NASA hopes retrofitted heater on cryogenic fuel line will remove debris danger and aims for July launch window
NASA has delayed launch of the Space Shuttle return-to-flight mission STS 114/Discovery to mid-July after deciding the risk of ice debris damaging the orbiter warranted retrofitting a heater to a cryogenic fuel line on the external tank.
The delay, from a launch window of 22 May-3 June to 12-31 July, was also the result of sensor and other anomalies observed during a 14 April fuelling test on the launch pad at Kennedy Space Center. Some of the time will be used for troubleshooting at the pad before Discovery is rolled back to the vertical assembly building for installation of the heater.
NASA is concerned that ice formed before launch on the upper expansion joint, or bellows, of the tank's liquid-oxygen feed pipe, and on brackets along its 21.3m (70ft) length, could break off and strike the orbiter. Impact tests show the damage to reinforced carbon-carbon is the same for hard or soft ice, but soft ice is more likely to shake loose during the ascent.
The space agency thought a "drip lip" modification to the bellows area would reduce the risk enough, but now believes it needs to eliminate the ice by installing a heater. NASA also had not believed shed ice could hit the Shuttle, but analysis revealed impacts on the orbiter and solid rocket boosters during previous launches.
"Ice can come off early in the flight and it doesn't have a transport mechanism to ever get to the vehicle…[but] there's a small region in there when you're at a particular Mach number that you have a transport mechanism that gets the ice to the vehicle," says Bill Parsons, Shuttle programme manager.
A piece of ice measuring 130 x 50 x 50mm (5 x 2 x 2in) was shaken free during ground acoustic-vibration rig tests, and NASA plans windtunnel tests of the chunk at Mach 3.
The agency had already decided to fit the bellows heater to the third tank, now in final assembly, and will modify the two delivered to Kennedy. Discovery's tank can be retrofitted in the assembly building with the orbiter still attached, says NASA.
NASA is also likely to install infrared lamps around the launch pad to help prevent ice formation. The agency is looking at protecting the bellows and brackets with insulating gel or beads in shrink-wrap plastic, but this is causing debris problems, says Parsons.
Following the tanking test, an intermittent fault with two of four hydrogen cut-off sensors that shut down the Shuttle main engines if tank contents drop below a certain level is also being investigated. Another concern is a potential problem in the tank's helium pressurisation pipework. Insulation blankets on Discovery's orbital manoeuvring system pods also have to be cleaned or replaced.
The second RTF flight, STS 121/Atlantis, has been pushed back to 9-24 September.
TIM FURNESS/LONDON
ADDITIONAL REPORTING BY GRAHAM WARWICK IN WASHINGTON DC
Source: Flight International
