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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.
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- A to-scale comparison of Falcon 1, Pegasus XL, MLV, and Falcon 9. (Teslarati/Stratolaunch/Wikipedia)
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- 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.
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- PGA’s first full-scale preburner seen during assembly. (Stratolaunch)
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- PGA’s first full-scale preburner seen during assembly. (Stratolaunch)
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- Jeff Thornburg stands in front of Stratolaunch’s NASA Stennis Space Center test stand. (Stratolaunch)
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- The PGA preburner seen after installation at Stennis. (Stratolaunch)
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- The control center. (Stratolaunch)
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- MLV is released from Stratolauncher. (Stratolaunch)
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- A concept video produced by Stratolaunch shows the Roc launching a Kraken rocket. (Stratolaunch, via Wired)
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Tesla rolls out xAI’s Grok to vehicles across Europe
The initial rollout includes the United Kingdom, Ireland, Germany, Switzerland, Austria, Italy, France, Portugal, and Spain.
Tesla is rolling out Grok to vehicles in Europe. The feature will initially launch in nine European territories.
In a post on X, the official Tesla Europe, Middle East & Africa account confirmed that Grok is coming to Teslas in Europe. The initial rollout includes the United Kingdom, Ireland, Germany, Switzerland, Austria, Italy, France, Portugal, and Spain, and additional markets are expected to be added later.
Grok allows drivers to ask questions using real-time information and interact hands-free while driving. According to Tesla’s support documentation, Grok can also initiate navigation commands, enabling users to search for destinations, discover points of interest, and adjust routes without touching the touchscreen, as per the feature’s official webpage.
The system offers selectable personalities, ranging from “Storyteller” to “Unhinged,” and is activated either through the App Launcher or by pressing and holding the steering wheel’s microphone button.
Grok is currently available only on Model S, Model 3, Model X, Model Y, and Cybertruck vehicles equipped with an AMD infotainment processor. Vehicles must be running software version 2025.26 or later, with navigation command support requiring version 2025.44.25 or newer.
Drivers must also have Premium Connectivity or a stable Wi-Fi connection to use the feature. Tesla notes that Grok does not currently replace standard voice commands for vehicle controls such as climate or media adjustments.
The company has stated that Grok interactions are processed securely by xAI and are not linked to individual drivers or vehicles. Users do not need a Grok account or subscription to enable the feature at this time as well.
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Tesla ends Full Self-Driving purchase option in the U.S.
In January, Musk announced that Tesla would remove the ability to purchase the suite outright for $8,000. This would give the vehicle Full Self-Driving for its entire lifespan, but Tesla intended to move away from it, for several reasons, one being that a tranche in the CEO’s pay package requires 10 million active subscriptions of FSD.
Tesla has officially ended the option to purchase the Full Self-Driving suite outright, a move that was announced for the United States market in January by CEO Elon Musk.
The driver assistance suite is now exclusively available in the U.S. as a subscription, which is currently priced at $99 per month.
Tesla moved away from the outright purchase option in an effort to move more people to the subscription program, but there are concerns over its current price and the potential for it to rise.
In January, Musk announced that Tesla would remove the ability to purchase the suite outright for $8,000. This would give the vehicle Full Self-Driving for its entire lifespan, but Tesla intended to move away from it, for several reasons, one being that a tranche in the CEO’s pay package requires 10 million active subscriptions of FSD.
Although Tesla moved back the deadline in other countries, it has now taken effect in the U.S. on Sunday morning. Tesla updated its website to reflect this:
🚨 Tesla has officially moved the outright purchase option for FSD on its website pic.twitter.com/RZt1oIevB3
— TESLARATI (@Teslarati) February 15, 2026
There are still some concerns regarding its price, as $99 per month is not where many consumers are hoping to see the subscription price stay.
Musk has said that as capabilities improve, the price will go up, but it seems unlikely that 10 million drivers will want to pay an extra $100 every month for the capability, even if it is extremely useful.
Instead, many owners and fans of the company are calling for Tesla to offer a different type of pricing platform. This includes a tiered-system that would let owners pick and choose the features they would want for varying prices, or even a daily, weekly, monthly, and annual pricing option, which would incentivize longer-term purchasing.
Although Musk and other Tesla are aware of FSD’s capabilities and state is is worth much more than its current price, there could be some merit in the idea of offering a price for Supervised FSD and another price for Unsupervised FSD when it becomes available.
Elon Musk
Musk bankers looking to trim xAI debt after SpaceX merger: report
xAI has built up $18 billion in debt over the past few years, with some of this being attributed to the purchase of social media platform Twitter (now X) and the creation of the AI development company. A new financing deal would help trim some of the financial burden that is currently present ahead of the plan to take SpaceX public sometime this year.
Elon Musk’s bankers are looking to trim the debt that xAI has taken on over the past few years, following the company’s merger with SpaceX, a new report from Bloomberg says.
xAI has built up $18 billion in debt over the past few years, with some of this being attributed to the purchase of social media platform Twitter (now X) and the creation of the AI development company. Bankers are trying to create some kind of financing plan that would trim “some of the heavy interest costs” that come with the debt.
The financing deal would help trim some of the financial burden that is currently present ahead of the plan to take SpaceX public sometime this year. Musk has essentially confirmed that SpaceX would be heading toward an IPO last month.
The report indicates that Morgan Stanley is expected to take the leading role in any financing plan, citing people familiar with the matter. Morgan Stanley, along with Goldman Sachs, Bank of America, and JPMorgan Chase & Co., are all expected to be in the lineup of banks leading SpaceX’s potential IPO.
Since Musk acquired X, he has also had what Bloomberg says is a “mixed track record with debt markets.” Since purchasing X a few years ago with a $12.5 billion financing package, X pays “tens of millions in interest payments every month.”
That debt is held by Bank of America, Barclays, Mitsubishi, UFJ Financial, BNP Paribas SA, Mizuho, and Société Générale SA.
X merged with xAI last March, which brought the valuation to $45 billion, including the debt.
SpaceX announced the merger with xAI earlier this month, a major move in Musk’s plan to alleviate Earth of necessary data centers and replace them with orbital options that will be lower cost:
“In the long term, space-based AI is obviously the only way to scale. To harness even a millionth of our Sun’s energy would require over a million times more energy than our civilization currently uses! The only logical solution, therefore, is to transport these resource-intensive efforts to a location with vast power and space. I mean, space is called “space” for a reason.”
The merger has many advantages, but one of the most crucial is that it positions the now-merged companies to fund broader goals, fueled by revenue from the Starlink expansion, potential IPO, and AI-driven applications that could accelerate the development of lunar bases.
Tesla rolls out xAI’s Grok to vehicles across Europe
Tesla ends Full Self-Driving purchase option in the U.S.
