New Tools and Procedures
Neither STIS nor ACS are particularly space-repair friendly, and developing a strategy to fix them presented engineers with many challenges. Planners needed answers to three main questions in order to accommodate the repairs in the mission schedule:
Knowing exactly what needs fixing is not enough to make repairs straightforward. To access the failed circuit boards on these two instruments, astronauts needed to remove 111 screws from the cover of STIS, and 32 screws from ACS, a lengthy process in an environment where time is a scarce commodity.
To overcome this challenge, Goddard engineers developed a high-speed power screwdriver with low torque, or twisting force. This combination of operational abilities means that the drill could speed up the removal process without breaking the screws or fasteners.
Hubble’s instruments are extremely delicate and any floating debris poses a real threat to exposed electronics. So, as astronauts open Hubble’s outer shell to make their repairs, they must exercise extreme caution.
To avoid floating debris during Servicing Mission 4's complex repairs, NASA engineers designed the fastener capture plate. Using the custom drill, astronauts first removed four outer screws to install the transparent capture plate over the electronic access panel. Tiny, labelled holes in the plate then allowed them to insert the drill bit and remove screws as the capture plate contained them. At the end, the entire capture plate could be taken off as one unit, safely taking the access panel and all the debris away from Hubble.
The second challenge was how to grasp the circuit boards. These are very thin, but the astronauts' suits, including their gloves, are bulky and pressurised. If you were to put on a pair of thick, woollen mittens before trying to grab a single piece of paper from the middle of a stack, you might have some idea of how difficult and time-consuming the task is for astronauts. NASA engineers got around this issue by developing a special card extraction tool that allowed the astronauts to easily grab and remove the circuit boards.
The last major challenge of the repair process was how to close the instruments once the repairs were complete. To conserve time, engineers designed a simplified version of the access panels. Two lever-like latches were all it took for the astronauts to securely lock the new STIS cover into place. A new panel was not required for ACS because the fresh electronic components are in a single box that easily slid into place and covered the open side of the instrument.