In theory, the SpaceX Falcon 9 v1.1 booster can be reused more than three-dozen times.
That's because the rocket's LOX/Kerosene Merlin 1D engine -- nine of which power its first stage -- has a cycle of 40, according to Stella Guillen, SpaceX director of business development.
"It's not obviously the entire system or the entire stage," she said during a space conference in Paris last month. “We don't know how many times we can fly the first stage. But the engines have a cycle of 40.”
As part of the company's goal to dramatically lower the cost of space launch, Hawthorne, Calif.-based SpaceX is slowly moving out on a strategy to test the reusable Falcon 9 core stage with the goal of eventually recovering and reflying the entire rocket, Guillen says.
But even without first-stage flyback, the Falcon 9 is already the lowest-cost rocket on the commercial market today. Able to throw 4,850 kg of payload to supersynchronous transfer orbit at roughly $60 million per launch, it bests even the Chinese Long March 3B at $70 million a pop.
Noting that the cost of fuel, oxygen and other expendable liquids in the rocket amounts to just 0.3% of the cost of a Falcon 9 mission, SpaceX CEO and founder Elon Musk says there is potential to further lower the cost by a factor of more than 100, assuming a high launch rate and the ability to fully and rapidly reuse the entire rocket, including its first and second stages.
“This is a difficult thing to achieve,” he told an annual U.S. Export-Import Bank conference in Washington April 25. “A lot of people in the aerospace industry think it's not possible, and most in industry have given up on it. But we think it's possible.”
Among the doubters is NASA Deputy Associate Administrator Dan Dumbacher, a former Space Shuttle engineer who leads the agency's exploration systems development. Dumbacher says the agency learned a lot from its experience with the orbiter's reusable Space Shuttle Main Engines (SSMEs).
“We tried to make the engines reusable for 55 flights,” he said in Paris last month. “Look how long and how much money it took for us to do that, and we still weren't successful for all parts.”
While NASA is using SSMEs to power its new heavy-lift Space Launch System (SLS), Dumbacher says the cost of refurbishing the engines means the agency has no plans to reuse them.
“All we're going to be doing is working on the manufacturability and getting the cost down,” Dumbacher said. “We're going to retain the reusability that's in there, but I'm actually going to throw them away.”
Dumbacher said unlike commercial airliners, rockets have limited flight opportunities, presenting a challenge to engineers in terms of measuring and understanding the environments in which launch vehicles operate.
“In the airline industry, you have lots of flights, lots of repetition, lots of opportunity to go learn, figure out what broke, and fix it,” he says. “With launch vehicles, we've launched a lot, but not to the order of magnitude we have with airplanes.”
Dumbacher said the Space Shuttle solid-rocket boosters were likewise problematic, requiring too much overhaul to justify the cost of reusing them.
“We'd pick them out of the ocean, but we spent a lot of time cleaning those things up when we got them back, inspecting the hardware to make sure that it was still good to go,” he said.
In addition to physics challenges, Dumbacher said the economics of launching reusable rockets is a key factor to consider in justifying the cost.
“You have to understand the economics of the situation and how that translates into flight rate and production rate to make the overall economic calculations for the system worthwhile,” Dumbacher says.
Christophe Bonnal of the launcher directorate at French space agency CNES, agrees.
“If you reuse, you stop producing, depending on the level of reusability,” he says. “So you end up with a permanent prototype, and to keep costs down you need to have a high rate of production.”
Barry Matsumori, SpaceX vice president of commercial sales and business development, says Falcon 9's 10 Merlin 1D engines, including nine on the core stage and a vacuum version flying on the second, mean SpaceX is already manufacturing a lot of motors.
“It doesn't take very many flights for us to hit rate production,” Matsumori said in April. “We are popping out engines at a pretty high rate.”
Bonnal said CNES has looked at “optimistic” flight rates of around 50 per year, assuming a single payload, but that, “Under the best conditions we could save 10% of the launch costs, plus or minus 15%,” he quipped.
