Risk tradeoffs
So, Nasa has decided not to repair damage to the Thermal Protection System (TPS) tiles on the underside of Endeavour. Much of the discussion they wert through was a tradeoff between risks. How did they reach the decision - and what options were available?
I have this knack for producing sexy, exciting titles to entries here....
They machined a test segment of tiling with the same damage that the shuttle has and subjected it to extreme theremal testing. The result was (picture from Nasa status briefing slides):
The first thing about the TPS damage is that shuttles have landed with worse damage - there was never a view that the magnitude of the damage would result in catastrophic breakup - but it might result in overhaul work to the underlying (aluminium honeycomb) structure. There's quite a bit of experience with this on early shuttle missions, some of which came back with quite large singed patches. However, the possibility of catastrophic failures caused by ice or foam debris during ascent was not fully appreciated until the loss of Columbia. this has led to the intensive imaging process on the Shuttle prior to docking with the ISS.
Also post-Columbia, various repair options have been developed (and there is always scope for improvisation - as the sewing down of a loose thermal blanket on the last mission demonstrated). Having precisely imaged the hole, NASA had options for filling it but after much analysis decided against it. Why not do this one when they stitched the blanket back on the last one?
There are several reasons - firstly, the underside of the shuttle is less accessible, and (overall) more thermally critical. Because the access is difficult (involving the astronaut being perched on a boom on the end of a manipulator arm with several joints being steered by camera) there's a much greater risk of the astronaut causing further damage. The thermal testing that has been performed after replicating the gouge indicates that although the adjacent tile might be damaged, there shouldn't be any structural issues (and Endeavour is not due for another flight until 2008, giving ample time for repair).
Furthermore, if the repair is not done perfectly, the change in airflow might have unexpected effects. Finally, there is a 1 in 2000 risk on each Extra Vehicular Activity (EVA) that the astronaut would be killed by a piece of orbital debris, so there is a reason for not doing it if you don't have to.
What would be the scenario if the shuttle had been more severely damaged so that there was serious doubt about a safe re-entry?
Again, post-Columbia the requirement is that another shuttle is available to rescue the astronauts, who can camp out on the international space station (ISS) until the so-called "launch-on-need" (LON) shuttle can reach them - there are sufficient consumables on the space station to do this. This procedure is Contingency Shuttle Crew Support(CSCS).
What happens to the shuttle in this case? There are two options. Firstly, a small amount of rewiring allows them to configure the remote closing of the docking hatch and undocking of the shuttle from the space station - the shuttle computers can be controlled from the ground, so the shuttle would be deorbited into an ocean (the payload bay doors would not be closed and the shuttle would enter the atmosphere with that ide down to accelerate disintegration).
There's a more interesting option, though, that NASA are more coy about. Further rewiring would allow the few manual functions (landing gear being the big one) in re-entry and landing to be automated from the ground as well - the shuttle could then make a remote-controlled landing (probably at Vandenberg in California - if the shuttle did break up on re-entry, the debris would land in the sea). the risk trade here is very interesting - it seems an obvious route to take but there is a risk in leaving the crew on the ISS.
While there are three permanent crew on the ISS, a three-seater Soyuz is always available as a refuge or escape module for the crew (the point at which someone officially becomes ISS crew is when their moulded seat-liner, which supports them during re-entry on the Soyuz, is put in place). The assumption is that a leak or fire would firstly involve isolating parts of the ISS and ultimately might involve the crew evacuating. The shuttle crew would not have an evacuation option. Of course, the likelihood is they wouldn't stay on the ISS for long before rescue, but it requires some balancing of the extent of damage and the risk of possible delay to the rescue mission.
Finally, there's the mission next year to service the Hubble Space Telescope - there will not be a station available to camp out in for that, so the requirement there is stricter - the LON shuttle will be on the other pad when the first once launches, and the shuttles will grapple each other with their manipulator arms while the crew move from one to the other in spacesuits. There was great hesitation in doing the Hubble mission at all, but the astronaut corps mad eit very clear that they were willing to take the risk in order to keep the telescope operating until its replacement is launched.
