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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 Robotaxi app download rate demolishes Uber, Waymo all-time highs

After two and a half months of testing with a group of hand-picked Tesla influencers and some media, the company has officially launched Robotaxi rides in both Austin, Texas, and the California Bay Area to the public.

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2 hours ago

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September 5, 2025

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Credit: @BLKMDL3 | X

Tesla launched its Robotaxi app to the general public yesterday, and the number of downloads is a testament to the platform’s high demand for testing.

After two and a half months of testing with a group of hand-picked Tesla influencers and some media, the company has officially launched Robotaxi rides in both Austin, Texas, and the California Bay Area to the public.

Tesla Robotaxi makes major expansion with official public app launch

Downloading the app is available to iOS users, so if you have an iPhone, you can get it and join the waitlist. Tesla has not yet launched the Robotaxi app for the Android platform, but did hint that it would be coming soon.

The testing phase with the group Tesla selected has gone well. In Austin, the City has only listed one “Safety Concern” with Robotaxi during the testing phase. For the most part, things have gone extremely well, and riders have had good things to say.

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Tesla is still operating with some safeguards in place, such as Safety Monitors and Safety Drivers, but these are precautionary and temporary; CEO Elon Musk has said they should be removed by the end of the year.

Elon Musk says Tesla will take Safety Drivers out of Robotaxi: here’s when

Even still, Tesla Robotaxi is something that many people want to experience, and the app downloads prove it.

The Tesla Robotaxi app was downloaded at a rate that exceeded all rolling 30-day periods of both Uber and Waymo, according to Brett Winton of ARK Invest. Tesla’s Robotaxi’s first day on the App Store exceeded Uber’s by 40 percent and Waymo’s best download day ever by six times:

The surge in downloads is a good indication of how in demand the Robotaxi suite was, as many people within the community had vocalized their requests to try the platform, but Tesla was not ready to expand it beyond its handpicked group.

The expansion of the program will result in more rides, provided Tesla continues to expand its fleet of vehicles. It has already admitted many of those who were initially placed on the waitlist.

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Elon Musk’s xAI expands to Seattle with salaries up to $440,000

The move was announced by the artificial intelligence startup and Elon Musk on social media platform X.

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3 hours ago

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September 5, 2025

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(Credit: xAI)

Elon Musk’s artificial intelligence startup xAI is opening a new office in Seattle as it accelerates its global expansion. 

The move was announced by the artificial intelligence startup and Elon Musk on social media platform X. xAI is also hiring for its first positions in the new site.

New Seattle office

As could be seen on xAI’s Careers webpage, the Seattle office is currently hiring for three engineering roles. Each of the three technical roles tied to the new site carry salaries ranging from $180,000 to $440,000. 

The new office adds to xAI’s growing presence, which now spans San Francisco, Austin, London, Dublin, New York, and Memphis. The Seattle-based roles focus on video and image generation systems, signaling Musk’s intent to challenge rivals like OpenAI and Meta in generative AI.

Pressures and challenges

Seattle also places xAI within reach of Microsoft’s headquarters in Redmond. Microsoft has emerged as a central player in the AI race through its multibillion-dollar partnership with OpenAI, making xAI’s move into the region notable. The competition for AI specialists has pushed salaries higher across the industry, with filings showing OpenAI staff earning up to $530,000 and Anthropic engineers as much as $690,000 annually, as noted by Insider.

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The startup has also seen some high-profile departures in recent months, including cofounder Igor Babuschkin and general counsel Robert Keele. Still, xAI continues to grow aggressively, and its Grok large language model has been gaining momentum among mainstream users. Work also continues to be underway to further build out the company’s Colossus supercomputer cluster. Reports have also suggested that xAI has moved into San Francisco offices in the Mission District, a site Musk initially leased during OpenAI’s early years.

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Investor's Corner

Elon Musk’s new pay plan ties trillionaire status to Tesla’s $8.5 trillion valuation

Shareholders are expected to vote on the proposal at the annual meeting on November 6.

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5 hours ago

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September 5, 2025

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USAFA_Hosts_Elon_Musk_(Image_1_of_17)_(cropped)
U.S. AIR FORCE ACADEMY, Colo. -- Tesla Inc. Chief Executive Officer Elon Musk poses for a photograph with U.S. Air Force Academy Cadets in Mitchell Hall during a tour hosted by Superintendent Lt. Gen. Richard Clark on April 7, 2022. Musk met with Academy senior leadership and delivered remarks to a crowd of cadets and faculty. (U.S. Air Force photo by Justin R. Pacheco)

Tesla’s board has proposed a new compensation package for CEO Elon Musk that could make him the world’s first trillionaire and Tesla the most valuable company in history. 

The 2025 CEO Performance Award, outlined in a securities filing on Friday, would be worth up to $900 billion in Tesla stock (NASDAQ:TSLA) if the automaker achieves a series of aggressive performance and valuation goals, according to the New York Times. 

Shareholders are expected to vote on the proposal at the annual meeting on November 6.

Tesla is aiming for an insane $8.5 trillion market cap

The package requires Musk to lift Tesla’s market capitalization from about $1.1 trillion today to $8.5 trillion over the next decade. At that level, Tesla would surpass every major public company in existence. Nvidia, currently the world’s most valuable firm, has a market cap of around $4.2 trillion today, as noted in a Motley Fool report. Microsoft and Apple follow at $3.8 and 3.6 trillion each, while Saudi Aramco is valued at around $1.5 trillion.

If Tesla achieves its $8.5 trillion target, it would be worth more than twice Nvidia’s present valuation and nearly eight times its current size. The compensation plan also requires Tesla’s operating profit to grow from $17 billion last year to $400 billion annually.

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Credit: Tesla

Elon Musk’s path to a trillionaire status

Apart from leading Tesla to become the world’s biggest company in history, Musk is also required to hit several product targets for the electric vehicle maker. These include the delivery of 20 million Tesla vehicles cumulatively, 10 million active FSD subscriptions, 1 million Tesla bots delivered, and 1 million Robotaxis in operation.

Tesla board chair Robyn Denholm and director Kathleen Wilson-Thompson said retaining Musk is “fundamental to Tesla achieving these goals and becoming the most valuable company in history.” If successful, the plan would raise Musk’s Tesla stake from 13% to about 25%, further consolidating his control. It would also result in the CEO earning $900 billion in TSLA stock, allowing him to effectvely become a trillionaire. 

The proposal mirrors a 2018 compensation plan that was invalidated in Delaware court earlier this year in the way that it is focused on very aggressive targets and operational milestones. Tesla has since shifted its corporate registration to Texas, where challenges from potential activist shareholders are less of a risk.

Tesla’s SEC filing can be viewed below.

www-sec-gov-Archives-edgar-data-1318605-000110465925087598-tm252289-4_pre14a-htm… by Simon Alvarez

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