Final Milstar communications spacecraft to launch Sunday

Posted: April 2, 2003

The Milstar 6 satellite being enclosed by the Titan rocket's tri-sector nose cone at the launch pad. Photo: Lockheed Martin
A 10,000-pound, $800 million satellite that will serve as an orbiting switchboard for secure communications between senior U.S. leaders and military forces around the world is slated for launch Sunday atop a Titan 4 rocket.

Liftoff is scheduled for 9:51 a.m. EDT (1351 GMT) from Complex 40 at Cape Canaveral Air Force Station, Florida. The day's available window extends four hours to 1:51 p.m. EDT (1751 GMT). (Editor's note: Sunday marks the start of Daylight Savings Time).

The $461 million Titan 4 with a Centaur upper stage will place the sixth and final Military Strategic and Tactical Relay spacecraft into geostationary orbit 22,300 miles above the planet about six-and-a-half hours after liftoff.

Controllers will position the satellite -- known as Milstar -- in view of the U.S. for testing. Its ultimate operational location has not been announced and is "under the cognizance of U.S. Strategic Command," the program office told Spaceflight Now.

Milstar 6 completes the series of Lockheed Martin-made satellites than began launching in 1994. Except for the family's third craft, which was stranded in a worthless orbit during a failed launch in 1999, the other birds were successfully delivered into space aboard Titan 4 rockets.

An artist's concept of a Milstar satellite in space. Photo: Lockheed Martin
Officials at the Los Angeles Air Force Base program office say the Milstar network, which achieved global coverage with last year's deployment of Milstar 5, is operating properly.

"The Milstar constellation health has an outstanding track record and continues to provide our forces with near perfect system availability."

Milstars act as relay stations in space, serving users on foot, ships, submarines and aircraft by transmitting voice communications, data, imagery and video.

"Milstar is the world's most secure, nuclear survivable, space-based communications system. It performs processing and network routing in space, eliminating the need for vulnerable land-based relay stations and reducing the chances of communications being intercepted on the ground," the program office said.

"Flight 6 will complete the Milstar satellite constellation which provides near-global coverage for the President, the nation's strategic forces, the Air Force's missile warning assets and our operationally deployed military forces. In addition, Flight 6 will significantly increase the tactical communications coverage for our forces."

File image of Titan 4 rocket igniting on the pad to place Milstar satellite in space. Photo: Russ Underwood/Lockheed Martin
This will be the first Titan 4 flight in more than a year. The last time one of the massive Lockheed Martin-built rockets was launched -- January 2002 -- it carried Milstar 5.

Sunday's mission was originally slated to occur in January. However, officials ordered a postponement to tweak Centaur upper stage software to prevent "oscillation instability" during the launch. The concern stems from the use of newer model Pratt & Whitney RL10 engines on this Centaur.

"Minor modifications to the Centaur navigation computer software and database allowed for a flight profile that avoided the possibility of cavitation upstream of the Centaur liquid oxygen pump. Without cavitation, there is no risk of the oscillation instability that was the original concern in January," the Titan launch program office told Spaceflight Now.

"The Titan team demonstrated excellent system engineering in innovatively modifying the pressure in the oxygen tank, along with varying the hydrogen/oxygen mixture ratio during portions of the three Centaur engine burns, to avoid cavitation.

"The resulting flight profile is almost identical to the previous flight plan. The team was able to accomplish this without sacrificing the excellent reserves of propellant and pressurant gas carried on the Centaur upper stage."

This specific Titan 4 core vehicle -- the two liquid-fueled stages -- and twin strap-on solid rocket motors were originally moved to launch pad for last June's planned spy satellite deployment mission for the National Reconnaissance Office.

But problems with that classified cargo significantly delayed the launch, prompting the Air Force to flip-flop the NRO and Milstar 6 flights. The Titan core and solid motors remained on the pad, while the Centaur earmarked for the NRO mission was replaced with the Centaur designed for the Milstar 6 launch.

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