CHAPTER 16 DAY IN THE LIFE: IN-FLIGHT MAINTENANCE 274 components. Ideally, everything would be fit checked on the ground prior to launch however, this not always possible due to schedule, cost limitations, and the fact that many of the hardware pieces are built in different countries around the world. In addition to the standard conditions of their intended use, flight hardware must endure the vibrations of launch, the change in pressure as hardware goes from Earth pressure to vacuum (for hardware kept outside on the ISS), and the widely changing thermal environment in Earth orbit. The thermal environment especially affects hardware located outside the space station, as that environment fluctuates with each orbit as well as with each season. Despite the best maintenance approaches and planning, as well as the best hardware design, two pieces of hardware can get stuck together and need to be separated. As sometimes happens when conducting repair work on Earth, the two pieces may need nothing more than a slight tap or even a good whack. Due to the cost of equipment on the ISS, this is usually an option of last resort. The need for a tap or a whack has played out a number of times on the ISS. On multiple occasions, hardware inside the space station needed some extra hammer taps to be convinced to come free from mounting locations they had occupied since launch. The same is true for hardware outside the space station. During an EVA in Expedition 6 (2003), astronauts Ken Bowersox and Don Pettit needed to move a light stanchion, or post, that was mounted to the ISS truss. The stanchion was held in place by a single bolt, and it sat in a type of tongue-and-groove interface on the truss. After the bolt was released, the stanchion refused to come free from the truss. After the astronauts tried multiple ways of wiggling and shaking to free the stanchion, it was decided to abandon the stanchion until the next EVA so that the ground team could come up with additional options and recommendations. After much discussion between the engineering and flight control teams on the ground, it was decided that the best option for the second EVA, if renewed wiggling and shaking did not work, was to use a hammer to tap the stanchion free. After a few small taps did not free the stanchion, the crew gave the stanchion a more reasonable “whack,” and the stanchion came loose. The STS-114/ISS-LF1 (2005) mission was the Return to Flight for the Space Shuttle Program after the loss of the Space Shuttle Columbia. This mission brought the MPLM to the space station to provide some much needed cargo resupply. The MPLM is a full-sized ISS module, flown up in the Space Shuttle cargo bay, and attached to one of the ISS Node modules. Once the MPLM was attached on this mission, the crew was ready to open its hatch and start transferring supplies. As happens with most new modules that are brought to the ISS, once power is available to the module, the internal fans are turned on to circulate the air within the module before the hatch is open. This draws any free-floating debris to the air filters so the crew members do not breathe in the debris or get any in their eyes when they enter the module for the first time. This air motion can generate a slight increase in pressure inside the module, which will effectively push the hatch closed. Given the large size of the hatches, even a small pressure differential across a hatch can create a large pressure force pushing the hatch against its seals, which would prevent the crew from being able to open the hatch. For example, a pressure differential of 0.98 mm Hg (0.019 psi) pushing the hatch closed will require the crew to put 220 N (50 lb) of force into the hatch handle to overcome the pressure. When the external side of a hatch is exposed to space vacuum, approximately 173,500 N (39,000 lb) of force press the hatch against the bulkhead seals. Hatch designers thought of this, of course, when the hatches were designed. All hatches have valves that crews open to allow the pressure on both sides of the hatch to equalize. Thus, to open any hatch on the space station, the crew must first open the equalization valve and wait for the pressures to balance. On STS-114/ISS-LF1, crew members opened the equalization valve and waited the appropriate amount of time to equalize the MPLM pressure with the ISS pressure. When they tried to open the hatch, it would not budge. As described in the “Hatches” section of Chapter 3, the hatch mechanism contains “kickers” that push against the bulkhead of the module when the crew turns the hatch handle. These kickers help crew members unseat the hatch. Even though crew members put extra force into the hatch handle to try to get these kickers to push harder against the bulkhead, the hatch would not open.