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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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Tesla Holiday Update is incoming, and the wishlist is Merry and Bright

There are a handful of big wishes, and we’ve seen a lot of different requests out there based on what owners are saying on social media. Nevertheless, what Tesla should bring and what Tesla will bring are two different things.

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14 hours ago

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November 17, 2025

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Tesla’s Holiday Update is going to be on its way soon, and although we have no idea what the company is planning to implement into vehicles with the 2025 iteration.

However, the wishlist is extensive, and owners are hoping to get a vast array of new features, both useful and artificial. That’s the fun thing about owning a Tesla — not everything is necessary, and it’s okay for your car to be fun.

There are a handful of big wishes, and we’ve seen a lot of different requests out there based on what owners are saying on social media. Nevertheless, what Tesla should bring and what Tesla will bring are two different things.

In past years, Tesla has brought both useful things and fun things with the Holiday Update. The Custom Lock Sound, new Light Shows, and even High Fidelity Park Assist have all come in past updates, among many other things. But for 2025, people want even more, and here’s what we have seen most frequently thus far:

More Streaming Platforms

This is a personal request of ours, and it’s something that we feel is long overdue.

Sure, Netflix, Disney+, and Hulu are all great — but there’s a lot of meat left on that bone. HBOMax, Paramount+, and even YouTube TV would be a great option for those of us who have subscriptions and want to watch Live Events while Supercharging or eating in our cars.

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The fact that Tesla has not added more platforms to its in-car Theater in a few years has been, dare I say, disappointing?

Full Self-Driving for Europe

This is something not even Santa can help with. Although his Elves are known for their high productivity, we’re not even sure they could convince European regulators to open the door for FSD’s entrance into the market.

Tesla deploys Unsupervised FSD in Europe for the first time—with a twist

FSD is definitely capable of handling European driving conditions, but regulators are truly dragging their feet through the mud with the approval process. Tesla has tested FSD in several countries in Europe, but nothing has been set in stone yet.

Deeper Grok Integration

Many owners have said something about how Grok is truly not super in-tune with the vehicles. This is something any owner will experience.

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It seems Grok should be capable of handling all in-car requests; everything from changing the A/C to a specific temperature to adding a stop within the Navigation should be handled by Grok.

Instead, Grok cannot handle those things currently. You have to speak to the car itself using the microphone button on the steering wheel.

Interestingly, some vehicles already have the Grok logo replacing the microphone. It is likely the most realistic request of all.

‘Learn’ Mode for Full Self-Driving Arrival Options

Although it is great for public destinations, FSD still does not allow you to choose a set parking spot at your residence. It also does not allow you to choose preferences for parking in large parking lots.

Renters, and even those who live in purchased townhomes, often have assigned parking spots. Full Self-Driving v14 has done a great job of doing half the work, but there have been too many times when I’ve arrived home, the car pulls me into a spot, and I’m forced to manually back out and park in my assigned space.

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Many people also do not like to park toward the entrance of a store, me included. Parking away from the front of a store eliminates parking congestion and usually is a safer bet for your vehicle to keep from being dinged by careless drivers who swing their doors open.

Navigation Adjustments

Sometimes you don’t want to turn left on the street the navigation chooses. Maybe you want to go a block down and check out that new Portuguese restaurant that just opened on the way to your next destination.

This is only possible currently by inputting a waypoint that would take you that way. Instead, the center screen could be opened, and the driver should be able to select an alternative route by simply touching a street they’d rather travel on.

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Investor's Corner

Tesla wins $508 price target from Stifel as Robotaxi rollout gains speed

The firm cited meaningful progress in Tesla’s robotaxi roadmap, ongoing Full Self-Driving enhancements, and the company’s long-term growth initiatives.

