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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.

 


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 admits he was ‘clearly wrong’ about Anthropic

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Ministério Das Comunicações, CC BY 2.0 , via Wikimedia Commons

Elon Musk posted a candid admission on his social media platform X on June 9, declaring that he had been “clearly wrong” about Anthropic. The statement marked a notable reversal from his earlier skepticism toward the AI company.

In September, Musk had written, “Winning was never in the set of possible outcomes for Anthropic,” reflecting his view at the time that the startup had lacked the foundation or even the trajectory to succeed in what is an incredibly intense race for advanced artificial intelligence.

Musk’s latest post came amid discussion of Anthropic’s reliance on external compute resources. He praised the company’s progress, stating that Anthropic is “obviously currently the leader in AI” and that “no company has released a model as good as Mythos/Fable,” with expectations of a strong follow-up in Mythos 2.

The tone shifted dramatically from dismissal to acknowledgement of superior performance.

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The context of Musk’s comments added significance. Anthropic has been operating under a recent compute deal with SpaceXAI, Musk’s AI infrastructure-focused venture. The pair entered a short-term GPU lease agreement initiated in May, providing Anthropic access to critical computing power for training and deploying its frontier models.

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SpaceXAI signs agreement with Anthropic for massive AI supercomputer access

Some observers had speculated that Musk could leverage this dependency to disadvantage a rival. Musk directly addressed the possibility, writing, “I would never cut them off in a way that hurt them badly, even as a competitor. That’s not my style.”

To support his commitment to ethical competition, Musk referenced concrete examples from his other companies. Tesla famously open-sourced its entire portfolio of electric vehicle patents in 2014. The move was designed to accelerate the global adoption of sustainable transportation technology rather than protect proprietary advantages.

Tesla also made its Supercharger network available to competing electric vehicle manufacturers, transforming what could have remained an exclusive charging ecosystem into a shared infrastructure that benefits the broader industry and reduces barriers for EV adoption.

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Musk further pointed to SpaceX’s practices, noting that the company launches satellites for competing commercial systems “with no increase in price or use of unfair terms.” He extended the principle to his social platform, observing that “even my worst enemies attack me on this platform,” underscoring preference for open discourse over retaliation.

These examples have illustrated Musk’s long-standing philosophy that long-term technological progress is best served by open competition and infrastructure sharing rather than leveraging market power to stifle rivals. In the fast-evolving AI sector, where compute resources and model capabilities determine leadership, Musk’s stance suggests a willingness to compete on innovation and performance alone.

Musk’s admission arrives as SpaceXAI itself advances its own frontier models while maintaining business relationships across the ecosystem. By publicly correcting his earlier assessment and reaffirming principles of fair play, Musk highlights a model of competition that prioritizes advancement of the field over short-term tactical advantages.

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Tesla analyst says Full Self-Driving is about to have its iPhone moment

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Credit: Tesla

A Tesla analyst believes the company’s Full Self-Driving suite is close to an “inflection point,” where people will finally realize that it is more than what it appears, similar to how many view the iPhone.

Pierre Ferragu, an analyst who has covered Tesla for many years at New Street Research, says the Full Self-Driving suite is one piece of evidence supporting the view that a Tesla is more than a car. He compared it to the iPhone and noted that the high price tag seemed like a lot for a phone early on. Then people realized the iPhone was more than just something you make calls with. It made their lives simpler.

Suddenly, that price tag was justified.

Tesla offers several models under the average transaction price for a new vehicle, which was above $49,000, according to Kelley Blue Book. However, that does not take into account that many people can still not afford a $35,000 vehicle. Ferragu offers his thoughts:

“Remember when the addressable market of the iPhone was 10 million units? Then people realized how good it was, and now, nearly 250m are sold every year.

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A similar evolution for Tesla is still on the table. A Tesla is not a car, the same way an iPhone was not a phone.

A model 3 at $35k + $100 per month is too expensive for most, but only as a car, the same way a $600 iPhone was too expensive for most, until most realized it was much more than a phone.

As a tool that gets you to work peacefully every morning, it is not expensive.”

This point is valid, especially considering the iPhone’s impact on the cell phone market. There are still a handful of players, but most people you know have an iPhone. The iPhone ties into Apple’s other ecosystem of products.

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This is how Tesla plans to infiltrate the automotive market, and once the company offers a fully autonomous suite, or something that can allow for unsupervised self-driving, more and more people will flock to Tesla.

Ferragu believes Tesla needs two additional quarters of development before things will truly change. He didn’t elaborate on what will happen in two quarters, but he said it will give us all time to “see where this is heading.”

It is really quite interesting to see people’s reactions when they find out what a Tesla is capable of. Full Self-Driving is a great tool for taking stress out of travel; I use it daily, and it has made it really difficult to consider taking any other car on a drive of practically any length.

To me, it is really hard to believe that people will not at least seriously consider a Tesla as their next car if they experience Full Self-Driving. This is a major point for those who argue that Tesla should advertise in some way.

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Investor's Corner

NASA taps SpaceX to launch the telescope that could unlock new worlds

NASA’s Roman Space Telescope heads to orbit this August aboard SpaceX’s Falcon Heavy with massive scientific ambitions.

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SpaceX is set to play a central role in one of NASA’s most anticipated science missions in years. The company’s Falcon Heavy rocket, currently the most powerful operational launch vehicle in the world, will carry the Nancy Grace Roman Space Telescope into orbit on August 30 from Kennedy Space Center in Florida. Roman is now in final preparations inside the Payload Hazardous Servicing Facility, where on June 26 technicians used a crane to lift the observatory into a specialized stand for fueling and pre-launch testing.

Roman is named after Nancy Grace Roman, NASA’s first chief of astronomy, whose career helped shape how the agency approaches space science.

NASA chose SpaceX Falcon Heavy because of Roman’s needs to reach a specific orbit far from Earth, well beyond where a standard Falcon 9 can deliver it. The Falcon Heavy, which first flew in 2018, has since become NASA’s go-to option for missions that need serious muscle without the cost and complexity of older launch systems.

Celebrating SpaceX’s Falcon Heavy Tesla Roadster launch, seven years later (Op-Ed)

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Roman will carry a field of view at least 100 times wider than the Hubble Space Telescope, meaning it can photograph enormous swaths of the universe in a single shot rather than the narrow slices Hubble captures. That difference in scale is significant. While Hubble reshaped our understanding of the cosmos over 30 years, Roman is built to work faster and wider, surveying hundreds of millions of galaxies at once.

One of Roman’s most compelling capabilities is its potential to discover and photograph planets orbiting stars outside our solar system, and with enough precision to directly image planets that would otherwise be lost. That means scientists could study the atmosphere and surface characteristics of distant worlds rather than simply confirming they exist. Combined with Roman’s sweeping field of view, the telescope could detect thousands of exoplanets, and some of those planets may be in habitable zones where liquid water could exist. No telescope currently in operation has this level of power and capability. That capability alone could change what we know about other worlds, and perhaps finally answer the question: are we the only intelligent lifeforms in existence? 

What Roman actually finds once it reaches orbit is an open question, and that is exactly what makes this launch worth watching.

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