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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 Q2 delivery consensus confirms this long-standing theory

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Credit: Joe Tegtmeyer/X

Tesla released what analysts believe the company will report in terms of deliveries and energy deployments for Q2, but the figures seem to confirm a long-standing theory on the company’s vehicle division.

For years, Tesla was just looked at as a car company. Now that it has established itself as a powerhouse in energy, AI, and tech as a whole, the company is now less hellbent on achieving quarterly growth, on a sequential basis, at least from a major standpoint.

Tesla topped out its annual deliveries in 2023 at 1.81 million, and in the two years since, the company has reported a decrease in deliveries for the entire 12-month term both times.

With Tesla delivering 358,023 cars in Q1, a 6.3 percent increase over Q1 2025, but falling short of Wall Street expectations at 365,000-370,000 units, the narrative around vehicle deliveries and their importance continued to change earlier this year. Some might say it is convenient, but others might say it is the typical evolution of a company that continues to change over time.

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For Q2, Tesla’s delivery consensus estimates sit at 406,024 units, analysts believe. They were surveyed from Daiwa, DB, Wedbush, Cowen, Canaccord, Baird, Wolfe, BMP Paribas, Goldman Sachs, RBC, Evercore ISI, Barclays, Bank of America, Wells Fargo, Morgan Stanley, Truist, UBS, Jefferies, JPM, Needham & Co., HSBC, and William Blair.

Credit: Tesla

Tesla is also expected to report deployments of 13.8 GWh this quarter.

The change to Tesla’s overall narrative now leans less on vehicle deliveries and more on its other projects. Most notably, Tesla’s Robotaxi project has taken the priority over most of its other business ventures, and investors and the public are more concerned about the deployment of vehicles into the fleet, the operation of a driverless ride-hailing service, Cybercab production and operation, and expansion into new cities.

Tesla analyst realizes one big thing about the stock: deliveries are losing importance

This big narrative switch happened when Tesla indicated it was looking at making transportation a service by launching a ride-hailing service that will operate using Tesla’s Full Self-Driving suite. Once unsupervised operation begins, Robotaxi could be a new way for people to get around, all without a driver in their car.

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Instead, they will rely on the billions of miles Tesla has accumulated from its real-world fleet.

It is important to note that Tesla remains significant in the automotive sector, and deliveries must continue as they have for years. Tesla still has a strong automotive business and needs to execute further on all facets to keep its investors happy.

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Tesla looks keen to bring larger Model Y L to the U.S.

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Credit: Tesla

Tesla launched the slightly larger Model Y L in China last year, and it became a hit in no time. The longer wheelbase, larger interior, and slightly more forgiving legroom area in the Model Y L became a sought-after possibility for U.S. buyers, who have been begging the company for a larger SUV.

Now, Tesla needs it more than ever, especially considering the Model X was discontinued alongside its Model S sibling earlier this year. It looks to be more likely than ever, and based on recent reports, it will fall in line with CEO Elon Musk’s prediction that it would arrive in the United States in late 2026.

Recent reports from Forbes and Not a Tesla App both have indicated Tesla plans to bring the Model Y L to the U.S. this year. The reports cite “credible sources,” and an analyst from AutoForecast Solutions named Sam Fiorani stated that the car would enter production later this year.

Fiorani said:

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“China, Australia, and India are supplied by the factory in China, which will not supply vehicles to the U.S. Production of the Model Y L is expected to begin in the U.S. in September, which will lead to sales beginning before the end of 2026.”

Production would take place at Gigafactory Texas.

Additionally, a few Model Y L units have been spotted under wraps in the United States, giving more indication that Tesla plans to bring the vehicle to the U.S. When Tesla is close to launching a vehicle in the U.S., it is not uncommon to see these models with the exact car covers that you see below:

It makes sense, especially considering Musk hinted the Model Y L would make it to the U.S. in late 2026, but it was up in the air. The CEO said the advent of self-driving might not warrant a larger SUV coming to the U.S. market specifically.

