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Ex-SpaceX engineer leads Stratolaunch to major rocket engine test milestone
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.
The team made amazing progress this week! Check out the #PGAEngine preburner’s first hot-fire test at @NASAStennis. #NewUSEngine pic.twitter.com/kKTnf0bj1S
— Stratolaunch (@Stratolaunch) November 6, 2018
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.
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- A to-scale comparison of Falcon 1, Pegasus XL, MLV, and Falcon 9. (Teslarati/Stratolaunch/Wikipedia)
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- A render of Stratolaunch’s impressive PGA engine. Note the giant nozzle relative to the throat. (Stratolaunch)
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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- PGA’s first full-scale preburner seen during assembly. (Stratolaunch)
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- PGA’s first full-scale preburner seen during assembly. (Stratolaunch)
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- Jeff Thornburg stands in front of Stratolaunch’s NASA Stennis Space Center test stand. (Stratolaunch)
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- The PGA preburner seen after installation at Stennis. (Stratolaunch)
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- The control center. (Stratolaunch)
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- MLV is released from Stratolauncher. (Stratolaunch)
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- A concept video produced by Stratolaunch shows the Roc launching a Kraken rocket. (Stratolaunch, via Wired)
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Elon Musk
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.
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.”
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.”
This is before supervised FSD is approved in China. Limiting factor is production output in Shanghai.
— Elon Musk (@elonmusk) April 19, 2026
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.
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.
Elon Musk
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.
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
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.
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.
Tesla (NASDAQ: TSLA) is set to hold its Earnings Call for the first quarter of 2026 on Wednesday, and there are a lot of interesting things that are swirling around in terms of speculation from investors.
With the company’s executives, including CEO Elon Musk, answering a handful of questions that investors submit through the Say platform, fans want to know a lot of things about a lot of things.
These five questions come from Retail Investors, who are normal, everyday shareholders:
- When will we have the Optimus v3 reveal? When will Optimus production start, since we ended the Model S and Model X production earlier than mid-year? What’s the expected Optimus production rate exiting this year? What are the initial targeted skills?
- What milestones are you targeting for unsupervised FSD and Robotaxi expansion beyond Austin this year, and how will that drive recurring revenue?
- How will Hardware 3 cars reach Unsupervised Full Self-Driving?
- When do you expect Unsupervised Full Self-Driving to reach customer cars?
- When will Robotaxi expand past its current limited rollout?
Additionally, these are currently the three questions that are slated to be answered by Institutional Firms, which also answer a handful of questions during the call:
- Now that FSD has been approved in the Netherlands and is expected to launch across Europe this summer, can you discuss your Robotaxi strategy for the region?
- What enabled you to finish the AI5 tapeout early and were there any changes to the original vision? Last week, Elon said AI5 will go into Optimus and the Supercomputer, but one month ago said it would go into the Robotaxi. Has AI5 been dropped from the vehicle roadmap?
- Given the recent NHTSA incident filings, can you update us on the Robotaxi safety data? If safety validation remains the primary bottleneck, why not deploy thousands of vehicles to accelerate the removal of the safety driver?
The questions range through every current Tesla project, including FSD expansion and Optimus. However, many of the answers we will get will likely be repetitive answers we’ve heard in the past.
This is especially pertinent when the questions about when Unsupervised FSD will reach customer cars: we know Musk will say that it will happen this year. Is Tesla capable of that? Maybe. But a more transparent answer that is more revealing of a true timeline would be appreciated.
Hardware 3 owners are anxiously awaiting the arrival of FSD v14 Lite, which was promised to them last year for a release sometime this year.
The Earnings Call is set to take place on Wednesday at market close.
Ford CEO Farley says Tesla is not who to look at for EV expertise
SpaceX wins its first MARS contract but it comes with a catch
