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The ejection mechanism assembly

Hello Everyone,

It is Arjun here again - this time to speak a bit about how the flight ejection mechanism is being assembled at the moment.

The components of the ejection mechanism:

As you might know, the ejection mechanism is composed of the rocket mounted unit (i.e. RMU, the blue cylinder in the picture below) inside which the free-falling unit (FFU, the grey box in the picture below) is housed while the sounding rocket ascends.

The RMU and the FFU

The FFU is held inside using a tensioned steel cable which acts against a compressed FFU ejection spring. The RMU is then covered with the hatch which is again held tight using another longer steel cable. This cable acts against four compressed hatch ejection springs mounted on the flange.

Ejection procedure:

Slightly before the apogee of the rocket, if the lateral pyrocutters do their job correctly (And we are confident they will), the hatch retention steel cable is cut and the hatch is ejected (literally thrown out) from the rocket. This will expose the FFU to the upper atmosphere for the first time. After that, the pyrocutter mounted on the back plate cuts the FFU retention steel cable. The FFU ejection spring, which now has to battle no opposing force, extends and pushes the FFU outside the rocket. This design is based on the TUPEX experiment developed by TU Berlin in 2018. The design and manufacturing of the B2D2 ejection mechanism has been done by Stefano Bassoli as a part of his Master’s thesis.

How are we assembling that?

My responsibility since the end of May has been to assemble the flight unit of the ejection mechanism. Flight unit, as the name suggests, is the unit meant to fly. This differs from the test units which were assembled and used earlier for the various testing phases. Ideally it should have been a quick assembly process void of problems - practically that was not the case.

All the parts had already been manufactured and we thought that two or three assemblies were enough to finalize it. This was not the case. Initially the FFU had considerable play (i.e. gap) between itself and the guiding rails on the inside of the RMU. This was a problem because the structure would not have been able to survive through the intense vibrations during the initial phase of the launch (vibration tests are conducted for this reason). This problem was noticed by Marci and we concluded that the rail spacing had to be changed. After experimenting with some permutations of shims, a good spacing allowing a snug fit of the FFU inside the RMU was found.

“Open” view of the ejection mechanism

Then I started the attempts to maintain tension on first the hatch retention cable and then on the FFU retention cable - both of which are non-trivial tasks. The cable terminals, after passing through the pyrocutters, go through either the lateral or the top holder (for the hatch retention and FFU retention cables respectively). Then a bronze tube is inserted around which a spring washer stack of an appropriate configuration is placed. The spring washer stack is then covered with the square spring washer holder to keep the stack aligned. Then a plain washer with an appropriate inner diameter is placed followed finally by the nut which engages with the terminal threads. These nuts (on all the three terminals) are tightened with predefined torque values using a torque wrench while ensuring that the terminal itself does not rotate. You can check the picture above to visualize all those elements.

It took me more than a couple of tries (and of course some back and forth talk with Stefano) to get the tensioning process right. In order for the tensioning to be complete and good, one condition is that, apart from attaining the required torque on the nut, the spring washer holders should be difficult to rotate by hand. It was this latter condition that took a lot of time to achieve. For now, the said holders are somewhat difficult to rotate by hand.

We also have problems with the FFU-rail contact - even though the FFU is snug with the RMU. One of the rails does not engage with it, this is the problem I am trying to solve at the moment.

So we still have some work to do, but step by step we are getting closer to our goal!

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