The first AEHF satellite looked like a goner, but the Air Force's unusual recovery effort pulled it back from the dead.
—Robert S. Dudney
This step forward almost did not happen.
Space Vehicle 1, launched Aug. 14, 2010, suffered many serious setbacks. The $2 billion spacecraft's main propulsion subsystem failed, it could have exploded, and it faced lethal space debris and radiation. The giant communications satellite could have died a quick death, but it didn't.
Instead, SV-1 was the beneficiary of a remarkable 14-month rescue effort. Last Oct. 24, against very long odds, SV-1 finally eased safely into its assigned orbit. The Air Force expects it to enter full operational service in March.
With the satellite safely in its proper orbit, Air Force Space Command officers have begun talking fairly openly about the rescue mission. It is an unusual tale.
The AEHF program, one of the largest space programs of the decade, is designed to augment and eventually replace the legacy Milstar satellite communications network.
Lockheed Martin is the prime contractor, Northrop Grumman built the payload, and everything is run by Space and Missile Systems Center at Los Angeles AFB, Calif. The constellation of four cross-linked AEHF satellites is expected to provide a communications capacity exceeding that of Milstar by a factor of 10.
However, the AEHF program fell behind schedule. A planned 2008 first launch was delayed by two years. For that reason, more than the usual anxiety attended its Aug. 14, 2010, blastoff.
A giant Atlas V rocket flawlessly lifted SV-1 from Cape Canaveral's Complex 41. Once in space, the booster and spacecraft separated exactly as planned. The 13,420-pound SV-1 went into a highly elliptical orbit, meaning its altitude varied greatly from apogee (the point farthest from Earth) to perigee (the one closest to Earth). In fact, the satellite swung from 31,000 miles above Earth at apogee to 143 miles above Earth at perigee.
No one intended SV-1 to stay on that unstable path. USAF planned to use three AEHF satellite propulsion systems to drive the perigee up and apogee down, in time creating a circular geosynchronous Earth orbit. SV-1 was to wind up 22,300 miles above the equator, hovering almost directly over the Galapagos Islands.
It was at this stage—the start of the satellite "transfer" from HEO to GEO—that things went haywire.
The mission profile called for operators to fire the AEHF satellite's hydrazine-fueled liquid apogee engine (LAE) several times. The thrust was supposed to raise SV-1's perigee to 11,800 miles in a short period. Smaller engines would then take over and continue the orbit circularization.
Alarm BellsHowever, disaster loomed on Aug. 15. USAF controllers and their contractor partners ignited the LAE and, after several seconds, the hydrazine engine failed. The AEHF satellite had detected a problem and shut the LAE down.
Operators were puzzled but not yet alarmed. Two days later, on Aug. 17, they gave it another go. In a few seconds, the LAE shut down again, this time with ominous signs of overheating.
Col. Michael L. Lakos, the MILSATCOM command lead at AFSPC, recalled thinking that it was "an Apollo 13 moment." The words that came to mind were, "Los Angeles, we have a problem." The space vehicle had no readily apparent way to reach its orbit.
Alarm bells went off all over Space Command. The burden of response fell on David W. Madden, the chief of SMC's MILSATCOM Systems Directorate and a recently retired Air Force colonel. His initial reaction was "that we'd lost the mission." At the time, he added, "there was huge uncertainty."
On Aug. 17, Madden moved to assemble four teams of handpicked experts. The first question to answer: What had happened to the LAE? Madden’s engineers rapidly worked through the telemetry and modelled the problem. They concluded—correctly—that the LAE had suffered a propellant-line blockage. Worse, they said, another firing could cause an explosion.
"They probably saved the satellite," said Madden of USAF's decision not to attempt a third firing. "We could have had combustion outside of the engine, which could have either totally damaged our payload or caused catastrophic damage to the vehicle."
"We're very, very fortunate that satellite didn't blow up," said Gen. William L. Shelton, AFSPC commander.
Thus warned, USAF sealed off the AEHF satellite's oxidizer tanks, rendering the LAE safe but unusable. Madden's experts then set to work on a makeshift strategy to raise SV-1's orbit. They proposed to use the AEHF satellite's two remaining propulsion systems, though in ways no one ever had tried.
As the project got under way, team members were told—politely, but firmly—they were to stay put and continue the work until they sorted out the critical issues and developed a get-well plan. "We literally were shoving pizza under the door so that these guys could keep working," said Madden.
