Updated May 25 at 10:18 a.m. Eastern.

WASHINGTON — SpaceX Chief Executive Elon Musk on May 10 went into detail on modifications made to the latest version of the Falcon 9, including redesigning a pressure vessel implicated in the rocket’s 2016 pre-launch explosion.

In a briefing with reporters hours before the scrubbed launch of the first Block 5 Falcon 9 rocket, Musk said the Block 5 is designed to be “the most reliable rocket ever built.”

“That is the design intent,” he said. “I hope fate doesn’t punish me for these words, but that is unequivocally the intent. And I think our most conservative customers would agree with that.”

A last-minute glitch May 10 postponed the Block 5 Falcon 9’s debut launch 24 hours to May 11. SpaceX blamed “a standard ground system auto abort” for halting the countdown 58 seconds before liftoff.  The rocket lifted off successfully May 11 at 4:14 p.m., landed its first stage 11 minutes later and deployed Bangladesh’s first telecom satellite, Bangabandhu-1, to geostationary transfer orbit just under 34 minutes later. 

Lessons from 2016

Musk said SpaceX put great effort into creating extremely reliable COPVs, or composite overwrapped pressure vessels, used to store helium to pressurize the propellant tanks in the launcher’s second stage. SpaceX has completed the new pressure vessels, but has yet to include them on the Block 5. The launch company plans to include the new pressure vessels no later than an August demonstration mission of an uncrewed Dragon V2 capsule. 

A SpaceX spokesperson told SpaceNews May 25 that the Block 5 Falcon 9’s first flight still “serves as an important milestone toward flying crew to the International Space Station later this year.”

“Starting with our first Block 5 launch, and working closely with NASA, SpaceX will evaluate the components, systems and operations from each Falcon 9 Block 5 mission to document the vehicle’s flight heritage and assure our development of the most advanced, reliable and safest human spaceflight system ever built,” the spokesperson said.

In September 2016, a Falcon 9 exploded during preparations for a static fire test and destroyed a telecom satellite for Israeli fleet operator Spacecom.

SpaceX traced the cause to liquid oxygen in the upper stage tank that got trapped between the COPV overwrap and liner and ignited either from friction or other mechanisms. SpaceX has since worked to redesign those pressure vessels in cooperation with NASA in order to address the agency’s concerns about using that design on later Falcon 9 commercial crew launches.

“This is by far the most advanced pressure vessel ever developed by humanity,” Musk said during the May 10 call. “It’s nuts. I’ve personally gone over the design; I can’t count how many times. The top engineering minds at SpaceX have agonized over this. We’ve tested the living daylights out of it. We’ve been in deep, deep discussions with NASA about this, and I think we’re in a good situation.”

Musk said the improved COPVs have a burst pressure “more than twice what they are actually loaded to on the pad.” SpaceX has a contingency design that would involve switching from high-strength carbon fiber with an aluminum liner to the superalloy Inconel, but that is “unlikely to be necessary,” Musk said.

New and improved

While block numbering would suggest this is the fifth iteration of the Falcon 9, Musk said the Block 5 “is arguably Falcon 9’s version 6” based on how improvements have been made over time.

“The word ‘block’ is a bit strange. We kind of adopted it from the Russians,” he said.

Musk said the each of the nine Merlin engines used to power the Falcon 9’s first stage now have an 8 percent increase in thrust at sea level to 190,000 pounds-force. The single vacuum-optimized Merlin engine on Falcon 9’s second stage has a 5 percent thrust increase to 220,000 pounds-force, he said.

By comparison, the Block 5 Falcon 9 is around twice as powerful as the Falcon 9 that first launched a demonstration resupply mission for NASA in 2010. The Merlin engines on that first version had 95,000 pounds of thrust for each first-stage engine and 92,500 pounds of thrust for the second-stage engine.

The first stage of Block 5 rockets are designed to be far more reusable than previous versions which so far have only flown twice before retirement.

“In principle, we could refly Block 4 probably upwards of 10 times, but with a fair amount of work between each flight,” Musk said. “The key to Block 5 is that it’s designed to do 10 or more flights with no refurbishment between each flight. The only thing that needs to change is to reload propellant and fly again.”

With some refurbishment, a Block 5 first stage should be able to launch 100 times, Musk said.

In addition to greater reusability, SpaceX’s Block 5 Falcon 9 is designed to meet NASA commercial crew requirements and Air Force national security launch criteria. It’s also designed for easier manufacturing.

Musk said the Falcon 9’s octaweb structure, used to support all nine first stage engines and provide compartmentalization in case one or more fails, is now much stronger. The octaweb is made with bolted instead of welded aluminum and has greater thermal protection to prevent melting, he said.

SpaceX put latch mechanisms on the Falcon 9’s landing legs so the vehicle doesn’t have to rely on external clamps for steadying on ocean-platform landings, Musk said.

SpaceX also upgraded the rocket’s avionics, and is keeping the titanium grid fins, he said. The Falcon 9 previously used aluminum fins for steering the Falcon 9 first stage back to Earth, but SpaceX changed those on the Block 3 to titanium after the aluminum fins caught fire during reentry.

Musk said the rocket’s interstage features a hydrophobic thermal protection developed by SpaceX that is highly reusable and doesn’t require paint. Placed between the first and second stages of the rocket, which are painted white, the jet-black carbon fiber interstage harkens back to SpaceX’s first rocket, the Falcon 1.

“Obviously, aesthetics are a minor factor in rocket design, but I still like the fact that we’ve returned for nostalgic reasons to having a black interstage,” Musk said.

Falcon 9 Block 6?

By keeping the Falcon 9 design static, SpaceX can devote more time and effort to its Big Falcon Rocket and Starlink satellite constellation. Though Musk and other SpaceX officials have repeatedly called the Block 5 the final design, Musk hinted that some minor improvements could still be made to the rocket.

SpaceX engineers might squeeze another 2 percent of additional thrust out of the first stage, and another 5 percent out of the second stage as compared to the Block 4, he said. Musk said the rocket could see “minor improvements” for better manufacturability, reflight and reliability “provided that they are supported by our key customers in commercial satellite launch, NASA and the Air Force.”

He emphasized that any further changes would be small.

“There will not be a Block 6,” Musk said. “We intend to stabilize on the Block 5 platform and have no major upgrades.”

Caleb Henry is a former SpaceNews staff writer covering satellites, telecom and launch. He previously worked for Via Satellite and NewSpace Global.He earned a bachelor’s degree in political science along with a minor in astronomy from...