WASHINGTON -- Space Exploration Technologies (SpaceX)
has declared its Falcon 1 rocket ready to begin launching satellites in September
despite a premature engine shut down that prevented the booster from reaching
orbit during a second demonstration flight last week.
"Having had several days to examine
the data, the second test launch of Falcon 1 is looking increasingly positive," SpaceX chief Elon Musk wrote in a Tuesday update posted on the El
Segundo, Calif.-based company's Web site. "Post flight review of telemetry has
verified that oscillation of the second stage late in the mission is the only
thing that stopped Falcon 1 from reaching full orbital velocity. The second
stage was otherwise functioning well and even deployed the satellite mass
simulator ring at the end of flight!"
SpaceX launched the two-stage Falcon 1 rocket March 20 from its Omelek Island launch site on the Pacific Ocean, but the rocket failed to reach its intended 425-mile (685-kilometer) orbit due to a roll control glitch.
Musk said
that no further demonstration flights are needed before the Falcon 1 is
entrusted with the Pentagon's experimental TacSat-1 remote sensing satellite.
"This confirms
the end of the test phase for Falcon 1 and the beginning of the operational
phase," he wrote. "The next Falcon 1 flight will carry the TacSat-1 satellite
for the U.S. Navy, with a launch window that begins in September, followed by
Razaksat for the Malaysian Space Agency in November. Beyond that, we have
another nine missions on manifest for [Falcon 1] and [Falcon 9]."
Musk's update
provides further details on the roll control anomaly that occurred late in the
second stage burn, causing the engine to shut down about 90 seconds before the
rocket had reached orbital velocity.
"Telemetry
shows that engine shutdown occurred only about a minute and a half before
schedule (roughly T +7.5 mins), due to the oscillations causing propellant to
slosh away from the sump," Musk wrote. "When the liquid level in the tank was
low, this effectively starved the engine of propellant."
Musk said
that Falcon 1's less-than-perfect stage separation -- visible in the flight video -- was a contributing factor.
"As the 2nd
stage nozzle exited the interstage, the first stage was rotating so fast that it smacked the niobium nozzle," Musk wrote. "There was no
apparent damage to the nozzle, which is not a big surprise given that niobium
is tough stuff."
While
pre-flight simulations led SpaceX to believe that the rocket's control system
would be able to damp out any fuel slosh, Musk said, the team "had not
accounted for the perturbations of a contact on the stage during separation,
followed by a hard slew to get back on track."
Musk said
that both the stage separation problem and tank slosh issue should be easy to
remedy.
"We
definitely intend to have both the diagnosis and cure vetted by third party
experts, however we believe that the slosh issue can be dealt with in short
order by adding baffles to our 2nd stage LOX tank and adjusting the control
logic. Either approach separately would do the trick (eg. the Atlas-Centaur
tank has no baffles), but we want to ensure that this problem never shows up
again," Musk wrote.
Musk said
SpaceX can avoid a repeat of the stage separation issue by initiating shutdown
of the first stage Merlin engine "at a much lower thrust level, albeit at some
risk to engine reusability."
"Provided we
have a good set of slosh baffles, even another nozzle impact at stage
separation would not pose a significant flight risk, although obviously we will
work hard to avoid that," he wrote.
Musk also
took issue with media reports that did not characterize Falcon 1's second demonstration
flight as a
successful launch.
"Although we
did our best at SpaceX to be clear about last week's launch, including naming
it DemoFlight 2 and explicitly not carrying a satellite, a surprising number of
people still evaluated the test launch as though it were an operational mission,"
Musk wrote. "This is neither fair nor reasonable. Test flights are used to
gather data before flying a "real" satellite and the degree of success is a
function of how much data is gathered."
"The reason
that flight two can legitimately be called a near complete success as a test
flight is that we have excellent data throughout the whole orbit insertion
profile, including well past second stage shutdown, and met all of the primary
objectives established before hand by our customer (The U.S Air Force and
Defense Advanced Research Projects Agency)," Musk continued. "This allows us to
wrap up the test phase of the Falcon 1 program and transition to the
operational phase, beginning with the TacSat mission at the end of summer. Let
me be clear here and now that anything less than orbit for that flight or any
Falcon 1 mission with an operational satellite will unequivocally be considered
a failure.
"This is not 'spin'
or some clever marketing trick, nor is this distinction an invention of SpaceX
-- it has existed for decades," Musk added. "The U.S. Air Force made the same
distinction a few years ago with the demonstration flight of the Delta 4 Heavy,
which also carried no primary satellite. Although the Delta 4 Heavy fell
materially short of its target velocity and released its secondary satellites
into an abnormally low altitude, causing reentry in less than one orbit, it was
still correctly regarded by Boeing and the Air Force as a successful test
launch, because sufficient data was obtained to transition to an operational phase."
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