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Air Force satellite's epic ascent should finish soon

Posted: October 9, 2011

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After 14 months in space and more than 450 maneuvers, the first satellite in the U.S. military's next-generation secure communications network now expects to complete its arduous journey to reach the correct orbit by late October and enter service in March.

An artist's concept of AEHF 1. Credit: Lockheed Martin
A manufacturing mishap prevented the Advanced Extremely High Frequency satellite's main propulsion system from firing once the craft reached space, prompting ground controllers to devise emergency plans for salvaging the mission.

Burning its exotic electric thrusters for the past year, AEHF 1 has reached an orbit of 24,000 by 22,300 miles. It should achieve the desired circular orbit with a 4.8-degree inclination by month's end, allowing payload activation and testing to occur through February in preparation for entering service at long last in March.

This has been a painstaking effort to save the spacecraft's life by working around the main engine problem and still climbing to the necessary orbital perch where it can match Earth's rotation to cover a large swath of planet.

The Lockheed Martin-built satellite was carried aloft on August 14, 2010 aboard a United Launch Alliance Atlas 5 rocket. But the original plan of maneuvering the spacecraft toward its operational orbit was aborted after the satellite's onboard Liquid Apogee Engine failed to accelerate AEHF 1 during two attempted burns.

The system detected that the proper boost was not being generated and immediately terminated the burns. According a Government Accountability Office report issued this summer, investigators probing the problem determined that debris mistakingly left inside the craft was the culprit.

The report cited "blockage in a propellant line that was most likely caused by a small piece of cloth inadvertently left in the line during the manufacturing process."

The Atlas 5 rocket successfully put the craft into a supersynchronous transfer orbit with a high point of 31,060 miles, low point of 170 miles and inclination of 22.1 degrees.

The satellite's main engine was supposed to produce 100 pounds of thrust while burning hydrazine fuel and nitrogen tetroxide during three firings to propel AHEF 1 into an intermediate orbit after launch.

The satellite's novel electric propulsion system using Hall Current Thrusters then would finish shaping the orbit into a circular, geosynchronous altitude about 22,300 miles high and inclined 4.8 degrees within 100 days of liftoff.

But with the Liquid Apogee Engine knocked out, the team designed a plan to use the craft's tiny five-pound-thrust steering engines to begin lifting the orbit higher. They would not reach the altitude target the Liquid Apogee Engine should have achieved, but the so-called Reaction Engine Assembly thrusters did deliver a dozen firings that resulted in boosting the orbit's low point to 2,900 miles and reducing inclination to 15.1 degrees.

The satellite's power-generating solar wings then unfurled and the Hall Current Thrusters were deployed. The 4.5-kilowatt HCT units that use electricity and xenon to produce thrust for maneuvering satellites in space.

Instead of simply finishing the altitude maneuvers as originally envisioned, the HCTs have been required to perform a much larger share of the work to position the satellite into a useful orbit.

The drawback for the electric system is the patience required to accumulate the whisper-like 0.06-pound-thrust into orbit-changing power over many months.

They burned about 12 hours per day from last October through this June. Near-continuous firings were underway this summer.

Controllers then paused the process for much of September, which resulted in delaying arrival in the final orbit from October 3 to sometime in late October.

"AEHF 1's change in orbit raising schedule was to optimize fuel and extend the expected mission life of the satellite," said a MILSATCOM Space and Missile Systems Center spokesperson.

Despite the extra-long wait to get the satellite into the proper orbit, the Air Force says the satellite will have enough remaining fuel to function for its full 14-year service life.

"The orbit raising has been a technical challenge. However the team of U.S. Air Force, Aerospace and Lockheed Martin have been able to balance space vehicle priorities of protecting the satellite from space debris and radiation while conserving fuel to maintain the full mission life of the space vehicle," the spokesperson said.

This and future AEHF spacecraft will replace the aging Milstar satellite fleet to provide faster communications simultaneously to more users through modernized technology.

One AEHF spacecraft has more capacity than Milstar's five-satellite constellation combined and its faster data rates will benefit tactical military communications, enabling higher quality maps, targeting data and live video to be transmitted without being detected by the enemy.

AEHF was envisioned to keep communications flowing between the military and civilian leadership in any extreme wartime environment, giving the U.S. information superiority.

Assuming the AEHF 1 on-orbit activation and checkout goes well in the coming months, the Air Force plans to launch the AEHF 2 satellite atop another Atlas 5 rocket from Cape Canaveral on April 27.