Patience required as AEHF 1 recovery begins new mode
BY JUSTIN RAY
Posted: October 17, 2010
Moving into the next phase of its orbital rescue, the Air Force's Advanced Extremely High Frequency satellite is warming up exotic electric thrusters to begin 10 months of propulsion-with-persistence that's needed to save the craft's life.
A United Launch Alliance Atlas 5 rocket successfully dispatched the craft into a supersynchronous transfer orbit with a high point of 31,200 miles, low point of 140 miles and inclination of 22.2 degrees. From there, the satellite's own Liquid Apogee Engine was supposed to fire three times to ascend into an intermediate orbit.
The electric propulsion system using Hall Current Thrusters then would finish shaping the orbit into a circular, geosynchronous altitude 22,300 miles high and inclined 4.8 degrees by December.
That was the plan. But a fault somewhere in the main propulsion system for the Liquid Apogee Engine meant the 100-pound-thrust motor couldn't generate acceleration.
Satellite-maker Lockheed Martin and the Air Force quickly scrambled to devise an alternate strategy using other ways to propel AEHF 1 into an operational orbit.
The emergency plan to salvage the satellite called upon the craft's tiny five-pound-thrust steering engines to begin lifting the orbit higher. They couldn't reach the altitude target the Liquid Apogee Engine should have achieved, but the so-called Reaction Engine Assembly thrusters would deliver a worthwhile step in the right direction.
By late September, AEHF 1 had performed a dozen firings that resulted in boosting the orbit's low point to 2,900 miles and reducing inclination to 15.1 degrees.
"I'm massively pleased with where we ended up," said Dave Madden, Military Satellite Communications Systems Wing program director at the Air Force's Space and Missile Systems Center.
"We actually did significantly better, I'd say 10-15 percent better, than we thought we were going to do. As we kept doing it, we found better ways to optimize how we were pulsing those thrusters to optimize getting the maximum orbit raising out of them based on the fuel we were using. So each time we burned, we actually got better at our modeling on how to get the most out of what we were doing to minimize fuel usage."
After the Reaction Engine Assembly phase of the rescue was completed, the satellite's power-generating solar wings were unfurled and the Hall Current Thrusters were deployed.
"We're getting exactly the power out of the solar panels that we would expect to get. We're just working on the preps to go into the Hall Current phase," Madden said in an interview October 14.
The HCTs, produced by Aerojet, are 4.5-kilowatt units that use electricity and xenon to produce thrust for maneuvering satellites in space.
Unlike conventional chemical engines that deliver substantial boosts with each brief firing, the electric system needs the stamina to operate for exceptionally long periods of time to harness its whisper-like 0.06-pound-thrust into orbit-changing power.
The divergent systems have their advantages and drawbacks. Although typical engines can maneuver satellites rapidly, they use large amounts of heavy fuel that in turn require a bigger, more expensive rocket to carry the spacecraft. Electric propulsion gives up timeliness for efficiency since its xenon fuel weighs a mere fraction of conventional hydrazine, but you must have patience to reap the rewards.
"The beauty of them is for size, weight and power, it's an extremely efficient system - if time is okay," Madden said.
Commercial communications satellites built by Boeing began using small ion thrusters in the late 1990s. The company's Wideband Global SATCOM satellites for the U.S. military also employ them. NASA's Deep Space 1 and the Dawn space probes have relied on large ion engines for interplanetary propulsion as well.
But it's the electric thrusters on AEHF 1, which are different and haven't flown before, that ultimately hold the key to saving the mission. The satellite cannot reach its intended orbit and operate for a full 14- year life using only conventional hydrazine with its tiny Reaction Engine Assembly thrusters.
Activities underway the past few weeks have focused on readying the HCTs for their extensive usage.
"To be able to utilize them, we have to go through a series of checkouts and conditioning of some of the components," Madden said.
"I would have loved to have seen us just jump right into HCT operations, turn them on and go. But knowing it's a new, first-of-a-kind system, we have to methodically work our way through this and get ourselves on our way."
Controllers have been commanding the thrusters on and off for short bursts, slowly working towards longer and longer burn times.
"Components in the system absorb moisture when they are at atmosphere. So we have to burn that stuff off because it causes perturbations in operations if we don't burn it off first. Right now, we're going through this phase called conditioning of the HCTs and making sure we have the incremental steps right to bring them into operation," Madden said.
"When we get done with doing that, hopefully in a week, a week-and-a-half, if the conditioning goes well, then we will go into a full 10-hour burn a day for probably about seven months."
The satellite's present elliptical orbit takes nearly 17 hours to complete a full revolution around the planet. The HCTs will burn for 10 straight hours per orbit every day through next spring.
This critical seven-month period will increase the altitude at the orbit's low point, or perigee, and reduce the orbital inclination closer to the equator.
"What we are going to do in the first seven months is push out the perigee, then bring down inclination. So the heavy lifting is in the first seven months. Then what we're going to be doing in the last three months is circularizing and aligning it," Madden said.
That subsequent three-month phase in late spring and early summer does what the original pre-launch plan for the HCTs envisioned by burning the thrusters non-stop to reduce the orbit's high and low points into a circular 22,300-mile, geosynchronous altitude where the satellite can match Earth's rotation.
"We're anxious to get into this seven-month phase so we can get into more of a rhythm with the vehicle," Madden said.
Despite the crisis that prompted the heroics to recover the satellite's mission, the Air Force remains confident the craft will achieve the correct orbit and have enough residual fuel for operations to fulfill its 14-year life to relay communications between the president, military commanders and troops on the battlefield.
"We have adequate hydrazine left over from the last burn to be able to do full operations, and all of our projections associated with the xenon that we have available for the Hall Current Thrusters (show) we will have adequate xenon available when we get on-orbit to also sustain a 14-year life or longer," Madden said.
After the cumulative 10 months of HCT thrusting concludes next summer, the satellite will be parked over the equator at 90 degrees West longitude for activation and testing of its communications package. Once that is accomplished, flight controllers will slide the craft east or west in geosynchronous orbit based on where the military needs the craft.
"The operators are trying to decide after we get to 90 degrees what final location that they want us put the satellite and what's the exact inclination they want to us to put it at. But right now all indications are we can go anywhere they want us to go and we can support any inclination they want and do that for a 14-year life.
"We just have got to get through this phase of getting our HCTs checked out," Madden said.
MISSION STATUS CENTER