National Aeronautics and Space Administration
proximate cause and no intermediate or root causes for this failure are yet known. The exact composition and source of this sublimator contamination have not been determined. Furthermore, there is no capability for on-orbit replacement of an EMU sublimator—suit was not designed for this replacement on-orbit, it has not been attempted before and no spare sublimators exist onboard ISS. EMU’s 3005, 3010 and 3011 were considered at low risk of the sublimator problem, based on satisfactory thermal performance during prior EVAs. The question arose, however, as to the status of EMUs 3005 and 3010 with respect to the leakage problem exhibited by EMU 3011. A screening test was devised to subject the water separator circuits of these units to the stress of operation under IVA conditions. The severity of the IVA condition arises from the fact that the ventilating circuit load on the fan motor is greater than that during EVA because of the increased density of the ventilating stream during the higher pressure of IVA (a factor of over 3.5). This results in a slower motor speed (17,300 rpm during IVA versus around 19,300 rpm during EVA) which means that the water collection drum spins more slowly. This translates to lower dynamic pressure at the pitot, and therefore less ability to pump water. Accordingly, on 8-14-13, sequential screening testing was carried out on EMU’s 3005 and 3010. The configuration of each EMU involved full-up suited unmanned operation at IVA pressure (approximately one psi above ambient) with helmets installed. TP B’s were closed, and the Item 125 pump priming valves were manually actuated (Figure 3-20) in order to assure that the water separator circuits were operating. Each EMU was left operating in this condition, and an examination for any quantity of water was carried out by the crew. No water was found at either the helmet vent pad (outlet of T2) or at the CCC outlet (fan inlet.) No change occurred in either the fan speed or the current, nor was any CO2 sensor fault noted, which indicated that the fan impeller and CO2 sensor were probably not exposed to water. The conclusion reached was that neither EMU 3005 nor EMU 3010 exhibited the failure condition present on EMU 3011 at the time of the test.
3.5.4 Specific Testing of the EMU 3011 Water Separator Circuit In looking at probable offenders in the water separator circuit, one candidate immediately suspected was the filter just upstream of the Item 134 check valve. This selection was due in large part to previous experiences with this filter becoming clogged as well as its location supporting a likely proximate cause scenario. Partial obstruction of this filter would prevent adequate flow through the Item 134 valve, and the result would be inability of the pitot pump to circulate all of the flow coming from the gas trap through the Item 125 valve (Figure 3-3). The excess water would then flood the water separator drum and enter the ventilation loop at the fan inlet (Figure 3-19). Failure of the Item 134 itself was a less likely possibility since check valves typically fail open. An open valve would not have caused the observed condition since flow through the valve is the normal operational condition. If the valve were to fail fully or partially closed, it could then have caused the failure, but the design of the valve (Figure 3-19) would make either of these failure conditions unlikely. In view of this, it was decided to develop a procedure for on-orbit removal and replacement of the Item 134 check valve and filter, and ship replacement items to the crew on orbit for a component substitution followed by retest. At this point, a decision was made to perform a repeat screening test of EMU 3011 using on-board highspeed data acquisition and display capabilities not previously used. The rationale for repeating the test was not to check again for the failure condition, but to acquire a better “signature” for the failure, and in
Published on Feb 27, 2014
Report of the NASA Mishap Investigation Board examining the high visibility close call event of July 16, 2013 when ESA astronaut Luca Parmit...