Extra Shuttle Flight
NASA space shuttle officials expect to add a second "return-to-flight" mission before resuming complex operations at the International Space Station, which would push major ISS assembly out a few months, to two missions beyond first flight.
The initial return-to-flight, now already extended to next summer from March 2004 (see p. 25), is Mission STS-114, and officials are talking of a following STS-114.1 to further address return-to-flight issues and ensure the ISS is sufficiently healthy for assembly to continue. Some future ISS work may be moved forward to this new mission.
Overall, shuttle productivity will be hurt by several post-Columbia changes:
*The spacecraft will carry less payload because of the extra weight of adding the normal remote manipulator system (RMS) arm on the left sill of the cargo bay, and a new 50-ft. extension boom stowed on the right sill. Adding the boom to the end of the RMS will make the total length 100 ft., allowing it to access the entire underside of the shuttle for thermal protection system inspection and repair. In the past, to gain extra ISS payload, the normal RMS was not being carried on ISS missions. It weighs about 1,000 lb. The boom is based on RMS structure and may weigh 800-1,000 lb., said Paul Hill, lead flight director for STS-114. There will also be the estimated 350-lb. weight of tile repair equipment. STS-114 is carrying a tile repair experiment that weighs about 500 lb.
*Scanning tiles in orbit will take one full day. If the initial scan shows a problem, about two more days would be needed for further inspection and repair. This would detract from ISS work and require leeway at the end of the mission.
*More conservative operation rules could hurt flexibility and slow decisions, giving a hard-to-quantify hit on productivity.
*Launch windows will be cut in half by the need to photograph each ascent to check for debris. This means the launch must be in daylight, and the external tank must also be in daylight for at least 6 min. after separation over the mid-Atlantic so the crew can photograph it. This particularly reduces launch windows in autumn and winter. Launches to the ISS are not possible in December, February and March, and only a few days are available in November and January, said Greg Oliver, chief of the Ascent/Descent Dynamics Branch here.
New hardware and rules require that some payload be deleted, that there is less ISS rendezvous opportunity, and that more time be reserved at the ISS, Hill said.
One step being taken to regain part of the payload is to let the ISS orbit decay from its nominal 205 mi. to 190 mi. at the time of shuttle rendezvous. This would increase shuttle uplift by 1,500 lb., Hill said. Afterward, the ISS could be reboosted to 205 mi. This would come at the cost of extra ISS drag and propellant usage in the lower atmosphere, but officials said the station is not facing a propellant shortage.
NASA morale was boosted by the recent electrical power-up of shuttle Discovery. It was the first shuttle activation since the Feb. 1 Columbia accident.
Agency officials began to detail the new mode of shuttle operations here last week, but stressed that their plans are still in flux. The return-to-flight implementation plan will be revised one or two times per month, and will soon add an appendix to address the Columbia Accident Investigation Board's "observations," contained in Chapter 10 of the CAIB report.
Thermal protection system damage is to be detected by several types of imaging. First, ground cameras will look for debris near the orbiter. Improvements since Columbia include the requirement to launch in good lighting, an increase in the number of cameras to about 23 from 14, better maintenance of the cameras, and possible addition of cameras farther out on a ship or on a pair of NASA WB-57F Canberra jets at 63,000 ft.
Second, the external tank (ET) will be photographed after separation both by cameras in the ET umbilical wells on the shuttle belly and by the astronauts out the windows. Improvements over past practice include the requirement to do this on every flight in good lighting conditions, more digital cameras in the cabin for quick downlink to ground analysts, and a digital camera in the right ET well. Also, cameras on the ET and the solid rocket boosters will be used on each flight and some upgraded to better quality and moved to better positions.
Once in orbit, the RMS, augmented by the 50-ft. boom, will carry a camera around the shuttle to photograph the tiles and reinforced carbon-carbon (RCC) pieces. Difficulties include detecting damage because of low contrast on the white tiles and black RCC, and the need to find small cracks in the RCC. Hill would like to use a laser radar camera with depth information, but so far the laser radar won't work while the arm is in motion, and it would take too long to constantly stop and start the arm while covering most of the shuttle. This might be fixed with software, but that is far from certain. Using a TV camera with polarizing filters to enhance contrast is the likely near-term technique. The entire process will take at least an entire day. These pictures will be transmitted to analysts on the ground, and they should have a map of any potential trouble spots by the fifth or sixth flight day.
As the shuttle approaches the station on the third flight day, it will stop 600 ft. away and do a pitch pirouette so ISS crewmembers can photograph the surface. The maneuver should take just 20 min., and will complement the pictures taken by the RMS camera. Neither set gives the full resolution desired. The ISS pictures also will be downlinked and analyzed on the ground.
If nothing serious is detected, that is the end of it. But if something is found, areas flagged by the analysts will be given closer inspection by the RMS. This time it might carry the laser radar because there's time to linger at the small number of target sites.
But the prospect of false alarms raises its head. The shuttle has an average of about 30 tile gouges at least 1 in. in size on every flight. Most of these are considered safe after inspection on the ground, but what will the new inflight system conclude?
If this second look indicates an area requires repair, the RMS, and boom if necessary, will place an astronaut at the site. This process will not be available on STS-114; repairs would be made from the ISS arm. The tile repair technique is well along in development, and fills the gouge with MA-25S, a silicone rubber-based ablator material. There's significant concern about making the repair stick to the dusty inside of a gouge. Another problem would be if the boom extension is used--the entire assembly is very limber, moving 16-20 in. with just 5 lb. of force, Hill said. The astronaut should expect a lot of unwanted motion as he presses the repair in place.
Repairs to RCC are not as developed as tile repairs. Four RCC techniques are under consideration, and one might be placed on STS-114 (AW&ST Sept. 1, p. 22).
Astronauts will experiment doing tile and possibly RCC repairs in the STS-114 payload bay. They will not be riding the RMS, and will be relatively stable next to the workpiece. Other items on the STS-114 agenda include replacing a ISS control moment gyro, which is required before major assembly continues with the addition of the P3/P4 Integrated Truss Segment with more solar array panel on ISS Flight 12A. This work temporarily creates an asymmetric ISS configuration that produces a large yaw force, and officials want to have at least three control moment gyros (CMGs) working to counter the yaw without having to immediately resort to thrusters. Now only two of the four CMGs are working, and STS-114 is to provide a third working gyro.
The mission carries about 10,000 lb. of cargo to be placed inside the ISS, primarily food and a Safer maneuvering backpack that serves as a safety backup in case an astronaut "falls" out of control during extravehicular activity. There also is an external storage pod to be placed on the outside of the ISS. This activity could be altered if time is needed at the end of the mission for tile inspection and repair.