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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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Investor's Corner

Tesla price target boost from its biggest bear is 95% below its current level

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1 hour ago

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January 7, 2026

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Credit: Tesla China

Tesla stock (NASDAQ: TSLA) just got a price target boost from its biggest bear, Gordon Johnson of GLJ Research, who raised his expected trading level to one that is 95 percent lower than its current trading level.

Johnson pushed his Tesla price target from $19.05 to $25.28 on Wednesday, while maintaining the ‘Sell’ rating that has been present on the stock for a long time. GLJ has largely been recognized as the biggest skeptic of Elon Musk’s company, being particularly critical of the automotive side of things.

Tesla has routinely been called out by Johnson for negative delivery growth, what he calls “weakening demand,” and price cuts that have occurred in past years, all pointing to them as desperate measures to sell its cars.

Johnson has also said that Tesla is extremely overvalued and is too reliant on regulatory credits for profitability. Other analysts on the bullish side recognize Tesla as a company that is bigger than just its automotive side.

Many believe it is a leader in autonomous driving, like Dan Ives of Wedbush, who believes Tesla will have a widely successful 2026, especially if it can come through on its targets and schedules for Robotaxi and Cybercab.

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Justifying the price target this week, Johnson said that the revised valuation is based on “reality rather than narrative.” Tesla has been noted by other analysts and financial experts as a stock that trades on narrative, something Johnson obviously disagrees with.

Dan Nathan, a notorious skeptic of the stock, turned bullish late last year, recognizing the company’s shares trade on “technicals and sentiment.” He said, “From a trading perspective, it looks very interesting.”

Tesla bear turns bullish for two reasons as stock continues boost

Johnson has remained very consistent with this sentiment regarding Tesla and his beliefs regarding its true valuation, and has never shied away from putting his true thoughts out there.

Tesla shares closed at $431.40 today, about 95 percent above where Johnson’s new price target lies.

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I subscribed to Tesla Full Self-Driving after four free months: here’s why

It has been incredibly valuable to me, and that is what my main factor was in considering whether to subscribe or not. It has made driving much less stressful and much more enjoyable.

Published

11 hours ago

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January 7, 2026

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Credit: Teslarati

I have been lucky enough to experience Tesla Full Self-Driving for the entire duration of my ownership experience for free — for four months, I have not had to pay for what I feel is the best semi-autonomous driving suite on the market.

Today, my free trial finally ran out, and I had two choices: I could go without it for a period until I felt like I absolutely needed it, or I could subscribe to it, pay $99 per month, and continue to experience the future of passenger transportation.

I chose the latter, here’s why.

Tesla Full Self-Driving Takes the Stress Out of Driving

There are a handful of driving situations that I don’t really enjoy, and I think we all have certain situations that we would just rather not encounter. This is not to say that I won’t ever experience them as someone who has driven a car for 15 years (it feels weird saying that).

I don’t love to drive in cities; I really don’t like driving on I-695 on my way to Baltimore, and I truly hate parallel parking. All three things I can do and have done, all three within the past few weeks, too.

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However, if I can avoid them, I will, and Tesla Full Self-Driving does that for me.

Tesla Full Self-Driving Eliminates the Monotony

I drive to my alma mater, Penn State University, frequently in the Winter as I am a season ticket holder to Wrestling and have been for 16 years now.

The drive to State College is over two hours and over 100 miles in total, and the vast majority of it is boring as I travel on Rt 322, which is straight, and there is a lot of nature to look at on the way.

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I am willing to let the car drive me on that ride, especially considering it is usually very low traffic, and the vast majority of it is spent on the highway.

The drive, along with several others, is simply a boring ride, where I’d much rather be looking out the windshield and windows at the mountains. I still pay attention, but having the car perform the turns and speed control makes the drive more enjoyable.

Tesla Full Self-Driving Makes Navigating Easier

Other than the local routes that I routinely travel and know like the back of my hand, I’ve really enjoyed Full Self-Driving’s ability to get me to places — specifically new ones — without me having to constantly check back at the Navigation.

Admittedly, I’ve had some qualms with the Nav, especially with some routing and the lack of ability to choose a specific route after starting a drive. For example, it takes a very interesting route to my local Supercharger, one that nobody local to my area would consider.

But there are many times I will go to a new palce and I’m not exactly sure where to go or how to get there. The Navigation, of course, helps with that. However, it is really a luxury to have my car do it for me.

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To Conclude

There was no doubt in my mind that when my Full Self-Driving trial was up, I’d be subscribing. It was really a no-brainer. I am more than aware that Full Self-Driving is far from perfect, but it is, without any doubt, the best thing about my Tesla, to me.

It has been incredibly valuable to me, and that is what my main factor was in considering whether to subscribe or not. It has made driving much less stressful and much more enjoyable.

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Tesla Diner becomes latest target of gloom and doom narrative

Published

14 hours ago

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January 7, 2026

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tesla diner
Credit: Tesla

The Tesla Diner has been subject to many points of criticism since its launch in mid-2025, and skeptics and disbelievers claim the company’s latest novel concept is on its way down, but there’s a lot of evidence to state that is not the case.

The piece cites anecdotal evidence like empty parking lots, more staff than customers during a December visit, removed novelty items, like Optimus robot popcorn service and certain menu items, the departure of celebrity chef Eric Greenspan in November 2025, slow service, high prices, and a shift in recent Google/Yelp reviews toward disappointment.

The piece frames this as part of broader Tesla struggles, including sales figures and Elon Musk’s polarizing image, calling it a failed branding exercise rather than a sustainable restaurant.

This narrative is overstated and sensationalized, and is a good representation of coverage on Tesla by today’s media.

Novelty Fade is Normal, Not Failure

Any hyped launch, especially a unique Tesla-branded destination blending dining, Supercharging, and a drive-in theater, naturally sees initial crowds taper off after the “Instagram effect” wears down.

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Tesla makes major change at Supercharger Diner amid epic demand

This is common for experiential spots in Los Angeles, especially pop-up attractions or celebrity-backed venues. The article admits early success with massive lines and social media buzz, but treats the return to normal operations as “dying down.”

In reality, this stabilization is a healthy sign of transitioning from hype-driven traffic to steady patronage.

Actual Performance Metrics Contradict “Ghost Town” Claims

  • In Q4 2025, the Diner generated over $1 million in revenue, exceeding the average McDonald’s location
  • It sold over 30,000 burgers and 83,000 fries in that quarter alone. These figures indicate a strong ongoing business, especially for a single-location prototype focused on enhancing Supercharger experiences rather than competing as a mass-market chain

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Conflicting On-the-Ground Reports

While the article, and other similar pieces, describe a half-full parking lot and sparse customers during specific off-peak visits, other recent accounts push back:

  • A January 2026 X post noted 50 of 80 Supercharger stalls were busy at 11 a.m., calling it “the busiest diner in Hollywood by close to an order of magnitude

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  • Reddit discussions around the same time describe it as not empty when locals drive by regularly, with some calling the empty narrative “disingenuous anti-Tesla slop.”

Bottom Line

The Tesla Diner, admittedly, is not the nonstop circus it was at launch–that was never sustainable or intended. But, it’s far from “dying” or an “empty pit stop.”

It functions as a successful prototype: boosting Supercharger usage, generating solid revenue, and serving as a branded amenity in the high-traffic EV market of Los Angeles.

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