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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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These Tesla, X, and xAI engineers were just poached by OpenAI

The news is the latest in an ongoing feud between Elon Musk and the Sam Altman-run firm OpenAI.

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2 hours ago

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July 8, 2025

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Credit: OpenAI | YouTube

OpenAI, the xAI competitor for which Elon Musk previously served as a boardmember and helped to co-found, has reportedly poached high-level engineers from Tesla, along with others from xAI, X, and still others.

On Tuesday, Wired reported that OpenAI hired four high-level engineers from Tesla, xAI, and X, as seen in an internal Slack message sent by co-founder Greg Brockman. The engineers include Tesla Vice President of Software Engineering David Lau, X and xAI’s head of infrastructure engineering Uday Ruddarraju, and fellow xAI infrastructure engineer Mike Dalton. The hiring spree also included Angela Fan, an AI researcher from Meta.

“We’re excited to welcome these new members to our scaling team,” said Hannah Wong, an OpenAI spokesperson. “Our approach is to continue building and bringing together world-class infrastructure, research, and product teams to accelerate our mission and deliver the benefits of AI to hundreds of millions of people.”

Lau has been in his position as Tesla’s VP of Software Engineering since 2017, after previously working for the company’s firmware, platforms, and system integration divisions.

“It has become incredibly clear to me that accelerating progress towards safe, well-aligned artificial general intelligence is the most rewarding mission I could imagine for the next chapter of my career,” Lau said in a statement to Wired.

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At xAI, Ruddarraju and Dalton both played a large role in developing the Colossus supercomputer, which is comprised of over 200,000 GPUs. One of the major ongoing projects at OpenAI is the company’s Stargate program,

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“Infrastructure is where research meets reality, and OpenAI has already demonstrated this successfully,” Ruddarraju told Wired in another statement. “Stargate, in particular, is an infrastructure moonshot that perfectly matches the ambitious, systems-level challenges I love taking on.”

Elon Musk is currently in the process of suing OpenAI for shifting toward a for-profit model, as well as for accepting an investment of billions of dollars from Microsoft. OpenAI retaliated with a counterlawsuit, in which it alleges that Musk is interfering with the company’s business and engaging in unfair competition practices.

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SpaceX share sale expected to back $400 billion valuation

The new SpaceX valuation would represent yet another record-high as far as privately-held companies in the U.S. go.

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2 hours ago

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July 8, 2025

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A new report this week suggests that Elon Musk-led rocket company SpaceX is considering an insider share sale that would value the company at $400 billion.

SpaceX is set to launch a primary fundraising round and sell a small number of new shares to investors, according to the report from Bloomberg, which cited people familiar with the matter who asked to remain anonymous due to the information not yet being public. Additionally, the company would sell shares from employees and early investors in a follow-up round, while the primary round would determine the price for the secondary round.

The valuation would represent the largest in history from a privately-owned company in the U.S., surpassing SpaceX’s previous record of $350 billion after a share buyback in December. Rivaling company valuations include ByteDance, the parent company of TikTok, as well as OpenAI.

Bloomberg went on to say that a SpaceX representative didn’t respond to a request for comment at the time of publishing. The publication also notes that the details of such a deal could still change, especially depending on interest from the insider sellers and share buyers.

READ MORE ON SPACEX: SpaceX to decommission Dragon spacecraft in response to Pres. Trump war of words with Elon Musk

SpaceX’s valuation comes from a few different key factors, especially including the continued expansion of the company’s Starlink satellite internet company. According to the report, Starlink accounts for over half of the company’s yearly revenue. Meanwhile, the company produced its 10 millionth Starlink kit last month.

The company also continues to develop its Starship reusable rocket program, despite the company experiencing an explosion of the rocket on the test stand in Texas last month.

The company has also launched payloads for a number of companies and government contracts. In recent weeks, SpaceX launched Axiom’s Ax-4 mission, sending four astronauts to the International Space Station (ISS) for a 14-day stay to work on around 60 scientific experiments. The mission was launched using the SpaceX Falcon 9 rocket and a new Crew Dragon capsule, while the research is expected to span a range of fields including biology, material and physical sciences, and demonstrations of specialized technology.

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Tesla Giga Texas continues to pile up with Cybercab castings

Tesla sure is gathering a lot of Cybercab components around the Giga Texas complex.

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7 hours ago

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July 8, 2025

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Credit: @JoeTegtmeyer/X

Tesla may be extremely tight-lipped about the new affordable models that it was expected to start producing in the first half of the year, but the company sure is gathering a lot of Cybercab castings around the Giga Texas complex. This is, at least, as per recent images taken of the facility. 

Cybercab castings galore

As per longtime drone operator Joe Tegtmeyer, who has been chronicling the developments around the Giga Texas complex for several years now, the electric vehicle maker seems to be gathering hundreds of Cybercab castings around the factory. 

Based on observations from industry watchers, the drone operator appears to have captured images of about 180 front and 180 rear Cybercab castings in his recent photos.

Considering the number of castings that were spotted around Giga Texas, it would appear that Tesla may indeed be preparing for the vehicle’s start of trial production sometime later this year. Interestingly enough, large numbers of Cybercab castings have been spotted around the Giga Texas complex in the past few months.

Cybercab production

The Cybercab is expected to be Tesla’s first vehicle that will adopt the company’s “unboxed” process. As per Tesla’s previous update letters, volume production of the Cybercab should start in 2026. So far, prototypes of the Cybercab have been spotted testing around Giga Texas, and expectations are high that the vehicle’s initial trial production should start this year. 

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With the start of Tesla’s dedicated Robotaxi service around Austin, it might only be a matter of time before the Cybercab starts being tested on public roads as well. When this happens, it would be very difficult to deny the fact that Tesla really does have a safe, working autonomous driving system, and it has the perfect vehicle for it, too.

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