I have this knack for producing sexy, exciting titles to entries here....
They machined a test segment of tiling with the same damage that the shuttle has and subjected it to extreme theremal testing. The result was (picture from Nasa status briefing slides):
The first thing about the TPS damage is that shuttles have landed with worse damage - there was never a view that the magnitude of the damage would result in catastrophic breakup - but it might result in overhaul work to the underlying (aluminium honeycomb) structure. There's quite a bit of experience with this on early shuttle missions, some of which came back with quite large singed patches. However, the possibility of catastrophic failures caused by ice or foam debris during ascent was not fully appreciated until the loss of Columbia. this has led to the intensive imaging process on the Shuttle prior to docking with the ISS.
Also post-Columbia, various repair options have been developed (and there is always scope for improvisation - as the sewing down of a loose thermal blanket on the last mission demonstrated). Having precisely imaged the hole, NASA had options for filling it but after much analysis decided against it. Why not do this one when they stitched the blanket back on the last one?
There are several reasons - firstly, the underside of the shuttle is less accessible, and (overall) more thermally critical. Because the access is difficult (involving the astronaut being perched on a boom on the end of a manipulator arm with several joints being steered by camera) there's a much greater risk of the astronaut causing further damage. The thermal testing that has been performed after replicating the gouge indicates that although the adjacent tile might be damaged, there shouldn't be any structural issues (and Endeavour is not due for another flight until 2008, giving ample time for repair).
Furthermore, if the repair is not done perfectly, the change in airflow might have unexpected effects. Finally, there is a 1 in 2000 risk on each Extra Vehicular Activity (EVA) that the astronaut would be killed by a piece of orbital debris, so there is a reason for not doing it if you don't have to.
What would be the scenario if the shuttle had been more severely damaged so that there was serious doubt about a safe re-entry?
Again, post-Columbia the requirement is that another shuttle is available to rescue the astronauts, who can camp out on the international space station (ISS) until the so-called "launch-on-need" (LON) shuttle can reach them - there are sufficient consumables on the space station to do this. This procedure is Contingency Shuttle Crew Support(CSCS).
What happens to the shuttle in this case? There are two options. Firstly, a small amount of rewiring allows them to configure the remote closing of the docking hatch and undocking of the shuttle from the space station - the shuttle computers can be controlled from the ground, so the shuttle would be deorbited into an ocean (the payload bay doors would not be closed and the shuttle would enter the atmosphere with that ide down to accelerate disintegration).
There's a more interesting option, though, that NASA are more coy about. Further rewiring would allow the few manual functions (landing gear being the big one) in re-entry and landing to be automated from the ground as well - the shuttle could then make a remote-controlled landing (probably at Vandenberg in California - if the shuttle did break up on re-entry, the debris would land in the sea). the risk trade here is very interesting - it seems an obvious route to take but there is a risk in leaving the crew on the ISS.
While there are three permanent crew on the ISS, a three-seater Soyuz is always available as a refuge or escape module for the crew (the point at which someone officially becomes ISS crew is when their moulded seat-liner, which supports them during re-entry on the Soyuz, is put in place). The assumption is that a leak or fire would firstly involve isolating parts of the ISS and ultimately might involve the crew evacuating. The shuttle crew would not have an evacuation option. Of course, the likelihood is they wouldn't stay on the ISS for long before rescue, but it requires some balancing of the extent of damage and the risk of possible delay to the rescue mission.
Finally, there's the mission next year to service the Hubble Space Telescope - there will not be a station available to camp out in for that, so the requirement there is stricter - the LON shuttle will be on the other pad when the first once launches, and the shuttles will grapple each other with their manipulator arms while the crew move from one to the other in spacesuits. There was great hesitation in doing the Hubble mission at all, but the astronaut corps mad eit very clear that they were willing to take the risk in order to keep the telescope operating until its replacement is launched.