Space shuttle Discovery's launch on mission STS-119 to the International Space Station remains on hold while engineers continue to study problematic hydrogen flow control valves in the orbiter's propulsion system. NASA managers opted against setting a launch date for the shuttle at today's Flight Readiness Review in order to give teams "quality time" to study the valve issue.
READ: MPS FLOW CONTROL VALVE FACT SHEET WATCH: FEB. 20 POST FLIGHT READINESS REVIWE BRIEFING At the marathon 13-hour meeting, NASA managers and engineers came close to moving forward with a decision to launch Discovery this month, but, in the end, weren't comfortable flying the shuttle "as-is" without understanding the valve issue better. "There was a just a sense of unease that we did not quite have the rigor that we typically expect for a question like this," said John Shannon, Space Shuttle Program Manager. Shuttle program officials hope to have a plan to move forward ready by next Wednesday that will permit Discovery's mission to proceed while NASA works on a new valve design to be implemented on future missions. Discovery was originally targeted to liftoff February 12, but that date was postponed to the 22nd and then 27th while engineers analyzed the problem and conducted extensive testing and computer simulations of the orbiter's propulsion system and valves. The purpose of the flow control valves during ascent is to maintain and regulate pressure in the liquid hydrogen and oxygen tanks of the ET. 
(Image Above: MPS Flow Control Valve Solenoid Assembly. Credit: NASA)
Reference material provided by NASA describes their operation in detail. "The MPS propellant management subsystem also contains two 2-inch-diameter manifolds, one for gaseous oxygen and one for gaseous hydrogen. Each manifold individually permits ground support equipment servicing with helium through the respective T-0 umbilical and provides initial pressurization of the external tank's liquid oxygen and liquid hydrogen orbiter/external tank disconnect umbilicals. Self-sealing quick disconnects are provided at the T-0 umbilical and the orbiter/external tank umbilical." "Six 0.63-inch-diameter pressurization lines, three for gaseous oxygen and three for gaseous hydrogen, are used after SSME start to pressurize the external tank's liquid oxygen and liquid hydrogen tanks." "In each SSME, a small portion of liquid oxygen is diverted into the engine's oxidizer heat exchanger, and the heat generated by the engine's high-pressure oxidizer turbopump converts the liquid oxygen into gaseous oxygen and directs it through a check valve to two orifices and a flow control valve for each engine. During SSME thrusting periods, liquid oxygen tank pressure is maintained between 20 and 22 psig by the orifices in the two lines and the action of the flow control valve from each SSME. The flow control valve is controlled by one of three liquid oxygen pressure transducers. When tank pressure decreases below 20 psig, the valve opens. If the tank pressure is greater than 24 psig, it is relieved through the liquid oxygen tank's vent and relief valve." "In each SSME, gaseous hydrogen from the low-pressure fuel turbopump is directed through two check valves to two orifices and a flow control valve for each engine. During the main engine thrusting period, the liquid hydrogen tank's pressure is maintained between 32 and 34 psia by the orifices and the action of the flow control valve from each SSME. The flow control valve is controlled by one of three liquid hydrogen pressure transducers. When tank pressure decreases below 32 psia, the valve opens; and when tank pressure increases to 33 psia, the valve closes. If the tank pressure is greater than 35 psia, the pressure is relieved through the liquid hydrogen tank's vent and relief valve. If the pressure falls below 32 psia, the LH 2 ullage press switch on panel R2 is positioned from auto to open , which will cause all three flow control valves to go to full open and remain in the full-open position." "The single gaseous hydrogen manifold repressurization line connects to the hydrogen line vent valve, which is controlled by the H 2 press line vent switch on panel R4. This valve is normally closed, and the switch is positioned to open when vacuum inerting the gaseous hydrogen pressurization lines after MECO and the liquid hydrogen dump. The gnd position allows the launch processing system to control the valve during ground operations." During Endeavour's launch last November on STS-126, a small piece broke off from the lip of the one of the GH2 valve poppets. While it had no impact to Endeavour's ascent, NASA managers want to make sure it wasn't a case of getting lucky one time. NASA is worried that if a valve breaks, debris could damage the plumbing in the main propulsion system and possibly cause a premature shutdown or explosion of the main engines. Engineers are currently working to redesign the valves and eliminate the problem permanently. That work is processing in tandem with the troubleshooting efforts that NASA hopes will allow Discovery to launch sometime in March or April. During this evening's press conference, Bill Gerstenmaier, NASA Associate Administrator for Space Operations, noted that this particular flaw has existed since the shuttle began flying in 1981. The problem only came to light with the advent of more powerful computers and sensors. Shuttle program officials plan to meet again next week to evaluate the progress of the troubleshooting efforts and possibly set a target date for Discovery's launch. When it lifts off, the shuttle will be carrying the fourth and final set of solar arrays for the orbiting outpost as NASA moves closer to completion of ISS next year. (The Spacearium / Space Media Corporation)
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