engineers will have some changes to make on the company's new Falcon 9 v1.1 launch vehicle before its next mission, following lessons learned from its Sept. 29 demonstration flight.
A preliminary test of an innovative reusable-booster concept intended to hold down costs showed the need for more work after an uncontrollable spin apparently hampered pre-spashdown braking. The vehicle also aborted a planned restart of its upper-stage engine during the otherwise successful debut of the upgraded rocket.
With more than $1 billion in commercial-launch backlog, restarting the Merlin 1D vacuum engine that powers Falcon 9's upper stage is critical to delivering government and commercial satellite payloads to geostationary transfer orbit (GTO), the destination of most commercial telecom spacecraft. But SpaceX CEO/Chief Technology Officer Elon Musk downplayed the upper-stage restart, characterizing it as an “optional” part of the flight profile.
“We essentially saw the engine initiate ignition, get up to about 400 psi, and then it encountered a condition it didn't like, and we think it may have been due to an extended spin start—maybe, this is speculative—and so it initiated an abort of the restart,” Musk said post-launch. “We have all the data from the restart, so I'm confident we'll be able to sort it out and address it before the next flight.”
That mission is expected to loft the new rocket's first GTO mission—the SES-8 telecommunications satellite for fleet operator SES—late this month.
Longer term, SpaceX wants to reuse hardware to hold down costs. The Falcon 9 v1.1 flight included an attempt to slow the hypersonic first stage for a controllable descent to the Pacific Ocean. Before launch, Musk gave this only a slim chance of working on the demo flight, but it seems it was at least partially successful.
Following main-engine cutoff, burnout and separation, with the stage likely slowing well below Mach 10, three of its nine engines were intended to fire to decelerate the booster for its initial descent. A single engine was then due to fire to perform a final braking maneuver just before the stage impacted the ocean at a slow, landing-like speed.
While the initial high-altitude retrofire sequence appeared to work to plan, Musk said the stage “actually ended up spinning to a degree that was greater than we could control with the gas thrusters, and it centrifuged the propellant. It caused the boost stage to run out of propellant before hitting the water. So it hit the water relatively hard.”
The stage lacked the landing gear that Musk says would help stabilize a reusable unit. SpaceX's reusability plan builds on a vertical landing and recovery system that has undergone initial tests at the company's facility in McGregor, Texas, using the Grasshopper demonstrator. Although only a foreshortened Merlin 1D-powered first stage, the Grasshopper's precise maneuvers and landings have bolstered confidence in the concept.
“Between this flight and Grasshopper tests, we now have all the pieces of the puzzle to bring the rocket back home,” said Musk. “If things go super well, then we will be able to re-fly a Falcon 9 stage before the end of next year.”
SpaceX plans to test a larger version of the Grasshopper, also dubbed the Falcon 9R (reusable)—with nine -1Ds more representative of the upgraded production launch vehicle—at a specially constructed site at Spaceport America in New Mexico.
As for the upper-stage abort, Musk said the Merlin 1D has been successfully reignited “dozens of times” during ground tests, but he conceded the company still needs to address minor technical issues associated with operating the engine in the vacuum of space.
“We just need to iron out the differences between restarting in vacuum and in microgravity versus restarting on the ground,” he noted. The issue could delay the launch of the SES-8 satellite by “a few extra weeks, to make sure we're super-confident of the second-stage restart,” he added.
Musk said the Falcon launch vehicle that will loft SES-8 is at SpaceX's Cape Canaveral AFS launch facility.
“We're ready to go with SES, with a steady cadence of launches thereafter,” he said, which would include the Thaicom 6 anticipated later this year, followed by SpaceX's third cargo resupply mission to the International Space Station (ISS) in February.
SES is discussing the reignition abort with SpaceX, and at press time it had not yet shipped the.-built spacecraft to the Cape, which ceased operations last week with the U.S. government shutdown.
Beyond reusability and upper-stage reignition, there were several other key milestones for the upgraded Falcon 9. It successfully delivered the first of six payloads into orbit 14 min. after it left the pad. These included the Canadian-built CAScade, Smallsat and IOnospheric Polar Explorer (Cassiope) satellite, Canada's first space environment sensor suite.
Major modifications to the new Falcon include the more powerful Merlin 1D engines, arranged for the first time in an “Octaweb” pattern with a central engine surrounded by eight other engines, rather than the former 3 X 3 configuration of previous Falcons. The Octaweb thrust structure also supports the higher thrust of the Merlin 1D, each of which develops 147,000 lb. thrust at sea level, rising to 161,000 lb. at altitude. Total thrust at launch is 1.3 million lb., increasing to 1.5 million lb. in vacuum.
Each kerosene-fueled engine is fed via a single-shaft, dual-impeller turbopump. The fuel is also cycled to provide regenerative cooling of the thrust chamber and expansion nozzle, as well as serving as the pressurized fluid to control the thrust vector actuators.
The 227-ft. vehicle incorporates extended fuel tanks as well as improved avionics and a larger, 43 X 17-ft.-dia. payload fairing. The demo was the first SpaceX flight from space launch complex SLC-4E, a former Titan 4 pad the company is leasing from the Air Force.
Aside from the faster-than-hoped-for splashdown of the first stage, the flight was not without its share of drama. Reports based on information from SpaceTrack, a satellite tracking and space situational awareness service, indicated a larger-than-expected debris field in the orbital path of the vehicle, with up to 20 discrete objects in view. The tracking data led to speculation that the second stage may have disintegrated shortly after payload deployment.
“We have no reason to believe there was an explosion of any kind,” SpaceX declared, noting that following insertion of the satellites to the correct orbit, “the Falcon 9 second stage underwent a controlled venting of propellants and the stage was successfully safed. During this process, it is possible insulation came off the fuel dome on the second stage, and is the source of what some observers incorrectly interpreted as a rupture in the second stage.”
The company adds that it “will continue to review to help identify the source of the extra debris, but our data confirms there was no rupture of any kind on the second stage.”