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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 CEO Elon Musk teases Optimus job that’s straight out of Robocop

“If somebody’s committed a crime, we might be able to provide a more humane form of containment of future crime. You now get a free Optimus, and it’s just going to follow you around and stop you from doing crime.”

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3 hours ago

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November 11, 2025

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Credit: @heydave7/X

Tesla CEO Elon Musk teased a potential job for the company’s Optimus robot last week that is straight out of the movie “Robocop.”

“Robocop” aimed to show a futuristic look at law enforcement in a Sci-fi thriller that was among the first iterations of how robots could be used for police work.

The 1987 film showcased an injured cop turning into an armed cyborg, and although Tesla’s Optimus won’t be a human-robot hybrid, Musk’s idea for the humanoid project is similar.

Musk said last week at the Annual Shareholder Meeting, where shareholders voted to approve his $1 trillion compensation plan, that Optimus could be the future of law enforcement, nearly revolutionizing the way criminals are prosecuted.

He hinted that Optimus could actually be used as a chaperone of sorts, arguing that it was a “more humane form of containment of future crime.” Musk said:

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“If somebody’s committed a crime, we might be able to provide a more humane form of containment of future crime. You now get a free Optimus, and it’s just going to follow you around and stop you from doing crime. Other than that, you get to do anything; it’s just going to stop you from committing crime. That’s really it. You don’t have to put people in prisons and stuff. It’s pretty wild to think of all the possibilities, but I think it’s clearly the future.”

Musk’s overall idea for Optimus is to change the way people are able to exist, from those law-abiding citizens to others who have their run-ins with the law. Instead, the Tesla CEO believes there could be a different way to handle everything, including punishment.

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It was not the only thing that Musk indicated could be changed significantly by the presence of humanoid robots, as he also said a universal basic income could be established with the help of products like Optimus.

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Elon Musk teases huge merger: ‘Trending towards convergence’

“My companies are, surprisingly in some ways, trending towards convergence.”

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5 hours ago

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November 11, 2025

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Elon Musk recently amplified the thoughts of Morgan Stanley analyst Adam Jonas, who had insight into the “Muskonomy” of his potentially interconnected ventures, something that was proposed at the recent Tesla Shareholder Meeting with xAI.

Musk’s words indicate a potential strategic fusion that could serve as a blueprint for future innovation–but it is dependent on a conglomeration between the many entities the CEO serves.

As Tesla grapples with scaling Optimus and preparing for its imminent production and the development of the Full Self-Driving suite, xAI’s computational edge could provide leverage for the millions of miles of data the company accumulates, providing a more stable and accurate development strategy for the autonomous and AI efforts it has put its chips all in on.

After Tesla Shareholders voted to deny Tesla and xAI’s potential financial partnership through an investment, Jonas said it was an issue that would have to be revisited due to its importance.

xAI has the opportunity to provide an incredible strategic and financial bolstering to Tesla, especially with how important a role data plays in the development of the company’s biggest products.

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Jonas wrote in a note to investors:

“They’re gonna have to revisit this. We don’t think investors understand just how important xAI is to Tesla and the broader Muskonomy. Tesla’s relationship with xAI (financially and strategically) is deterministic to the long-term success of Tesla due in part to the natural synergies of data, software, hardware, and manufacturing in recursive loops. The values (and value systems) of both Tesla and xAI are endowed by the values of their shared creator. We believe this co-determination becomes more obvious in the next phases of physical AI/ autonomy for Tesla in the year ahead.”

Musk said, in response to Jonas’ note, that his companies are “surprisingly in some ways, trending toward convergence.”

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Mergers and shared ecosystems between companies are not new moves out of Musk’s playbook, as it has been done in the past, especially with Tesla acquiring other entities.

It did it with SolarCity in 2016 and with Maxwell Technologies in 2019. Investments between Musk companies have occurred before, too, as SpaceX dumped $2 billion into xAI last July.

