• mrgboucher

Rise of the tape springs

Hello Everyone!

With the advent of spring here in beautiful Sweden, there is no better time to talk about springs of our own: Our composite tape springs.

As you readers might recall from our previous blogs, the tape springs serve as the deployment structure of our magnetometer boom assembly. Think of them like a measuring tape which is stable when coiled and stable when extended. They are made up of a composite material called Glass Fibre Reinforced Polymer (GFRPs), and the behavior of these springs can be customized by the designer/engineer. The raw material for these springs is called prepreg (fibres pre-impregnated with resin) and was kindly provided to us by a local Swedish composite manufacturing company named Marström.

These springs are being manufactured locally by our teammates at an external composite manufacturing facility with the help of Johan Rasmussen. Johan Rasmussen is an engineer from the Sundbro Flying club in Uppsala and works on his own composite gliders in his spare time. If anyone who is reading this already knows about how composite parts are manufactured, bear with me as I explain it to the layman.

First, the prepreg raw material, which is stored in freezers, is left to thaw. Then it is cut according to the specified layup configuration and slowly, with the help of a heat gun, is arranged layer by layer on a metal mold. The whole setup is then placed under tight vacuum conditions and the material is left to harden under a temperature of about 80°C. Once it has hardened overnight, the setup is taken out and the tape springs are cut according to the given dimensions, either manually or by using a special band-saw.

First steps of the process

So far, the manufacturing process has been focused on successfully making smaller samples, measuring 250 mm each in length. You can see one in the picture below. The goal is to nail down a manufacturing procedure with all the little details, and to identify and iron out any kinks early on. One of the early difficulties was the proper alignment of the different glass fibre layers. When they were misaligned, it led to improper twisting of the manufactured samples. We did not want that!

So, a clever solution was devised by our teammate Danai. She created the template below using clear acrylic to help mark the centerline of the cut pieces and to indicate the layer angle.

Another problem was an unusual resin buildup while the setup was under vacuum. The resin buildup made the surfaces uneven which was undesirable as well. The team decided to add a layer of “peel ply” over the assembled glass fibre layers. A “peel ply”, in composite manufacturing terms, is a material which is placed over the composite part during vacuum bagging to absorb some of the extra resin. As the name suggests, this material can be peeled off once the composite part has been fully hardened.

After all these corrections and improvements, a tape spring sample was successfully manufactured and performed according to our requirements and expectations. You can see the result just here.

Now, the work is being focused on manufacturing the full scale 1m long tape springs for our Magnetometer boom assembly structure. A 1m long metal mold is being manufactured at the student workshop at KTH, after which we will be able to start the full scale manufacturing at Johan’s workshop. The plan currently, is to manufacture 15 full sized tape springs which will be used for various types of tests and which will be finally integrated into the full assembly.

Thanks to our readers who continue to excitedly follow our project’s progress. We will be back with an exciting blog next week, so stay tuned. Until then, enjoy the beautiful spring weather and stay safe in these uncertain times! That is it from the desk of B2D2 and Kaushik Iyer.

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