News
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.
-
- A to-scale comparison of Falcon 1, Pegasus XL, MLV, and Falcon 9. (Teslarati/Stratolaunch/Wikipedia)
-
- 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.
-
- PGA’s first full-scale preburner seen during assembly. (Stratolaunch)
-
- PGA’s first full-scale preburner seen during assembly. (Stratolaunch)
-
- Jeff Thornburg stands in front of Stratolaunch’s NASA Stennis Space Center test stand. (Stratolaunch)
-
- The PGA preburner seen after installation at Stennis. (Stratolaunch)
-
- The control center. (Stratolaunch)
-
- MLV is released from Stratolauncher. (Stratolaunch)
-
- A concept video produced by Stratolaunch shows the Roc launching a Kraken rocket. (Stratolaunch, via Wired)
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!
Tesla CEO Elon Musk posted a photo of himself standing with the Optimus production team inside Tesla’s Fremont factory, arms crossed amid workers in hard hats and safety vests. The image captures a pivotal industrial shift: the same facility space once dedicated to building Tesla’s flagship Model S sedan and Model X SUV is now home to the company’s humanoid robot manufacturing line.
Walking the Optimus production line in Fremont pic.twitter.com/ABS0tuRibW
— Elon Musk (@elonmusk) July 1, 2026
Tesla’s Fremont Factory, acquired in 2010 from the former NUMMI joint venture between Toyota and GM, has been the company’s original U.S. manufacturing hub since Model S production began in 2012.
The Model X followed soon thereafter. These premium vehicles offered lower annual volumes, recently around 30,000 combined, compared to the high-volume Model 3 and Model Y lines that continue around the site. Over their combined run, the S and X accounted for roughly 610,000 units.
In late January 2026, during Tesla’s Q4 2025 earnings call, Elon Musk announced the end of Model S and Model X production in Q2 2026. The final vehicles rolled off the line in early May. Rather than retooling for another vehicle, Tesla chose to convert the dedicated S/X assembly area into a dedicated Optimus Gen 3 production line.
Model 3 and Y manufacturing remains unaffected. Tesla’s official Fremont Factory page now lists Optimus alongside the 3 and Y as core products.
The conversion was executed with remarkable speed. After production stopped, crews dismantled the existing vehicle line and installed entirely new modular equipment—including lines sourced from Germany and dozens of sub-lines for actuators, batteries, and other components—in roughly four months.
Musk described the timeline as “insanely fast,” noting it would be unprecedented for any other manufacturer. Initial Optimus output is expected to ramp slowly due to the robot’s roughly 10,000 unique parts and the brand-new production processes involved. The Fremont line targets an eventual capacity of 1 million Optimus units per year.
Tesla isn’t joking about building Optimus at an industrial scale: Here we go
Optimus Development Timeline
- August 19, 2021: Optimus (then called Tesla Bot) formally announced at Tesla’s first AI Day. A concept video showed a person in a suit demonstrating the vision for a general-purpose humanoid capable of dangerous, repetitive, or boring tasks using the same AI architecture as Full Self-Driving.
- 2022: Early prototypes displayed. At the second AI Day in September, semi-functional units demonstrated walking across a stage and basic arm movements
- 2023: September videos showed improved capabilities, including sorting colored blocks, precise limb awareness, and holding a Yoda pose.
- 2024-early 2025: Factory integration videos showed Optimus navigating workspaces and handling objects like battery cells.
- January 2026: Gen 3 mass-production activities began at Fremont, with reports of over 1,000 Gen 3 units already operating inside the factory for real-world learning and AI training
- April 2026: Musk confirms Optimus production on converted Fremont line would begin in late July or August 2026. The Gen 3 reveal, originally eyed for Q1, was pushed closer to production start. A second, much larger Optimus factory at Giga Texas is under construction, with volume production targeted for Summer 2027 and long-term capacity of 10 million units annually
- July 1, 2026: Musk’s on-site visit and team photo confirm the Optimus line is operational and the transition is actively progressing
Tesla positions Optimus as potentially its largest project ever, leveraging vertical integration, AI expertise, and car-like manufacturing know-how to scale humanoid robots first for its own factories and later for broader industrial and consumer use.
