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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’s Semi truck factory is open with a detail that changes everything

Tesla’s dedicated Nevada Semi factory has opened, targeting 50,000 trucks per year as fleet adoptions accelerate nationwide.

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2 hours ago

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April 10, 2026

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Nearly nine years after Elon Musk unveiled the Tesla Semi in November 2017, the company is now opening a dedicated factory just outside of Reno, Nevada, and ramping toward mass production of 50,000 trucks per year.

Volume production began in March 2026 at the new Tesla Semi factory, with the competitive advantage not being the factory itself. Rather, it’s where Tesla built it. By constructing the 1.7 million square foot facility directly adjacent to Gigafactory Nevada in Sparks, Tesla closed the one supply chain loop that had delayed the Semi program for years. The 4680 battery cells that power the Semi are manufactured in the same complex, which significantly streamlines supply logistics. That single decision eliminates the bottleneck that forced Tesla to prioritize battery supply for passenger cars over the Semi throughout 2020, 2021, and 2022, which is precisely why the first deliveries slipped three years past the original target. Every other electric truck manufacturer sources its battery cells from a separate supplier, ships them to a separate factory, and absorbs the cost and delay that comes with that. Tesla built its Semi factory around its battery factory, and that vertical integration is what makes 50,000 trucks per year a realistic number rather than an aspirational one.

At the 2025 Annual Shareholder Meeting, Musk was direct about where things stood, stating “Starting next year, we will manufacture the Tesla Semi. We already have a lot of prototype Semis in operation – PepsiCo and other companies have been using them for some time. But in 2026, we’ll begin volume production at our Northern Nevada factory.” Full ramp to volume output is targeted before June 30, 2026.


The first limited deliveries happened in December 2022 to PepsiCo, which eventually doubled its fleet to 50 trucks out of its California distribution facility. Since then the Semi has been showing up in more corporate fleets. As Teslarati noted in March, a Ralph’s Supermarkets branded Semi was spotted on a Los Angeles highway, confirming Kroger’s partnership with Tesla to deploy up to 500 electric Semis. Walmart, Costco, Sysco, US Foods, DHL, Hight Logistics and WattEV are among the companies actively running or receiving units. DHL logged real-world efficiency of 1.72 kWh per mile under a full 75,000 pound load over 388 miles, matching Tesla’s targets closely.

The 2026 production model arrives with meaningful upgrades over the original, with a 1,000 pound weight reduction, updated aerodynamics, and support for 1.2 MW Megacharger speeds that can restore 60% of range in around 30 minutes during a mandatory driver rest break. Tesla opened its first public Megacharger in Ontario, California in March, positioned near the I-10 and I-15 interchange serving the Ports of Los Angeles and Long Beach. The company plans 37 Megacharger sites by end of 2026 and 66 total across 15 states by early 2027, with construction beginning at the nation’s largest truck stop operator in the first half of this year.

Tesla reveals various improvements to the Semi in new piece with Jay Leno

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Musk has described the Semi’s economics as a straightforward case. “The Semi is a TCO no-brainer,” he said, noting the total cost of ownership is “much, much cheaper than any other transportation you could have.” At under $300,000, the truck costs roughly double a comparable diesel, but California’s $200,000 per vehicle subsidy has driven over 1,000 state orders alone. As Teslarati has tracked, the prototype fleet accumulated over 13.5 million miles with 95% fleet uptime before production ever scaled. The factory opening now turns that proof of concept into a production program.

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Tesla Full Self-Driving gets first-ever European approval

Tesla owners in the Netherlands with a Full Self-Driving subscription will receive a software update “shortly,” the company said, activating the operation of the company’s semi-autonomous driving tech for the first time in Europe.

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5 hours ago

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April 10, 2026

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Credit: Tesla AI | X

Tesla Full Self-Driving (Supervised) got its first-ever European approval, as the Netherlands gave the suite the green light to begin operation.

Tesla owners in the Netherlands with a Full Self-Driving subscription will receive a software update “shortly,” the company said, activating the operation of the company’s semi-autonomous driving tech for the first time in Europe.

The Dutch vehicle authority RDW granted the type approval after more than 18 months of rigorous testing on both closed tracks and public roads. FSD Supervised complies with UN R-171 standards and benefits from Article 39 exemptions under EU Regulation 2018/858. Importantly, it is not a fully autonomous vehicle.

The RDW stressed that the driver remains fully responsible and must maintain attention at all times. “Safety is paramount for the RDW,” the authority stated. “Proper use of this driver assistance system contributes positively to road safety.” Sensors monitor driver alertness, issuing warnings if eyes leave the road or hands are unavailable to take control immediately.

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CEO Elon Musk also commented on the approval in a post on X, saying:

“First (supervised) FSD approval in Europe! Congratulations to the Tesla team and thank you to the regulatory authorities in the Netherlands for all of the hard work required to make this happen.”