Published

16 hours ago

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November 17, 2025

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Credit: Joe Tegtmeyer/X

Tesla received another round of bullish analyst updates this week, led by Stifel, raising its price target to $508 from $483 while reaffirming a “Buy” rating. The firm cited meaningful progress in Tesla’s robotaxi roadmap, ongoing Full Self-Driving enhancements, and the company’s long-term growth initiatives. 

Robotaxi rollout, FSD updates, and new affordable cars

Stifel expects Tesla’s robotaxi fleet to expand into 8–10 major metropolitan areas by the end of 2025, including Austin, where early deployments without safety drivers are targeted before year-end. Additional markets under evaluation include Nevada, Florida, and Arizona, as noted in an Investing.com report. The firm also highlighted strong early performance for FSD Version 14, with upcoming releases adding new “reasoning capabilities” designed to improve complex decision-making using full 360-degree vision.

Tesla has also taken steps to offset the loss of U.S. EV tax credits by launching the Model Y Standard and Model 3 Standard at $39,990 and $36,990, Stifel noted. Both vehicles deliver more than 300 miles of range and are positioned to sustain demand despite shifting incentives. Stifel raised its EBITDA forecasts to $14.9 billion for 2025 and $19.5 billion for 2026, assigning partial valuation weightings to Tesla’s FSD, robotaxi, and Optimus initiatives.

TD Cowen also places an optimistic price target

TD Cowen reiterated its Buy rating with a $509 price target after a research tour of Giga Texas, citing production scale and operational execution as key strengths. The firm posted its optimistic price target following a recent Mobility Bus tour in Austin. The tour included a visit to Giga Texas, which offered fresh insights into the company’s operations and prospects. 

Additional analyst movements include Truist Securities maintaining its Hold rating following shareholder approval of Elon Musk’s compensation plan, viewing the vote as reducing leadership uncertainty.

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@teslarati Tesla Full Self-Driving yields for pedestrians while human drivers do not…the future is here! #tesla #teslafsd #fullselfdriving ♬ 2 Little 2 Late – Levi & Mario
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Elon Musk

Tesla CEO Elon Musk teases autobiography following fallout with Isaacson

“I need to tell my story myself and highlight lessons that I learned along the way that would be useful to others.”

Published

17 hours ago

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November 17, 2025

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Credit: xAI

Tesla CEO Elon Musk teased the potential for an autobiography following his fallout with author Walter Isaacson, who wrote a book on the serial entrepreneur in the past.

Isaacson met with C-SPAN’s American’s Book Club on November 13, and went into detail as to why he disagreed with Musk’s place in politics, especially with how he handled Department of Government Efficiency (DOGE):

“It’s a shame because had he gone into government and focused on what he’s good at…He could have changed the government for good, but instead…he started, you know, let’s get rid of this part of USAID and firing people.”

It’s sort of a shame to see Isaacson cast stones in the direction of Musk, whom he spent a significant amount of time with, aiming to put forth an accurate and realistic depiction of perhaps the greatest entrepreneur in the modern era.

However, Musk did not come back at Isaacson. Instead, he highlighted the need for what could potentially be his autobiography, aiming to “highlight lessons that [he] learned along the way that would be useful to others.”

Musk’s time in government was met with harsh criticisms from many, but there was a lot of support for the work that he did during his time with DOGE as well. Eventually, Musk’s responsibilities with DOGE started to wind down, and he pledged to step back from government to focus on his companies.

A Musk-written biography would potentially be a great book for those who are interested in his story, but also those who plan to enter entrepreneurism, tech, or the sciences, as there truly could be some excellent lessons within.

However, Musk’s recently approved compensation package and the tranches that could pay him $1 trillion in shares will likely take up most of his time. Tesla also has a lofty goal of launching Robotaxi and expanding the ride-hailing service to other areas of the country in the coming months.

@teslarati With a pedestrian in the crosswalk, Tesla Full Self-Driving shows off its courtesy. Human drivers? Not so much. #tesla #teslafsd #fullselfdriving ♬ AMERICAN HEART – Maxwell Luke

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