The
Atlas rocket program looks to return to its successful ways this week,
launching a powerful U.S. military communications satellite Wednesday night
with a tried-and-true engine valve instead of a newer design blamed for
creating serious problems during the last ascent four months ago.
Atlas enjoyed a spotless
14-year consecutive success record covering 80 flights, but that score card was
hit with a major blemish June 15 when a leaky fuel valve caused an Atlas 5
rocket to fall
short of its intended altitude.
A pair of classified U.S.
national security spacecraft deployed from the rocket's Centaur upper stage
into a lower-than-planned orbit that day. Government officials said afterward,
in essence, that the launch wasn't perfect but it wasn't a complete failure
either.
Throughout the summer,
sky-watching hobbyists who routinely track all sorts of satellites reported the
secret duo from the botched launch were gradually boosting themselves higher
and higher. Ted Molczan, a respected satellite observer, says the craft have
made up about half the distance toward the expected launch altitude so far.
While the satellite
recovery work has been underway, rocket-maker United Launch Alliance, engine
builder Pratt & Whitney Rocketdyne and the Air Force have investigated the
ascent incident and ordered a corrective fix to ensure upcoming launches won't
suffer a similar mishap.
Several factors had
coincided to create the problem, officials concluded, including an unusually
lengthy firing by the cryogenic RL10 main engine during the launch sequence.
But the liquid hydrogen fuel valve design -- a new version with only a
half-dozen previous flights to its credit -- was at the heart of the problem.
"The investigation
determined that the fuel inlet valve on the RL10 engine system failed to
completely close after the first main engine cutoff on the Centaur upper stage.
That happened because its closing force was not sufficient," Jim Sponnick,
ULA's vice president for Atlas programs, said in an interview Friday.
"We learned that we
had a combination of higher than expected friction forces inside the valve, as
well as the effects of a very cold temperature on the helium that is used to
operate this valve. This particular mission required a longer than average time
for the first burn, which resulted in the liquid hydrogen flowing to the valve
for a longer than typical period of time, which then decreased the temperature
and increased the frictional forces within this valve.
"And those factors in
combination led to the point where the valve did not have sufficient force to
close itself."
After the Atlas 5 rocket's
first stage had fired and dropped away, the Centaur ignited its engine four
minutes into flight as the booster soared northeastward from Cape Canaveral on
a trajectory experts believed illustrated that the clandestine payload was two
ocean surveillance spacecraft.
The initial firing by the
RL10 powerplant lasted about 15 minutes to reach a temporary parking orbit. As
the cryogenic engine shut down, the fuel valve tried but failed to close
completely.
The valve reached
"near-closure but not full-closure," Sponnick said.
That left a pathway for the
rocket's precious liquid hydrogen fuel supply to slowly escape as the Centaur
quietly coasted in space for nearly an hour.
When the Centaur restarted
the engine to deliver a final propulsive push for its payload, there wasn't
enough fuel left in the tank to complete all of the mission's second burn.
Although the firing seemed only fractionally short, roughly four seconds, the
rocket simply could not achieve the desired orbit.
The satellites separated
from the rocket and officials in the Atlas control center cheered, not fully
aware that the launch had been crippled by a fuel leak.
"We did have some
indications of that condition in real-time. There were several parameters on
the vehicle that were indicating temperature differences from normal. It was
not known at that time that there would be an affect on vehicle
performance," Sponnick recalled.
But the realization soon
hit the team and efforts commenced to figure out what went wrong.
"We've literally spent
thousands of hours conducting very high fidelity tests to thoroughly understand
the cause, as well as to test the replacement valve," said Sponnick,
adding that the testing occurred at Pratt & Whitney Rocketdyne facilities
in West Palm Beach and ULA's sites in Denver.
"We both developed
very high fidelity test rigs to subject the valves to very flight-like
conditions. With that high fidelity testing, we were able to replicate the
failure-to-close scenario. And we've also been able to use those test
capabilities to test the valves that will be flown."
The remedy ordered for
upcoming Atlas 5 and Delta 4 rocket launches -- both rocket families use
versions of RL10 engines to power their upper stages -- is reverting back the
previous fuel inlet valve design that officials fully trust.
"The upcoming missions
are not flying a new valve design, it's actually the valve that has flown for
many years on prior Atlas flights and also on the Delta flights that have flown
to date. Our testing, as a result of the investigation, has shown that the
heritage valve design has larger closing force margins," said Sponnick.
"And we've also taking
several additional measures. We are performing a very high fidelity test of the
heritage valve design that ensures that we are confirming prior to flight all
of the opening and closing force margins for the valve. So we're doing a very
specific test and we're also implementing some additional cycles of the valve
in flight to provide even greater margin to close following main engine
burns."
Some may question why there
was a switch from the trusty old design to the new valve in the first place.
But officials explained that they were forced to find another valve because the
vendor was stopping production of the old device.
"The manufacturer that
had supplied the valve in the past was essentially phasing out that product
line. So we were developing a new design," Sponnick said.
"For the next several
flights we do have in inventory the heritage valves that are being tested in
the manner we've described. We are developing several options for longer
term."
The Atlas 5 rocket will
resume launches Wednesday evening with a planned 8:22 p.m. EDT (0022 GMT) liftoff
from Cape Canaveral's Complex 41. Riding atop
the 19-story rocket will be the U.S. military's first Wideband Global
SATCOM communications satellite.
"Mission success is
our absolute No. 1 priority. And that's exactly what we applied to this
corrective action. We continue to be driven by technical success, not launch
dates. So we are doing what we feel, jointly, is the right thing to make sure
our customer -- the Wideband Global SATCOM program -- is going to get a great
launch come (Wednesday)," Col. Michael Moran, the Air Force's Atlas group
commander, said in an interview Friday.
"From my vantage
point, I am more confident now than ever on the capability of that valve to
perform as it's expected in flight. I'm absolutely confident that the
processing that we've gone through, the reviews that we've gone through and the
technical analysis that we've gone through responding to the anomaly in June
has been comprehensive and effective. I have absolutely zero concern about the
performance of that vehicle making its way to orbit."
"And I'll echo
that," said Sponnick. "We're extremely confident in the understanding
of the anomaly and the thoroughness of the correction actions that have been
put in place."
This will be flight No. 11
for the Atlas 5 rocket fleet, and everyone involved hopes and expects to see a
successful outcome when the massive satellite is deployed into a
supersynchronous transfer orbit a half-hour after liftoff.
You can follow Wednesday's
countdown and launch in our Mission Status
Center. We'll have live play-by-play reports throughout the evening.
The status center also has
an update on the launch delay from Tuesday to Wednesday.
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