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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 UK sales see 14% year-over-year rebound in June: SMMT data

The SMMT stated that Tesla sales grew 14% year-over-year to 7,719 units in June 2025.

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12 hours ago

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July 4, 2025

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Credit: Tesla

Tesla’s sales in the United Kingdom rose in June, climbing 14% year-over-year to 7,719 units, as per data from the Society of Motor Manufacturers and Traders (SMMT). The spike in the company’s sales coincided with the first deliveries of the updated Model Y last month.

Model Y deliveries support Tesla’s UK recovery

Tesla’s June performance marked one of its strongest months in the UK so far this year, with new Model Y deliveries contributing significantly to the company’s momentum. 

While the SMMT listed Tesla with 7,719 deliveries in June, independent data from New AutoMotive suggested that the electric vehicle maker registered 7,891 units during the month instead. However, year-to-date figures for Tesla remain 2% down compared to 2024, as per a report from Reuters.

While Tesla made a strong showing in June, rivals are also growing. Chinese automaker BYD saw UK sales rise nearly fourfold to 2,498 units, while Ford posted the highest EV growth among major automakers, with a more than fourfold increase in the first half of 2025.

Overall, the UK’s battery electric vehicle (BEV) demand surged 39% to to 47,354 units last month, helping push total new car sales in the UK to 191,316 units, up 6.7% from the same period in 2024.

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EV adoption accelerates, but concerns linger

June marked the best month for UK car sales since 2019, though the SMMT cautioned that growth in the electric vehicle sector remains heavily dependent on discounting and support programs. Still, one in four new vehicle buyers in June chose a battery electric vehicle.

SMMT Chief Executive Mike Hawes noted that despite strong BEV demand, sales levels are still below regulatory targets. “Further growth in sales, and the sector will rely on increased and improved charging facilities to boost mainstream electric vehicle adoption,” Hawes stated.

Also taking effect this week was a new US-UK trade deal, which lowers tariffs on UK car exports to the United States from 27.5% to 10%. The agreement could benefit UK-based EV producers aiming to expand across the country.

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Tesla Model 3 ranks as the safest new car in Europe for 2025, per Euro NCAP tests

Despite being on the market longer than many of its rivals, the Tesla Model 3 continues to set the bar for vehicle safety.

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18 hours ago

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July 4, 2025

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Credit: Tesla Asia/X

The Tesla Model 3 has been named the safest new car on sale in 2025, according to the latest results from the Euro NCAP. Among 20 newly tested vehicles, the Model 3 emerged at the top of the list, scoring an impressive 359 out of 400 possible points across all major safety categories.

Tesla Model 3’s safety systems

Despite being on the market longer than many of its rivals, the Tesla Model 3 continues to set the bar for vehicle safety. Under Euro NCAP’s stricter 2025 testing protocols, the electric sedan earned 90% for adult occupant protection, 93% for child occupant protection, 89% for pedestrian protection, and 87% for its Safety Assist systems.

The updated Model 3 received particular praise for its advanced driver assistance features, including Tesla’s autonomous emergency braking (AEB) system, which performed well across various test scenarios. Its Intelligent Speed Assistance and child presence detection system were cited as noteworthy features as well, as per a WhatCar report.

Other notable safety features include the Model 3’s pedestrian-friendly pop-up hood and robust crash protection for both front and side collisions. Euro NCAP also highlighted the Model 3’s ability to detect vulnerable road users during complex maneuvers, such as turning across oncoming traffic.

Euro NCAP’s Autopilot caution

While the Model 3’s safety scores were impressive across the board, Euro NCAP did raise concerns about driver expectations of Tesla’s Autopilot system. The organization warned that some owners may overestimate the system’s capabilities, potentially leading to misuse or inattention behind the wheel. Even so, the Model 3 remained the highest-scoring vehicle tested under Euro NCAP’s updated criteria this year.

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The Euro NCAP’s concerns are also quite interesting because Tesla’s Full Self-Driving (FSD) Supervised, which is arguably the company’s most robust safety suite, is not allowed for public rollout in Europe yet. FSD Supervised would allow the Model 3 to navigate inner city streets with only minimal human supervision.

Other top scorers included the Volkswagen ID.7, Polestar 3, and Geely EX5, but none matched the Model 3’s total score or consistency across categories. A total of 14 out of 20 newly tested cars earned five stars, while several models, including the Kia EV3, MG ZS, and Renault 5, fell short of the top rating.

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Why Tesla’s Q3 could be one of its biggest quarters in history

Tesla could stand to benefit from the removal of the $7,500 EV tax credit at the end of Q3.

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1 day ago

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July 3, 2025

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(Credit: Tesla)

Tesla has gotten off to a slow start in 2025, as the first half of the year has not been one to remember from a delivery perspective.

However, Q3 could end up being one of the best the company has had in history, with the United States potentially being a major contributor to what might reverse a slow start to the year.

Earlier today, the United States’ House of Representatives officially passed President Trump’s “Big Beautiful Bill,” after it made its way through the Senate earlier this week. The bill will head to President Trump, as he looks to sign it before his July 4 deadline.

The Bill will effectively bring closure to the $7,500 EV tax credit, which will end on September 30, 2025. This means, over the next three months in the United States, those who are looking to buy an EV will have their last chance to take advantage of the credit. EVs will then be, for most people, $7,500 more expensive, in essence.

The tax credit is available to any single filer who makes under $150,000 per year, $225,000 a year to a head of household, and $300,000 to couples filing jointly.

Ending the tax credit was expected with the Trump administration, as his policies have leaned significantly toward reliance on fossil fuels, ending what he calls an “EV mandate.” He has used this phrase several times in disagreements with Tesla CEO Elon Musk.

Nevertheless, those who have been on the fence about buying a Tesla, or any EV, for that matter, will have some decisions to make in the next three months. While all companies will stand to benefit from this time crunch, Tesla could be the true winner because of its sheer volume.

If things are done correctly, meaning if Tesla can also offer incentives like 0% APR, special pricing on leasing or financing, or other advantages (like free Red, White, and Blue for a short period of time in celebration of Independence Day), it could see some real volume in sales this quarter.

Tesla is just a shade under 721,000 deliveries for the year, so it’s on pace for roughly 1.4 million for 2025. This would be a decrease from the 1.8 million cars it delivered in each of the last two years. Traditionally, the second half of the year has produced Tesla’s strongest quarters. Its top three quarters in terms of deliveries are Q4 2024 with 495,570 vehicles, Q4 2023 with 484,507 vehicles, and Q3 2024 with 462,890 vehicles.

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