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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 Boring Company has big plans for Las Vegas by year’s end

Elon Musk’s Boring Company says Vegas Loop stations will double by year end once again.

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10 hours ago

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September 14, 2026

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the boring company's vegas loop entrance

The Boring Company says the Vegas Loop’s station count will double by the end of the year, tying the target to a hiring push for drivers and operations managers in Las Vegas. “Vegas Loop is getting bigger – the station count will double by end of year!” the company wrote in a post on X, attaching listings for a Loop driver and a senior Loop operations manager.

The number checks out against what’s already public, with the Vegas Loop currently running 14 operational stations, while the Boring Company’s own project page lists 28 as the target for the end of 2026.

Much of that growth is tied to tunnels that are already built and waiting on an opening date. A roughly two-mile dual tunnel system under Paradise Road, connecting Westgate to a planned station at 4744 Paradise Road, is expected to open in stages over the coming weeks, Las Vegas Convention and Visitors Authority chief executive Steve Hill told the Review-Journal last week. New stations at 4744 Paradise, Virgin Hotels Las Vegas, and the former Gordon Biersch site would come online with it, several of them built to speed up rides to Harry Reid International Airport ahead of Formula 1’s Las Vegas Grand Prix.

Clark County entitled Vegas Loop for 123 stations after approving 19 more in August, as Teslarati reported at the time. Entitlement and construction move at different speeds on this project, so county approval alone does not guarantee a station opens on any particular schedule.

Clark County approved 18 additional stations back in 2023, part of a plan that pushed the system’s target to 69 stations across 65 miles, doubling the network on paper for the first time. The target kept climbing after that, to roughly 93 stations by the end of that year and 104 by last year, before August’s vote pushed it to 123. This week’s announcement is the first time that doubling language has been attached to stations actually running rather than stations merely approved on a county map.

Ridership gives some sense of what a denser network could carry. Boring Company executive Mike Baier said in July that the Vegas Loop already moves around 40,000 passengers on busy convention days, a total that tops most light rail systems in the country despite the system running on a fraction of its planned tunnel mileage. Company leadership has projected ridership could triple or quadruple once the airport connector tunnels fully open.

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Boring Company did not say which stations beyond those already under construction would open by year end, or whether the hiring push points to a fleet expansion alongside the new stops.

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Tesla looks to expand into new Asian market, strengthening presence

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18 hours ago

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September 14, 2026

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Credit: Tesla Asia | X

Tesla is looking to expand into a new Asian market, strengthening its presence in a region that has been bullish on electric vehicles as a whole.

Tesla officially filed to establish a subsidiary of its business in Vietnam, a report from Reuters suggests. Tesla named the entity “Tesla Motors Vietnam Limited Liability Company.”

The planned entrance into the Vietnamese market is a good sign and move for Tesla, as it has become one of the fastest-growing EV markets in Southeast Asia. It is already among the leaders in the region in both volume and electrification rate. In the first half of this year, Vietnam led Southeast Asia in battery-electric passenger car sales at about 116,000 units, up about 71 percent year over year.

Currently, Vietnamese EV drivers rely on VinFast’s V-Green network, which has about 150,000 ports, but these are primarily reserved for VinFast vehicles. Public third-party charging is fragmented and unreliable for those who do not own chargers that are dedicated to a certain manufacturer’s vehicles.

Tesla has had mixed results in Asia as a whole, and as China remains the core part of its story in Asia, the company is evidently working on expanding its footprint on the continent. Tesla’s domestic retail deliveries fell about 12 percent year over year through the first eight months of 2026.

Model Y remains a standout individual product, holding its position as one of, if not the, best-selling vehicles in the world. However, Model 3 has been weaker than it has been in past years.

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Gigafactory Shanghai, the company’s Chinese production facility, still performs very well. Wholesale volumes in terms of exports have more than doubled and now exceed domestic retail sales; Giga Shanghai builds vehicles for Europe, South Korea, Japan, Australia, and other markets. South Korea has been an explicit bright spot, with registrations doubling year-to-date and Tesla frequently appearing as the top imported brand.

Tesla just did something in South Korea that no foreign carmaker has ever done

Tesla’s entrance into Vietnam signals a broader effort to take over the Asian market and grab more market share from rivals.

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Elon Musk

Elon Musk’s companies made up with Apple but OpenAI still on the hook

Elon Musk’s X Corp and SpaceXAI dropped their Apple antitrust suit, leaving OpenAI as defendant.

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22 hours ago

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September 14, 2026

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X Corp and SpaceXAI, Elon Musk’s social platform and AI venture, have dropped Apple from the antitrust lawsuit that they filed against the iPhone maker and OpenAI last year. In a filing in the U.S. District Court for the Northern District of Texas, attorneys for X and SpaceXAI moved to dismiss the Apple portion of the case, first reported by Reuters. The filing does not explain why the companies are dropping Apple or say whether a settlement was reached.

X and SpaceXAI say they intend to keep pursuing the case against OpenAI, which remains a defendant. That resolves the dispute with one company while leaving the core allegation intact against the other, with no public accounting of what changed in between.

The lawsuit dates to August 2025, when xAI and X sued Apple and OpenAI, arguing that Apple’s decision to make ChatGPT the only generative AI chatbot built into iOS gave OpenAI an unfair structural advantage. The complaint claimed ChatGPT controlled roughly 80 percent of the chatbot market at the time, while Grok held only a few percent. It sought billions of dollars in damages and asked the court to unwind the arrangement.

Elon Musk’s xAI and X file antitrust suit against Apple and OpenAI over AI exclusivity

The filing followed weeks of Musk publicly complaining that Grok and X weren’t appearing in Apple’s “Must Have” App Store section, though Grok ranked second in the Productivity category and X ranked first in News at the time. He accused Apple of “playing politics” and warned of immediate legal action before following through days later.

Apple and OpenAI tried to get the case thrown out, but a federal judge denied both motions in November, ruling the dispute was better suited to summary judgment than an early dismissal. That decision sent the case into discovery, which is presumably what led to Monday’s filing.

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The timing is notable given how Musk’s sentiment toward Apple has shifted, with Musk noting that he was open to letting Grok power a revamped Siri after a user suggested Apple replace its aging assistant with xAI’s model.

xAI, the AI venture Musk folded into X Corp last year, has since combined with SpaceX under the SpaceXAI brand. That structure now puts X, Grok and SpaceX’s rocket and satellite businesses under one roof as Musk pushes his AI ambitions beyond chatbots.

OpenAI remains the sole defendant going forward, and Musk’s companies have not said if there’s any changes to those original claims. Apple and OpenAI did not immediately respond to requests for comment on the filing.

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