Engineers will test a redesigned part of the Minotaur 4 rocket's third stage later this month to ensure it eliminates unintended thrust from the motor, potentially clearing the way for a foursome of military launches this spring and summer, an Air Force spokesperson said.

The Minotaur 4 pathfinder vehicle on the pad in California. Credit: Orbital Sciences

Teams from ATK, Orbital Sciences Corp., Northrop Grumman Corp., and the Air Force Space Development and Test Wing developed a diffuser for the third stage of the four-stage rocket.

Taken from retired Peacekeeper missile stockpiles, the SR120 third stage is considered government-furnished equipment. The Minotaur 4 is propelled into space by three Peacekeeper stages, then an Orion 38 fourth stage motor inserts payloads into orbit.

The first flight of the new rocket was delayed from last year after analyses indicated the gas generator device that powers the third stage steering system would produce unintended thrust after the motor burns out.

Officials were concerned cold gas attitude control thrusters would attempt to compensate for the unintended thrust.

"It was possible before the diffuser fix mentioned above was implemented that this moment would eventually cause the depletion of this cold gas during long coast periods, possibly causing loss of control," an Air Force Space Development and Test Wing spokesperson said.

The diffuser will be installed on all Minotaur 4 third stages to reduce the gas generator's unintended thrust to acceptable levels well within the control abilities of the cold gas thrusters, according to the Air Force.

A Peacekeeper SR120 motor outfitted with the diffuser will be ignited during a ground test March 31.

"This static fire will provide additional confidence that the implemented diffuser solution is functioning as designed," the Air Force said in a statement.

If the test goes well, a suborbital three-stage Minotaur 4 rocket will launch a Defense Advanced Research Projects Agency high-speed glider around April 20. The Hypersonic Technology Vehicle, or HTV, will be boosted to a precise point in space from Vandenberg Air Force Base, Calif., then follow a series of programmed range and cross-range maneuvers before impacting near Kwajalein Atoll in the Pacific Ocean.

The third stage diffuser will fly on the HTV mission as a demonstration, although the suborbital flight does not require the new system.

Three satellite launches are scheduled as soon as this summer, beginning with a multi-payload mission for the Air Force Space Test Program no earlier than May 28. Launching from Kodiak Island, Alaska, the Minotaur 4 rocket will shoot a cache of military, NASA and university satellites into orbit.

The 2,200-pound Space Based Space Surveillance satellite is also on the Minotaur manifest. The $800 million mission, also called SBSS Block 10, features an optical telescope to track objects circling in geosynchronous orbit 22,300 miles above Earth.

SBSS could launch as soon as July, pending the results of ground and flight tests of the third stage gas generator diffuser, the Air Force said. A specific target launch could be determined in the next two weeks.

The SBSS launch has been pushed back about three years, partially due to rocket issues. In a March 10 statement to the Senate Armed Services Committee's Subcommittee on Strategic Forces, a Government Accountability Office manager stated SBSS program officials and contractors are studying the feasibility of launching the payload on a larger Delta 2 rocket.

The Air Force spokesperson said the Minotaur 4 vehicle remains baselined to launch SBSS.

Alaska will be the starting point for another Minotaur 4 flight around August, when the solid-fueled rocket will launch the military's experimental TacSat 4 satellite. TacSat 4 will test new tactical reconnaissance and surveillance sensors.

At least eight Minotaur 4 rockets are slated to launch orbital and suborbital missions over the next few years.