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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

Tesla Full Self-Driving set to get an awesome new feature, Elon Musk says

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6 hours ago

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February 21, 2026

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Credit: Teslarati

Tesla Full Self-Driving is set to get an awesome new feature in the near future, CEO Elon Musk confirmed on X.

Full Self-Driving is the company’s semi-autonomous driving program, which is among the best available to the general public. It still relies on the driver to ultimately remain in control and pay attention, but it truly does make traveling less stressful and easier.

However, Tesla still continuously refines the software through Over-the-Air updates, which are meant to resolve shortcomings in the performance of the FSD suite. Generally, Tesla does a great job of this, but some updates are definitely regressions, at least with some of the features.

Tesla Cybertruck owner credits FSD for saving life after freeway medical emergency

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Tesla and Musk are always trying to improve the suite’s performance by fixing features that are presently available, but they also try to add new things that would be beneficial to owners. One of those things, which is coming soon, is giving the driver the ability to prompt FSD with voice demands.

For example, asking the car to park close to the front door of your destination, or further away in an empty portion of the parking lot, would be an extremely beneficial feature. Adjusting navigation is possible through Grok integration, but it is not always effective.

Musk confirmed that voice prompts for FSD would be possible:

Tesla Full Self-Driving is a really great thing, but it definitely has its shortcomings. Navigation is among the biggest complaints that owners have, and it is easily my biggest frustration with using it. Some of the routes it chooses to take are truly mind-boggling.

Another thing it has had issues with is being situated in the correct lane at confusing intersections or even managing to properly navigate through local traffic signs. For example, in Pennsylvania, there are a lot of stop signs with “Except Right Turn” signs directly under.

This gives those turning right at a stop sign the opportunity to travel through it. FSD has had issues with this on several occasions.

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Parking preferences would be highly beneficial and something that could be resolved with this voice prompt program. Grocery stores are full of carts not taken back by customers, and many people choose to park far away. Advising FSD of this preference would be a great advantage to owners.

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Cybertruck

Elon Musk clarifies Tesla Cybertruck ’10 day’ comment, fans respond

Some are arguing that the decision to confirm a price hike in ten days is sort of counterproductive, especially considering it is based on demand. Giving consumers a timeline of just ten days to make a big purchase like a pickup truck for $60,000, and basically stating the price will go up, will only push people to make a reservation.

Published

8 hours ago

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February 21, 2026

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Credit: Tesla

Elon Musk has clarified what he meant by his comment on X yesterday that seemed to indicate that Tesla would either do away with the new All-Wheel-Drive configuration of the Cybertruck or adjust the price.

The response was cryptic as nobody truly knew what Musk’s plans were for the newest Tesla Cybertruck trim level. We now have that answer, and fans of the company are responding in a polarizing fashion.

On Thursday night, Tesla launched the Cybertruck All-Wheel-Drive, priced competitively at $59,990. It was a vast improvement from the Rear-Wheel-Drive configuration Tesla launched last year at a similar price point, which was eventually cancelled just a few months later due to low demand.

Tesla launches new Cybertruck trim with more features than ever for a low price

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However, Musk said early on Friday, “just for 10 days,” the truck would either be available or priced at $59,990. We can now confirm Tesla will adjust the price based on more recent comments from the CEO.

Musk said the price will fluctuate, but it “depends on how much demand we see at this price level.”

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Some are defending the decision, stating that it is simply logical to see how the Cybertruck sells at this price and adjust accordingly.

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Others, not so much.

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Some are arguing that the decision to confirm a price hike in ten days is sort of counterproductive, especially considering it is based on demand. Giving consumers a timeline of just ten days to make a big purchase like a pickup truck for $60,000, and basically stating the price will go up, will only push people to make a reservation.

Demand will look strong because people want to lock in this price. The price will inevitably go up, and demand for the trim will likely fall a bit because of the increased cost.

Many are arguing Musk should have kept this detail internal, but transparency is a good policy to have. It is a polarizing move to confirm a price increase in just a week-and-a-half, but the community is obviously split on how to feel.

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Cybertruck

Tesla Cybertruck’s newest trim will undergo massive change in ten days, Musk says

It appears as if the new All-Wheel-Drive trim of Cybertruck won’t be around for too long, however. Elon Musk revealed this morning that it will be around “only for the next 10 days.”

Published

1 day ago

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February 20, 2026

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(Credit: Tesla)

Tesla’s new Cybertruck trim has already gotten the axe from CEO Elon Musk, who said the All-Wheel-Drive configuration of the all-electric pickup will only be available “for the next ten days.”

Musk could mean the price, which is $59,990, or the availability of the trim altogether.

Last night, Tesla launched the All-Wheel-Drive configuration of the Cybertruck, a pickup that comes in at less than $60,000 and features a competitive range and features that are not far off from the offerings of the premium trim.

Tesla launches new Cybertruck trim with more features than ever for a low price

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It was a nice surprise from Tesla, considering that last year, it offered a Rear-Wheel-Drive trim of the Cybertruck that only lasted a few months. It had extremely underwhelming demand because it was only $10,000 cheaper than the next trim level up, and it was missing a significant number of premium features.

Simply put, it was not worth the money. Tesla killed the RWD Cybertruck just a few months after offering it.

With the news that Tesla was offering this All-Wheel-Drive configuration of the Cybertruck, many fans and consumers were encouraged. The Cybertruck has been an underwhelming seller, and this seemed to be a lot of truck for the price when looking at its features:

  • Dual Motor AWD w/ est. 325 mi of range
  • Powered tonneau cover
  • Bed outlets (2x 120V + 1x 240V) & Powershare capability
  • Coil springs w/ adaptive damping
  • Heated first-row seats w/ textile material that is easy to clean
  • Steer-by-wire & Four Wheel Steering
  • 6’ x 4’ composite bed
  • Towing capacity of up to 7,500 lbs
  • Powered frunk

It appears as if this trim of Cybertruck won’t be around for too long, however. Musk revealed this morning that it will be around “only for the next 10 days.”

Musk could mean the price of the truck and not necessarily the ability to order it. However, most are taking it as a cancellation.

If it is, in fact, a short-term availability decision, it is baffling, especially as Tesla fans and analysts claim that metrics like quarterly deliveries are no longer important. This seems like a way to boost sales short-term, and if so many people are encouraged about this offering, why would it be kept around for such a short period of time?

Some are even considering the potential that Tesla axes the Cybertruck program as a whole. Although Musk said during the recent Q4 Earnings Call that Cybertruck would still be produced, the end of the Model S and Model X programs indicates Tesla might be prepared to do away with any low-volume vehicles that do not contribute to the company’s future visions of autonomy.

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The decision to axe the car just ten days after making it available seems like a true head-scratcher.

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