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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
The Boring Company’s newest tunnel vehicle runs on Tesla parts and no one is driving it
The Boring Company’s new tunnel vehicle runs on Tesla Model 3 batteries and drive units.
The Boring Company just introduced a new piece of hardware, and it runs on parts pulled straight from a Tesla showroom. Liner Truck 3, unveiled in a post from the tunneling company’s official X account, is an all electric vehicle built around Tesla Model 3 battery packs and drive units, purpose built to move concrete tunnel segments to the boring machine face without a single person underground.
Introducing Liner Truck 3 — our latest fully electric tunnel vehicle.
– Tesla Model 3 battery and drive units
– Transports 22,000+ lb of concrete segments to the boring machine
– 28 miles of range
– 12 mph max operating speed
– Remotely piloted from Global OCC in Texas, with… pic.twitter.com/XB7FgSXnpy— The Boring Company (@boringcompany) August 7, 2026
The job itself is unglamorous but critical. Each precast segment run weighs more than 22,000 pounds, roughly the load of a full cement mixer, and Liner Truck 3 hauls that weight repeatedly between the surface staging area and wherever the Prufrock machine happens to be cutting.
The Boring Company said Liner Truck 3 is piloted remotely out of its Global Operations Control Center in Texas, extending the Zero-People-In-Tunnel approach the company has spent years building toward. An earlier version of a ZPIT liner truck was already tested at the company’s Bastrop, Texas research tunnels, and a factory tour released last month showed an employee flying a fully loaded liner truck with a PlayStation controller. Liner Truck 3 looks like the production version of that same idea, cleaned up and pushed into daily use.
The timing lines up with a company digging in more places than it ever has before. The Boring Company now has multiple Prufrock machines active or arriving in Nashville, where Music City Loop construction has been accelerating since February, and its Vegas Loop network keeps adding tunnel mileage on a near monthly basis. Every one of those projects depends on getting concrete segments to the cutting face fast enough to keep the boring machine from idling, which is exactly the bottleneck Liner Truck 3 is designed to remove.
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It also reinforces something Tesla owners have watched happen gradually across Musk’s companies: passenger car hardware finding a second life in heavy equipment. Model 3 drive units already move people through the Vegas Loop, and now the same components are hauling concrete underground in Nashville and wherever The Boring Company digs next. Whether that kind of component reuse extends further into TBC’s equipment lineup, or into other Musk owned industrial hardware, is the next thing worth watching.
SpaceX stock did the opposite of what most of Wall Street expected this week, when the day designed to be its most dangerous turned into a rally, and the rally kept going.
Thursday marked the first major lockup expiration since SpaceX’s June IPO, making roughly 911.5 million insider held shares eligible to trade for the first time, more than doubling the company’s public float. Analysts and short sellers had spent weeks bracing for a flood of selling, especially after the stock fell 13 percent following its first earnings report as a public company on Tuesday. Instead, shares rose 6.1 percent Thursday to close at $114.92, and by Friday they were trading near $129, up more than another 12 percent on the day.
SpaceX shorts get warned by Musk ally, echoing Tesla’s early struggles
The setup made the outcome notable. Short interest had climbed to roughly 34 percent of the float heading into earnings, among the highest of any large cap stock, with about 95 percent of available shares to borrow already on loan. CEO Elon Musk warned short sellers twice in the weeks before the lockup, writing on X that “the survival probability of firms who maintain a significant short position in SpaceX over time is very low,” then following up on the morning of earnings with “I try to warn them, but they just double down.”
When the newly unlocked shares hit the market and the selloff never showed up, some of that short position appears to have started unwinding. TipRanks reported that options activity shifted toward bullish strategies like put selling and risk reversals following the rally, with roughly $600 million in options premium trading Thursday alone. Retail buyers also stepped in during the earnings dip, according to Vanda Research.
The fundamentals behind the stock have not changed much in a week. SpaceX’s revenue nearly doubled year over year to $7.8 billion, with Starlink subscribers doubling to 12 million and the company’s AI segment growing 247 percent. What spooked investors on Tuesday was the spending side. Capital expenditures jumped to more than $18 billion for the quarter, up from $2.8 billion a year earlier, with AI investment alone rising from $749 million to $15.8 billion. Wall Street remains split on whether that spending is building infrastructure SpaceX needs or outrunning what the business can currently support, a debate Teslarati has tracked since shares first came under pressure.
None of that resolves the bigger question hanging over the stock. Thursday’s release was only the first of nine staggered lockup tranches, with roughly $800 billion worth of additional shares scheduled to become eligible through October, and Musk’s own stake stays locked until next June. If this week is any indication, the market is treating that supply as something it can absorb rather than something to fear, at least for now.
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News
The Boring Company’s newest Vegas Station has a permit quietly waiting behind it
Sahara Las Vegas opened a new Vegas Loop station, joining an exclusive two resort transit club.
Sahara Las Vegas opened a new Vegas Loop station Thursday, giving The Boring Company’s underground transit system its northernmost stop yet on the Strip. The station sits at Sahara’s Paradise Road entrance, on the southeast corner of Las Vegas Boulevard and Sahara Avenue, and connects riders to the Las Vegas Convention Center, other Strip resorts on the network and, eventually, Harry Reid International Airport.
The addition makes Sahara the second resort, after Fontainebleau opened its own station in January, to get a stop built at street level rather than tucked into the property itself. Sahara now joins Westgate as the only two Strip resorts offering both a Vegas Loop station and a stop on the Las Vegas Monorail, giving guests two separate ways to get around without leaving the property.
The Boring Company just doubled its tunneling power in Nashville
The bigger news buried in Thursday’s announcement is what comes next. Boring Company has already secured its first permit to tunnel north of Sahara Avenue, extending the network beyond where it currently ends, even though permits to push the Loop toward downtown Las Vegas still haven’t been granted. Crews are also working on a two mile dual tunnel line running from Westgate to a planned station at 4744 Paradise Road, just north of Tropicana Avenue, that Las Vegas Convention and Visitors Authority CEO Steve Hill has said the company hopes to open in time for November’s Las Vegas Grand Prix.
Ridership has grown alongside the buildout. The Loop moved roughly 82,000 passengers during CONEXPO in early March, a total the company highlighted on its own X account at the time, and the system has now carried more than 4 million passengers through 11 open stations since it began running in 2021. The airport connector tunnels, meant to give the Loop a direct link to Harry Reid, have slipped past their original first quarter target and remain under construction, with Boring Company director Mike Baier saying that a full opening is still a few months out.
For Sahara, the calculation is straightforward. Convention traffic drives a large share of Loop ridership, and a station at the property’s front door gives conventiongoers one more reason to book rooms on the Strip’s north end instead of closer to the convention center itself.
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Inside Tesla’s secretive $10 Billion “Project Crystal Sun” filing
The Boring Company’s newest tunnel vehicle runs on Tesla parts and no one is driving it
