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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.

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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Tesla rolling out Robotaxi pilot in SF Bay Area this weekend: report

Similar to the Austin pilot, the Robotaxi rides will reportedly be a paid service.

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8 hours ago

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July 25, 2025

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Credit: @AdanGuajardo/X

Tesla is reportedly preparing to launch a Robotaxi pilot program in the Bay Area this weekend, with invites to a select number of customers reportedly being sent out as early as this Friday.

The update was shared in a report from Insider, which cited an internal memo from the electric vehicle maker.

New Robotaxi service launch

According to Insider, the Robotaxi service in the Bay Area is set to launch as soon as Friday. Thus, some Tesla owners in the area should receive invites to use the driverless ride-hailing service. Similar to the Austin pilot, the Robotaxi rides will reportedly be a paid service.

The publication noted that the Robotaxi service’s geofence in its Bay Area launch will be quite large, as it will include Marin, much of the East Bay, San Francisco, and San Jose. This is not surprising as California has long been saturated with Teslas, and it is home to several of the electric vehicle maker’s key facilities.

Unlike the Austin pilot, the Tesla Robotaxi service’s pilot in the Bay Area will use safety drivers seated in the driver’s seat. These drivers will be able to manually take over using the steering wheel and brakes as needed. As per a spokesperson from the California DMV, the agency recently met with Tesla but the company is yet to submit a formal application to operate fully driverless cars. 

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Tesla Robotaxi expansion

Interestingly enough, Tesla did tease the release of its Robotaxi service to the Bay Area in its second quarter earnings call. While discussing the service, Tesla VP of Autopilot/AI Software Ashok Elluswamy mentioned that the company will initially be rolling out Robotaxis with safety drivers in the San Francisco Bay Area. He did, however, also highlight that the electric vehicle maker is working hard to get government permission to release the service for consumers.

“The next thing to expand would be in the San Francisco Bay Area. We are working with the government to get approval here and, in the meanwhile, launch the service without the person in the driver seat just to expedite and while we wait for regulatory approval,” he stated.

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Tesla is ready with a perfect counter to the end of US EV tax credits

Tesla executives have mentioned that these more affordable models would resemble the company’s current lineup.

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10 hours ago

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July 25, 2025

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Credit: Tesla Asia/X

The United States’ electric vehicle tax credit is coming to an end at the end of the third quarter. Tesla, the country’s leading electric vehicle maker, is ready to meet this challenge with a rather simple but clever counter. 

Tesla executives outlined this strategy in the recently held Q2 2025 earnings call.

End of the US EV tax credit

While Elon Musk has always maintained that he prefers a market with no EV tax credit, he also emphasized that he supports the rollback of any incentives given to the oil and gas industry. The Trump administration has not done this so far, instead focusing on the expiration of the $7,500 EV tax credit at the end of the third quarter.

Tesla has been going all-in on encouraging customers to purchase their vehicles in Q3 to take advantage of lower prices. The company has also implemented a series of incentives across all its offerings, from the Cybertruck to the Model 3. This, however, is not all, as the company seems to be preparing a longer-term solution to the expiration of the EV tax credit.

Affordable variants

During the Q2 2025 earnings call, Vice President of Vehicle Engineering Lars Moray stated that Tesla really did start the production of more affordable models in June. Quality builds of these vehicles are being ramped this quarter, with the goal of optimizing production over the remaining months of the year. If Tesla is successful, these models will be available for everyone in Q4. 

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“We started production in June, and we’re ramping quality builds and things around the quarter. And given that we started in North America and our goal is to maximize production with a higher rate. So starting Q3, we’re going to keep pushing hard on our current models to avoid complexity… We’ll be ready with new, more affordable models available for everyone in Q4.,” Moravy stated. 

These comments suggest that Tesla should be able to offer vehicles that are competitively priced even after the EV tax credit has been phased out. Interestingly enough, previous comments from Tesla executives have mentioned that these more affordable models would resemble the company’s current lineup. This suggests that the more affordable models may indeed be variations of the Model Y and Model 3, but offered at a lower price.

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Elon Musk

Elon Musk reveals Tesla’s next Robotaxi expansion in more ways than one

Tesla Robotaxi is growing in more ways than one. Tesla wants to expand and hopes to reach half the U.S. population by the end of the year.

Published

1 day ago

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July 24, 2025

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Credit: Tesla

Tesla CEO Elon Musk revealed the company’s plans for its next expansion of the Robotaxi in terms of both the geofence in Austin and the platform overall, as it looks to move to new areas outside of Texas.

Tesla launched the Robotaxi platform last month on June 22, and has since expanded both the pool of users and the area that the driverless Model Y vehicles can travel within.

The first expansion of the geofence caught the attention of nearly everyone and became a huge headline as Tesla picked a very interesting shape for the new geofence, resembling male reproductive parts.

The next expansion will likely absolve this shape. Musk revealed last night that the new geofence will be “well in excess of what competitors are doing,” and it could happen “hopefully in a week or two.”

Musk’s full quote regarding the expansion of the geofence and the timing was:

“As some may have noted, we have already expanded our service area in Austin. It’s bigger and longer, and it’s going to get even bigger and longer. We are expecting to greatly increase the service area to well in excess of what competitors are doing, hopefully in a week or two.”

The expansion will not stop there, either. As Tesla has operated the Robotaxi platform in Austin for the past month, it has been working with regulators in other areas, like California, Arizona, Nevada, and Florida, to get the driverless ride-hailing system activated in more U.S. states.

Tesla confirmed that they are in talks with each of these states regarding the potential expansion of Robotaxi.

Musk added:

“As we get the approvals and prove out safety, we will be launching the autonomous ride-hailing across most of the country. I think we will probably have autonomous ride-hailing in probably half the population of the US by the end of the year.”

We know that Tesla and Musk have been prone to aggressive and sometimes outlandish timelines regarding self-driving technology specifically. Regulatory approvals could happen by the end of the year in several areas, and working on these large metros is the best way to reach half of the U.S. population.

Tesla said its expansion of the geofence in Austin is conservative and controlled due to its obsession with safety, even admitting at one point during the Earnings Call that they are being “paranoid.” Expanding the geofence is necessary, but Tesla realizes any significant mistake by Robotaxi could take it back to square one.

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