Precautionary modifications solve oscillation problems in rocket's Centaur upper stage

Lockheed Martin is on schedule to launch the delayed final US Milstar-2 military communications satellite on 6 April following precautionary modifications to the Titan IVB's Pratt & Whitney RL10-powered Centaur upper stage.

Launch of Milstar Flight 6, originally due to complete the five-satellite constellation in February, was postponed while changes were made to reduce the chance of excessive vibration on the RL10. Vibration was first noticed during an Atlas Centaur launch last October, and prompted an RL10/ Centaur hot-fire test programme by Lockheed Martin. "We found a very small possibility of vibratory exceedence on the Milstar and we thought it possibly a risk to the configuration," says Titan programmes vice-president Tom Scanlan. The third Milstar-2 was lost in 1999 due to a Centaur malfunction.

"In recognition of the $1.2 billion cost and critical importance [of Milstar], we recommended to the government we stand down to understand this more fully," adds Scanlan. Tank pressures were adjusted to slow the fuel feed and burn rates, which "prevented the oscillations that started in the oxygen pump". Tests also demonstrated the booster retained adequate net performance margin for the launch mission.

Lockheed Martin says the modifications are unique to this mission, as none of the remaining five Titan IV launches (all with US Department of Defense reconnaissance payloads) are of the same configuration. The last launch is scheduled from Vandenberg, California, in early 2005.

Following launch, Milstar 6 is expected to be checked out in record time to join the current constellation, which provides high-speed anti-jam communications to primarily mobile forward and special forces. Milstar works with the Lockheed Martin-built Defence Satellite Communications System (DSCS), a constellation now supporting communications in the Gulf. A fifth DSCS III was launched from Cape Canaveral, Florida, on a Boeing Delta IV on 10 March, and is being checked out to enter service over the Indian Ocean. DSCS provides point-to-point services via ground stations, and is used in US unmanned air vehicle operations.

Source: Flight International