Part of the preparation for the upcoming ATom campaign is to fit everything in out flight rack. The flight rack is a large metal rack designed to fit on the DC-8. Everything we fly has to be secured in it with aircraft grade hardware so that it cannot shake loose or be propelled out of it in case of a crash. The rack is then screwed to the seat tracks of the plane, where the seats are screwed down in a passenger jet).
The engineer on our team, Frank, had been modeling how everything would fit in the rack , not only in terms of space, but also weight and over-turning moment. Putting in the instruments, computers, flow controllers, power boxes etc. was therefore simple, having a a very comprehensive plan to follow.
Plumbing was more of a challenge. As were are interested in particles that we suck through an inlet line that sticks out of the plane, we want to lose as few as possible on their journey into the instruments. They get lost on the way because they can diffuse to the walls of the tubes in which they’re traveling and then stick on the walls. Therefore tubing has to be as short as possible and we have to take care to avoid turbulent flow (which can be caused by too large flow traveling down too small tubes). For larger particles long stretches of horizontal tubing should be avoided because particles fall under gravity, and instead all tubing should be pointing slightly downhill in the direction of flow. Flows that are too slow mean that the air spends a long time inside the tubing (we call this residence time) and again particle losses are larger. We therefore spent time precisely cutting, bending and attaching sections of stainless-steel tubing to connect the instruments to the inlet flow in a very precise way.
The plumbing being sorted we addressed another challenging issue – wiring. The plane supplies power in 60Hz and 400Hz to power boxes on our rack. As we ready for a flight, the power on the plane is on before take-off, for us to start and and test instruments, then the plane has to power down just before take-off to switch from ground to engine power. While a lot of the instruments and controllers on our rack can handle being turned off and on again just before a flight, others cannot, for example, instruments where parts are heated/cooled and take a while to reach a stable temperature. These have to be connected via a universal power supply, which chargers a battery while power is on, and this sustains the power supply when the power turns off.
With all the instruments on the rack this involves a lot of cables, and without proper organization this quickly becomes a jumbled mess. So as each cable is put in, both ends have to be labeled and then the cable carefully bundled together with other cables and routed along the rack frame. These bundles are secured to the rack with clamps, which also stop them from rubbing against corners of the metal rack and risking abrasion from vibrations in flight. Then they are cable tied together. You can imagine if you’ve completed a bundle, and the suddenly realize a single cable has to be added or removed, it’s a lot of work undoing and redoing everything you’ve just done. Careful planning is require, but even more, a lot of patience with yourself.
When everything is finally in the rack we tested it to make sure there were no leaks in the plumbing, all the wiring was correctly connected and functions, the computers and instruments could all talk to each other properly, particles were being transmitted well to the instruments etc. etc. It was a fine moment to see everything finally come together like this and to see how neat and (at least to my eyes) impressive our rack looked.