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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.

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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.

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“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.

 

For prompt updates, on-the-ground perspectives, and unique glimpses of SpaceX’s rocket recovery fleet check out our brand new LaunchPad and LandingZone newsletters!

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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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Elon Musk

Elon Musk announces disappointing Tesla Optimus update

In a post on X on March 31, Musk stated that Optimus 3 is mobile but requires some finishing touches before it is ready to be shown to the world. This update comes on the final day of the first quarter, a period when Tesla had previously signaled expectations for a Gen 3 reveal.

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5 hours ago

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March 31, 2026

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Credit: Tesla China

Elon Musk announced a disappointing update to the unveiling of Tesla Optimus and its third-generation iteration, missing a timeline it aimed to hit in the first quarter of the year.

Musk has confirmed that the highly anticipated Optimus Gen 3 humanoid robot is already walking around and operational, yet the public unveiling will face a short delay as the company applies final refinements.

In a post on X on March 31, Musk stated that Optimus 3 is mobile but requires some finishing touches before it is ready to be shown to the world. This update comes on the final day of the first quarter, a period when Tesla had previously signaled expectations for a Gen 3 reveal.

The announcement follows reports of Optimus Gen 3 appearing at the Tesla Diner in Los Angeles, where it was observed serving and moving about until sunset. Images and videos shared by observers captured the robot in action, highlighting its progress in real-world mobility.

Tesla had aimed to showcase the production intent version of Optimus Gen 3 during the first quarter of 2026, positioning it as a major step toward factory deployment and eventual commercial availability. Musk has described the robot as featuring advanced capabilities, including highly dexterous hands with significant degrees of freedom, powered by Tesla’s AI systems for complex tasks.

This minor postponement aligns with Tesla’s iterative approach to development. Earlier statements from Musk indicated that Gen 3 would represent the most advanced humanoid robot yet, designed primarily for internal factory use before scaling to external customers.

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Elon Musk’s $10 Trillion robot: Inside Tesla’s push to mass produce Optimus

Production timelines point toward low-volume output starting in the summer of 2026, with volume ramp-up targeted for 2027. The delay underscores the company’s commitment to quality over speed, ensuring the robot meets rigorous standards for safety and performance in practical environments.

Optimus represents a cornerstone of Tesla’s long-term vision beyond electric vehicles. Musk has repeatedly emphasized that successful humanoid robotics could transform industries by addressing labor shortages and enabling new forms of productivity.

Competitors in the space continue to advance their own platforms, yet Tesla’s vertical integration, from custom actuators to end-to-end AI training, positions Optimus as a potential leader. Community reactions on social media range from excitement over visible progress to impatience with shifting timelines, a familiar pattern in Tesla’s innovation journey.

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Investors and enthusiasts view Optimus as critical to Tesla’s valuation, potentially surpassing its automotive business in scale. With the robot already demonstrating walking and basic interactions, the finishing touches likely involve software polishing, hardware fine-tuning, and reliability enhancements.

Musk’s update suggests the reveal could arrive in the coming weeks or months, maintaining momentum toward broader deployment.

As Tesla pushes the boundaries of physical artificial intelligence, this latest development keeps Optimus in the spotlight. The company continues to prioritize rapid iteration while delivering on its promises to shareholders and customers. The robotics revolution at Tesla appears closer than ever, promising profound impacts on manufacturing, services, and daily life in the years ahead.

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Countdown: America is going back to the Moon and SpaceX holds the key to what comes after

NASA’s Artemis II launches Wednesday, sending humans near the Moon for the first time since 1972.

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9 hours ago

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March 31, 2026

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For the first time since Apollo 17 touched down on the lunar surface in December 1972, the United States is sending humans back toward the Moon. NASA’s Artemis II mission is set to launch as early as this week from Kennedy Space Center in Florida, carrying four astronauts on a 10-day journey around the Moon and back to Earth. It will not land anyone on the surface this time, but it is the first crewed flight in over half a century to travel beyond low Earth orbit, and it sets the stage for Elon Musk’s SpaceX missions to follow.

The mission uses NASA’s Space Launch System rocket and the Orion spacecraft, which will fly around the Moon before splashing down in the Pacific Ocean around April 10. For context, an uncrewed Artemis I flew the same path in 2022, proving the hardware worked. Artemis II now tests it with people aboard.

According to NASA’s official countdown blog, launch preparations are on track with an 80 percent chance of favorable weather. “Hey, let’s go to the moon!” Commander Wiseman told reporters upon arriving at Kennedy Space Center.

Source: NASA

Beyond Artemis II lies the lander question, and that is where SpaceX enters directly. In 2021, NASA awarded SpaceX a $2.89 billion contract to develop the Starship Human Landing System, a modified version of Starship designed to ferry astronauts from lunar orbit to the surface. The original plan called for SpaceX to deliver that lander for Artemis III, which was to be the first crewed lunar landing. Timing for Starship development, however, caused NASA to restructure the mission sequence entirely.

