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Ex-SpaceX engineer leads Stratolaunch to major rocket engine test milestone

Stratolaunch has successfully completed the first full-scale test of its 200,000-lbf thrust PGA rocket engine. (Stratolaunch)

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8 years ago

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Led by rocket propulsion expert Jeff Thornburg, Stratolaunch – famous for owning the largest fixed-wing aircraft ever built – has completed the first hot-fire test of a full-scale rocket engine component known as the preburner, a major milestone in the development of any launch vehicle or propulsion system.

Despite the significant size and power of the component, destined to support an engine that will generate 200,000 pounds (~900 kN) of thrust, Thornburg and his team of engineers and technicians have managed to go from designing the preburner to successfully hot-firing a full-scale test article, an extraordinary achievement by any measure.

Aside from SpaceX, Blue Origin, and Aerojet-Rocketdyne, Stratolaunch is the only private entity developing – let alone testing full-scale parts for – a liquid-fueled rocket engine as large as PGA. Shorthand for the Stratolaunch’s late founder and bankroller Paul G. Allen, PGA is a fuel-rich staged combustion cycle engine that uses liquid hydrogen and oxygen (hydrolox) fuel and oxidizer, typically resulting in high efficiency. In terms of scale and thrust, PGA is very closely comparable to SpaceX’s Merlin 1D engine, which uses kerosene instead of hydrogen but produces roughly 190,000 lbf (850 kN) of thrust and stands 4 feet (1.2m) wide and ~10 feet (~3m) tall.

Another major difference between PGA and Merlin 1D is the fact Merlin 1D’s nozzle is largely optimized for sea level while PGA is being built for a rocket that will be “launched” from a massive plane flying around 35,000 feet (~10.5 km), ultimately resulting in a nozzle that is much wider and longer, featuring nearly the same proportions as fully vacuum-optimized engines like SpaceX’s MVac. By widening the nozzle relative to the rest of the engine, rocket engines are able to operate far more efficiently at higher altitudes, where Earth’s atmosphere thins and exerts less pressure on the escaping exhaust gases. This is visualized well by the visible expansion of rocket exhausts during launches, morphing from a straight cylinder to a massive teardrop-shaped plume. At lower altitudes (and thus higher atmospheric pressures), wider nozzles can produce extreme turbulence and will ultimately shake themselves to destruction, preventing their usage on ground-launched rocket boosters.

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Judging from official renders of the engine, PGA’s in-atmosphere variant appears to utilize a form of regenerative nozzle cooling very similar to that used on M1D, where liquid propellant flows through thin capillaries sandwiched between two or more layers of metal to cool the nozzle much like cold water chills the skin of an uninsulated water bottle.

Testing rocket engine preburners

In the case of staged combustion cycle hydrolox rocket engines, a small portion of liquid oxygen and all of the liquid hydrogen (hence “fuel-rich”) are mixed and combusted to generate hot gas that then spools up the engine’s primary turbopump(s), ultimately drawing fuel and oxidizer into the combustion quickly enough to ignite the engine and generate sustained thrust. The components that get those main turbopumps started are known collectively as the preburner, which is what Stratolaunch successfully tested – at full-scale – for the first time ever last week. For any liquid rocket engine that cannot solely rely on propellant tank pressure to deliver fuel to the combustion chamber, full-scale tests of preburners or gas-generators effectively mark the moment that engines truly become real.

“This is the first step in proving the performance and highly efficient design of the PGA engine. The hot-fire test is an incredible milestone for both the propulsion team and Stratolaunch.” – Jeff Thornburg, VP of Propulsion, Stratolaunch

Stratolaunch’s propulsion team will continue to test the preburner for longer durations and at higher power levels over the next several months, likely optimizing operations and tweaking or upgrading the preburner’s hardware as real tests produce valuable lessons-learned. Built entirely with additive manufacturing (3D printing), the team should be able to rapidly iterate on the physical design of the engine, a rarity in a field where traditional fabrication methods can take weeks or months to produce complex turbomachinery components with mercilessly strict tolerances.

