|The Heat is On! New Heater Added to Space Shuttle's Fuel Tank||
A new heater added to the Space Shuttle's fuel tank will significantly reduce the potential for ice and frost buildup and improve the safety of the tank.
The 15-story tank is filled with more than a half-million pounds of super-cold liquid oxygen and liquid hydrogen fuel. Because ice sometimes forms on portions of the tank's exterior and can break off during launch and potentially damage the Shuttle's Orbiter, NASA made the decision on April 29 to re-schedule the Shuttle's Return to Flight mission to July so the heater could be installed on the Return to Flight tank.
Image at right: ET-121 heads to the barge for shipment to Kennedy Space Center. Image credit: Lockheed Martin/NASA Michoud
The belt-like heater wraps around the top joint of the External Tank's liquid oxygen propellant line, which runs from the tank's midsection to its base. NASA already had planned to add the heater to the liquid oxygen feedline bellows -- a fuel line joint that feeds the Shuttle's main engines -- for the Shuttle's third mission after returning to flight. It has now been added to External Tank-121, originally slated to fly with Atlantis on mission STS-121, but which will now help carry Discovery into space on the upcoming STS-114 mission.
The bellows heater is a copper-nickel alloy metal strip heater, similar to heaters used on the Solid Rocket Motor joints, which will keep the bellows area slightly warmer than freezing, about 40 degrees Fahrenheit. The heater strips are about 53 inches long -- the circumference of the bellows -- and about 0.5 inches wide. The two heater strips are covered and joined by a silicone gasket that allows the heater to be bonded between the bellows rain shield and end shield. Tabs placed at intervals on the heater assist in its placement on the bellows and allow tests to verify the strength of the adhesive bond.
There are five bellows along the 70-foot-long, 17-inch-diameter liquid oxygen feedline assembly that runs externally along the right side of the liquid hydrogen tank up and into the intertank. The bellows allow the feedline to move or flex during fueling of super-cold liquid prior to launch. The topmost bellows –near the top of the hydrogen tank -- poses the main concern for potential ice debris because of its position above the Orbiter.
Although testing has proved ice released in this area typically occurs at relatively low speeds -- during the initial stages of liftoff and the climb to orbit due to vibration -- the potential still exists for ice and frost to shed later when it would be a more hazardous debris source. Ice released later is more of a danger because of the speed of the vehicle. This information prompted a redesign of the bellows that includes adding a "drip lip" and a heater to promote condensate runoff and to prevent the formation of ice.
Initially, a foam skirt around the bellows rain shield was extended to divert condensate. This modification, called the “drip lip,” squared off the edge of the rain shield and allowed the condensate to "drip off." Though it was decided that the drip-lip configuration would be flown on STS-114, engineers in the External Tank Project Office continued to pursue the option of a heater for the bellows.
The heater will be turned on shortly after the liquid oxygen tank begins to fill approximately 5 hours and 10 minutes before launch, and turned off 1 minute and 52 seconds before launch.
The Return to Flight mission will take Shuttle Commander Eileen Collins and six crew members to the International Space Station. The mission is the first of two test flights to evaluate new inspection and repair techniques of the Thermal Protection System, the Shuttle's heat shield, and to deliver supplies to the Station.
Martin Jensen, Marshall Space Flight Center