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Examiner exclusive: NASA pressure builds as critical valves recycle old problems, schedules slip
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Space shuttle Discovery is temporarily grounded at NASA's Kennedy Space Center as engineers struggle to understand a major problem with critical flow control valves (FCV) for the orbiter's massive fuel tank. In this exclusive, the Examiner will detail how this is not the first time that the valves have raised a red flag--and why damage to one of Endeavour's valves during launch in November 2008, have brought intense focus on the issue.
During the November, 2001 Flight Readiness Review for Endeavour's STS-108 mission, problems with valves were upgraded to a Criticality 1 level. A CIL (Critical Items List) 1 rating means a "Single failure point that could result in loss of vehicle or personnel." It's the highest rating that's applied to any component onboard the space shuttle and its systems.
The image, right, is from Oklahoma State University, which participated in a NASA study to evalue the use of passive neural networks to better analyze and understand the functioning of the valves:
The gaseous hydrogen flow control vale [sic] GH2FCV; see Figure 1) is a critical space shuttle component that controls the pressure of the hydrogen external tank during launch and ascent to orbit. The valve is one of the components that are to be monitored by the Integrated Vehicle Health Monitoring (IVHM) system. The GH2FCV is a two-position valve that is actuated by electromagnetic force acting against a return spring. In the de-energized position, the gaseous
hydrogen flow is high; when the valve is energized, flow is reduced. There are three such valves on the shuttle, one connected to each of the three main engines and manifolded together to return hydrogen gas to the external tank.The valves are precision manufactured but, in spite of this, each exhibits slightly different operating characteristics from the others.
In the post-Columbia environment, NASA is under pressure to ensure that any potential problems, especially CIL 1, are thoroughly evaluated. They've applied that philosophy to the upcoming launch, even though further slips will impact an already-conflicted shuttle/Ares test pad scheduling problem.
Originally scheduled to launch on Feb. 12, the STS-119 mission to the International Space Station (ISS) now will be pushed back to no earlier than Feb.22. Even that date is speculative, as special studies continue to determine the risk associated with a set of new flow control valves.
Working poppet-style, like your garden sprinklers, the valves are used to regulate the flow of gaseous hydrogen from the shuttle's three main engines to the external tank (ET) as the shuttle powers into flight. It's a complex and critical system.
The external tank feeds fuel into the main engines. In a carefully-calibrated quid pro quo, the engines process the flow and return gaseous hydrogen back to the ET to keep it properly pressurized so that fuel flow is normal during liftoff.
That vital pressure was not maintained during space shuttle Endeavour's November launch. Instead, one valve allowed extra, volatile hydrogen to flow through faster than expected.
The other two valves acted in tandem to stabilize the system, and the flight-to-orbit proceeded without problems. However, post-flight study showed not only a small crack in the errant valve, but also some missing metal.
At risk: the control of fuel flow as well as potential dangers should metal bits get loose and into the fuel system. Although NASA mission managers had been expected to approve a Feb. 12 launch date during a recent Flight Readiness Review (FRR) at KSC, the decision instead was to delay at least a week, to Feb 19.
Today NASA confirmed the scuttlebutt: Discovery's liftoff will be delayed more than the original week.
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