Some technicalities about the mechanical design of the radiation measurement aboard PR4

For the ones interested in a little technical story, here is a "small" description of one of the design cases of the PR? Space project, which I hope some of you might find interesting ;).

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In the cover picture our module can be seen that was part of the Rexus 31 rocket (REXUS/BEXUS). We conducted two experiments, a radio interferometry experiment (the smaller housing with the white sticker) and a radiation measurement experiment (the bigger housing in the heart of the module). The design of the radio interferometry had to be finalised, but my main focus after joining the team has been the mechanical design of the radiation measurement.

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Pictured above is the basic concept of the radiation measurement experiment. Scintillator blocks are used to detect cosmic rays, of which 8 are placed in an array to enable the determination of the direction of origin of the cosmic rays. These had to be placed inside a 2U cubesat format (200x100x100 mm), along with the electronics. The design becomes challenging when including the requirements on the mechanics: it must be light weight, operate in a vacuum, have a sufficiently high resonance frequency, survive the shock and vibrations of the rocket launch and operate in a thermal range from the outside temperature in the arctic circle up to the temperatures reached during flight.

This last requirement is particularly challenging when considering that the used scintillator material has a rather big thermal expansion coefficient, about three times that of aluminium. The scintillator blocks can therefore not simply be put in an aluminium slot, as in low temperatures they would shrink, get loose and devastate the frequency response, while in high temperatures big strains occur quickly inducing stresses on the block and easily damaging the fragile coating on the blocks.

The finalised radiation measurement experiment is shown above. A piece of standard sized aluminium box section is used as outer housing, very easy to obtain and according to the 2U cubesat size, it provides a rigid frame.

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The scintillator blocks have been placed in a stack, which allows for modular placement of the blocks when using different sized spacer blocks. The stack can be assembled separately from the housing, as it is held in a sleeve of folded aluminium. The idea to mitigate the issue of the thermal expansion (and contraction), while maintaining a rigid fixation, is to apply a pre-load on the stack with an elastic element in line, providing a spring force.

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A cutaway view of the components in the stack is shown. The scintillator blocks can be recognised by the small green PCB's on top. They are placed in a 6-2 configuration to provide the most useful measurements, conveniently leaving space for an elastic element and an adjustable element to set the pre-load. The dark brown layers in the stack are made of cork and the light brown layers are balsa wood. Surprisingly, the use of cork is apparently not very unique in the space industry, but contrary to this, the use of balsa wood has allegedly not often been seen in Rexus projects. Yet, due to its very low specific mass it was very useful to stay within our weight budget. Likewise, the central spacer is hollow.

Cork is surprisingly well suited for the application as elastic element since it has an Poissons ratio of +-0 until a compression of 15%, meaning it effectively acts as a linear spring and is mechanically very predictive when compressed. (Most notably in contrast to the use of rubber as elastic element, having a Poissons ratio of almost 0.5 would make it useless in this application.)

Surprising to me were the magnitude of the forces that would appear when compressing a layer of cork (assuming an E-modulus of 20 MPa). Calculated according to an initial design of a single solid cork layer, a temperature increase of 50 degrees would result in a force in the stack of almost 4 kN, i.e. 400 kg force, way too much. (neglecting deformation of other parts as cork is about 100 times less stiff to the other parts). To limit the force the area of the cork layer is reduced (by the cutout of the centre shown in the picture) and doubled the layers to half the forces (since they are essentially springs in series). The cut in the aluminium is to evacuate the air out of the hollow spaces when the ambient pressure drops.

The grey coloured part in the cutaway view next to the cork layer is the so called wedgeblock. With this block the pre-load on the stack can be adjusted after it is installed in the housing. Two bolts from the top to the bottom of the block can be reached from above, tightening these adjustment bolts pulls the wedges on top and bottom together, consequently pushing the wedges left and right out, compressing the cork layers. After the pre-load is set, the adjustment bolts can be locked by placement of the locking plate. This plate is secured by a bolt again secured by locktite, making the adjustment vibration resilient. To save weight the wedgeblock is hollowed out as much as possible.

(I'm sorry for my lecturer, but the idea of the wedgeblock came into existence in my notebook during the lectures of a very certain programming course... )

The pre-load is set by applying a calculated amount of compression on the cork layers. After the free play is evaded from the stack by lightly tightening the wedgeblock, the initial distance over the cork layers is measured over two bespoke tabs. Tightening the wedgeblock, the distance must be reduced by the calculated distance, only a mere 0.1 mm. Not ideal with non-digital callipers, but doable. The small amount of compression is again to limit forces on the stack while it should be enough to counteract a possible shrinkage of the scintillator blocks.

After the adjustment was set, only the securing bolt needed to be torqued and sealed by the lid to declare the module flight ready! Finally it was time to sit back and watch the rocket go.

I'm eagerly awaiting the scientific results that still need to be concluded, and I'm hoping that they will be valuable for future research. I'll be very happy to share the anticipated papers on the project, so stay tuned if you're particularly interested in the science.

I must say it has been an amazing time working on the project, it has been a real privilege to participate in the Rexus project and to travel to Esrange for two weeks! Many thanks to all the coaches along the way and to the team members for the collaboration!

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