One of the most challenging aspects of reusability, he said, is the weight penalty added by hardware and propellant. He says the latter means reserving 30% of first-stage fuel in order to return a booster to the launch site.
“You end up designing much larger vehicles, with landing gear, with legs or wings, so it's heavier and you need more propulsion, at least 25-30% more propulsion on the stage,” Bonnal said, adding that a previous study by CNES and Russian space agency Roscosmos looked at the feasibility of making the Ariane 5 solid-rocket boosters liquid-fueled and reusable, but scrapped the idea after the hardware grew too large.
“The thing that shocked me was that at the beginning, this reusable flyback booster was just a cylinder with engines and little wings, just a turbo fan in the back,” he said. “And three years later these were complete Airbuses in terms of size with four engines in each of them.”
SpaceX President Gwynne Shotwell says Falcon 9's reusability is already designed into the rocket's first stage, including the weight of the landing legs that would otherwise detract from the rocket's performance. She also said Falcon 9 retains 30% performance margin over the company's advertised mass-to-orbit capability of 4,850 kg to GTO – margin SpaceX is using to conduct operational trials of a reusable Falcon 9 first stage.
“The mass of the recovery hardware is not a dramatic impact on payload performance,” Shotwell said in a February interview. “Conservatively, if I put 1,000 lbs on the first stage, I'm only losing 100 pounds of payload or so to orbit. What impacts is the fuel we need to reserve to execute the reentry and the landing burns.”
However, Musk says he sees a Falcon 9 with a reusable core stage targeting the small- to medium-size weight category for missions to GTO, with the larger Falcon Heavy now in development expected to do the heavy lifting.
"Where I basically see this netting out is Falcon 9 will do satellites to roughly up to 3.5 tonnes with full reusability of the boost stage, and Falcon Heavy will do satellites up to 7 tonnes with full reusability of all three boost stages," Musk told Aviation Week in February. "Now Falcon Heavy could double its payload, almost; if, for example, we went expendable on the center core, we could do 14 tonnes to GTO."
But beyond performance, says Bonnal is the impact of rocket reusability on ground installations. As an example, he said CNES has found that safety requirements would make return of a boost stage problematic at Europe's South American space port in Kourou, French Guiana.
“I will be very interested in seeing the three Falcon Heavy boosters coming back to Vandenberg with propellant sloshing,” he said, referring to SpaceX plans to start flying a heavy version of the Falcon 9 from the U.S. Air Force's California launch installation next year. “In terms of safety, it must be quite challenging.”
Paul Eckert, manager of budget, policy, and international affairs at the FAA Office of Commercial Space Transportation says regulators are prepared to work with SpaceX to ensure the company can proceed with its reusability initiative.
“There are always risk factors, there are certain criteria, and the company and the government are both well aware, so we don't anticipate any problem,” he said in Paris last month. “I'm sure we can work it out.”
With telemetry gleaned from a simulated soft-landing of a Falcon 9 first stage in the Atlantic Ocean last month, SpaceX says it is chipping away at its goal. But as Musk has emphasized, the economics of reflying launch vehicles are most promising if the entire rocket is reusable.
“The most fundamental breakthrough is a rapidly and fully reusable rocket,” Musk said in February. “It doesn't help if it's partially reusable.”
Shotwell concedes that second stage-recover presents a different set of challenges, notably the performance offset required for additional propellant.
“The second stage mass trade is pound for pound,” Shotwell says. “Whatever incremental mass I add, the same has to come off the payload.”
When asked if SpaceX has the technology in hand to achieve second-stage flyback, Guillen declined to comment, though she said “it is in our plans to recover the entire rocket.”
Matsumori agreed that recovering a Falcon 9 upper stage is more difficult than returning a core booster.
“The thing goes to the other side of the world; bringing it back is another challenge,” he said.