PASADENA, Calif. — As SpaceX launched another Falcon 9 with a previously-flown first stage May 22, both the company and its competitors are seeing a growing acceptance of reusable vehicles in the overall market.

The Falcon 9 that launched five Iridium Next satellites and two GRACE-FO Earth science satellites from Vandenberg Air Force Base used a first stage that first flew in January, carrying the classified Zuma payload. That booster was the 12th first stage to be reflown, counting the two used as side boosters in the inaugural Falcon Heavy launch in February.

Most of those Falcon 9 missions with reflown boosters have been for commercial customers, enticed at least in part by the modest discounts SpaceX has offered for using previously-flown stages. NASA has flown two Dragon cargo missions to the International Space Station on reflown boosters, but the agency says it evaluates the use of such vehicles on a case-by-case basis.

“I think government customers are taking a very careful approach, which you would expect them to do” about accepting reused vehicles, said Josh Brost, senior director for government business development at SpaceX, during a panel discussion at the Space Tech Expo conference here shortly before the Falcon 9 launch.

Brost said he expects that attitude to change as SpaceX brings into the service the Block 5 version of the Falcon 9, which made its first flight May 11. That vehicle incorporates lessons learned from earlier versions of the Falcon 9 intended to make to improve the vehicle’s reusability, with a goal of performing at least 10 flights of the first stage without requiring significant maintenance.

“There’s lots of little things that we learned from each flight about how to make our rocket more reliable and more rapidly reusable,” he said of the changes that went into the Block 5. Those lessons ranged from improvements to the heat shield on the bottom of the stage to how to more efficiently guide the stage back to landing.

He predicted that, once the Block 5 achieves that ten-flight goal, government and other customers will be more willing to fly on it. “We see a future where the most risk-averse customers are likely to prefer to fly on the second flight of a booster rather than the first flight,” he said. “Once you demonstrate you can fly it many times, you can see that first flight as essentially a check flight.”

Brost said that SpaceX was working with “other government entities” about the use of previously-flown boosters. That’s likely to include the U.S. Air Force, which has not yet certified reflown Falcon 9 vehicles for its missions.

Col. Jon Strizzi, chief engineer of the Launch Directorate of the Air Force’s Space and Missile Systems Center, didn’t directly address reusability in his comments on another conference panel, but did note that the service was figuring out how to incorporate innovative approaches to launch services.

“As we move into the future, with new ways to build things, produce things and operate them in different commercial landscapes and different military landscapes, keeping the secret recipe, the ‘secret sauce’ for our success is going to be an interesting opportunity, and we have to work closely with our providers to do that,” he said.

Other companies are also pursuing reusability because of its cost and operational benefits. Blue Origin has demonstrated reusability with its New Shepard suborbital vehicle and plans to land and reuse the first stage of its New Glenn rocket.

Ariane Cornell, New Glenn commercial sales director in the Americas for Blue Origin, noted that the company’s vision is for millions of people to be living and working in space. “The only way that you can do that is by bringing down the cost of access to space, so obviously reusability has been central to our design and our philosophy from the very beginning.”

European agencies and companies, once skeptical of the benefits of reusability, are now growing more interested in the development of reusable launchers.

“The question of reusability is whether it’s really a cost breakthrough for such relative small numbers of launches,” said Andreas Rittweger, director of the Institute of Space Systems at the German space agency DLR. “We came, in our investigations, to the conclusion that 10 launches per year indeed makes it sensible for reusability.”

Studies of reusable technologies by DLR and the French space agency CNES are still in their early stages, he said, including work the Prometheus engine and concepts to demonstrate both vertical landing of stages, as done by SpaceX and Blue Origin, as well as winged flyback boosters.

“We are investigating different architectures,” he said, including the different landing modes as well as propellant combinations. The goal is to show a cost reduction of at least 30 percent with 10 launches per year.

SpaceX’s goals are more ambitious in terms of cost reductions and flight rates. “Our end goal is to make humanity interplanetary,” Brost said. “That only happens if getting to space is much less expensive than it’s ever been before. And the reason getting to space has been so expensive is because every time you went to space you threw your vehicle away.”