Trained on billions of kilometers of real-world driving data, FSD Supervised allows the vehicle to handle residential streets, dense city traffic, and highways under constant supervision. Tesla’s post declared:

“It can drive you almost anywhere under your supervision – from residential roads to city streets & highways. No other vehicle can do this.”

The company added that it is “excited to bring FSD Supervised to more European countries soon.”

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This national approval paves the way for broader EU adoption. Other member states can recognize the Dutch certification individually, with a potential bloc-wide rollout via European Commission committee vote anticipated by this Summer. The decision underscores Europe’s stricter safety and documentation requirements compared to U.S. self-certification.

Tesla Europe shares FSD test video weeks ahead of launch target

The Netherlands’ approval represents a pivotal step for Tesla in Europe, where complex regulations and mixed traffic have delayed rollout. Musk added that the RDW was “rigorous” in its assessment of FSD.

By proving the system’s safety in one of the continent’s most bicycle- and tram-heavy nations, Tesla positions itself to transform mobility across the EU—delivering greater convenience while keeping drivers firmly in control.

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As the first domino falls, anticipation builds for FSD Supervised to reach additional countries soon.

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Tesla is using a redesigned Cybertruck battery cell to mitigate Semi challenges

It is perhaps the most recent example of Tesla using unique engineering prowess and cross-pollinating vehicle elements to solve common problems, something it does better than most companies out there.

Published

14 hours ago

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April 10, 2026

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Credit: Tesla

Tesla revealed that it is utilizing redesigned Cybertruck battery cells in its Long Range Semi to mitigate some pertinent challenges that come with long-haul logistics.

It is perhaps the most recent example of Tesla using unique engineering prowess and cross-pollinating vehicle elements to solve common problems, something it does better than most companies out there.

Tesla’s long-awaited Semi truck is entering production at its Nevada Gigafactory, and fresh factory footage reveals a clever evolution in its battery technology.

The Long Range variant, designed for up to 500 miles of real-world range, relies on a structural battery pack that uses the same 4680-form-factor cells found in the Cybertruck.

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However, Tesla engineers have completely redesigned the pack’s architecture—shifting from the flat, pancake-style modules typical in passenger vehicles to a compact, vertical cubic layout. This change isn’t just about cramming more energy into the chassis; it’s a targeted solution to one of electric trucking’s biggest headaches: range loss in cold climates.

Dan Priestley, Head of the Tesla Semi program, said:

“We’re using essentially the same cell out of Cybertruck, but our cars packs are more like a pancake. Whereas these are more like a cube. You get a lot of energy stored in a small space. You can only do this if you design the vehicle to be electric from the ground up.”

In conventional EVs, battery packs are laid out horizontally in wide, flat arrays to fit under the floor. While this works for cars and even the Cybertruck’s structural pack, it exposes a large surface area to the elements.

Heat escapes quickly, especially overnight when the truck is parked. Cold temperatures slow chemical reactions inside lithium-ion cells, reducing available energy and forcing the vehicle to expend extra power warming the battery and cabin.

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Real-world tests on vehicles like the Cybertruck show winter range losses of 20-40 percent, depending on conditions. For long-haul truck drivers operating in Canada, Scandinavia, or the northern U.S., this “silent killer” means unplanned stops, reduced payloads, and higher operating costs.

From personal experience, cold weather still impacts EV batteries even with various inventions and strategies that companies have come up with. In the cold Pennsylvania winter, charging was much more frequent for me due to range loss due to temperatures.

Tesla’s cubic battery pack flips the script. By arranging the 4680 cells in tall, dense vertical stacks, the pack minimizes external surface area relative to its volume—essentially turning the battery into its own thermal blanket.

Factory video from the Semi assembly line shows these large, yellow-green structural modules mounted directly onto the chassis, forming a near-cube shape.

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The reduced exposure helps the pack retain heat generated during operation, keeping cells closer to their optimal temperature even after hours in sub-zero conditions.

The design doesn’t stop there. Tesla pairs the cubic pack with an advanced heat pump system that actively recycles thermal energy from the motors, brakes, and even ambient air.

Tesla reveals various improvements to the Semi in new piece with Jay Leno

Unlike passive systems in earlier EVs, this architecture transfers waste heat back into the battery, maintaining readiness for morning departures without draining the pack.

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Executives have noted that the combination, cubic geometry plus intelligent thermal management, dramatically cuts overnight cooldown and range degradation, making the Semi viable for 24/7 fleet operations in harsh winters.

Beyond cold-weather performance, the redesigned pack integrates structurally with the truck’s frame, enhancing rigidity while simplifying assembly. Production footage shows workers installing the massive modules early in the line, signaling that the Semi’s battery is now a core chassis component rather than an add-on.

Using proven 4680 cells keeps costs down and leverages Tesla’s scaled manufacturing know-how from Cybertruck and Model Y lines.

Tesla’s focus on ramping up Semi output will lean on small innovative steps like this one. Truckers are not immune to traveling in cold weather conditions, and changes like this one will help make them more effective while also increasing output by logistics operators who choose to go all-electric with the Tesla Semi.

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