According to Thornburg, the ultimate goal is to continue that additive-manufacturing-only strategy throughout the development of this rocket engine, theoretically enabling unprecedented design flexibility while also slashing production time throughout. PGA will ultimately power the creatively-named Medium Launch Vehicle (MLV), a small-ish air-launched rocket designed to place a respectable 3400 kg into low Earth orbit (LEO) as early as 2022, as well as a Heavy version of MLV and, potentially, a reusable spaceplane somewhere down the line.

 

For prompt updates, on-the-ground perspectives, and unique glimpses of SpaceX’s rocket recovery fleet check out our brand new LaunchPad and LandingZone newsletters!

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Eric Ralph is Teslarati's senior spaceflight reporter and has been covering the industry in some capacity for almost half a decade, largely spurred in 2016 by a trip to Mexico to watch Elon Musk reveal SpaceX's plans for Mars in person. Aside from spreading interest and excitement about spaceflight far and wide, his primary goal is to cover humanity's ongoing efforts to expand beyond Earth to the Moon, Mars, and elsewhere.

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Elon Musk’s surprise addition to the X Takeover lineup has fans talking

Elon Musk will join Saturday’s X Takeover at Giga Texas for a live virtual interview.

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4 hours ago

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October 9, 2026

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Credit: Tesla Owners Silicon Valley
Credit: Tesla Owners Silicon Valley

Elon Musk will join X Takeover at Giga Texas on Saturday for a live virtual interview, according to Sawyer Merritt, who shared the news late Thursday. Musk will not be on stage in Austin. The conversation is set to stream for free on the @teslaownersSV account on X.

Organizers had kept expectations in check. In a September update, Tesla Owners Silicon Valley said Musk had appeared at the event twice before but was not promising a third appearance, even as fans hoped he would walk the Giga Texas grounds in person. A virtual spot matches 2024, when Musk gave a surprise interview of about an hour to the crowd in San Luis Obispo, as Teslarati reported at the time.

This year’s edition is a first in several ways. It is the first X Takeover held outside California and the first at a Tesla facility, with tickets selling out in eight days. Tesla provides the venue, but the event is produced independently by Tesla Owners Silicon Valley. The main event runs from 10 a.m. to 6 p.m. CT, followed by a drone and light show at 9 p.m. Maye Musk is the keynote speaker, Franz von Holzhausen is set for a virtual keynote, and Nicki Minaj is the special guest. Joe Tegtmeyer, whose drone footage Teslarati used to track the Optimus factory steel frame at Giga Texas, is also on the speaker list.

Musk’s interview topics have not been revealed, but the backdrop is busy. Tesla doubled its Cybercab fleet in Austin in late September, and last week Musk explained why Robotaxi hours only moved from 10 p.m. to 11 p.m.. Merritt also reported Thursday that Texas DMV records now show 319 registered Cybercabs, up from 169. NHTSA’s deadline for Tesla’s sworn answers on Cybercab certification is October 30, and Tesla reports third quarter earnings on October 21.

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Fans who cannot make it to Austin can watch the livestream on X. Musk tends to say more in unscripted settings than he does in prepared remarks, which is the reason this one is worth having open on Saturday.

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It’s official: SpaceX takes aim at Verizon, AT&T, and T-Mobile

SpaceX is buying 800 MHz spectrum from Grain to turn Starlink Mobile into a carrier.

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16 hours ago

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October 8, 2026

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Starlink D2D direct to device vs Verizon, AT&T (Concept render by Grok)

SpaceX has agreed to buy a nationwide block of low band wireless spectrum, a deal the company says will let Starlink Mobile operate as a full US carrier rather than a satellite add-on for someone else’s network.

The company announced the agreement on X on Thursday afternoon, saying it will “pave the way for @Starlink to become a major mobile carrier in the US.” The seller is Grain Management, a private investment firm that confirmed in a statement that SpaceX will acquire 100% of its nationwide 800 MHz portfolio. That covers up to 14 MHz of paired spectrum in the 817 to 824 MHz and 862 to 869 MHz bands. Neither side disclosed a price, and the deal still needs FCC approval.

Grain only recently picked up the licenses itself. It bought the portfolio from T-Mobile in a transaction that closed in August, paying cash plus its own 600 MHz spectrum. Rival AST SpaceMobile had been testing satellites on the same bands before SpaceX stepped in.