The Fremont conversion serves as a critical proving ground for this ambitious new chapter in Tesla’s already-rich history.
Investor's Corner
Tesla gets its latest short from Michael Burry: ‘Happy it jumped back to this level’
Tesla short seller Michael Burry, the subject of the film “The Big Short,” where he was portrayed by Steve Carell, has revealed he has opened a new bet against the stock.
In a new update to his Substack newsletter in a post titled “Trading Post June 30, 2026,” Burry revealed a new set of bets against Tesla, Caterpillar, NVIDIA, Applied Materials Inc., and the iShares Semiconductor ETF.
In regard to Tesla, Burry wrote:
“And finally I shorted Tesla at 416.22. Happy it jumped back to this level.”
This means Burry likely opened his new short position after the company’s recent rally on Wall Street, which saw Tesla shares sink in mid-May, only to recover to well over the $400 mark. Currently, shares trade at around $427.
The company saw a big Tuesday as shares climbed considerably, over 10 percent. The size of the Tesla short was not provided, nor did Burry give any information on the position’s structure, the number of shares, dollar value, or whether options were used in the short.
The Tesla and SpaceX merger everyone is talking about is quietly building
Over the years, Burry has been one of the more vocal critics of Tesla, calling its share price “media inflated,” and saying it was “ridiculously overvalued” as recently as December.
The company has largely transitioned away from being known as an automotive company and instead is much more widely regarded as an AI play, mostly due to its Full Self-Driving efforts, Optimus robot development, and data collection related to both.
This has not pulled those skeptics away from being vocal about their distaste for how Tesla is valued, but there’s no denying that the company is a global force in many things, including sustainable energy, automotive, and AI.
Wedbush Securities is initiating stock coverage on SpaceX (NASDAQ: SPCX), marking the first comments on the company since it went public several weeks ago. Wedbush and its analyst handling coverage, Dan Ives, are widely bullish on fellow Musk company Tesla (NASDAQ: TSLA).
Ives wrote his first note initiating coverage of SpaceX shares on Wednesday with a $190 price target and an ‘Outperform’ rating. The firm believes the company is well positioned off of its IPO because of its wide array of projects, including AI compute power and infrastructure, connectivity projects, and launches.
“We view SpaceX as one of the most differentiated assets within the tech market with a strong footprint across its three core markets, with Starlink driving success with connectivity,” Ives wrote, “Starship launches leading to a demand flywheel and increasing deal flow for its Colossus clusters.”
Elon Musk called it Epic: The full story of SpaceX’s Starship Flight 12
Wedbush leans heavily on Starlink, which they say is the “profitability driver given the strength of its recurring revenue base of ~12 million subscribers as of June 5th.” Ives believes Starlink is still in the “early innings” of penetrating the global telecommunications and broadband market, as it only holds less than a 1 percent share. However, this number is sure to increase over time.
It also highlights the importance of Starship, which it says is an “essential layer” of SpaceX’s overall success. SpaceX developing and displaying the ability to reuse rockets is a major cost and reliability advantage “as it reduces the necessary hardware launch costs while generating a feedback loop for future flights to improve their launch flight rate without accelerating capex spend.”
Finally, SpaceX’s recent AI/Compute projects are also very elementary, Ives writes. It is worth mentioning Wedbush said its $190 price target is derived from a valuation forecast that sees the company yielding roughly $2.48 trillion of implied enterprise value.
There are also some factors that Wedbush did not take into account with its initial coverage. The firm wrote in the note:
“We note that there is optional value coming from Starship’s accelerating scale towards sub-$200/kg unit economics, orbital data centers, and enterprise AI monetization as these factors could drive meaningful upside but these face major hurdles, so we do not take that into account with our valuation.”
SpaceX shares are down just over 2 percent today, trading at around $167 at the time of publication.
Tesla Optimus project fires up as Musk sees production line progress
Tesla gets its latest short from Michael Burry: ‘Happy it jumped back to this level’
