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Ex-SpaceX engineer leads Stratolaunch to major rocket engine test milestone
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.
The team made amazing progress this week! Check out the #PGAEngine preburner’s first hot-fire test at @NASAStennis. #NewUSEngine pic.twitter.com/kKTnf0bj1S
— Stratolaunch (@Stratolaunch) November 6, 2018
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.
- A to-scale comparison of Falcon 1, Pegasus XL, MLV, and Falcon 9. (Teslarati/Stratolaunch/Wikipedia)
- A render of Stratolaunch’s impressive PGA engine. Note the giant nozzle relative to the throat. (Stratolaunch)
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.
- PGA’s first full-scale preburner seen during assembly. (Stratolaunch)
- PGA’s first full-scale preburner seen during assembly. (Stratolaunch)
- Jeff Thornburg stands in front of Stratolaunch’s NASA Stennis Space Center test stand. (Stratolaunch)
- The PGA preburner seen after installation at Stennis. (Stratolaunch)
- The control center. (Stratolaunch)
- MLV is released from Stratolauncher. (Stratolaunch)
- A concept video produced by Stratolaunch shows the Roc launching a Kraken rocket. (Stratolaunch, via Wired)
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‘Tesla tax’ could be no more in United Kingdom
A tax on vehicles in the United Kingdom is set to be adjusted after automakers have blamed it for struggling sales.

A controversial tax set on cars that are above the £40,000 price threshold in the United Kingdom could be abandoned, as the “Tesla Tax,” as it is commonly referred to, is taking the blame for poor EV sales.
The tax has been imposed on ICE vehicles since 2017, but on April 1, 2025, the “Expensive Car Supplement,” or ECS, was applied to new EVs sold after that date, a move that was initiated in an act of fairness.
Tesla best-rated car brand in UK, beats Toyota in reliability: survey
However, the tax is now being blamed for sluggish EV sales across various parts of Europe. In the UK, manufacturers are blaming the tax for making it more difficult to reach sales quantities for binding green car sales targets. Missing these sales goals could cost manufacturers millions or even billions of dollars.
A leaked letter seen by This Is Money and MailOnline shows the UK is ready to reconsider the tax, which is combined with a Vehicle Excise Duty (VED) on cars in the second year after they are registered.
The tax could be applied to vehicles at a higher price point, or it could be eliminated altogether. However, Ben Maguire, a Lib Dem MP for one region in the UK, said:
“We will consider raising the threshold for zero-emission cars only at a future fiscal event to make it easier to buy electric cars.”
Several companies said an adjustment to the tax would be “a move in the right direction,” and one of the major new car retailers in the UK said sales targets are “unrealistic” with the tax currently set at where it is.
Even still, without the adjustment, retailers are concerned that EVs are not at a spot where consumers truly can justify them as a purchase. One company said “cost, lack of incentives, and lack of a public charging infrastructure” are the biggest bottlenecks in the adoption of EVs.
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Tesla’s new Model S and X spotted, but they leave a lot to be desired
The Model S and Model X testing mules spotted by The Kilowatts have few minor visual changes.

