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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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7 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.

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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.

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.

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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’s six-seat extended wheelbase Model Y L sold out for January 2026

Estimated delivery dates for new Tesla Model Y L orders now extend all the way into February 2026.

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31 minutes ago

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December 4, 2025

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Credit: Tesla China

The Tesla Model Y L seems to be in high demand in China, with estimated delivery dates for new orders now extending all the way into February 2026. 

This suggests that the Model Y L has been officially sold out from the rest of 2025 to January 2026. 

Model Y L estimated delivery dates

The Model Y L’s updated delivery dates mark an extension from the vehicle’s previous 4-8 week estimated wait time. A detailed chart shared by Tesla data tracker @Tslachan on X shows the progressions of the Model Y L’s estimated delivery dates since its launch earlier this year. 

Following its launch in September, the vehicle was given an initial October 2025 estimated delivery date. The wait times for the vehicle were continually updated over the years, until the middle of November, when the Model Y L had an estimated delivery date of 4-8 weeks. This remained until now, when Tesla China simply listed February 2026 as the estimated delivery date for new Model Y L orders.

Model Y demand in China

Tesla Model Y demand in China seems to be very healthy, even beyond the Model Y L. New delivery dates show the company has already sold out its allocation of the all-electric crossover for 2025. The Model Y has been the most popular vehicle in the world in both of the last two years, outpacing incredibly popular vehicles like the Toyota RAV4. In China, the EV market is substantially more saturated, with more competitors than in any other market.

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Tesla has been particularly kind to the Chinese market, as it has launched trim levels for the Model Y in the country that are not available anywhere else, such as the Model Y L. Demand has been strong for the Model Y in China, with the vehicle ranking among the country’s top 5 New Energy Vehicles. Interestingly enough, vehicles that beat the Model Y in volume like the BYD Seagull are notably more affordable. Compared to vehicles that are comparably priced, the Model Y remains a strong seller in China. 

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NVIDIA CEO Jensen Huang commends Tesla’s Elon Musk for early belief

“And when I announced DGX-1, nobody in the world wanted it. I had no purchase orders, not one. Nobody wanted to buy it. Nobody wanted to be part of it, except for Elon.”

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6 hours ago

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December 3, 2025

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Credit: NVIDIA

NVIDIA CEO Jensen Huang appeared on the Joe Rogan Experience podcast on Wednesday and commended Tesla CEO Elon Musk for his early belief in what is now the most valuable company in the world.

Huang and Musk are widely regarded as two of the greatest tech entrepreneurs of the modern era, with the two working in conjunction as NVIDIA’s chips are present in Tesla vehicles, particularly utilized for self-driving technology and data collection.

Nvidia CEO Jensen Huang regrets not investing more in Elon Musk’s xAI

Both CEOs defied all odds and created companies from virtually nothing. Musk joined Tesla in the early 2000s before the company had even established any plans to build a vehicle. Jensen created NVIDIA in the booth of a Denny’s restaurant, which has been memorialized with a plaque.

On the JRE episode, Rogan asked about Jensen’s relationship with Elon, to which the NVIDIA CEO said that Musk was there when nobody else was:

“I was lucky because I had known Elon Musk, and I helped him build the first computer for Model 3, the Model S, and when he wanted to start working on an autonomous vehicle. I helped him build the computer that went into the Model S AV system, his full self-driving system. We were basically the FSD computer version 1, and so we were already working together.

And when I announced DGX-1, nobody in the world wanted it. I had no purchase orders, not one. Nobody wanted to buy it. Nobody wanted to be part of it, except for Elon.

He goes ‘You know what, I have a company that could really use this.’ I said, Wow, my first customer. And he goes, it’s an AI company, and it’s a nonprofit and and we could really use one of these supercomputers. I boxed one up, I drove it up to San Francisco, and I delivered it to the Elon in 2016.”

The first DGX-1 AI supercomputer was delivered personally to Musk when he was with OpenAI, which provided crucial early compute power for AI research, accelerating breakthroughs in machine learning that underpin modern tools like ChatGPT.

Tesla’s Nvidia purchases could reach $4 billion this year: Musk

The long-term alliance between NVIDIA and Tesla has driven over $2 trillion in the company’s market value since 2016.

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GM CEO Mary Barra says she told Biden to give Tesla and Musk EV credit

“He was crediting me, and I said, ‘Actually, I think a lot of that credit goes to Elon and Tesla…You know me, Andrew. I don’t want to take credit for things.”

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17 hours ago

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December 3, 2025

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General Motors CEO Mary Barra said in a new interview on Wednesday that she told President Joe Biden to credit Tesla and its CEO, Elon Musk, for the widespread electric vehicle transition.

She said she told Biden this after the former President credited her and GM for leading EV efforts in the United States.

During an interview at the New York Times Dealbook Summit with Andrew Ross Sorkin, Barra said she told Biden that crediting her was essentially a mistake, and that Musk and Tesla should have been explicitly mentioned (via Business Insider):

“He was crediting me, and I said, ‘Actually, I think a lot of that credit goes to Elon and Tesla…You know me, Andrew. I don’t want to take credit for things.”

Back in 2021, President Biden visited GM’s “Factory Zero” plant in Detroit, which was the centerpiece of the company’s massive transition to EVs. The former President went on to discuss the EV industry, and claimed that GM and Barra were the true leaders who caused the change:

“In the auto industry, Detroit is leading the world in electric vehicles. You know how critical it is? Mary, I remember talking to you way back in January about the need for America to lead in electric vehicles. I can remember your dramatic announcement that by 2035, GM would be 100% electric. You changed the whole story, Mary. You did, Mary. You electrified the entire automotive industry. I’m serious. You led, and it matters.”

People were baffled by the President’s decision to highlight GM and Barra, and not Tesla and Musk, who truly started the transition to EVs. GM, Ford, and many other companies only followed in the footsteps of Tesla after it started to take market share from them.

Elon Musk and Tesla try to save legacy automakers from Déjà vu

Musk would eventually go on to talk about Biden’s words later on:

“They have so much power over the White House that they can exclude Tesla from an EV Summit. And, in case the first thing, in case that wasn’t enough, then you have President Biden with Mary Barra at a subsequent event, congratulating Mary for having led the EV revolution.”

In Q4 2021, which was shortly after Biden’s comments, Tesla delivered 300,000 EVs. GM delivered just 26.

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