The launch date of US Space Shuttle Atlantis on mission STS-122 which had been put back to the 10th of January has slipped once more while remedial work is carried out on the external tank ECO sensors.

Atlantis’ original December launch date was scrubbed due to the simultaneous failure of two of the four ECO sensors during tanking.

As we explained before, the ECO (Engine Cut Off) sensors reside in the bottom of the liquid-hydrogen portion of the shuttle external tank. Their job is to detect the exhaustion of liquid hydrogen prior to it running out completely and signal the Space Shuttle Main Engines (SSMEs) to shut down. Normally, the Space Shuttle Main Engines would be commanded to shut down by the flight avionics with some liquid hydrogen to spare but should a fuel-line run dry at flight-power, the powerful turbopumps which force fuel and oxidiser through the engines would suddenly spin up beyond their rated rpm and fail catastrophically. Due to their proximity to vital systems such as APUs and avionics in the rear of the shuttle, the consequences of such a failure are not something NASA are willing to risk.

NASA appear to have now isolated the fault to the connections that lead from the External Tank carrying the ECO sensor signals. They intend to repair these connectors with the vehicle on the pad to make sure all of the sensors are functional for launch. No revised launch date has yet been set for STS-122

Such prudence with regard to safety is undoubtedly a good thing but it does raise an interesting problem for the remaining shuttle flights. The shuttles are due to be retired from service by mid-2010 (Atlantis is due to retire in mid-2008) which means that the ten remaining ISS construction missions will have to be flown at an average rate of one every 3 months, a rate which NASA has been unable to achieve in recent years. There is also a Hubble service mission scheduled for 2008 which, due to the different orbit and the inability to reach the safe-harbour of the ISS, will require a second Shuttle ready on the other pad for a contingency launch. It is planned that immediately after this flight, pad 39B will begin conversion for use with the new Ares I booster system which 39A is used for the remainder of the shuttle flights.

Personally, I’d be inclined to look at building Pad 39C which was projected but never constructed during the Apollo era, it would form the third pad in a row to the North of 39B and 39A which were themselves converted to shuttle use after the Apollo program. While 39C was never built, a short stub of crawler-way points toward it’s intended location from the dog-leg of 39B’s crawler-way. The construction of 39C would remove the constraint on Ares-I project testing as it is already intended that the VAB be fitted out to handle both Ares I and STS configurations at the same time during transition. The creation of 39C would also allow for more complex logistics when the Ares-V boosters come into service later. For the record, there were two further pads planned to the North of 39C but whether the VAB could ever need to service five vehicles at the same time seems unlikely.

I make the assumption that the construction of pads is a relatively insignificant cost in relation to the rest of the space program.