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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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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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ARK’s SpaceX IPO Guide makes a compelling case on why $1.75T may not be the ceiling

ARK Invest breaks down six reasons SpaceX’s $1.75 trillion IPO valuation may be justified.

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ARK Invest, which holds SpaceX as its largest Venture Fund position at 17% of net assets, has published a detailed investor guide to why a SpaceX IPO may be grounded in a $1.75 trillion target valuation.

The financial case starts with Starlink, SpaceX’s satellite internet constellation, which has surpassed 10 million active subscribers globally as of early 2026, with 2026 revenue projected to exceed $20 billion. ARK’s research puts the total satellite connectivity market opportunity at roughly $160 billion annually at scale, and Starlink is adding customers faster than any telecom network in history. That growth alone would justify a substantial valuation.

Additionally,  ARK notes that SpaceX has reduced the cost per kilogram to orbit from roughly $15,600 in 2008 to under $1,000 today through reusable Falcon 9 hardware. A fully operational Starship targeting sub-$100 per kilogram would represent a significant cost decline and open markets that do not currently exist. SpaceX executed a staggering 165 missions in 2025 and now accounts for approximately 85% of all global orbital launches. That infrastructure position took decades to build and would be nearly impossible to replicate at comparable cost.

SpaceX officially acquires xAI, merging rockets with AI expertise

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The February 2026 merger with xAI added a layer to the valuation that straightforward financial models struggle to capture. ARK argues that at sub-$100 launch costs, orbital data centers could deliver compute roughly 25% cheaper than ground-based alternatives, without power grid delays, permitting friction, or land constraints. Musk has stated a goal of deploying 100 gigawatts of AI computing capacity per year from orbit.

The $1.75 trillion figure itself is not a conventional earnings multiple. At roughly 95x trailing revenue, it prices in Starlink’s adoption curve, Starship’s cost trajectory, and the orbital compute thesis together. The public S-1 prospectus, due at least 15 days before the June roadshow, will give investors their first complete look at the financials to test those assumptions. ARK’s position is that the track record earns the benefit of the doubt. Fully reusable rockets were considered unrealistic for years. Starlink was considered financially unviable. Both happened on timelines that surprised skeptics.

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Ford CEO Farley says Tesla is not who to look at for EV expertise

Interestingly, Farley has been one of the most hellbent CEOs in terms of a legacy automaker standpoint to push the EV effort. It did not go according to plan, as Ford took a $19.5 billion charge and retreated from its EV push in late 2025.

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Ford CEO Jim Farley said in a recent podcast interview that Tesla is not who Americans should look at to beat Chinese carmakers.

The comments have sparked quite a bit of outrage from Tesla fans on X, the social media platform owned by Elon Musk.

Farley said that Chinese automakers are better examples of how to beat competitors. He said (via the Rapid Response Podcast):

“If you’re an American and you want us to beat the Chinese in the car business, you’re all going to want to pay attention, not necessarily to Tesla. Nothing against Tesla—they’ve been doing great—but they really don’t have an updated vehicle. The best in the business for us, cost-wise and competition-wise, supply chain, manufacturing expertise, and the I.P. in the vehicle, was really BYD. In this next cycle of EV customers in the U.S., they want pickups and utilities and all these different body styles. But they want them at $30,000, not $50,000. Like the first inning, they want them affordably.”

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Despite Farley’s synopsis, it is worth mentioning that Tesla had the best-selling passenger vehicle in the world last year, and in China in March, as the Model Y continued its global dominance over other vehicles.

Musk responded to Farley’s comments by stating:

“This is before Supervised FSD is approved in China. Limiting factor is production output in Shanghai.”

Interestingly, Farley has been one of the most hellbent CEOs in terms of a legacy automaker standpoint to push the EV effort. It did not go according to plan, as Ford took a $19.5 billion charge and retreated from its EV push in late 2025.

Ford cancels all-electric F-150 Lightning, announces $19.5 billion in charges

Instead, Ford is “doubling down on its affordable” EVs and said it would pivot from its previous plans.

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Reaction from Tesla fans was pretty much how you would expect. Many said they have lost a lot of respect for Farley after his comments; others believe he is the last CEO anyone should be taking advice on EVs from.

Nevertheless, Farley’s plans are bold and brash; many consider Tesla the most ideal company to replicate EV efforts from. It will be interesting to see if Ford can rebound from this big adjustment, and hopefully, Farley’s plans to replicate efforts from BYD work out the way he hopes.

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SpaceX wins its first MARS contract but it comes with a catch

NASA awarded SpaceX a $175 million Mars rover contract while the White House proposes cutting the mission.

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NASA just signed a $175.7 million contract with SpaceX to launch a Mars rover that the White House is simultaneously trying to defund. The contract, awarded on April 16, 2026, tasks SpaceX’s Falcon Heavy with launching the European Space Agency’s (ESA) Rosalind Franklin rover from Kennedy Space Center in Florida, no earlier than late 2028. It would mark the first time SpaceX has ever sent a payload to Mars.

Under NASA’s Rosalind Franklin Support and Augmentation project, known as ROSA, the agency is providing braking engines for the rover’s descent stage, radioisotope heater units that use decaying plutonium to keep the rover warm on the Martian surface, additional electronics, and a mass spectrometer instrument, as noted by SpaceNews.

Those nuclear heating units are the reason an American rocket was required at all. U.S. export controls on radioisotope technology mean any payload carrying them must launch on a domestic vehicle, which narrowed the field to SpaceX and United Launch Alliance. Falcon Heavy’s pricing made it the practical choice.

SpaceX is quietly becoming the U.S. Military’s only reliable rocket

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Falcon Heavy debuted in February 2018 and has 11 launches to its record. The rocket has not flown since October 2024, when it sent NASA’s Europa Clipper toward Jupiter. The three-core design, built from modified Falcon 9 first stages, gives it the lift capacity needed for deep space planetary missions that a single Falcon 9 cannot reach.

The Rosalind Franklin rover has been sitting in storage in Europe for years. It was originally due to launch in 2022 as a joint mission with Russia, but Russia’s invasion of Ukraine ended that partnership, leaving the rover built but stranded without a launch vehicle or landing hardware. NASA stepped back in through a 2024 agreement with ESA to rescue the mission. The rover is designed to drill up to two meters below the Martian surface in search of evidence of past life, a science objective no previous mission has attempted at that depth.

The contradiction at the center of this story is hard to ignore. The White House’s fiscal year 2027 budget proposal included no funding for ROSA and did not mention the mission at all in the detailed congressional justification document released April 3.

Musk has long argued that reaching Mars is not optional. “We don’t want to be one of those single planet species, we want to be a multi-planet species.” Whether this particular mission survives Washington’s budget fight, the Falcon Heavy contract means SpaceX is now formally on record as the rocket that could get humanity’s next Mars science mission off the ground.

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The timing of this contract carries extra weight given that SpaceX filed confidentially with the SEC in early April and is targeting an IPO roadshow in the week of June 8. It would be the largest public offering in history.

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