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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.

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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 ramps Robotaxi public street testing as vehicle enters mass production queue

Recent sightings on public roads and growing fleet activity at Giga Texas signal Tesla’s accelerating push toward the Cybercab’s commercial launch.

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52 minutes ago

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March 10, 2026

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Tesla Cybercab spotted in San Jose, CA testing on public roads with Robotaxi validation equipment [Credit: Nic Cruz Patane via X]

Tesla Cybercab is being spotted with increasing frequency both on public roads and across the grounds of Gigafactory Texas, suggesting that the company’s road testing and validation program is ramping meaningfully ahead of mass production.

A total of 25 Cybercab units were recently observed across three separate locations at Giga Texas by drone observer Joe Tegtmeyer — with 14 metallic gold units parked in a tight formation outside the factory exit, nine more at the crash testing facility undergoing structural and safety validations, and two additional units at the west end-of-line area for final checks.

The activity on public roads is just as telling. The Cybercab was spotted testing on public roads for the first time last October, near Tesla’s Engineering Headquarters in Los Altos, California, marking a significant development in the vehicle’s progression toward commercial readiness. As expected at that early stage, a safety driver was present in the seat.

Since then, sightings have only become more frequent. Community observers on X have posted fresh footage of Cybercabs navigating public streets in Silicon Valley, with each new clip adding to a growing body of evidence that Tesla’s validation efforts are well underway. The production backdrop supports the momentum. Tesla’s production line at Giga Texas moved into a higher volume early in March, representing what observers are calling the largest single-day grouping of Cybercabs seen to date.

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Tesla Cybercab spotted testing on public roads in Los Gatos, CA – March 10, 2026 [Credit: Osman Sarood via X]

CEO Elon Musk has been clear-eyed about what to expect from the ramp. “It’s an all-new product and radical redesign of car manufacturing to achieve ~5X higher production rate, which means the output S-curve will be very slow in the beginning, but ultimately super high volume,” Musk wrote on X. “For Cybercab and Optimus, almost everything is new, so the early production rate will be agonizingly slow, but eventually end up being insanely fast.”

Tesla ramps Cybercab test manufacturing ahead of mass production

Musk has also stated that Tesla is aiming for at least 2 million Cybercab units per year across more than one factory, with a potential ceiling of 4 million annually.

With testing activity on public roads accelerating and factory output visibly increasing week over week, the coming months at Giga Texas are set to be pivotal in determining how quickly Tesla can bring the Cybercab from validation to volume.

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Tesla opens Supercharging Network to other EVs in new country

Tesla’s Supercharging infrastructure is the most robust in the world, and it has done a wonderful job of keeping things up and running for the millions of owners out there. As it expanded access to non-Tesla EVs a couple years back, it has still managed to keep things pretty steady, although the need for more charging is apparent.

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11 hours ago

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March 10, 2026

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Kia EV6, EV9 and Niro Owners Gain Access to Over 21,500 Tesla Superchargers

Tesla has started opening its Supercharging Network, which is the most expansive in the world, to other EVs in a new country for the first time.

After expanding its Supercharging offerings to other car companies in the United States a few years ago, Tesla is still making the move in other markets, as it aims to make EV ownership easier for everyone, regardless of what manufacturer a consumer chose to purchase from.

Tesla’s Supercharging infrastructure is the most robust in the world, and it has done a wonderful job of keeping things up and running for the millions of owners out there. As it expanded access to non-Tesla EVs a couple years back, it has still managed to keep things pretty steady, although the need for more charging is apparent.

Now, Tesla is expanding access to the Supercharger Network to non-Tesla EVs in Malaysia. The automaker just opened up a charging stie at the Pavilion KL Mall in Kuala Lumpur to non-Tesla owners, giving them eight additional Superchargers to utilize with a charging speed of up to 250 kW.

