Imagine you would have to destroy your front doors lock every time you want to enter your house. It would be tedious to replace the broken parts, but that’s what happens most times during tests of deployment systems for satellites. So this week it’s about mechanisms.
Satellites which need to deploy a certain appendage (e.g. solar panels or antennas) need a deployment system. A deployment system in general incorporates a Hold-Down-Release-Mechanism (HDRM). This mechanism is used to hold the appendage in its stowed state during launch and release it in orbit. Most HDRM are single-use mechanisms, meaning that at least one part or the whole mechanism itself needs to be replaced after it has been used.
This was also the case for the HDRM used on MOVE‑IIs predecessor First‑MOVE. Naturally the HDRM only needs to be used once during the satellites mission, but during testing of the mechanism this has to done many times. With a mechanism that has to be refurbished, this is time consuming and the mechanism flown is not exactly the mechanism that was tested. Either parts had to be replaced or the mechanism is an entirely new one, in both ways you can’t be completely sure that the mechanism will function the same as tested.
To address this issue a group of students at the TU München started a project back in 2013 and developed a new type of HDRM. The idea was to use Shape-Memory-Alloy (SMA) as a reusable actuator for the mechanism. The project was called SMARD (Shape Memory Alloy Reusable Deployment Mechanism) and it resulted in the identically called HDRM, intended for use on MOVE‑II. The SMARD development was successfully finished with the test flight on the REXUS 18 mission in March 2015. For more information about the SMARD Experiment visit their webpage: http://smard-rexus.de/
The mechanism was then developed further into the eSMARD mechanism during the summer of 2015 to fit the geometric requirements of an application on MOVE‑II. It is a smaller version of the SMARD mechanism with 40 % reduced height compared to its predecessor (4 – 4.7 mm compared to 7 mm).
But how does it work? A hook, which is fixed to the appendage is restrained by a slider. The slider is held in position by a normal coil pull spring. When activated a SMA spring is heated up through electric current passing through it. At a certain activation temperature, the SMA spring contracts and pulls the slider into opening position. The hook and therefore the appendage is released. Once the SMA spring is cooled down, the pull spring is able to pull the slider back into the start position. This process can be repeated several times. The mechanism can be reset with ease by simply activating it again and placing the hook back in its original position.
Right now the some final parts of the mechanism are to be finished. In the next weeks we look forward to test the first functional prototype of the mechanism.
During the development (two developments) we had great support from the company Ingpuls GmbH. They helped and provided us with the SMA springs needed for the mechanisms. Special thanks to them!