vilán Vilán, J.A., Aguado Agelet, F., Barandiarán Mancheño, J., López Estévez,  M., Martínez Fernández, C., Sarmiento Ares, F.  "Xatcobeo: Small Mechanisms for CubeSat Satellites  – Antenna and Solar Array Deployment" Proceedings of the 40th Aerospace Mechanisms Symposium,   (2010), 415-430.
May 2004 NASA/CP-2004-212073 

37th Aerospace Mechanisms Symposium 

Compiled by:  Edward A. Boesiger          

      Proceedings of a symposium held at  Moody Gardens Hotel, Galveston, TX  Hosted by the Johnson Space Center and  Lockheed Martin Space Systems Company   Organized by the Mechanisms Education Association 

May 19-21, 2004
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May 2004 NASA/CP-2004-212073 

37th Aerospace Mechanisms Symposium 

Compiled by:  Edward A. Boesiger                      

Proceedings of a symposium held at  Moody Gardens Hotel, Galveston, TX  Hosted by the Johnson Space Center and  Lockheed Martin Space Systems Company   Organized by the Mechanisms Education Association 

May 19-21, 2004