Space Picture of the Space Shuttle Main Engines (SSME) During the Launch of Atlantis During STS-115 September 9, 2006.

This latest improvement is to the controller, the on-engine computer that monitors and controls all main engine operations. The enhancement allows an engine to shut down during launch if vibration levels exceed safe limits, according to NASA. AHMS consists of advanced digital signal processors, radiation-hardened memory and new software.

The Space Agency says that AHMS first flew on Shuttle Discovery's STS-116 mission in December 2006 with a single controller on one engine, but in monitor-only mode, meaning AHMS collected and processed vibration data but could not shut down the engine. AHMS is operating in active mode - the ability to shut down an engine if an anomaly is detected - on a single engine during the STS-117 mission, and is scheduled to fly in active mode on all three engines during the STS-118 mission later in 2007.

In the event of an engine shut down, the Shuttle has several options available to abort the ascent:

• Return to the launch site
• Transatlantic abort landing
• Landing at an alternate site in the United States
• Rendezvous with the International Space Station

Each scenario would depend on when an engine shuts down during flight, the mission trajectory and mission specific requirements, such as payloads, says NASA.

The system uses data from three existing sensors, or accelerometers, mounted on each of the high pressure turbopumps to measure how much each pump is vibrating. High-pressure fuel and high-pressure oxidizer turbopumps rotate at approximately 34,000 revolutions per minute and 23,000 revolutions per minute, respectively. To operate at such extreme speeds, the turbopumps are equipped with highly specialized bearings and precisely balanced components. Output data from the accelerometers is routed to the new digital signal processors installed in the main engine controller. These processors analyze the sensor readings 20 times each second, looking for vibration anomalies that are indicative of impending failure of rotating turbopump components such as blades, impellers, inducers and bearings. When working correctly, if the magnitude of any vibration anomaly exceeds safe limits, the upgraded main engine controller would shut down the unhealthy engine immediately, according to NASA.

AHMS is the sixth major upgrade to space shuttle main engines since the first shuttle flight in 1981. The series of shuttle main engine enhancements have increased safety and reliability through such improvements as the addition of a two-duct powerhead, a single-coil heat exchanger, a new high-pressure oxidizer turbopump, a large-throat main combustion chamber and a new high-pressure fuel turbopump.

The shuttle's three main engines start approximately 6.5 seconds prior to lift-off. Once running, the solid rocket boosters ignite, and lift-off occurs. During ascent, the solid rocket boosters burn for approximately 120 seconds and are then jettisoned. The shuttle, still attached to the external tank with three main engines running, continues ascent until main engine cut-off (MECO) at 510 seconds, or eight and one-half minutes after liftoff - long enough to burn more than 500,000 gallons of fuel. The engines shut down just before the shuttle, traveling at about 17,000 mph, reaches orbit. The three main engines are never restarted during the mission, re-entry or landing. The shuttle returns to Earth as a glider.

NASA says the Space Shuttle main engines operate at greater temperature extremes than any mechanical system in common use today. These powerful engines are clustered at the aft end of the shuttle and have a combined thrust of more than 1.2 million pounds. Each is 14 feet long, is seven and one-half feet in diameter at the nozzle exit, weighs approximately 7,750 pounds and generates more than 12 million horsepower.

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