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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 got a brash response from Ryanair CEO Michael O’Leary, who said in a press conference on Wednesday afternoon that the Tesla frontman’s criticism of the airline not equipping Starlink has increased bookings for the next few months.
The two have had a continuing feud over the past several weeks after Musk criticized the airline for not using Starlink for its flights, which would enable fast, free, and reliable Wi-Fi on its aircraft.
Tesla CEO Elon Musk trolls budget airline after it refuses Starlink on its planes
Musk said earlier this week that he was entertaining the idea of purchasing Ryanair and putting someone named Ryan in charge, which would oust O’Leary from his position.
However, the barbs continued today, as O’Leary held a press conference, aiming to dispel any beliefs about Starlink and its use case for Ryanair flights, which are typically short in length.
O’Leary said in the press conference today:
“The Starlink people believe that 90% of our passengers would happily pay for wifi access. Our experience tells us less than 10% would pay; He (Elon) called me a retar*ed twat. He would have to join the back of a very, very long queue of people that already think I’m a retar*ed twat, including my four teenage children.”
He then went on to say that, due to Musk’s publicity, bookings for Ryanair flights have increased over the past few days, up 2 to 3 percent:
“But we do want to thank him for the wonderful boost in publicity. Our bookings are up 2-3% in the last few days. So thank you to Mr. Musk, but he’s wrong on the fuel drag. Non-European citizens cannot own a majority of European airlines, but if he wants to invest in Ryanair, we think it would be a very good investment.”
O’Leary didn’t end there, as he called Musk’s social media platform X a “cesspit,” and said he has no concern over becoming a member of it. However, Ryanair has been very active on X for several years, gaining notoriety for being comical and lighthearted.
🚨 Ryanair CEO’s comments on X and Starlink today at the planned presser.
Strange comments here, it just feels like it’s time to end all this crap https://t.co/NYeG95bM82
— TESLARATI (@Teslarati) January 21, 2026
The public spat between the two has definitely benefited Ryanair, and many are calling for it to end, especially those who support Musk, as they see it as a distraction.
Nevertheless, it is likely going to end with no real movement either way, and is more than likely just a bit of hilarity between the two parties that will end in the coming days.
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Tesla CEO Elon Musk outlines expectations for Cybercab production
“…initial production is always very slow and follows an S-curve. The speed of production ramp is inversely proportionate to how many new parts and steps there are. For Cybercab and Optimus, almost everything is new, so the early production rate will be agonizingly slow, but eventually end up being insanely fast.”
Tesla CEO Elon Musk outlined expectations for Cybercab production as the vehicle is officially set to start rolling off manufacturing lines at the company’s Giga Texas factory in less than 100 days.
Cybercab is specifically designed and catered to Tesla’s self-driving platform and Robotaxi ride-hailing service. The company has been pushing hard to meet its self-set expectations for rolling out an effective self-driving suite, and with the Cybercab coming in under 100 days, it now needs to push for Unsupervised Self-Driving in the same time frame.
Tesla CEO Elon Musk confirms Robotaxi is set to go unsupervised
This is especially pertinent because the Cybercab is expected to be built without a steering wheel or pedals, and although some executives have said they would build the car with those things if it were necessary.
However, Musk has maintained that the Cybercab will not have either of those things: it will have two seats and a screen, and that’s it.
With production scheduled for less than 100 days, Musk broke down what people should expect from the initial manufacturing phases, being cautiously optimistic about what the early stages will likely entail:
“…initial production is always very slow and follows an S-curve. The speed of production ramp is inversely proportionate to how many new parts and steps there are. For Cybercab and Optimus, almost everything is new, so the early production rate will be agonizingly slow, but eventually end up being insanely fast.”
Musk knows better than most about the challenges of ramping up production of vehicles. With the Model 3, Musk routinely refers to it as “production hell.” The Cybertruck, because of its polarizing design and stainless steel exterior, also presented challenges to Tesla.
With the important caveat that initial production is always very slow and follows an S-curve.
The speed of the production ramp is inversely proportionate to how many new parts and steps there are.
For Cybercab and Optimus, almost everything is new, so the early production…
— Elon Musk (@elonmusk) January 20, 2026
The Cybercab definitely presents an easier production process for Tesla, and the company plans to build millions of units per year.
Musk said back in October 2024:
“We’re aiming for at least 2 million units a year of Cybercab. That will be in more than one factory, but I think it’s at least 2 million units a year, maybe 4 million ultimately.”
When April comes, we will find out exactly how things will move forward with Cybercab production.
Tesla has revealed an awesome Model 3 and Model Y incentive to help consumers make the jump to one of its affordable mass-market vehicles, but it’s ending soon.
Tesla is offering one free upgrade on eligible inventory of the Model 3 and Model Y until February 2.
This would help buyers receive the most expensive paid option on the vehicle at no additional cost, meaning white interior or a more premium paint option will be free of charge if you take delivery on or before February 2.
Tesla states on its website for the offer:
“Only for limited inventory while supplies last. Price displayed on inventory listings already deducts the cost of the free option.”
Tesla says its one free upgrade offer on eligible U.S. inventory for the Model 3 and Model Y ends February 2.
With this incentive, buyers receive the most expensive paid option on the vehicle at no additional cost (up to $2k in savings). pic.twitter.com/IhoiURrsDI
— Sawyer Merritt (@SawyerMerritt) January 21, 2026
This latest incentive is just another advantage Tesla has by selling its vehicles directly and not using some sort of dealership model that relies on approvals from higher-ups. It is important to note that these programs are offered to help stimulate demand and push vehicles into customers’ hands.
It is not the only incentive Tesla is currently offering, either. In fact, there is a much larger incentive program that Tesla is working on, and it has to do with Full Self-Driving transfers, which could result in even more sales for the company through Q1.
Tesla is ending its FSD Transfer program on March 31, as it plans to transition to a Subscription-only basis with the self-driving suite for anyone who has not already purchased it outright.
This could help drive some on-the-fence buyers to new vehicles, but it remains to be seen. Given the timing of the program’s demise, it appears Tesla is hoping to use it to add additional sales and bolster a strong Q1 2026.
Interior and exterior paint colors can add up to $2,000 if you choose the most premium Ultra Red body color, or an additional $1,000 for the Black and White interior option. The discount, while small, could help get someone their preferred design configuration, instead of settling for something that is not quite what they want.
Elon Musk gets brash response from Ryanair CEO, who thanks him for booking increase
Tesla CEO Elon Musk outlines expectations for Cybercab production
