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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