Tuesday, August 31, 2010

Faults In Space: Glitchy AEHF To Reach Orbital Slot Next Summer

Aug 31, 2010

By Amy Butler AVWK

The U.S. Air Force’s newest protected satellite communications spacecraft will likely reach operational status 7-8 months later than planned, after a liquid apogee engine designed for orbit raising was deemed useless after two failed burn attempts.

The first Lockheed Martin-built Advanced Extremely High Frequency (AEHF) satellite was expected to reach its testing position at 90 deg. West in geosynchronous orbit in November, about three months after the Aug. 14 launch date, says Dave Madden, director of the Air Force’s military satellite communications program office. Now, however, a series of burns designed to carry the satellite about 22,000 mi. into geosynchronous orbit will require 10-12 months, Madden says. He spoke Aug. 30 with the media via telecom.

At issue is how to compensate for a failure of the liquid apogee engine (LAE) on AEHF-1. Air Force and company officials tried two burns of the LAE, which was designed to produce about 100 lb. of thrust, before declaring it unusable (Aerospace DAILY, Aug. 27, Aug. 30).

Now, Air Force officials are planning to execute a four-phased orbit-raising strategy. First, the Air Force will increase the satellite’s perigee (or the lowest point that it reaches in orbit). This is needed to reduce drag on the spacecraft from the Earth’s atmosphere.

For this phase, which began Aug. 29 at 7 a.m. PDT, the Air Force is using the third and final propulsion system on the satellite, a reaction engine assembly (REA) — which has six 5-lb. and 12 0.2-lb. thrusters).

This system was designed for use in maintaining yaw attitude control and for in-orbit operations. In the next three phases of the orbit-raising plan, additional REA apogee burns will be conducted to more than double the current perigee. Hall Current Thrusters (HCTs) will then be used at apogee and, finally, HCTs will conduct continuous maneuvers to reach the final orbital slot.

Madden says he expects this approach will not affect the satellite’s planned design life of 14 years .

The satellite’s design allows for the REA to tap into the LAE’s hydrazine fuel store during the orbit-raising process, Madden says. “We basically got lucky,” he says, noting the operational requirement was to reach orbit in 90 days. “We put enough fuel on board to pretty much fire those thrusters continuously.”

Discussions are ongoing with U.S. Strategic Command, which will be the operational owner of the satellite. Madden says that while the Milstar constellation now in orbit is aging, so far operators do not expect operational necessity to force them to tap into the fuel designated for on-orbit stationkeeping or maneuvers to get the spacecraft into orbit faster.

A team of Air Force, Lockheed Martin and Aerospace Corp. engineers is studying the root cause of the failure, and findings are expected in about three weeks. Because the overall propulsion performance was not as expected on the LAE, the “system shut it down,” Madden says. Possible causes could range from “a bad valve in the system all the way up to the propulsion wasn’t being cooled or heated properly ... or we could have a bad engine,” he says.

No comments:


Blog Widget by LinkWithin