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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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Tesla Full Self-Driving expands to another European country

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37 minutes ago

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September 21, 2026

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Credit: Tesla

Tesla’s Full Self-Driving (Supervised) is heading to Czechia after the Czech Ministry of Transport recognised the Dutch RDW’s provisional type approval, making the country the seventh EU member state to clear the system for public roads. Tesla Europe announced on 21 September 2026 that “FSD Supervised is now approved in Czechia” and that rollout “will begin soon.”

The decision marks a notable reversal. Earlier in 2026, Prague had declined to automatically recognise the Netherlands’ April approval, citing concerns over speed-limit compliance, traffic-sign recognition and driver-attention monitoring, and arguing that a coordinated EU approach was preferable. Officials said months of expert review, talks with Tesla and other member states, and real-world data from countries already using the system resolved those issues.

“Safety remains the top priority,” the ministry stated.

FSD Supervised remains a Level 2 driver-assistance system: the driver must stay engaged and is legally responsible. Eligible vehicles need AI4, the company’s most up-to-date hardware version. Tesla is expected to push the feature over the air in the coming days, following the pattern seen after earlier national approvals.

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Europe’s rollout began when Dutch regulator RDW issued a provisional EU type approval on 10 April 2026 after extensive testing. Mutual recognition then produced a rapid cascade: Lithuania (20 May), Estonia (29 May), Denmark (9 June), Belgium (10 June) and Slovenia (7 September). Czechia now completes that list of seven.

The approvals cover only a modest share of the EU population, but they add political weight ahead of a 6 October vote by the Technical Committee on Motor Vehicles. A qualified majority, at least 15 of 27 member states representing 65 percent of the EU population, could open the remaining markets, including large ones such as Germany, France, Italy and Spain that have so far preferred to wait for a bloc-wide decision.

For Czech Tesla owners, the immediate prize is access to the same supervised highway and city driving already available in the other six countries. For Tesla, each new market generates additional European driving data and strengthens the case that FSD Supervised can operate safely under the continent’s varied road rules. The Czech approval is therefore both a local milestone and another incremental step toward a wider European launch.

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Tesla Roadster event requires restricted airspace, and the FAA obliges

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1 hour ago

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September 21, 2026

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Credit: @BLKMDL3/Twitter

The Federal Aviation Administration (FAA) has established a Temporary Flight Restriction over SpaceX’s McGregor, Texas, rocket development and test facility, a move widely viewed as preparation for Tesla’s October 1 Roadster reveal. The restriction took effect September 18 and runs through October 2.

NOTAM FDC 6/3825 covers a 1.5-nautical-mile radius around the site near Waco and extends from the surface to 10,000 feet above ground level. The FAA cited hazards under 14 CFR 91.137(a)(3) and barred aircraft and drones from the zone. Tesla’s invitation to reservation holders already placed the event in Waco, about 20 minutes from McGregor, making the timing and location more than coincidental.

What stands out is the altitude. Typical recent TFRs at McGregor for engine static fires and component tests have used far lower ceilings, often around 2,000 feet. Raising the limit to 10,000 feet is unusually high even compared with some Starbase restrictions and signals operations that go beyond a standard ground-level engine test.

That extra airspace has fueled speculation about the long-promised SpaceX Package for the Roadster. Elon Musk has described cold-gas thrusters that could deliver sub-one-second 0-60 times and, more dramatically, brief lift-off. Reports earlier this year indicated Tesla planned a remote-controlled demonstration at McGregor in which the car would leave the ground with no one inside; spectators kept hundreds of yards away because of the noise.

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The 10,000-foot envelope would give operators a large safety buffer even if the vehicle only hovers a short distance.

Tesla has not confirmed a flight demo. The company has only used the phrase “Go for launch” and posted a teaser image of the car on what looks like a launch pad. The TFR itself mentions only hazards. Still, closing airspace this high and this close to the reveal date strongly suggests the event will include more than a static display.

Whether the Roadster actually hovers on October 1 remains to be seen. What is certain is that the FAA has cleared a large vertical slice of Texas sky for whatever Tesla and SpaceX intend to show. Reservation holders heading to Waco will be among the first to find out if the car can do more than drive.

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Elon Musk drops a surprise update on Boring Company’s next big dig

Musk says Boring Company could shrink the Austin to San Antonio drive to just minutes.

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11 hours ago

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September 20, 2026

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Elon Musk says The Boring Company is working on what he called “a simple, precursor Hyperloop” tunnel connecting Austin and San Antonio, targeting speeds above 200 mph and cutting a drive that can take up to two and a half hours down to a consistent under 30 minutes. Musk posted the idea on X Sunday, in a reply to a repost of an AI generated video imagining a science fiction future with human colonies on other worlds, which he shared with the line “This is the future we shall bring into being.”


The Boring Company’s own account picked up the idea in the same thread, adding a detail about how the trip would actually work: “Because Loop/Hyperloop is express (i.e. no intermediate stops), one could travel from an Austin parking lot to a favorite San Antonio restaurant in about 30 minutes. As long as they both have Loop stations.” That framing ties the proposed intercity link to the same station model the company already runs in Las Vegas, where riders enter the tunnel network through small, garage style stops rather than one central terminal.

This is not the company’s first run at the Austin to San Antonio corridor. Boring Company floated tunnels between the two cities as far back as 2021, and later competed for a separate San Antonio Loop project tied to the airport before that specific bid stalled. Pitches for tunnels in Chicago, Los Angeles, and a New York to Washington corridor have followed a similar pattern of big announcement without a shovel in the ground.

What is different this time is the balance sheet, especially since The Boring Company closed a 3 billion dollar funding round led by investors in the United Arab Emirates earlier this month at a valuation near 23 billion dollars, giving the tunneling company more capital to chase speculative projects than it had during its earlier Texas pitches. The company is also mid-build on two other intercity systems it has actually broken ground on, inc;luding a Nashville tunnel linking downtown to the airport, where a second boring machine finished commissioning in June, and its Las Vegas network, where the station count keeps climbing on paper faster than tunnels get dug.

That gap between announcement and execution is the reason to treat Sunday’s post as an opening bid rather than a project. A tunnel spanning roughly 80 miles between two metro areas, running at speeds Boring Company has not demonstrated over any real distance, would dwarf anything the company has built. For now, the Austin to San Antonio Hyperloop exists as a caption under an AI generated space video.

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