Atlantis Returns to Pad
Space Shuttle Atlantic (STS-117) is once again making the 3.5 mile trip from the VAB to launch pad 39A. It may look unlike what everyone has come to envision as a space shuttle - but NASA's engineers have determined it is both safe and ready to fly.
The Space Shuttle system actually consists of the orbiter, two reusable solid rocket boosters and the large orange external tank. Or at least, it used to be a solid orange external tank. STS-117's external tank looks unlike any tank ever seen before. Instead of that nice solid orange color - this tank is pockmarked with a myriad of white spots - where engineers sprayed, scraped and filled fresh foam into more than 2600 pits acquired during a freak hail storm in February.
STS-117 should have launched March 15 of this year. Atlantis was on the pad and ready to go when a freak hail storm struck on February 26. Wind gusts up to 62 knots were recorded and hail up to 1.5 inches in diameter was found around the pad. The rotating service structure was around the shuttle at the time but this does not provide protection for the entire system.
The upper, liquid oxygen portion of Atlantis' external tank bore the brunt of damage, suffering about 4200 dings, dents, gouges and pits to the orange foam insulation. The orbiter also suffered very minor scrapes to twenty six heat shield tiles on the orbiter's left wing. Most fortunate - detailed inspections revealed no damage to the fragile carbon composite wing leading edges.
The foam insulation on the external tank is used to prevent ice build-up from the super cold fuels inside. Pieces of ice falling during launch could cause severe damage although falling pieces of foam have caused problems itself - including damage that led to the loss of Columbia.
After the hail storm, engineers conducted an inspection and found that damage to the tank was so severe that managers decided Atlantis would have to come off the pad and be returned to the VAB for repairs. "I'll never forget the day of the hail storm itself and then the first time I saw the external tank in the VAB," launch director Mike Leinbach reflected. "I was really wondering if we were going to be able to fix this tank or not."
Of the 4200 impacts from hail, close to 1500 were clustered near the very top of the external tank. The dings were clustered so closely together that the entire area was sanded down and resprayed with the foam insulation. The area then had to be milled to the proper slope and thickness. Another 450 dings were located on one side of the oxygen tank and repaired with a second large area of foam.
About 1100 individual pits were fixed using a pourable foam and another thousand fixed by just sanding them down. Around 412 were so minor it was determined they could "fly as is" with no repairs needed.
Similar repairs to the foam had been made before - though none on so grand a scale. As a safety measure, engineers reassessed flight performance of previous repairs as well as testing simulated repairs at Marshall Space Flight Center. The first few minutes of flight places the most stress on the external tank. During ascent, friction from the atmosphere causes temperatures in some areas of the tank to rise as high as 650 degrees. The foam has an ablation process which can allow heat to get to the aluminum-lithium shell of the tank and weaken it structurally. No such repairs have ever failed on a mission, although such repairs do pose some slight additional risk to flight.
The repairs were a tremendous challenge to the engineers, who worked around the clock conducting analysis, testing and repairs. The next launch window for Atlantis opens June 8th and NASA would like to make that date. There are three weeks work to do once the shuttle reaches the pad - which leaves only three days for any unexpected issues. "It's tight, but the guys feel we have a good plan in place," NASA Launch Director Mike Leinbach said. "If the weather hits us out at the pad, that hurts. But barring something else -- some other technical issue -- we have a good shot at June 8."
During the delay, while Atlantis was in the VAB, technicians removed Atlantis' three main engines to check for tiny pieces of rubber in propellant lines. A post launch inspection of Discovery last December reveled several pieces. A minuscule piece of rubber was removed from a propellant line in Atlantis.
STS-117 and her crew of seven will be transporting a new set of solar power-producing wings for the space station. Originally Atlantis had a crew of six, but the delay made NASA revise its crew rotation timetable on the ISS. It was decided to add Clayton Anderson to the crew, who will be replacing Suni Williams on the ISS. Williams will then return with Atlantis.
Launch is targeted for 7:37:56 p.m. on June 8. On May 30 and 31, NASA will conduct a two day flight readiness review to assess progress and set an official launch date.
Copyright © 1995 - 2008
Kathy Miles, Author, and Chuck Peters, Systems Administrator