As NASA tries to understand problems with flight-critical main propulsion system flow control valves, space shuttle Discovery remains grounded at Kennedy Space Center. The liftoff of STS-119 now has slipped from Feb. 12 to no earlier than Feb. 22, but even that date is speculative--with up to a two-month delay being rumored.
The crux of the problem: the reliability of three poppet-style valves that control the flow of gaseous hydrogen from the shuttle's three main engines to the external tank during liftoff. The valves must maintain an exact pressure within the hydrogen tank in order to provide fuel to the main engines.
During Endeavour's launch in November 2008, one valve allowed extra amounts of hydrogen to flow through. The other two valves compensated, but now NASA engineers want to know why the problem occurred--and how it could affect future flights.
With that, a full-bore investigation is underway at KSC. Shown above: a diagram of the complex space shuttle main engine. Outside this diagram: a flow control valve (FCV) for each main engine.
In Part I of this series I detailed the background of this serious problem. Part 2 focused on the dangers of valve failure, rated as a Criticality 1 (could cause lose of vehicle and crew) in 2001 and metal cracks in valve poppets analyzed back in 1992.
Here's how the main engines operate, according to NASA:
The SSME uses a two-stage combustion process. Liquid hydrogen and liquid oxygen are pumped from the external tank and burned in two preburners. The hot gas from the preburners drives two high-pressure turbopumps - one for liquid hydrogen (fuel) and one for liquid oxygen (oxidizer).
The high-pressure pumps inject the fuel and oxidizer into the main combustion chamber, where the second stage of combustion occurs. The expansion of the hot gases through the chamber and the nozzle produces thrust. The resulting blue-white exhaust is mainly water vapor (super-hot steam).
In approximately 8 minutes, 40 seconds, the three SSMEs burn over 1.6 million pounds of propellant (approximately - 528,000 gallons).
Temperatures inside the main combustion chamber reach 6,000 degrees Fahrenheit, hot enough to melt steel. Meanwhile, liquid hydrogen circulates through miles of tubing at -423 degrees to cool the engines.
Turbomachinery in the pumps rotate at up to 37,000 revolutions per minute. These speed and temperature extremes are unmatched by any other machine in the world!
While all that is going on, with computers and sensors monitoring every minute action and bit of data, the main engines, via the flow control valves are also sending gaseous hydrogen back to the external tank. The valves must respond to the ullage--amount left--of fuel in the tank, and properly flow fuel amounts to the main engines to keep them working properly--and safely.
The space shuttle and its systems are actually an engineering marvel of plumbing, electrical connections, hydraulics, sensors, computers, and millions of parts, all powered into space with volatile fuels. The importance of even a small valve recalls the old nursery rhyme that issues a caution about the importance of small items: for want of a nail, a shoe was lost; for want of a shoe, a horse was lost, etc.--all the way to the loss of a major battle for want of a nail.
Along with the STS-126 FCV problems, the 2001 citing of the valves as Crit 1 items, and the 1992 study of mysterious cracking on valve flanges, the Flight Readiness Review for STS-114 revealed another valve problem. The June, 2005 FRR included this item: an"unplanned" replacement of the "MPS GH2 Flow Control Valves (3). No further information is provided in the report.
How important can "just a valve" be? In 1984, a sluggish fuel valve in Discovery's main engine #3 malfunctioned and caused an abort at four seconds before the orbiter's planned maiden launch. A fuel leak led to a fire on the launch pad. All the astronauts evacuated safely, but the incident underscored how crucial just one valve can be. STS-41D eventually launched later that summer.
The current Discovery delay will add complexity to an already-conflicted schedule. KSC will need to turn over one of its two shuttle launch pads to the Ares project later this month if the new rocket's first test flight is to meet its mid-July schedule.
However, NASA had planned to have a second shuttle on standby for the May 12 Hubble mission. Either NASA figures out a way to launch one shuttle, then roll another to the same pad, and get it ready for a possible emergency flight, or else planners reserve both pads for shuttles.
In that case, the Ares test flight would most likely slip to October or later.
Image credit: Encyclopedia Britannica & NASA
Comments