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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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Tesla Cybercab display highlights interior wizardry in the small two-seater

Photos and videos of the production Cybercab were shared in posts on social media platform X.

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Credit: Tesla Robotaxi/X

The Tesla Cybercab is currently on display at the U.S. Department of Transportation in Washington, D.C., and observations of the production vehicle are highlighting some of its notable design details. 

Photos and videos of the production Cybercab were shared in posts on social media platform X.

Observers of the Cybercab display unit noted that the two-seat Robotaxi provides unusually generous legroom for a vehicle of its size. Based on the vehicle’s video, the compact two-seater appears to offer more legroom than Tesla’s larger vehicles such as the Model Y, Model X, and Cybertruck.

The Cybercab’s layout allows Tesla to dedicate nearly the entire cabin to passengers. The vehicle is designed without a steering wheel or pedals, which helps maximize interior space.

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Footage from the display also highlights the Cybercab’s large center screen, which is positioned prominently in front of the passenger bench. The display appears intended to provide entertainment and ride information while the vehicle operates autonomously.

Images of the vehicle also show an additional camera integrated into the Cybercab’s C-pillar. The extra camera appears to expand the vehicle’s field of view, which would be useful as Tesla works toward fully unsupervised Full Self-Driving.

Tesla engineers have previously explained that the Cybercab was designed to be highly efficient both in manufacturing and in operation. Cybercab Lead Engineer Eric E. stated in 2024 that the Robotaxi would be built with roughly half the number of parts used in a Model 3 sedan.

“Two seats unlocks a lot of opportunity aerodynamically. It also means we cut the part count of Cybercab down by a substantial margin. We’re gonna be delivering a car that has roughly half the parts of Model 3 today,” the Tesla engineer said.

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The Tesla engineer also noted that the Cybercab’s cargo area can accommodate multiple golf bags, two carry-on suitcases, and two full-size checked bags. The trunk can also fit certain bicycles and a foldable wheelchair depending on size, which is quite impressive for a small car like the Cybercab.

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Elon Musk’s xAI wins permit for power plant supporting AI data centers

The development was reported by CNBC, citing confirmation from the Mississippi Department of Environmental Quality (MDEQ).

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Mississippi regulators have approved a permit allowing Elon Musk’s artificial intelligence company xAI to construct a natural gas power plant in Southaven. The facility is expected to support the company’s expanding AI infrastructure tied to its Colossus data center operations near Memphis.

The development was reported by CNBC, citing confirmation from the Mississippi Department of Environmental Quality (MDEQ).

According to the report, regulators “voted to approve the permit” of xAI subsidiary MZX Tech LLC to construct a power plant featuring 41 natural gas-burning turbines “after careful consideration of all public comments and community concerns.”

The Mississippi Department of Environmental Quality stated that the permit followed a regulatory review process that included public comments and community input. Jaricus Whitlock, air division chief for the MDEQ, stated that the project met all applicable environmental standards.

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“The proposed PSD permit in front of the board today not only meets all state and federal permitting regulations, but goes above and beyond what is required by law. MDEQ and the EPA agree that not a single person around our facilities will be exposed to unhealthy levels of air pollution,” Whitlock stated.

The planned facility will help provide electricity for xAI’s AI computing infrastructure in the Memphis region.

The Southaven project forms part of xAI’s efforts to scale computing capacity for its artificial intelligence systems.

The company currently operates two major data centers in Memphis, known as Colossus 1 and Colossus 2, which provide computing power for xAI’s Grok AI models. xAI is also planning to build another large data center in Southaven called Macrohardrr, which would be located in a warehouse previously used by GXO Logistics.

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Large-scale AI training requires substantial computing power and electricity, prompting technology companies to develop dedicated energy infrastructure for their data centers.

SpaceX President Gwynne Shotwell previously stated that xAI plans to develop 1.2 gigawatts of power capacity for its Memphis-area AI supercomputer site as part of the federal government’s Ratepayer Protection Pledge. The commitment was announced during an event with United States President Donald Trump.

“As part of today’s commitment, we will take extensive additional steps to continue to reduce the costs of electricity for our neighbors. xAI will therefore commit to develop 1.2 GW of power as our supercomputer’s primary power source. That will be for every additional data center as well. We will expand what is already the largest global Megapack power installation in the world,” Shotwell said.

“The installation will provide enough backup power to power the city of Memphis, and more than sufficient energy to power the town of Southaven, Mississippi where the data center resides. We will build new substations and invest in electrical infrastructure to provide stability to the area’s grid.”

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Tesla China teases Optimus robot’s human-looking next-gen hands

The image was shared by Tesla AI’s account on Weibo and later reposted by Tesla community members on X.

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Credit: Tesla China

A new teaser shared by Tesla’s China team appears to show a pair of unusually human-like hands for Optimus. 

The image was shared by Tesla AI’s account on Weibo and later reposted by Tesla community members on X.

As could be seen in the teaser image, the new version of Optimus’ hands features proportions and finger structures that look strikingly similar to those of a human hand. Their appearance suggests that they might have dexterity approaching that of a human hand.

If the image reflects a new generation of Optimus’ hands, it could indicate Tesla is continuing to refine one of the most critical components of its humanoid robot.

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Hands are widely viewed as one of the most difficult engineering challenges in robotics. For Optimus to perform complex real-world work, from manufacturing tasks to household activities, its hands would need to be the best in the industry.

Elon Musk has repeatedly described Optimus as Tesla’s most important long-term product. In posts on social media platform X, Musk has stated that Optimus could eventually become the first real-world Von Neumann machine.

In theory, a Von Neumann machine is a self-replicating system capable of building copies of itself using available materials. The concept was originally proposed by mathematician John von Neumann in the mid-20th century.

“Optimus will be the first Von Neumann machine, capable of building civilization by itself on any viable planet,” Musk wrote in a post on X.

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If Optimus is expected to carry out complex work autonomously in the future, high levels of dexterity will likely be essential. This makes the development of advanced robotic hands a key step towards Musk’s long-term expectations for the product.

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