Connecting the Dots
Faintly visible attached to the base of the Falcon 9 v1.1 booster as it lifts off, the four 25-ft.-long landing legs are deployed for landing using helium.
While (SpaceX) and monitored the Falcon 9 second stage as it powered a Dragon cargo spacecraft on the third cargo resupply (CRS-3) mission to the International Space Station (ISS) from on April 18, another group of launch specialists was looking elsewhere. They were tracking the descent of the first stage to a controlled, gentle splashdown and demonstration of what they hope will clear the way for routine powered-booster recoveries on land.
The test was another step toward SpaceX’s aim of fundamentally reducing launch costs by employing completely reusable rocket boosters that can be quickly refueled for another flight without rebuild or refurbishment. To achieve this target, which could see the first test flight of a reused booster as early as 2015, the company plans to guide a discarded first stage to a powered landing at a yet-to-be-determined site along the Florida coastline this year.
While SpaceX remains coy about the exact fate of the modified Falcon 9 first stage, the overall results appear to be encouraging despite the apparent breakup of the booster after it landed in heavy seas. Additional encouragement comes from the successful first vertical launch and recovery test flight of the Falcon 9 Reusable (F9R) development unit 1 at SpaceX’s rocket facility in McGregor, Texas, the day before the CRS-3 flight.
“We are starting to connect the dots,” says SpaceX CEO and chief designer Elon Musk, who confirmed on Twitter shortly after launch that the “landing in Atlantic was good.” The “dots” Musk refers to range from development and testing of specific guidance, navigation, control and landing technology using dedicated test vehicles such as the F9R Dev 1 and a scaled predecessor called the Grasshopper, to key tests of reusable technology during actual launches. However, Musk cautions that all the dots need to be joined before SpaceX can claim success: “To be reusable, it must be both rapid and complete, like an aircraft or a car.” Having to replace parts or refurbish stages between flights will defeat the object of saving costs. “The only thing that changes is reloading propellant and expendables, and the vehicle is designed for that,” he adds.
The April 17 F9R Dev 1 flight, which lasted under 1 min., was the first vertical landing test of a production-representative recoverable Falcon 9 v1.1 first stage, while the April 18 cargo flight to the ISS was the first opportunity for SpaceX to evaluate the design of foldable landing legs and upgraded thrusters that control the stage during its initial descent. “With the design of the landing legs and greater progress of the booster stage on this flight, there are only a few dots needed to make all this work. I think we’ve got a decent chance of bringing a stage back this year,” says Musk.
The latest flight test follows an unsuccessful recovery attempt on Sept. 29, 2013, when the first upgraded Falcon 9 v1.1 was launched from Vandenberg AFB, Calif. Although three of its nine Merlin 1D engines fired to slow the initial descent ahead of a single engine firing for final braking, the vehicle’s spinning motion could not be controlled. As a result of aerodynamic torque, the fuel centrifuged inside the tank, flaming out the engine and sending the stage crashing into the sea.
The latest recovery test began when the initial braking relight commenced a short time after stage separation, which occurred 164 sec. after launch, and 3 sec. after main engine cutoff. “We were able to control the first stage to a zero roll rate,” says Musk. “With more powerful nitrogen thrusters, we were able to null the roll rate.”
In addition, data showed the legs were able to cope with the maximum dynamic loads associated with ascent and the descent from hypersonic speeds around Mach 10 to deceleration through transonic conditions. After data collected by SpaceX’s tracking aircraft were analyzed, “flight computers continued transmitting for eight seconds after reaching the water,” Musk says, adding that the transmissions “stopped when the booster went horizontal.”
Following initial tests of the vertical landing and recovery system at McGregor using the Grasshopper demonstrator, further work is planned using the F9R Dev 1 in Texas in addition to a second F9R Dev 2, which will fly at SpaceX’s recently completed site at Spaceport America in New Mexico. Dev 1 will conduct testing up to restricted altitudes of 10,000 ft. over Texas, while Dev 2 will be used for higher-altitude, exoatmospheric testing up to 300,000 ft. and above. In addition, full-up recovery attempts will continue as part of further launches over the course of 2014.
“This year we will recover the booster and maybe re-fly it next year. That will complete the picture as far as the booster stage is concerned,” Musk says.