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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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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.

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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.

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.

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“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 puts Giga Berlin in Plaid Mode with new massive investment

The facility, Tesla’s first in Europe, opened in 2022 and has become a cornerstone for Model Y production and, increasingly, in-house battery manufacturing. Recent announcements highlight a dual focus on scaling vehicle output and advancing vertical integration through 4680 battery cells.

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Credit: Tesla

Tesla is pushing forward with significant upgrades at its Gigafactory Berlin-Brandenburg in Grünheide, Germany, signaling renewed confidence in its European operations despite past market challenges.

The facility, Tesla’s first in Europe, opened in 2022 and has become a cornerstone for Model Y production and, increasingly, in-house battery manufacturing. Recent announcements highlight a dual focus on scaling vehicle output and advancing vertical integration through 4680 battery cells.

In April, plant manager André Thierig announced a 20 percent increase in Model Y production starting in July, following a record Q1 output of more than 61,000 vehicles. To support the ramp-up, Tesla plans to hire approximately 1,000 new employees beginning in May and convert 500 temporary workers to permanent positions.

The move is expected to lift weekly production significantly, addressing rebounding demand in Europe after a challenging 2025.

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The expansion builds on earlier progress. In 2025, Tesla secured partial approvals to add roughly 2 million square feet of factory space, raising potential annual vehicle capacity from around 500,000 toward 800,000 units, with longer-term ambitions approaching one million vehicles per year. Logistical improvements, new infrastructure, and battery-related facilities are already underway on company-owned land.

Battery production is the latest major focus. On May 12, Thierig revealed an additional $250 million investment in the on-site cell factory. This more than doubles the planned 4680 battery cell capacity to 18 gigawatt-hours annually—up from the 8 GWh target set in December 2025—while creating over 1,500 new battery-related jobs.

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Total cell investments at the site now exceed previous figures, bringing the factory closer to full vertical integration: cells, packs, and vehicles produced under one roof. Tesla describes this as unique in Europe and a step toward stronger supply chain resilience.

The plans come amid regulatory and community hurdles. Earlier expansion proposals faced protests over environmental concerns and water usage, leading to phased approvals beginning in 2024. Tesla has navigated these by emphasizing sustainable practices and economic benefits, including thousands of local jobs in Brandenburg.

With nearly 12,000 employees already on site and production steadily climbing, Gigafactory Berlin is poised for growth. The combined vehicle and battery expansions position the plant as a key hub for Tesla’s European ambitions, potentially making it one of the continent’s largest manufacturing complexes if local support continues.

As EV demand recovers, these investments underscore Tesla’s commitment to scaling efficiently in Germany while addressing regional supply chain needs.

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Honda gives up on all-EV future: ‘Not realistic’

Mibe believes the demand for its gas vehicles is certainly strong enough and has changed “beyond expectations.” As many drivers went for EVs a few years back, hybrids are becoming more popular for consumers as they offer the best of both worlds.

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honda logo with red paint
Ivan Radic, CC BY 2.0 , via Wikimedia Commons

Honda has given up on a previous plan to completely changeover to EVs by 2040, a new report states. The company’s CEO, Toshihiro Mibe, said that the idea is “not realistic.”

Mibe believes the demand for its gas vehicles is certainly strong enough and has changed “beyond expectations.” As many drivers went for EVs a few years back, hybrids are becoming more popular for consumers as they offer the best of both worlds.

Mibe said (via Motor1):

“Because of the uncertainty in the business environment and also the customer demand, is changing beyond our expectation and, therefore, we have judged that it’ll be difficult to achieve. That ratio [100-percent electric in 2040] is not realistic as of now. We have withdrawn this target.”

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Instead of going all-electric, Honda still wants to oblige by its hopes to be net carbon neutral by 2050. It will do this by focusing on those popular hybrid powertrains, planning to launch 15 of them by March 2030.

Honda will invest 4.4 trillion yen, or almost $28 billion, to build hybrid powertrains built around four and six-cylinder gas engines.

There are so many companies abandoning their all-electric ambitions or even slowing their roll on building them so quickly. Ford, General Motors, Mercedes, and Nissan have all retreated from aggressive EV targets by either cancelling, delaying, or pausing the development of electric models.

Hyundai’s 2030 targets rely on mixed offerings of electric, hybrid & hydrogen vehicles

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Early-decade pledges from multiple brands proved overly ambitious as infrastructure lags, battery costs remain high in some markets, and many buyers prefer hybrids for their convenience and range. Toyota has long championed hybrids, while others have quietly extended internal-combustion timelines.

For Honda—historically known for reliable gasoline engines—this shift leverages its core strengths while buying time to refine electric technology. Whether the hybrid-heavy strategy will protect market share in an increasingly competitive landscape remains to be seen, but one thing is clear: the gas engine is far from dead at Honda, unfortunately.

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Delta Airlines rejects Starlink, and the reason will probably shock you

In a pointed exchange on X, Elon Musk defended SpaceX’s uncompromising approach to Starlink’s in-flight internet service, explaining why Delta Air Lines walked away from a deal.

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Delta Airlines Airbus photographed April 2024 Delta-owned. No expiration date, unrestricted use.

SpaceX frontman Elon Musk explained on Wednesday why commercial airline Delta got cold feet over offering Starlink for stable internet on its flights — and the reason will probably shock you.

In a pointed exchange on X, Elon Musk defended SpaceX’s uncompromising approach to Starlink’s in-flight internet service, explaining why Delta Air Lines walked away from a deal.

Delta rejected Starlink because it insisted on routing all connectivity through its branded “Delta Sync” portal rather than allowing a simple Starlink experience.

Instead, the airline partnered with Amazon’s Project Kuiper—rebranded as Amazon Leo—for high-speed Wi-Fi on up to 500 aircraft, with rollout targeted for 2028. At the time of the announcement, Kuiper had roughly 300 satellites in orbit, while Starlink operated more than 10,400.

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The use of the “Delta Sync” portal would not work for SpaceX, as Musk went on to say that:

“SpaceX requires that there be no annoying ‘portal’ to use Starlink. Starlink WiFi must just work effortlessly every time, as though you were at home. Delta wanted to make it painful, difficult and expensive for their customers. Hard to see how that is a winning strategy.”

Musk doubled down in a follow-up post:

“Yes, SpaceX deliberately accepted lower revenue deals with airlines in exchange for making Starlink super easy to use and available to all passengers.”

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SpaceX has structured its airline agreements to prioritize zero-friction access—no captive portals, no SkyMiles logins, no paywalls or ads blocking basic connectivity.

While this means forgoing higher-margin deals that would let carriers monetize the service more aggressively, it ensures Starlink feels like home broadband at 35,000 feet. Passengers on partner airlines such as United, Qatar Airways, and Air France have already praised the service for enabling seamless video calls, streaming, and work mid-flight without interruptions.

Delta’s choice reflects a different philosophy. By keeping Wi-Fi behind its Delta Sync ecosystem, the airline aims to drive loyalty program engagement and control the digital passenger journey. Yet, critics argue this short-term control comes at the expense of immediate competitiveness.

Airlines already installing Starlink are pulling ahead in customer satisfaction surveys, while Delta passengers face years of reliance on slower, legacy systems until Leo launches.

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SpaceX’s decision to trade revenue for simplicity will pay off in the longer term, as Starlink is already positioning itself as the default high-speed option for carriers that value passenger satisfaction over incremental fees.

Musk’s focus on creating not only a great service but also a reasonable user experience highlights SpaceX’s prowess with Starlink as it continues to expand across new partners and regions.

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