The pivotal show-and-tell moment came on Saturday, Aug. 21—a mere one week after launch and four days after the last LAE burn. Lt. Gen. John T. Sheridan, then SMC commander, held a meeting in Los Angeles, assembling senior experts from the contractors, program office, and AFSPC. Madden presented a notional plan, based on four distinct phases of action.
The stars of the plan would be SV-1's two smaller engine types—hydrazine-fueled reaction engine assemblies (REAs) and tiny xenon-fueled Hall Current Thrusters (HCTs) of about 0.05 pounds of thrust.
In a vote of confidence, Sheridan gave a go-ahead, stepped back, and let his experts work the problem.
"The trust was significant," Madden said. "They gave us a lot of rope. Early on, that rope was critical. . . . We were encountering problems real time. We weren't having to report on the crisis of the hour."
"It was not a 'mother, may I' thing," Lakos added. "It was basically an Air Force Space Command thing. It was, 'Let's go off and do this.' We didn’t want to study something to death."
Phase 1 began right away and lasted only a few days. In it, Madden's team sought to quickly blunt the two most immediate dangers to the spacecraft. One was Earth's gravitational pull. The second was orbital debris.
"The way this thing was put into its initial orbit, it was very low—a roughly 140-mile entry point," said Madden. The drag exerted by gravity caused the satellite to lose more than three miles of altitude every day.
Equally worrisome was the prevalence of space debris at that orbital altitude. "That's a pretty nasty area," noted Madden. On several occasions, in fact, controllers had to maneuver SV-1 to avoid a collision with speeding space junk. This burned up valuable—and limited—fuel.
To get the satellite out of this danger zone, Air Force controllers on Aug. 29 began firing the spacecraft's REA thrusters. These motors had been designed to help stabilize the AEHF satellite, not propel it.
By early September, ground controllers had conducted four burns and the perigee had risen to some 600 miles above Earth. It was the end of Phase 1.
Phase 2 was essentially an extension of Phase 1, after a brief pause to assess results. USAF ground controllers continued to use the hydrazine-based REA thrusters to move the satellite. The goal was to raise the perigee from 600 miles to about 3,000 miles. That would prevent orbital decay, among other things.
Unusual BurnsNew problems emerged. For one thing, the REA system was never expected to burn for hours at a time. During this time, SV-1 was held in fixed position, such that it was exposed to significant solar heating and potential damage.
In response, the team devised a technique that allowed ground controllers to occasionally flip over the spacecraft, thus giving exposed panels an opportunity to cool. Madden's experts had to devise a whole strategy to carry out this maneuver while keeping the spacecraft on course.
The biggest problem of all: fuel usage. To minimize it, engineers had to write and upload new flight software to enable the plan to work and to save every ounce of fuel. This allowed the REA thrusters to be used in new, more efficient ways and allowed controllers to properly position the satellite using its onboard reaction wheels instead of fuel.
"We had to do the calculation to make sure that each time we burned, we knew exactly where we were going to end up," Madden said. "It's kind of like in 'Star Wars'—if you're going to jump to light speed, you had better know what's in your way."
During all of this, Air Force Space Command's 50th Space Wing worked the orbital aspects of these unusual burns. "It was almost like you’re doing a launch," said Madden. "We had to do the orbital aspects every time we did a burn plan—every day, for a month. All of this was extra work, just absorbed by the 50th Space Wing guys," even as they handled regular Milstar operations. In addition, 14th Air Force personnel handled the mission's collision avoidance work, all of which was unplanned and taken up on an emergency basis.
In Phase 2, the daily burns of the thrusters tapped into the store of hydrazine once reserved for the LAE. The Madden team had calculated how much hydrazine could be used to try to get up to 3,000 miles at perigee, and still leave enough to do the mission once the satellite reached GEO—if it did.
On Sept. 22, 2010, the spacecraft reached a perigee of more than 2,900 miles. "That was the optimum place to stop [the firing of the REA thrusters] and our use of that fuel," said Madden. The team had reached the end of Phase 2.
Phase 3 turned out to be by far the longest and perhaps most innovative part of the AEHF satellite rescue. It began in October 2010 and did not end until June 2, 2011. Those eight months saw the beleaguered satellite get far along in its trek to GEO.
Propulsion now was provided by the spacecraft's exotic Hall Current Thrusters, small motors that use electricity and xenon gas as propellant. The thrust of an HCT is far less than that of chemical-fueled power plants—it puts out small puffs of power—but a thruster can fire for thousands of hours.
