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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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SpaceX just got the green light Starship has waited years for

The FAA has cleared Starship Flight 14, setting up SpaceX’s first orbital attempt on Monday.

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2 hours ago

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September 27, 2026

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SpaceX has cleared the last regulatory hurdle standing between Starship and its first trip to orbit. The Federal Aviation Administration issued the launch license for Starship Flight 14 late Saturday, keeping the mission on track for liftoff Monday, September 28, from Pad 2 at Starbase, Texas.

The 75 minute launch window opens at 7:15 a.m. Central, and Boca Chica Beach closures are also scheduled for September 29 and 30 as backup dates. This will be Starship’s first revenue generating mission.

The license was the missing piece after SpaceX completed a full wet dress rehearsal with Booster 21 and Ship 41 on September 24. At the time, the company said the flight remained on track pending regulatory approval. Because Flight 14 flies an orbital profile, the FAA had to sign off on a modified license that met its safety, payload and financial responsibility requirements.

Observers combing through the new FAA paperwork also noticed that lightning no longer appears among Starship’s listed launch hazards. If that holds, it matters more for where Starship is headed than for Monday’s attempt. Florida and Louisiana, home to LC-39A and the planned Starbase Louisiana site, see some of the most frequent lightning in the United States.

SpaceX tells the FCC that Starship Flight 14 is going to orbit

Flight 14 is the mission SpaceX has been building toward for months. Ship 41 will carry 26 Starlink V3 satellites, the first operational V3 units to be deployed, and attempt roughly six orbits at about 275 kilometers over a flight lasting just under 10 hours. SpaceX says the ship will only perform its orbital insertion burn after flight controllers confirm enough hardware redundancy remains for the deorbit burn at the end of the mission. Ship 41 is targeting a splashdown in the Pacific west of Chile, while Super Heavy will return to the Gulf of Mexico.

The date carries some symbolism as well. A Monday launch would come 10 years and one day after Elon Musk first presented the Interplanetary Transport System, the design that became Starship, at the International Astronautical Congress in Guadalajara, Mexico.

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SpaceX has not announced what comes next, but air traffic planning slides reported this week, list Flight 15 no earlier than October 19 and a first Starship launch from LC-39A in Florida no earlier than October 30. Both dates depend on how Monday goes.

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SpaceX completes another secret Pentagon launch, adding to suspected Starshield buildout

SpaceX launched the classified USSF-385 mission from Vandenberg, landing its booster on a tenth flight.

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1 day ago

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September 26, 2026

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US Golden Dome space defense system (Concept render by Grok)

SpaceX launched another classified mission for the U.S. Space Force from California early Saturday morning, and the Falcon 9 booster that carried it landed on a drone ship in the Pacific for the tenth time. The USSF-385 mission lifted off from Space Launch Complex 4E at Vandenberg Space Force Base at 7:00 a.m. PT.

Booster B1100 touched down on Of Course I Still Love You roughly eight and a half minutes after liftoff. It was the booster’s tenth flight and tenth successful landing, following the NROL-95 national security mission and eight Starlink launches. Its previous flight, a Starlink Group 15 mission on August 22, came just 35 days earlier. SpaceX ended its livestream shortly after the landing, which is standard for classified payloads, and neither the company nor the Space Force has said what the rocket carried.

USSF-385 is the fourth Space Force launch from the same Vandenberg pad in roughly six weeks, following USSF-366 on August 15, USSF-153 on September 10, and USSF-259 on September 17. When SpaceX flew USSF-366 in August, independent trackers noted that the rocket’s stage drop zones matched SpaceX’s Starlink Group 15 missions, pointing to Starshield, the government version of the Starlink satellite bus. The Space Force later cataloged 23 satellites after both USSF-366 and USSF-153, while USSF-259 placed 17 satellites into a different orbital plane, per KeepTrack. Launch databases describe USSF-385 the same way, though the payload remains officially unidentified.

Starlink’s Starshield wins contract with US Space Force

The cadence lines up with the contracts, because in July, the Space Force awarded SpaceX $1.6 billion in task orders for 18 Falcon 9 missions from Vandenberg through the end of 2027. SpaceX also holds contracts to build pieces of that same network, which pushed its Pentagon contract total for 2026 past $8 billion.

Saturday’s flight was also the sixth and final Falcon 9 launch from Vandenberg in September, according to Spaceflight Now, while only one Falcon 9 flew from the East Coast this month as SpaceX shifts its Florida infrastructure toward Starship. Launch trackers list it as SpaceX’s 112th mission of 2026 and the 108th Falcon 9 flight of the year, with SLC-4E turned around about six and a half days after its previous launch.

The West Coast pad will not stay quiet for long, considering SpaceX has another Starlink mission scheduled from SLC-4E on September 30. Meanwhile, in Texas, the company is two days away from Starship Flight 14, which is targeting Monday at 7:15 a.m. CT for the vehicle’s first attempt to reach orbit.

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Tesla hints at new Roadster design in surprise clip

Tesla ended its Semi event with a Roadster teaser revealing a new front light bar.

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2 days ago

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September 25, 2026

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Tesla Battery Day event (Credit: Ryan McCaffrey/Twitter)

Tesla closed out its Semi event in Nevada on Thursday night with a nod to its own history, dropping a short Roadster teaser that suggests the production car will look noticeably different from the prototype first shown in 2017.

“We can’t have a Semi event without the Roadster,” Tesla engineering executive Lars Moravy told the crowd before the clip played. The line was a deliberate callback. Tesla first revealed the next generation Roadster in November 2017 by driving it out of the back of a Semi trailer at the truck’s original unveiling in Hawthorne, California.

The new video opens on trailer doors swinging apart in the dark. A thin white light bar glows across what appears to be the nose of the car, Tesla and SpaceX logos flash over the frame, and the Roadster name appears before the clip ends on “See you next week.” Tesla posted the nine second clip on X after the livestream wrapped.

The light bar is the most concrete design detail so far. The 2017 prototype used two separate curved headlamp pods, while a connected front light strip would bring the Roadster in line with the Cybertruck, Cybercab, Semi, and refreshed Model Y. Sawyer Merritt was among the first to point out what looked like part of a SpaceX logo in the video, something Tesla has not addressed.

That logo fits the buildup around the optional SpaceX Package, which Elon Musk has long said would use cold gas thrusters to improve acceleration and possibly allow the car to briefly leave the ground. Tesla’s “Go for launch” post on September 12 set the October 1 date, and invitations sent to reservation holders place the event in Waco, Texas, at 8:30 p.m. Eastern. Waco sits roughly 20 minutes from SpaceX’s McGregor rocket test site, where the FAA has put a temporary flight restriction in place from September 18 through October 2, covering a 1.5 nautical mile radius from the surface up to 10,000 feet.

Tesla is also taking money ahead of the reveal. The company reopened Roadster reservations earlier this week with a $5,000 refundable card payment, followed by a $45,000 wire transfer due within 10 days. That puts buyers at $50,000 committed before Tesla has published a price.

The original pitch set a high bar: 0 to 60 mph in 1.9 seconds before any upgrades, 620 miles of range, a top speed above 250 mph, and production in 2020. That timeline has slipped repeatedly, and Tesla has since pointed to production at Gigafactory Texas no earlier than 2027. The company has said next Thursday’s event will include pricing, specifications, and production targets, the three details original reservation holders have been waiting on for nearly nine years.

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