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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
SpaceX secures win as US labor board drops oversight case
The NLRB confirmed that it no longer has jurisdiction over SpaceX.
SpaceX scored a legal victory after the National Labor Relations Board (NLRB) decided to dismiss a case which accused the company of terminating engineers who were involved in an open letter against founder Elon Musk.
The NLRB confirmed that it no longer has jurisdiction over SpaceX. The update was initially shared by Bloomberg News, which cited a letter about the matter it reportedly reviewed.
In a letter to the former employees’ lawyers, the labor board stated that the affected employees were under the jurisdiction of the National Mediation Board (NMB), not the NLRB. As a result, the labor board stated that it was dismissing the case.
As per Danielle Pierce, a regional director of the agency, “the National Labor Relations Board lacks jurisdiction over the Employer and, therefore, I am dismissing your charge.”
The NMB typically oversees airlines and railroads. The NLRB, on the other hand, covers most private-sector employers, as well as manufacturers such as Boeing.
The former SpaceX engineers have argued that the private space company did not belong under the NMB’s jurisdiction because SpaceX only offers services to “hand-picked customers.”
In an opinion, however, the NMB stated that SpaceX was under its jurisdiction because “space transport includes air travel” to get to outer space. The mediation board also noted that anyone can contact SpaceX to secure its services.
SpaceX had previously challenged the NLRB’s authority in court, arguing that the agency’s structure was unconstitutional. Jennifer Abruzzo, the NLRB general counsel under former United States President Joe Biden, rejected SpaceX’s claims. Following Abruzzo’s termination under the Trump administration, however, SpaceX asked the labor board to reconsider its arguments.
SpaceX is not the only company that has challenged the constitutionality of the NLRB. Since SpaceX filed its legal challenge against the agency in 2024, other high-profile companies have followed suit. These include Amazon, which has filed similar cases that are now pending.
News
Tesla accuses IG Metall member of secretly recording Giga Berlin meeting
The union has denied the electric vehicle maker’s allegations.
Police seized the computer of an IG Metall member at Tesla Giga Berlin on Tuesday amid allegations that a works council meeting was secretly recorded.
The union has denied the electric vehicle maker’s allegations.
In a post on X, Gigafactory Berlin plant manager André Thierig stated that an external union representative from IG Metall attended a works council meeting and allegedly recorded the session. Thierig described the event as “truly beyond words.”
“What has happened today at Giga Berlin is truly beyond words! An external union representative from IG Metall attended a works council meeting. For unknown reasons he recorded the internal meeting and was caught in action! We obviously called police and filed a criminal complaint!” Thierig wrote in his post on X.
Police later confirmed to local news outlet rbb24 that officers did seize a computer belonging to an IG Metall member at the Giga Berlin site on Tuesday afternoon. Tesla stated that employees had contacted authorities after discovering the alleged recording.
IG Metall denied Tesla’s accusations, arguing that its representative did not record the meeting. The union alleged that Tesla’s claim was simply a tactic ahead of upcoming works council elections.
The next works council election at Giga Berlin is scheduled for March 2 to 4, 2026. The facility’s management had confirmed the dates to local news outlets. The official announcement marks the start of the election process and campaign period.
Approximately 11,000 employees are eligible to participate in the vote.
The previous works council election at the plant took place in 2024, and it was triggered by a notable increase in workforce size. Under German labor law, regular works council elections must be held every four years between March 1 and May 31.
Elon Musk
Elon Musk’s xAI plants flag in Bellevue AI hotspot
The lease places xAI’s new office in one of the region’s fastest-growing tech hubs.
Elon Musk’s artificial intelligence company xAI has leased a full floor at Lincoln Square South in downtown Bellevue, WA, as per city permit filings.
The lease places xAI’s new office in one of the region’s fastest-growing tech hubs.
Public records indicate that xAI leased roughly 24,800 square feet in Lincoln Square South. The location was previously occupied by video game company Epic Games. Lincoln Square South is part of the Bellevue Collection, which is owned by Kemper Development Co.
The lease was first referenced in January by commercial real estate firm Broderick Group, which noted that an unnamed tenant had secured the space, as stated in a report from the Puget Sound Business Journal. Later filings identified xAI as the occupant for the space.
xAI has not publicly commented on the lease.
xAI hinted at plans to open an office in the Seattle area back in September, when the startup posted job openings with salaries ranging from $180,000 to $440,000. At the time, the company had narrowed its location search to cities on the Eastside but had not finalized a lease.
xAI’s Bellevue expansion comes as Musk continues consolidating his businesses. Last week, SpaceX acquired xAI in a deal that valued the artificial intelligence startup at $250 billion. SpaceX itself is now valued at roughly $1.25 trillion and is expected to pursue an initial public offering (IPO) later this year.
Musk already has a significant presence in the region through SpaceX, which employs about 2,000 workers locally. That initiative, however, is focused largely on Starlink satellite development.
Bellevue has increasingly become a center for artificial intelligence companies. OpenAI has expanded its local office footprint to nearly 300,000 square feet. Data infrastructure firms such as Crusoe and CoreWeave have also established offices downtown.
SpaceX secures win as US labor board drops oversight case
Tesla accuses IG Metall member of secretly recording Giga Berlin meeting