Before SpaceX’s Starship Human Landing System (HLS) can put anyone on the Moon, it has to solve a problem no rocket has demonstrated at scale, which is refueling in orbit. Because the Starship HLS requires approximately ten tanker launches worth of propellant loaded into a depot in low Earth orbit before it has enough fuel to reach the lunar surface, SpaceX plans to conduct this refueling process using its upgraded V3 Starship. And until that demonstration flies and succeeds, the Starship moon lander remains a question mark.

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SpaceX’s Starship V3 is almost ready and it will change space travel forever

In February 2026, NASA Administrator Jared Isaacman confirmed that Artemis III, now planned for mid-2027, and will instead test lunar landers in low Earth orbit, with the actual landing pushed to Artemis IV that’s targeted for 2028.

Musk responded to earlier criticism of SpaceX’s schedule by posting on X that his company is “moving like lightning compared to the rest of the space industry,” and added that “Starship will end up doing the whole Moon mission.” The contract competition was also reopened in October 2025 by then NASA chief Sean Duffy, who cited Starship’s delays and said the agency needed speed given China’s own stated goal of landing astronauts on the Moon by 2030.


Artemis came from the first Trump administration’s 2017 Space Policy Directive 1, which directed NASA to return humans to the Moon. The program picked up pace through the 2020s, with the Orion spacecraft and SLS taking years to develop at enormous costs. SpaceX entered the picture in 2021 as the chosen lander contractor, tying the commercial space sector into what had historically been an all government undertaking.

Whether SpaceX’s Starship ultimately carries astronauts to the lunar surface or shares that role with Blue Origin’s competing lander, this week’s Artemis II launch is the necessary first step. Getting four humans to the Moon’s vicinity and back safely is the proof of concept everything else depends on.

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Elon Musk debunks latest rumors about SpaceX IPO

Musk has swiftly put to rest circulating reports suggesting that SpaceX would exclude popular retail brokerages Robinhood and SoFi from its highly anticipated initial public offering. In a direct response posted on X on March 31, Musk stated simply, “These reports are false,” addressing widespread speculation fueled by a Reuters article.

Published

10 hours ago

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March 31, 2026

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(Credit: SpaceX)

Tesla and SpaceX CEO Elon Musk debunked the latest rumors about the space exploration company’s initial public offering (IPO), which has been the subject of a wide array of speculation over the last few weeks.

With SpaceX likely heading to Wall Street to become a publicly-traded stock in the coming months, there is a lot of speculation surrounding how it will happen, whether the company will potentially combine with Tesla, and more.

Tesla and SpaceX to merge in 2027, Wall Street analyst predicts

But the latest rumors have to do with where SpaceX will list the stock.

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Musk has swiftly put to rest circulating reports suggesting that SpaceX would exclude popular retail brokerages Robinhood and SoFi from its highly anticipated initial public offering.

In a direct response posted on X on March 31, Musk stated simply, “These reports are false,” addressing widespread speculation fueled by a Reuters article.

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The Reuters report, published March 30, claimed that Morgan Stanley’s E*Trade was in talks to lead the sale of SpaceX shares to small U.S. investors.

Sources indicated that Robinhood and SoFi, despite pitching for roles, faced potential exclusion from the retail allocation, with Fidelity also competing for a piece of the action. The story quickly spread across financial media, raising concerns among retail investors eager to participate in what could be one of the largest IPOs in history.

SpaceX has a reported valuation nearing $1.75 trillion, and Musk’s plan to allocate up to 30 percent of shares to individual investors — far above the typical 5-10% — had generated massive excitement.

Musk’s concise denial immediately calmed the narrative. The original X post quoting the rumor garnered significant engagement, with users expressing relief that everyday investors would not be sidelined.

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This episode reflects Musk’s hands-on approach to SpaceX’s public debut.

Earlier reporting revealed plans for an unusually large retail slice to leverage Musk’s dedicated fan base and stabilize post-IPO trading. SpaceX aims to file potentially as early as this period, building on momentum from its Starship program and Starlink growth.

The IPO could mark a transformative moment, potentially elevating Musk’s status further while democratizing access to a company long reserved for accredited investors and institutions.

The rumor’s quick debunking also revives debates about retail access in high-profile listings. Robinhood gained popularity during the 2021 meme-stock surge but faced criticism for past trading restrictions.

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SoFi has positioned itself as a modern financial platform for younger investors. Excluding them could have limited participation from tech-savvy retail traders who form a core part of Musk’s supporter base across Tesla and SpaceX.

While details remain fluid, Musk’s intervention reinforces commitment to broad accessibility. As preparations advance, investors await official filings. For now, the message is clear: rumors of restricted retail access were overstated, keeping the door open for widespread participation in SpaceX’s public chapter.

This development comes amid broader market enthusiasm for space and technology stocks. Musk’s transparency through X continues to shape public perception, distinguishing SpaceX’s path from traditional Wall Street norms. With retail allocation potentially reaching 30 percent, the IPO promises to be both commercially massive and culturally significant.

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