As the nation's Evolved Expendable Launch Vehicle fleet prepares for its 50th flight next month, the U.S. Air Force on Monday announced the purchase of an Atlas 5 and a Delta 4 for future military satellite deployments.

Credit: Justin Ray/Spaceflight Now LAUNCH PHOTO GALLERY

Rocket services provider United Launch Alliance was awarded the $398 million firm-fixed-price contract to launch the Navy's Mobile User Objective System 4 spacecraft, MUOS 4, aboard the most-powerful version of the Atlas 5 rocket from Cape Canaveral, Florida, and a GPS navigation bird aboard a Delta 4 booster.

An Atlas 5 successfully deployed the first MUOS satellite in February, beginning construction of a new mobile communications satellite constellation to create a 3G cellular telephone network for voice, data and video services to military troops on the move.

The Atlas 5-551 rocket needed to haul the hefty MUOS spacecraft features a five-meter nose cone, five solid-fuel boosters and single-engine Centaur.

Atlas 5 has been tapped to launch MUOS 2 and MUOS 3 as well. The orbiting system is designed to include four primary satellites and one in-space spare.

Read all about MUOS in our previous launch story.

The other launch in Monday's contract covers a Global Positioning System satellite atop a Delta 4 rocket. The announcement did not specify which spacecraft in the GPS sustainment it would cover.

A pair of Delta 4 rockets have launched GPS 2F satellites from Cape Canaveral in the past two years, flying in the Medium+ (4,2) configuration with a four-meter upper stage and two strap-on solid motors.

See our previous GPS launch story for details on the spacecraft.

Atlas 5 and Delta 4 were born in the 1990s as modular systems that would carry all of the military's satellite fleets -- from the smallest to gigantic craft -- into space. The Air Force sought, developed, funded and now enjoys the fruits of the EELV program.

The maiden missions for both rockets occurred 10 years ago under the direction of their original parent companies -- Lockheed Martin for Atlas and Boeing for Delta.

But in subsequent years, the Air Force pushed for the creation of United Launch Alliance to operate both rocket lines, ensuring they remained viable and alive, while reducing overhead costs and erasing duplication in efforts between the two aerospace giants.

The 50th EELV launch is scheduled for June 18 from the Cape when an Atlas 5 flies the classified NROL-38 mission for the National Reconnaissance Office. A Delta 4-Heavy on the 51st mission follow closely behind on June 28 for another NRO deployment from Florida.