Tesla is also opening up the four-Supercharger site in Shah Alam, a four-Supercharger site at the IOI City Mall, and a six-Supercharger site in Gamuda Cove Township.

Electrive first reported the opening of these Superchargers in Malaysia.

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The initiative from Tesla helps make EV ownership much simpler for those who only have access to third-party charging solutions or at-home charging. While at-home charging is the most advantageous, it is not an end-all solution as every driver will eventually need to grab some range on the road.

Tesla has been offering its Superchargers to non-Tesla EVs in the United States since 2024, as Ford became the first company to gain access to the massive network early that year when CEO Elon Musk and Ford frontman Jim Farley announced it together. Since then, Tesla has offered its chargers to nearly every EV maker, as companies like Rivian and Lucid, and even legacy car companies like General Motors have gained access.

It’s best for everyone to have the ability to use Tesla Superchargers, but there are of course some growing pains.

Charging cables are built to cater to Tesla owners, so pull-in Superchargers are most advantageous for non-Tesla EVs currently, but the company’s V4 Superchargers, which are not as plentiful in the U.S. quite yet, do enable easier reach for those vehicles.

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Tesla Semi expands pilot program to Texas logistics firm: here’s what they said

Mone said the Tesla Semi it put into its fleet for this test recorded 1.64 kWh per mile efficiency, beating Tesla’s official 1.7 kWh per mile target and delivering a massive leap over conventional diesel trucks.

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13 hours ago

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March 10, 2026

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Credit: Mone Transport

Tesla has expanded its Semi pilot program to a new region, as it has made it to Texas to be tested by logistics from Mone Transport. With the Semi entering production this year, Tesla is getting even more valuable data regarding the vehicle and its efficiency, which will help companies cut expenditures.

Mone Transport operates in Texas and on the Southern border, and it specializes in cross-border U.S.-Mexico freight operations. After completing some rigorous testing, Mone shared public results, which stand out when compared to efficiency metrics offered by diesel vehicles.

“Mone Transport recently had the opportunity to put the Tesla Semi to the test, and we’re thrilled with the results! Over 4,700 miles of operations at 1.64 kWh/mile in our Texas operation. We’re committed to providing zero-emission transportation to our customers!” the company said in a post on X.

Mone said the Tesla Semi it put into its fleet for this test recorded 1.64 kWh per mile efficiency, beating Tesla’s official 1.7 kWh per mile target and delivering a massive leap over conventional diesel trucks.

Comparable Class 8 diesel semis, typically achieving 6-7 miles per gallon, consume roughly 5.5 kWh per mile in energy-equivalent terms, meaning the Semi uses three to four times less energy while also producing zero tailpipe emissions.

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Tesla Semi undergoes major redesign as dedicated factory preps for deliveries

The performance of the Tesla Semi in Mone Transport’s testing aligns with data from other participants in the pilot program. ArcBest’s ABF Freight Division logged 4,494 miles over three weeks in 2025, averaging 1.55 kWh per mile across varied routes, including a grueling 7,200-foot Donner Pass climb. The truck “generally matched the performance of its diesel counterparts,” the carrier said.

PepsiCo, which operates the largest known Semi fleet, recorded 1.7 kWh per mile in North American Council for Freight Efficiency testing. Additional pilots showed similar gains: DHL hit 1.72 kWh per mile, and Saia achieved 1.73 kWh per mile.

These metrics underscore the Semi’s ability to slash operating costs through superior efficiency, lower maintenance, and zero-emission operation. As charging infrastructure scales and production ramps toward 2026 targets, participants like Mone Transport are proving electric semis can seamlessly integrate into freight networks, accelerating the industry’s shift to sustainable, high-performance trucking.

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Tesla continues to prep for a more widespread presence of the Semi in the coming months as it recently launched the first public Semi Megacharger site in Los Angeles. It is working on building out infrastructure for regional runs on the West Coast initially, with plans to expand this to the other end of the country in the coming years.

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