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April 2009

NASA: Flow control valve delays shuttle

By the third week of March, NASA had delayed Discovery’s STS-119 mission to the International Space Station five times.

Four of the delays came because of a problem with the shuttle’s hydrogen-flow control valves. The shuttle has three of the metering devices.

The concern harkened back to damage to one of the valves during Endeavour’s November 2008 STS (Shuttle Transport System)-126 mission.

NASA explained the problem this way: Gaseous Hydrogen (GH2) flow control valves are part of the main propulsion system on the space shuttle. There are three valves within the system, one dedicated to each of the shuttles main engines.

The valve’s function is to regulate the flow of gaseous hydrogen from the main engines to the external fuel tank so the tank will maintain structural integrity and deliver liquid hydrogen to the engines at the correct pressure.

The GH2 flow control valve has two flow positions, high and low. During ascent, the valve moves between the two positions about 15 times. Also during ascent, the external fuel tank supplies the main engines with liquid oxygen and hydrogen propellants.

As the engines consume propellant, the liquid level within each tank drops and causes an empty space above the liquid. To fill the void within the liquid hydrogen (LH2) tank, the engines turn LH2 into gaseous hydrogen and return this gas to the external tank.

The role of the GH2 flow control valves is to regulate the amount of gaseous hydrogen that flows back to the tank.

For the shuttle engines to run correctly and for the hydrogen tank to maintain structural integrity, the LH2 tank pressure should be 33 psi. For single flow-control-valve failures, the other two valves can compensate by moving to either the high or low flow position as required. In some cases, the other valves cannot compensate and the tank pressure can reach 35 psi. In this event, a vent valve on the tank can open to relieve the pressure.

During space shuttle Endeavour’s STS-126 mission in November 2008, flight controllers identified GH2 was flowing from one of the shuttle’s engines at a higher than normal rate.

To compensate, the other two gaseous hydrogen-flow control valves reduced the amount of their flow, and there were no problems during launch. After landing, engineers inspected the main propulsion system, and engineers discovered the GH2 flow-control valve poppet on the suspect line was cracked and a small piece was missing.

The poppet on the valve acts like a pop-up on a sprinkler to let the GH2 flow.

They removed the damaged valve from Endeavour and shipped it to the vendor, Vacco (http://www.vacco.com) for disassembly. It then went to the Boeing Co. where engineers determined metal fatigue caused the crack.

The concerns were whether a failed poppet or poppets could cause:

1. A rupture in the gaseous hydrogen line, resulting in loss of pressure to the external tank’s hydrogen tank. This could result in a main engine shutdown.

2. Over pressurization of the hydrogen tank, forcing open a vent line that could expel hydrogen into an oxygen-filled area.

After finding the failure, teams of engineers worked to understand what caused the fatigue. As part of the investigation, the flow control valves from all three shuttles and three flight spares underwent rigorous inspection.

The team also assessed the risk to the shuttle in the event another poppet breaks and a piece comes off. They conducted impact testing at multiple NASA centers including Glenn Research Center, Cleveland, Ohio; Stennis Space Center, Miss.; and White Sands Test Facility, Las Cruces, N.M.

Engineers at Marshall Space Flight Center, Huntsville, Ala., are constructing computer models to better understand the conditions that would result in a fatigue failure.

A hydrogen leak caused the last delay, and mechanics fixed that problem on the launch pad before the shuttle lifted off on 15 March.