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Testing the Heat Transfer Host 1 Instrument during a Parabolic Flight

Cedric Goossens – Systems Engineer

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At risk of stating the obvious, one of the main challenges of designing and testing an experiment that should be run on-board the International Space Station (ISS) is microgravity. You may well be building something that runs smoothly on ground but will it work in orbit? This is mainly tackled by theorising about it and working to avoid an error in building up that theory.

That said, theorising does not necessarily lead to a proper conclusion and the need to test arises. This in itself can be a challenge as it is not an easy or cheap operation. However, for our development of the Heat Transfer Host 1 (HOST1) it was agreed with the customer, the European Space Agency (ESA), that this course of action was indeed needed.

Well, to be more precise, we were planning to test a small breadboard (like a prototype) on the ground when the science team responsible of the Enhanced Evaporator experiment of HOST1 asked us whether we had something that they could use for a parabolic flight. The obvious reply was yes - the breadboard and my full support! I was scheduled to be on board flight PFC77 in October 2021 – which delivers the parabolic flight.

To give a bit more technical background as to why a parabolic flight is required sometimes, we need to focus on the experiment itself. What Enhanced Evaporators aim to do is in the name itself: aiming to optimise the evaporator structure of a heat pipe. In order to investigate that, one builds a cell with the structure to be investigated, attaches a heating element to it, and connects it to a condenser (cooled zone) through an adiabatic (thermally not controlled) zone. One would also need to know that the lifetime of the experiment is six months and that in this lifetime, fluid will leak from the experiment. To alleviate the issue of ending up with not enough liquid at a certain fraction of the lifetime, an overfilling element was introduced.

The condenser surface was expanded by adding ribs to it which, together with the wettability of copper (of which the condenser is made), would enable the experiment to store more liquid than needed in one experiment run but only allowing the needed amount to flow to the evaporator zone through capillary effect. The evaporator would thus act as a sort of sponge only pulling the liquid from the condenser, acting as the bucket with liquid, which it needed to be saturated. There were some doubts on the concept of the overfilling and as this could not be tested on ground because the ribs extended above the evaporator zone, it was agreed that a parabolic flight was needed.

The road there was still tricky and getting the medical all clear from Novespace, the flight operator, still stood between me and floating around. The first hurdle was my general practitioner who had absolutely no problem with me going there. She indicated that there were no issues to further discuss with the Novespace doctor which, and I cannot stress this enough, is the absolute truth. However, Novespace was made aware that I am wheelchair-bound and of course, issues arose. After some phone calls and some very strange questions, such as whether I could lie on my back or whether I had problems urinating, that hurdle was taken as well. The only problem remaining was staying healthy to be able to leave for Bordeaux, France, but I was able to keep fit, and together with my colleague Jeroen Peeters, destination ‘plumet to the ground really fast without being fully aware’ was set.

Two notable things are still to be mentioned before reaching a conclusion. Firstly, in order to not get sick during the flight, you get an injection of scopolamine; luckily that worked like a charm. Secondly, a measure that Novespace insisted on, was that I had my own safety person. I don’t think the measure was necessary, but I did get to meet Alain Legendre (he has 9000 parabolas on his book and was once thrown out of a plane by Jean-Claude Van Damme for a film!).

Being part of the PFC77 has been one of the better experiences I’ve had in my life, both personally and professionally. It is of course not all about just having fun and floating. Seeing hardware that you built and seeing it in action in the conditions for which it was meant does add an extra dimension to the work you are doing, as well as providing you with those additional insights that make perfect sense once in microgravity conditions.

A first conclusion is that sadly, pending misinformation, I was the first wheelchair user that was part of a parabolic flight under ESA flag. Now, it is nice to be the first but I shouldn’t have been. The challenges faced to reach the zero g plane should not have been challenges at all because there is no issue. As a society, we still have a long way to go but that discussion is not one to be held on this platform. In any case, I’ll continue with my arduous complaining to get the second, third… parabolic flight. Hopefully it doesn’t last four years this time. Spoiler: there might be a chance for the second in October of this year.

A final conclusion is to be reached on the technological results of the flight itself. As you may derive from the spoiler above, the overfilling did not function as intended. The wettability of the ribs did what they were supposed to do but this wettability extended to the adiabatic zone causing a ‘spillage’. On top, the science team was not fully happy with the overall functioning of their concept.

As the adiabatic zone is not fully thermally decoupled from the condenser, some parasitic condensation can be seen there, something they want to avoid. As such, the scientists want to move away from both overfilling (going to a reservoir which adds liquids when needed) and an adiabatic zone (going to tubing and active liquid displacement by including a pump). A redesign is at hand and this, of course, needs to be tested.

To infinity and beyond.