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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.

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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Elon Musk’s Boring Co goes extra hard in Nashville with first rock-crushing TBM

The Boring Company’s machine for the project is now in final testing.

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Credit: The Boring Company/X

The Boring Company is gearing up to tackle one of its toughest projects yet, a new tunnel system beneath Nashville’s notoriously tough limestone terrain. Unlike the soft-soil conditions of Las Vegas and Austin, the Music City Loop will require a “hard-rock” boring machine capable of drilling through dense, erosion-resistant bedrock. 

The Boring Company’s machine for the project is now in final testing.

A boring hard-rock tunneling machine

The Boring Company revealed on X that its new hard-rock TBM can generate up to 4 million pounds of grip force and 1.5 million pounds of maximum thrust load. It also features a 15-filter dust removal system designed to keep operations clean and efficient during excavation even in places where hard rock is present.

Previous Boring Co. projects, including its Loop tunnels in Las Vegas, Austin, and Bastrop, were dug primarily through soft soils. Nashville’s geology, however, poses a different challenge. Boring Company CEO and President Steve Davis mentioned this challenge during the project’s announcement in late July.

“It’s a tough place to tunnel, Nashville. If we were optimizing for the easiest places to tunnel, it would not be here. You have extremely hard rock, like way harder than it should be. It’s an engineering problem that’s fairly easy and straightforward to solve,” Davis said.

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Nashville’s limestone terrain

Experts have stated that the city’s subsurface conditions make it one of the more complex tunneling environments in the U.S. The Outer Nashville Basin is composed of cherty Mississippian-age limestone, a strong yet soluble rock that can dissolve over time, creating underground voids and caves, as noted in a report from The Tennessean.

Jakob Walter, the founder and principal engineer of Haushepherd, shared his thoughts on these challenges. “Limestone is generally a stable sedimentary bedrock material with strength parameters that are favorable for tunneling. Limestone is however fairly soluble when compared to other rack materials, and can dissolve over long periods of time when exposed to water. 

“Unexpected encounters with these features while tunneling can result in significant construction delays and potential instability of the excavation. In urban locations, structures at the ground surface should also be constantly monitored with robotic total stations or similar surveying equipment to identify any early signs of movement or distress,” he said.

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Elon Musk

Elon Musk shares ridiculous fact about Optimus’ hand demos

It appears that Optimus’ V3 iteration is still very much under wraps.

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Elon Musk recently revealed something quite shocking about the Optimus demonstration hand that was showcased at the 2025 Annual Shareholder Meeting. As per the CEO, the complex robotic hand that impressed the event’s attendees was not a component of Optimus V3 at all. 

Needless to say, it appears that Optimus’ V3 iteration is still very much under wraps. 

Optimus’s hand

Even in Tesla’s We, Robot event last year, the company showcased a robotic hand that seemed capable of performing complex tasks. A similar hand was showcased at the recent investor event. It was then no surprise that some attendees and EV community members assumed that the robotic component, which was very dexterous, was a preview of Optimus V3’s hand. 

As per Elon Musk in a recent post on X, however, this was not the case. While the robotic hand that Tesla showcased at the 2025 Annual Shareholder Meeting was already very impressive, it was still a V2 component. In response to a quote post from his mom Maye Musk, who noted that “Elon told me a few times that the hand is the most difficult part of the robot,” Elon Musk clarified that the impressive component was still from Optimus V2.

“This is just the V2 Optimus hand. The V3 hand is another level beyond this. Exquisite engineering,” Musk wrote in his post on X.

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Not like Tesla

Tesla is designing Optimus to be a potential replacement for humans in some of the world’s most delicate tasks, such as surgery. It is then extremely important for Optimus’ hand to be very dexterous and refined in its movements. This is something that even companies that are also producing humanoid robots have yet to accomplish fully. Musk highlighted this during the Annual Shareholder Meeting, when he discussed how Tesla is really the only company that can scale humanoid robots properly.

“You will see certainly many companies showing demonstration robots. There’s really three things that are super difficult about robots. One is the engineering of the forearm and hand because the human hand is an incredible thing, actually. It’s super dexterous. 

“So, engineering the hand really well, the real-world AI, and then volume manufacturing. Those are generally the things that are missing. One or more of those things are missing from other companies. So Tesla is the only one that has all three of those,” Musk said.

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Energy

Tesla starts hiring efforts for Texas Megafactory

Tesla’s Brookshire site is expected to produce 10,000 Megapacks annually, equal to 40 gigawatt hours of energy storage.

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Tesla's Megapack Factory in Lathrop, CA (Credit: Tesla)

Tesla has officially begun hiring for its new $200 million Megafactory in Brookshire, Texas, a manufacturing hub expected to employ 1,500 people by 2028. The facility, which will build Tesla’s grid-scale Megapack batteries, is part of the company’s growing energy storage footprint. 

Tesla’s hiring efforts for the Texas Megafactory are hinted at by the job openings currently active on the company’s Careers website.

Tesla’s Texas Megafactory

Tesla’s Brookshire site is expected to produce 10,000 Megapacks annually, equal to 40 gigawatt hours of energy storage, similar to the Lathrop Megafactory in California. Tesla’s Careers website currently lists over 30 job openings for the site, from engineers, welders, and project managers. Each of the openings is listed for Brookshire, Texas.

The company has leased two buildings in Empire West Business Park, with over $194 million in combined property and equipment investment. Tesla’s agreement with Waller County includes a 60% property tax abatement, contingent on meeting employment benchmarks: 375 jobs by 2026, 750 by 2027, and 1,500 by 2028, as noted in a report from the Houston Business Journal. Tesla is required to employ at least 1,500 workers in the facility through the rest of the 10-year abatement period. 

Tesla’s clean energy boom

City officials have stated that Tesla’s arrival marks a turning point for the Texas city, as it highlights a shift from logistics to advanced clean energy manufacturing. Ramiro Bautista from Brookshire’s economic development office, highlighted this in a comment to the Journal

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“(Tesla) has great-paying jobs. Not just that, but the advanced manufacturing (and) clean energy is coming to the area,” he said. “So it’s not just your normal logistics manufacturing. This is advanced manufacturing coming to this area, and this brings a different type of job and investment into the local economy.”

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