Currently classed as one of the main reasons for the delay to Space Shuttle Discovery’s Return to Flight mission (STS-114), the redesigned External Tank (ET) has been defended by Hal Simoneaux, director of production operations at NASA’s Michoud Assembly Facility in New Orleans.
Building 420 at the New Orleans facility bears the legacy of the loss of Columbia and her crew of seven, with a wall-length poster of the Orbiter and signatures of astronauts who have visited the huge production plant in the months since the tragedy adorn the building.
“In both cases (Challenger and Columbia) you feel a great deal of loss because they (the astronauts) are family,” Simoneaux added. “Knowing that the tank was blamed for the second one made it hurt that much more. (However,) the (Columbia Accident Investigation) Board also said it found no indications of negligence or faulty workmanship in the application of the external tank thermal protection system.”
Following errors noted by two engine cut-off (ECO) sensors during the recent tanking test, and concerns raised by reviews looking into icing and foam shedding, the importance of eliminating as much of the ‘threat’ from the tank as possible has been paramount for NASA managers in the run-up to STS-114, now slated for the second half of July.
While foam shedding is a familiar problem, having been the root cause in the loss of Columbia when a piece of shed insulation foam struck and breached the Orbiter’s left wing on assent, the ECO sensor issue is less well known. The sensors (there are four in the hydrogen tank and four LOX sensors in the LOX feedline in the aft fuselage of the Orbiter) are designed to protect the main engines from the damage that would occur if the propellants ran low. During the tanking test, two of the sensors behaved erratically, if this had happened during ascent the likely result would have been a launch abort. KSC engineers have been investigating the cause of the ECO sensor problem, but as yet have been unable to fully nail down the cause. Engineers currently believe that the problem isn’t with the sensors themselves, but with one of the electronics boxes on the Orbiter or with a connector.
Following what now appears to be a likely second tanking test to and subsequent roll-back of Discovery to the Vertical Assembly Building (VAB), engineers are currently planning to perform a rarely executed “stack swap” – they will de-stack the Shuttle and place the Orbiter on Atlantis’ stack. This shuffling of the stacks is being contemplated since access to the ECO sensors and connectors will require cutting into the ET insulation which has already been ‘closed out’ for flight. Swapping the stacks will buy engineers additional time.
The ECO sensor is not the only potential trouble-spot highlighted as a result of the tanking test. During the test, the tank performed largely as expected but there were a few areas of concern. Concerns which were raised again in the recent Debris Verification Review (DVR) – such as ice building up on the tank.
The supercold propellants inside the 154 foot long ET – more than 390,000 gallons of liquid hydrogen at minus 423 degrees Fahrenheit and 145,000-plus gallons of liquid oxygen at minus 297 degrees Fahrenheit – cause ice to form on the outside of the tank as the shuttle is prepared to launch. Insulation, applied as a foam at Michoud, reduces the amount of ice. However, more ice than was predicted formed in some areas of the tank.
The ET’s for both STS-114 and Atlantis’ STS-121 (STS-300) will have an extra modification and extra heaters will placed over areas of concern. The main new modification to be made to the two tanks at the Kennedy Space Centre is the installation of a heater on the 70-foot liquid oxygen propellant line – again reducing potential for ice build – ice that could potentially fall off and strike the Orbiter on assent. There is an outside possibility that due to the high humidity during the summer months, Discovery could see her launch delayed yet further, to later in the year, when the chance of high levels of moisture in the air turning to ice are lessened.
Back at the Michoud plant where the ET’s began their journey, Simoneaux noted that large foam wedges that once covered the bipod ramp’s metal fixtures have been replaced with thinner base layers of foam, representing an 80 percent reduction in foam in that area. Also new is a copper plate plugged into heating coils that sits below the metal fixtures, a move to prevent ice formation.
“That (the heaters) worked real well during the tanking test,” Simoneaux noted. “NASA tested the tank to check the performance of the new ice-preventing heaters and other post-Columbia modifications. As part of the ET redesign, metal joints – called bellows – along an outside liquid oxygen fuel line where ice tends to form were covered with insulation encased in a fibreglass cloth. Also, bolts along a tank seam have been reversed so air pockets cannot form behind them when foam is sprayed over the seam.”
Inside Building 420, the foam on the tank also is X-rayed. “That’s a new thing we did starting with ET-120,” Simoneaux said.
Michoud shipped tank ET-121 to Florida in early March. Nine more existing tanks must be retrofitted to meet the new requirements.
“The post-Columbia modifications to the external tank will increase the cost of the $40 million tank and add a month or two to the normal 20-month construction period,” added Simoneaux.
A mandate of 28 launches has been mentioned until the Shuttle Fleet is retired.