Tesla has been hinting for a few months now that the flagship Model S and Model X would be getting some attention in 2025 as the vehicles continue to be sold in extremely low volumes.
Both models seem to be under the knife, especially as their newest versions were spotted in California earlier this week.
However, images of the vehicles seem to show that Tesla is not planning a major overhaul, which begs the question: why even do it in the first place?
Tesla makes a decision on the future of its flagship Model S and Model X
The Model S and Model X are grouped with the Cybertruck in Tesla’s quarterly delivery releases, and Q1 saw just 12,881 units of the three cars delivered. The Cybertruck likely made up the majority of this number, as some outlets reported around 6,400 deliveries of the all-electric pickup in Q1.
This is unconfirmed.
The Model S and Model X have stuck around for “sentimental reasons,” according to CEO Elon Musk, who said back in 2021:
“I mean, they’re very expensive, made in low volume. To be totally frank, we’re continuing to make them more for sentimental reasons than anything else. They’re really of minor importance to the future.”
However, the cars seem to be in need of a serious refresh. As Tesla changed up the exterior aesthetic on both the Model 3 and Model Y, recent images captured of the Model X by The Kilowatts seem to show this is not the strategy with the Model X or Model S:
— The Kilowatts 🚗⚡️ (@klwtts) May 22, 2025
As we can see, the overall aesthetic of the X, if this is what Tesla plans to release, has literally no changes from a purely visual standpoint. There is the addition of the front bumper camera, which was first implemented on the Cybercab unveiled in October 2024, and then on the new Model Y this year.
There are some new 20″ wheels, and the interior has been fitted with ambient lighting.
The Model S looked to be relatively the same, other than these few hardware changes, including a rear diffuser on this Plaid that was spotted:
Tesla is definitely doing some things 👀 pic.twitter.com/qchMiAWEoT
— The Kilowatts 🚗⚡️ (@klwtts) May 22, 2025
While these changes are welcome and should be beneficial, they don’t seem like they will encourage major sales growth, which might be something Tesla is okay with.
Admitting the two cars are low volume and not contributors to the company’s long-term goals, Musk is likely willing to just upgrade things to make these more compatible and better functioning with the Full Self-Driving suite.
Earlier this year, VP of Vehicle Engineering Lars Moravy said the S and X were not going anywhere and would get “some love” before the end of 2025:
“Just give it a minute. We’ll get there. The upgrade a couple of years ago was bigger than most people thought in terms of architecture and structure of the car got a lot better, too. But, we’ll give it some love later this year and make sure it gets a little bit…you know, with the stuff we’ve been putting in 3 and Y. Obviously, with 3 and Y, the higher volume stuff, you’ve gotta focus there.”
It seems these strategies have held true — the S and X appear to be getting what the 3 and Y got with the ambient lighting and front bumper camera (at least on the Model Y).
Elon Musk
Tesla set for ‘golden age of autonomous’ as Robotaxi nears, ‘dark chapter’ ends: Wedbush
Tesla is set to win big from the launch of the Robotaxi platform, Wedbush’s Dan Ives said.

Tesla (NASDAQ: TSLA) is set to kick off its own “golden age of autonomous growth” as its Robotaxi platform nears launch and a “dark chapter” for the company has evidently come to a close, according to Wedbush analyst Dan Ives.
Ives has jostled his price target on Tesla shares a few times already this year, usually switching things up as the market sways and the company’s near-term outlook changes. His price target on Tesla has gone from $550 to $315 to $350 back to $500 this year, with the newest adjustment coming from a note released early on Friday.
🚨 Wedbush’s Dan Ives is raising his price target on Tesla $TSLA from $350 to $500 as the “golden age of autonomous” nears:
“We believe the golden age of autonomous is now on the doorstep for Tesla with the Austin launch next month kicking off this key next chapter of growth for…
— TESLARATI (@Teslarati) May 23, 2025
As CEO Elon Musk has essentially started to dwindle down his commitment to the Department of Government Efficiency (DOGE) altogether, Ives believes that Tesla’s “dark chapter” has come to a close:
“First lets address the elephant in the room…2025 started off as a dark chapter for Musk and Tesla as Elon’s role in the Trump Administration and DOGE created a life of its own which created brand damage and a black cloud over the story….but importantly those days are in the rear-view mirror as we are now seeing a recommitted Musk leading Tesla as CEO into this autonomous and robotics future ahead with his days in the White House now essentially over.”
Ives believes Tesla’s launch of Robotaxi should be the company’s way to unlock at least $1 trillion in value alone, especially as the Trump White House will fast-track the key initiatives the automaker needs to get things moving in the right direction:
“The $1 trillion of AI valuation will start to get unlocked in the Tesla story and we believe the march to a $2 trillion valuation for TSLA over the next 12 to 18 months has now begun in our view with FSD and autonomous penetration of Tesla’s installed base and the acceleration of Cybercab in the US representing the golden goose.”
There are some concerns moving forward, but none of which relate to the AI/autonomous play that Ives primarily focuses on within the Friday note. Instead, they are related to demand in both Europe and Asia, as Ives said, “there is still wood to chop to turn around Model Y growth” in both of those markets.
Nevertheless, the big focus for Ives is evidently the launch of Robotaxi and the potential of the entire autonomous division that Tesla plans to offer as a ride-sharing service in the coming months. Ives also believes a 50 percent or more penetration of Full Self-Driving could totally transform the financial model and margins of Tesla moving ahead.
Aware of the setbacks Tesla could encounter, Ives still believes that Tesla will establish itself as “the true autonomous winner over” and that investors will recognize the AI vision the company has been so bullish on.
Ives pushed his price target to $500. Tesla shares are down just under 1% at the time of publication. They are trading at $337.88 at 11:45 on the East Coast.
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