delusionofgrandeur

After a successful mission to the International Space Station, Shuttle Endeavour has undocked and is preparing to return to Earth on Tuesday.

STS-118 was due to continue for a further day on orbit but was cut short due to Hurricane Dean which is currently threatening Jamaica and could pose a threat to normal operations at NASA MCC Houston (Texas) on Wednesday. The storm reaching Houston would necessitate a temporary command centre being set up elsewhere which controllers are keen to avoid.

Therefore Endeavour will be aiming to land at Kennedy Space Center in Florida at 16:32 ZULU on Tuesday with the weather alternate site being Edwards AFB near Los Angeles, California. The White Sands Space Harbour in New Mexico is not under consideration as a landing site for this mission.

During the on-orbit inspection of the Orbiter Thermal Protection System, a ‘gouge’ was found at the junction of a couple of tiles. This gouge was apparently caused during launch by a piece of foam from the external tank which broke free and struck the underside of the orbiter. The Mission Management Team do not feel that this gouge goes right through the Thermal Protection System and they believe that the underlying aluminium structure of the airframe will not be adversely affected by the heating of re-entry through the atmosphere.

I was asked by someone with regards to this, why the crew did not make an EVA to “replace the tile”.

Firstly, there is no means of replacing entire tiles on orbit. The tiles themselves are actually unique, each with slight differences in shape which would be impossible to replicate properly without ground facilities. The crew are instead provided with a compound based ‘patch kit’ with which they would fill any damaged areas to improve thermal and aerodynamic performance during re-entry.

However, using such a patch kit would be far from easy. To reach the underside of the space shuttle, an astronaut would be required to don their EVA suit and then attach themselves to a long extension bar fitted to the end of the robotic Canadarm. The robotic arm would them move the astronaut around to the underside of the shuttle and ‘present’ them to the working area.

This method of personally inspecting the underside of the shuttle has been tried previously on-orbit during STS-114 (Discovery). During that EVA, only a ‘gap filler’ was removed, no actual repair was made to the TPS. The motions that would be involved in a repair were simulated with the astronaut in situ and it was found that the combined length of the extension bar and the Canadarm was rather flexible and the astronaut found himself ‘bouncing around’ making work somewhat difficult.

So, NASA has had to make what they term a ‘judgement call’ and decide whether any risk due to the gouge is greater than the risk of having an astronaut bouncing around on an arm in close proximity to the TPS. Further, any such repair attempt may directly cause more damage to the TPS or problems with the way the compound is laid may actually hinder thermal protection during re-entry.

It should, of course, be noted that most shuttle flights prior to the loss of Columbia have involved some damage to the TPS with no major adverse effects on re-entry. The damage caused to Columbia during it’s final launch was significantly greater than that seen here on Endeavour and occurred on the reinforced carbon-carbon leading edge of the wing where heating is greatest.

This entry was posted on Monday, August 20th, 2007 at 12:36 am and is filed under Space.You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.