The problem is, consumers do not want to hear that. They love Tesla’s tech, FSD, and other features, but they need more space for growing families. The Model X is gone, and the most anyone can fit in a Tesla right now is seven people in the seven-seat Model Y. That back row is truly only large enough to fit small children comfortably.

Tesla fans have requested a full-size SUV, and the company has made some hints that it could be in the plans.

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The Model Y and Model Y L differ noticeably in size, with the Model Y L being a stretched, six-seat variant designed for great interior room. The Standard Model Y measures approximately 4,790mm in length, 1,982 mm in width with the mirrors folded, 1,624mm in height, and 2,890mm in wheel base.

In contrast, the Model Y L extends to be about 4,969–4,976mm long (roughly 179mm or 7 inches longer), stands 1,668mm tall (+44mm), and features a significantly longer 3,040 mm wheelbase (+150mm), while maintaining the same width.

This elongation primarily benefits rear passenger space and enables a 2+2+2 seating layout with captain’s chairs, though it slightly reduces maximum cargo capacity behind the rearmost seats and adds a bit of overall mass and turning radius. The result is a more spacious family hauler that still shares the core footprint and agile character of the original Model Y.

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One of Tesla’s biggest threats just got banned in the U.S.

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In a major development that will inevitably strengthen Tesla’s dominant position in the American EV market, Polestar has been effectively banned from selling new vehicles in the United States, starting with the 2027 model year.

The U.S. Department of Commerce denied Polestar authorization under the Connected Vehicle Rule, which prohibits vehicles containing certain connected technologies (Cellular, Wi-Fi, Bluetooth, etc.) linked to China or Russia due to national security risks, including potential data collection on American drivers.

Polestar, which is majority-owned by China’s Geely Holding, could not obtain the required exemption despite producing some models domestically.

Polestar confirmed it will sell off any remaining inventory of the Polestar 3 and Polestar 4 models, while continuing service and warranty support for existing customers. No new models or major refreshes will reach U.S. buyers, and the company is pivoting its growth strategy to Europe, where it already generates the vast majority of its sales.

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The outcome removes a direct premium EV competitor that had positioned itself as a stylish, performance-oriented alternative to Tesla’s lineup. The Polestar 2 challenged the Model 3, while the Polestar 3 and 4 targeted segments overlapping with the Model Y and upcoming Tesla offerings. Polestar’s U.S. sales had already been sluggish amid intense competition and slower demand, representing just 6 percent of its global volume in the first quarter of 2026.

While Polestar was not on Tesla’s level in the U.S., it still places a dent in the evergrowing field of Tesla competitors in the country, where it has long dominated EV sales.

Tesla faces none of these hurdles. As a U.S.-founded and U.S.-headquartered company with major manufacturing in Fremont, Austin, and Nevada, Tesla’s vehicles are built with compliant domestic and allied supply chains. Its Full Self-Driving technology, over-the-air software updates, and vertically integrated ecosystem were developed entirely in-house without foreign ownership entanglements that trigger national security reviews, at least in the U.S.

Of course, it did face a similar threat in China a few years back:

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Elon Musk responds to reports of Tesla ban among China’s military over security concerns

The Connected Vehicle Rule, first advanced under the prior administration and upheld under the current one, is part of a broader U.S. effort to protect the domestic auto industry and critical technology from Chinese influence. High tariffs on Chinese-made EVs and related restrictions have already reshaped the market. Tesla benefits directly: it avoids these barriers while continuing to lead in U.S. EV sales volume, Supercharger network expansion, and energy storage integration.

By clearing Polestar from the new-vehicle playing field, the policy reduces competitive pressure in the premium and performance EV segments where Tesla has invested billions. American consumers seeking cutting-edge electric vehicles now have one fewer option tied to foreign adversaries — and one clearer path to the market leader that has driven the EV transition from the start.

For Tesla, this is more than regulatory relief. It is a strategic tailwind that reinforces its position as America’s premier EV innovator at a time when domestic manufacturing and technological independence matter most.

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