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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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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 posts Optimus’ most impressive video demonstration yet

The humanoid robot was able to complete all the tasks through a single neural network.

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Credit: Tesla Optimus/X

When Elon Musk spoke with CNBC’s David Faber in an interview at Giga Texas, he reiterated the idea that Optimus will be one of Tesla’s biggest products. Seemingly to highlight the CEO’s point, the official Tesla Optimus account on social media platform X shared what could very well be the most impressive demonstration of the humanoid robot’s capabilities to date.

Optimus’ Newest Demonstration

In its recent video demonstration, the Tesla Optimus team featured the humanoid robot performing a variety of tasks. These include household chores such as throwing the trash, using a broom and a vacuum cleaner, tearing a paper towel, stirring a pot of food, opening a cabinet, and closing a curtain, among others. The video also featured Optimus picking up a Model X fore link and placing it on a dolly.

What was most notable in the Tesla Optimus team’s demonstration was the fact that the humanoid robot was able to complete all the tasks through a single neural network. The robot’s actions were also learned directly from Optimus being fed data from first-person videos of humans performing similar tasks. This system should pave the way for Optimus to learn and refine new skills quickly and reliably.

Tesla VP for Optimus Shares Insight

In a follow-up post on X, Tesla Vice President of Optimus (Tesla Bot) Milan Kovac stated that one of the team’s goals is to have Optimus learn straight from internet videos of humans performing tasks, including footage captured in third person or by random cameras.

“We recently had a significant breakthrough along that journey, and can now transfer a big chunk of the learning directly from human videos to the bots (1st person views for now). This allows us to bootstrap new tasks much faster compared to teleoperated bot data alone (heavier operationally).

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“Many new skills are emerging through this process, are called for via natural language (voice/text), and are run by a single neural network on the bot (multi-tasking). Next: expand to 3rd person video transfer (aka random internet), and push reliability via self-play (RL) in the real-, and/or synthetic- (sim / world models) world,” Kovac wrote in his post on X.

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Starship Flight 9 nears as SpaceX’s Starbase becomes a Texan City

SpaceX’s launch site is officially incorporated as Starbase, TX. Starship Flight 9 could launch on May 27, 2025. 

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(Credit: Jenny Hautmann/Wikimedia Commons)

SpaceX’s Starbase is officially incorporated as a city in Texas, aligning with preparations for Starship Flight 9. The newly formed city in Cameron County serves as the heart of SpaceX’s Starship program.

Starbase City spans 1.5 square miles, encompassing SpaceX’s launch facility and company-owned land. A near-unanimous vote by residents, who were mostly SpaceX employees, led to its incorporation. SpaceX’s Vice President of Test and Launch, Bobby Peden, was elected mayor of Starbase. The new Texas city also has two SpaceX employees as commissioners. All Starbase officials will serve two-year terms unless extended to four by voters.

As the new city takes shape, SpaceX is preparing for the Starship Flight 9 launch, which is tentatively scheduled for May 27, 2025, at 6:30 PM CDT from Starbase, Texas.

SpaceX secured Federal Aviation Administration (FAA) approval for up to 25 annual Starship and Super Heavy launches from the site. However, the FAA emphasized that “there are other licensing requirements still to be completed,” including policy, safety, and environmental reviews.

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On May 15, the FAA noted SpaceX updated its launch license for Flight 9, but added: “SpaceX may not launch until the FAA either closes the Starship Flight 8 mishap investigation or makes a return to flight determination. The FAA is reviewing the mishap report SpaceX submitted on May 14.”

Proposed Texas legislation could empower Starbase officials to close local highways and restrict Boca Chica Beach access during launches. Cameron County Judge Eddie Trevino, Jr., opposes the Texas legislation, insisting beach access remain under county control. This tension highlights the balance between SpaceX’s ambitions and local interests.

Starbase’s incorporation strengthens SpaceX’s operational base as it gears up for Starship Flight 9, a critical step in its mission to revolutionize space travel. With growing infrastructure and regulatory hurdles in focus, Starbase is poised to become a cornerstone of SpaceX’s vision, blending community development with cutting-edge aerospace innovation.

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The Boring Company accelerates Vegas Loop expansion plans

The Boring Company clears fire safety delays, paving the way to accelerating its Vegas Loop expansion plans.

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Credit: The Boring Company/X

After overcoming fire safety hurdles, the Boring Company is accelerating its Vegas Loop expansion. The project’s progress signals a transformative boost for Sin City’s transportation and tourism.

Elon Musk’s tunneling company, along with The Las Vegas Convention and Visitors Authority (LVCVA) and Clark County, resolved fire safety concerns that delayed new stations.

“It’s new. It’s taken a little time to figure out what the standard should be,” said Steve Hill, LVCVA President and CEO, during last week’s board meeting. “We’ve gotten there. We’re excited about that. We’re ready to expand further, faster, than we have.”

Last month, the company submitted permits for tunnel extensions connecting Encore to a parcel of land owned by Wynn and Caesars Palace. The three tunnels are valued at $600,000 based on country records.

Plans for a Tropicana Loop are also advancing, linking UNLV to MGM Grand, T-Mobile Arena, Allegiant Stadium, Mandalay Bay, and the upcoming Athletics’ ballpark. Downtown extensions from the convention center to the Strat, Fremont Street Experience, and Circa’s Garage Mahal are also in the permitting process.

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“Those are all in process,” Hill noted. “We’ve got machines that are available to be put in the ground. I think we’ve reached a framework for how these projects are going to work and how they’ll be permitted from a safety standpoint, as well as a building standpoint.”

The Boring Company has six boring machines, with three currently active in Las Vegas. Last week, TBC announced that it successfully mined continuously in a Zero-People-in-Tunnel (ZPIT) configuration, enabling it to build more tunnels faster, safer, and at a more affordable rate.

Tunneling under Paradise Road is underway as The Boring Company works on the University Center Loop. The University Center Loop is expected to connect to the Las Vegas Convention Center within two months, linking to the Westgate tunnel. The full Vegas Loop will span 104 stations and 68 miles. Even though The Boring Company’s tunnel network in Las Vegas isn’t nearly finished, it has already become a key attraction in the city.

“It’s such a great attraction for shows that are looking at this building (convention center) and we’re going to be connected to everybody in town,” Hill said. “It’s a real difference-maker.”

A few Vegas Loop stations are already operational, including those connected to Resorts World, Westgate, Encore, and all the Las Vegas Convention Center Loop stations. The Downtown Loop, which connects to the downtown area, and the Riviera Station, the hub that leads to Resorts World with Westgate destinations, are also operational.

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As The Boring Company accelerates the Vegas Loop, its tunnels are poised to redefine mobility and tourism in Las Vegas, blending cutting-edge technology with practical urban solutions.

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