The HCTs were designed mostly for station-keeping, so the Madden team would use them in an untested way. "The HCTs, at this power level, . . . it's the first time that they have been flown in space," said Madden.
Moreover, the spacecraft was much heavier than it should have been at that point. Because the LAE had been shut down, most of the system's oxidizer—about 1,000 pounds' worth—remained in the sealed tanks.
A new danger arose. The AEHF satellite was now in the Van Allen Belts of radiation, a zone of energetic charged particles held in place by the Earth's magnetic field. These particles can damage a satellite, which must shield its sensitive components if it spends much time there.
The AEHF satellite needed to extend its solar-panel "wings," which until now had remained stowed against the side of the satellite. HCTs run on electricity produced from sunlight collected by the solar wings. However, they are sensitive to radiation.
The AEHF satellite's power-generating solar wings were unfurled and HCTs were deployed. "Every hour we were in that environment," recalled Madden, "we were beating up our solar panel, which would harm our ability to get power once we got to our final geosynchronous location. We did not want our solar panels to degrade." After much deliberation, the team came up with a burn strategy that got the satellite rapidly out of the Van Allen zone so that it could operate for longer periods.
From late October 2010 to June 2011, the HCTs burned for 10 to 12 hours per day. The motors were optimized to fire at the apogee of the AEHF satellite's orbit, so as to drive up the perigee. By last summer, nearly continuous firings were taking place. The HCTs had never been used in such a fashion in zero gravity conditions. The team began to see some features they had never seen before. For instance, they needed a warm-up period to operate at maximum efficiency.
"It seemed like every month or two, we thought we had the equation down for how to do it, and all of a sudden we'd see another hiccup," said Madden. "We'd get a fluctuation." He continued, "They're like a finicky old car, one that you've got to constantly adjust to get it to optimize. There's no instruction manual for how to do that. It's basically an art."
That was a big challenge for the engineers. As the system got older, it exhibited unexpected variation. The team thus had no choice but to collect data, review operations on a regular basis, and make adjustments on the fly. Complicating the operation was another computational task. The HCTs had not only to drive the perigee up; by this time, they were also needed to make major changes in the inclination of the satellite's orbit. This was a complex task.
As Madden put it, "We had to get the inclination down, because that enables us to 'see' more of the Earth. The higher the inclination, the smaller the amount of the Earth that can talk to your spacecraft. That's really where most of the energy was used, trying to drive down that inclination."
In Phase 3, ground controllers conducted burns only at the apogee of the AEHF satellite's flight. The HCTs had been able to raise the low point of the orbit from 3,000 miles to more than 17,000 miles. The phase ended on June 2, 2011.
The Circle Was FormedThe last part of the rescue—Phase 4—saw USAF take steps to change not only the AEHF satellite's perigee but also its apogee. The latter was 32,145 miles above Earth, which was far too high. It had to be reduced by a whopping 10,000 miles.
At the same time, the perigee had to go up from 17,000 miles to about 22,000 miles.
Finally, the satellite's inclination had to be driven down closer to alignment with the equator.
The HCTs again were firing for long periods every day. The forces they produced induced a convergence of the apogee and perigee altitudes. These maneuvers had many complex orbital aspects. Madden said the shifts were timed to optimize beneficial effects of Earth's gravitational pull, thereby conserving valuable fuel.
The AEHF satellite's perigee finally reached its required altitude of 22,300 miles in early August. The declining apogee reached proper altitude Oct. 24. The circle was formed.
The sophisticated recovery campaign, entailing about 500 propulsion burns, was over.
When the spacecraft reached its orbit slot, USAF deployed the payload. It had been stowed to allow it to fit within the nosecone of the Atlas rocket. A long checkout of the antennas and other mission-critical equipment then commenced. Plans call for this checkout to take about three and a half months, after which SMC will turn over control to 50th Space Wing.
Madden said, as a result of the careful efforts to husband SV-1's hydrazine and xenon fuel during the orbit-raising phases, there will be no reduction in its planned 14-year life.
Moreover, the Madden team learned quite a few tricks along the way. "When we hand it over to the operator, we will show him how to use the system efficiently to make sure he gets at least 14 years of life."
Early in the rescue drama, Air Force leaders decided to postpone the launch of follow-on AEHF satellites. They wanted to have a chance to check out SV-1 before proceeding.
Barring the discovery of more problems, the Air Force will launch SV-2 in late April.
"I'm feeling very good with the fuel that we have on board the vehicle," said Madden.
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