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SpaceX said its 2 GHz spectrum will handle high bandwidth capacity, while the new 800 MHz layer “ensures Starlink Mobile’s signal penetrates through obstacles, such as walls, and can provide service to customers’ devices even when they are in buildings.” The company added that most existing phones already support the band, so customers would not need new hardware to use it.

That 2 GHz spectrum came from SpaceX’s EchoStar acquisition last year, which gave the company exclusive S band rights in the US and global Mobile Satellite Service licenses. The Grain spectrum is different in an important way: it is tailored for service from ground towers, not satellites. SpaceX said that combination would make Starlink Mobile “the first network operator to deploy both satellite and terrestrial spectrum.”

The announcement also follows a key regulatory win. Earlier this week, the FCC approved SpaceX’s plan to deploy 15,000 second generation Starlink Mobile satellites, which the company has said will carry up to 100 times the data density of the current system, as Teslarati previously reported.

Shares of AT&T, Verizon and T-Mobile fell in extended trading after the announcement. T-Mobile is currently SpaceX’s launch partner for Starlink Mobile in the US, which makes its position the most complicated of the three.

SpaceX has not been subtle about its plans. During the company’s August earnings call, President and COO Gwynne Shotwell said she expected Starlink Mobile to win over customers from the major carriers. “I anticipate us to be able to acquire quite a few of their customers because I think our service will be better,” she said, pointing to dead zone coverage and resilience during disasters. Shotwell also described plans for low cost cellular base stations that could pair with existing Starlink dishes.

SpaceX has targeted 2027 for deployment of its next generation Starlink Mobile satellites, with upgraded service expected by the end of that year. The FCC review of the Grain deal now determines when the terrestrial half of that network can come online.

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Trump presents Elon Musk with National Medal of Science, calls him “modern-day Thomas Edison”

Trump gave Elon Musk the National Medal of Science and compared him to Thomas Edison.

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23 hours ago

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October 8, 2026

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Elon Musk holds the National Medal of Science alongside President Donald Trump at the “Science: A New Golden Age” summit in Washington on October 8, 2026.
Elon Musk holds the National Medal of Science alongside President Donald Trump at the “Science: A New Golden Age” summit in Washington on October 8, 2026.

Elon Musk now has the highest honor the U.S. government gives to scientists and engineers. President Donald Trump presented him with the National Medal of Science on Thursday at the White House’s “Science: A New Golden Age” summit, held at the Institute of Peace in Washington, according to the White House.

Trump called Musk a “national treasure” and “our modern-day Thomas Edison.” “Few Americans in history have done more to advance America’s national interest than Elon in one field after another,” the president said. He also described Musk as an “industrial titan, brilliant engineer and one of the greatest technology founders to ever live.”

The official citation credits Musk “for engineering achievements of extraordinary ambition and scale that have pushed the frontiers of space exploration, intelligent systems, communications, and beyond, renewing American technological leadership and opening a new chapter in American manufacturing.” Tesla is not named in it, but “intelligent systems” and “American manufacturing” cover a lot of ground that Tesla and the Terafab chip project occupy.

Google co-founder Sergey Brin, Nvidia CEO Jensen Huang and AMD CEO Lisa Su received the same medal. Microsoft CEO Satya Nadella and Dell Technologies CEO Michael Dell were given the National Medal of Technology and Innovation. It was the first time Trump has handed out either award in his two terms. As Teslarati noted when the list was first reported on Wednesday, the National Science Foundation says 529 people have received the Medal of Science since Congress created it in 1959.


The ceremony closes a loop that opened in 2025, when Musk left DOGE and criticized the “Big Beautiful Bill,” sparking a public feud. Al Jazeera noted that Musk was the largest contributor to Trump’s 2024 campaign, and CNBC reported that all of the honorees have given financially to Trump or Republican causes in some form. The reconciliation has moved quickly in recent weeks. Musk sat at Trump’s left at the September 29 White House AI lunch, was named to help lead the Pentagon’s Project Meridian study on the future of warfare, and adopted the administration’s “super intelligence” wording when he said SpaceXAI will become SpaceXSI.

For Tesla and SpaceX shareholders, the practical question is what the closer relationship means for federal approvals. Tesla’s Robotaxi and Cybercab expansion depends on national autonomy rules, and SpaceX’s Starship and Starlink plans run through the FAA, FCC and NASA. Thursday’s ceremony did not change any of that directly.

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