He’s also said on several occasions that he could eventually bring everything together into some sort of single entity. In July 2024, he said:

“I’m not opposed to the idea in principle, but I’m not sure there is a pragmatic or legal way to merge them. There is also value in equity incentives of people at the companies being tied to that company’s accomplishments.”

This point is especially relevant now with Musk’s recently approved compensation package.

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He also said in June, during an interview with CNBC , that “It’s not out of the question” for xAI to merge with Tesla, but it would have to be approved by shareholders. Just a few days later, he said he would not support xAI merging with Tesla; however, he put it in investors’ hands.

It’s more than just a deal; it’s symbiotic. Musk being at the helm of various companies, all intertwined with one another, helps foster recursive innovation. Despite these advantages, there are still a handful of things to consider, especially from a regulatory perspective.

However, it is not competition; it’s convergence. In Musk’s universe, especially from a business sense, mergers are not endpoints, but instead launchpads for ambitions that aim to take each company from Earth to lands beyond our atmosphere.

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Tesla makes Elon Musk’s new compensation package official

This is an important thing to note, as much of the media coverage regarding Musk’s pay package seems to indicate that the company and the shareholders are simply giving the CEO the money. He has to come through on each of these tranches to unlock the $1 trillion.

Published

7 hours ago

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November 11, 2025

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Credit: @JoeTegtmeyer/X

Tesla has made CEO Elon Musk’s new compensation package official, as it filed a Form 4 with the Securities and Exchange Commission (SEC) on Monday.

The package officially gives Musk the opportunity to acquire over 423 million shares of Tesla stock (NASDAQ: TSLA), dependent on his ability to achieve twelve performance-based tranches that will bring growth to the company and its shareholders.

Tesla (TSLA) shareholders officially approve Elon Musk’s 2025 performance award

Musk’s new compensation package was approved by investors last Thursday at the company’s Annual Shareholder Meeting, as over 75 percent of voters supported the CEO’s new plan, which could be valued at over $1 trillion if he is able to come through on all twelve tranches.

The twelve tranches include growth goals related to vehicle deliveries, the Optimus humanoid robot project, and Tesla’s valuation. If Musk is able to achieve each tranche, he would help Tesla achieve an over $8 trillion market cap.

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The 12 tranches include:

  1. $2 trillion market cap + Deliver 20 million Tesla vehicles cumulatively
  2. $2.5 trillion market cap + Reach 10 million active Full Self-Driving (FSD) subscriptions
  3. $3 trillion market cap + Deliver 1 million Optimus humanoid robots
  4. $3.5 trillion market cap + Operate 1 million Robotaxis commercially
  5. $4 trillion market cap + Hit $50 billion in adjusted EBITDA (earnings before interest, taxes, etc.)
  6. $4.5 trillion market cap + Hit $80 billion in adjusted EBITDA
  7. $5 trillion market cap + Hit $130 billion in adjusted EBITDA
  8. $5.5 trillion market cap + Hit $210 billion in adjusted EBITDA
  9. $6 trillion market cap + Hit $300 billion in adjusted EBITDA
  10. $6.5 trillion market cap + Hit $400 billion in adjusted EBITDA
  11. $7.5 trillion market cap + Hit $400 billion in adjusted EBITDA for four straight quarters in a row
  12. $8.5 trillion market cap + Hit $400 billion in adjusted EBITDA for four straight quarters in a row

Achieving the twelve levels of the new compensation package would also give Musk what he’s really after: a larger ownership share in Tesla, which would help him achieve more control, something he feels is necessary for the rollout of the Optimus robot “army.”

Musk does not earn a dime if he does not achieve any of the tranches above.

This is an important thing to note, as much of the media coverage regarding Musk’s pay package seems to indicate that the company and the shareholders are simply giving the CEO the money. He has to come through on each of these tranches to unlock the $1 trillion.

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