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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.

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 Cybercab undergoes winter testing as Elon Musk reiterates production start date

CEO Elon Musk confirmed the timeline in a recent post on X, while Tesla’s official social media accounts separately revealed that Cybercab prototypes are now undergoing winter testing in Alaska.

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1 hour ago

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January 25, 2026

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Credit: Tesla Robotaxi/X

Tesla has reiterated that production of its fully autonomous Cybercab is set to begin in April, even as the company continues expanding real-world testing of the vehicle. 

CEO Elon Musk confirmed the timeline in a recent post on X, while Tesla’s official social media accounts separately revealed that Cybercab prototypes are now undergoing winter testing in Alaska.

Musk confirms April Cybercab initial production

In a post on X, Musk reiterated that Cybercab production is scheduled to begin in April, reiterating his guidance about the vehicle’s manufacturing timeline. Around the same time, Tesla shared images showing the Cybercab undergoing cold-weather testing in Alaska. Interestingly enough, the Cybercab prototypes being tested in Alaska seemed to be equipped with snow tires. 

Winter testing in Alaska suggests Tesla is preparing the Cybercab for deployment across a wide range of climates in the United States. Cold temperatures, snow, ice, and reduced traction present some of the most demanding scenarios for autonomous systems, making Alaska a logical proving ground for a vehicle designed to operate without a human driver.

Taken together, Musk’s production update and Tesla’s testing post indicate that while the Cybercab is nearing the start of manufacturing, validation efforts are still actively ramping to ensure reliability in real-world environments.

What early Cybercab production might look like

Musk has previously cautioned that the start of Cybercab manufacturing will be slow, reflecting the challenges of launching an all-new vehicle platform. In a recent comment, Musk said initial production typically follows an S-curve, with early output constrained by how many new parts and processes are involved.

According to Musk, both Cybercab and Optimus fall into this category, as “almost everything is new.” As a result, early production rates are expected to be very deliberate before eventually accelerating rapidly as manufacturing processes mature.

“Initial production is always very slow and follows an S-curve. The speed of production ramp is inversely proportionate to how many new parts and steps there are. For Cybercab and Optimus, almost everything is new, so the early production rate will be agonizingly slow, but eventually end up being insanely fast,” Musk wrote in a post on X.

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Tesla to increase Full Self-Driving subscription price: here’s when

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24 hours ago

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January 24, 2026

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Credit: Tesla

Tesla will increase its Full Self-Driving subscription price, meaning it will eventually be more than the current $99 per month price tag it has right now.

Already stating that the ability to purchase the suite outright will be removed, Tesla CEO Elon Musk said earlier this week that the Full Self-Driving subscription price would increase when its capabilities improve:

“I should also mention that the $99/month for supervised FSD will rise as FSD’s capabilities improve. The massive value jump is when you can be on your phone or sleeping for the entire ride (unsupervised FSD).”

This was an expected change, especially as Tesla has been hinting for some time that it is approaching a feature-complete version of Full Self-Driving that will no longer require driver supervision. However, with the increase, some are concerned that they may be priced out.

$99 per month is already a tough ask for some. While Full Self-Driving is definitely worth it just due to the capabilities, not every driver is ready to add potentially 50 percent to their car payment each month to have it.

While Tesla has not revealed any target price for FSD, it does seem that it will go up to at least $150.

Additionally, the ability to purchase the suite outright is also being eliminated on February 14, which gives owners another reason to be slightly concerned about whether they will be able to afford to continue paying for Full Self-Driving in any capacity.

Some owners have requested a tiered program, which would allow people to pay for the capabilities they want at a discounted price.

Unsupervised FSD would be the most expensive, and although the company started removing Autopilot from some vehicles, it seems a Supervised FSD suite would still attract people to pay between $49 and $99 per month, as it is very useful.

Tesla will likely release pricing for the Unsupervised suite when it is available, but price increases could still come to the Supervised version as things improve.

This is not the first time Musk has hinted that the price would change with capability improvements, either. He’s been saying it for some time. In 2020, he even said the value of FSD would “probably be somewhere in excess of $100,000.”

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Tesla starts removing outright Full Self-Driving purchase option at time of order

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1 day ago

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January 24, 2026

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(Credit: Tesla)

Tesla has chosen to axe the ability to purchase Full Self-Driving outright from a select group of cars just days after CEO Elon Musk announced the company had plans to eliminate that option in February.

The company is making a clear-cut stand that it will fully transition away from the ability to purchase the Full Self-Driving suite outright, a move that has brought differing opinions throughout the Tesla community.

Earlier this week, the company also announced that it will no longer allow buyers to purchase Full Self-Driving outright when ordering a pre-owned vehicle from inventory. Instead, that will be available for $99 per month, the same price that it costs for everyone else.

The ability to buy the suite for $8,000 for a one-time fee at the time of order has been removed:

This is a major move because it is the first time Tesla is eliminating the ability to purchase FSD outright for one flat fee to any of its vehicles, at least at the time of purchase.

It is trying to phase out the outright purchase option as much as it can, preparing people for the subscription-based service it will exclusively offer starting on February 14.

In less than a month, it won’t be available on any vehicle, which has truly driven some serious conversation from Tesla owners throughout the community.

There’s a conflict, because many believe that they will now lose the ability to buy FSD and not pay for it monthly, which is an attractive offer. However, others believe, despite paying $8,000 for FSD, that they will have to pay more money on top of that cost to get the unsupervised suite.

Additionally, CEO Elon Musk said that the FSD suite’s subscription price would increase over time as capabilities increase, which is understandable, but is also quite a conflict for those who spent thousands to have what was once promised to them, and now they may have to pay even more money.

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