

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)
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Tesla Robotaxi just got a big benefit from the U.S. government
The NHTSA is looking to help streamline the application process for companies developing driverless vehicles.

Tesla Robotaxi just got a big benefit from the U.S. Government, as the National Highway Traffic Safety Administration (NHTSA) is looking to ease some rules and streamline the application process that could hinder the development and licensing of autonomous vehicles.
Tesla is set to launch its Robotaxi platform in the coming days or weeks, but regulation on autonomous vehicles is incredibly slim, so automakers are left in a strange limbo as permissions to operate are usually up to local jurisdictions.
The NHTSA still has the ultimate say, but it is now adopting a new strategy that will see companies gain an exemption from federal safety standards and streamline the entire application process.
The agency is authorized to grant exemptions to permit manufacturers to produce vehicles over a two or three-year period that might not comply with certain Federal Motor Vehicle Safety Standards (FMVSS). Robotaxi, for example, will eventually not have a steering wheel or pedals, through the Cybercab that Tesla unveiled last October.
The exemption program the NHTSA announced today would be possible through Part 555 of the National Traffic and Motor Vehicle Safety Act:
“NHTSA may grant a Part 555 exemption if at least one of four bases listed in the statute is met and NHTSA determines that the exemption is consistent with the public interest and the Safety Act. The statute also authorizes NHTSA to subject an exemption to terms the agency deems appropriate and requires that NHTSA publish notice of the application and provide an opportunity to comment.”
The rapid and non-stop innovation that is being performed is tough to keep up with from a legal standpoint. The NHTSA recognizes this and says current legislation is appropriate for traditional vehicles, but not for the self-driving cars companies are producing now:
“The current Part 555 process was designed for traditional vehicles. As currently applied, this process is not well suited for processing exemptions involving ADS-equipped vehicles in a timely manner or overseeing the unique complexities involving their operations. This has resulted in long processing times for applications for ADS-equipped vehicles. NHTSA must improve its Part 555 processing times substantially to keep pace with the rapid innovation of the ADS industry and to ensure that exemptions remain effective tools for nurturing groundbreaking safety technologies.”
Now, the NHTSA will be “enhancing application instructions” to help manufacturers understand the requirements involved in the application process. This will streamline the entire process by “reducing the need for NHTSA to request additional information from the manufacturer,” the agency says.
First Tesla driverless robotaxi spotted in the wild in Austin, TX
Next, the NHTSA is going to have a more flexible approach to evaluating exemptions for ADS-equipped vehicles:
“To build flexibility into the Part 555 process while also accounting for the unique aspects of those exemptions, NHTSA intends to develop terms that could be included in Part 555 exemption grants, when appropriate, to condition operations of exempted ADS-equipped vehicles on enhanced and continuing oversight from NHTSA. NHTSA would expect to administer this enhanced oversight through letters, which could be updated over time, mirroring real-world ADS development. This will enable NHTSA to focus its initial review during the application stage and align the Part 555 oversight approach more closely to exemptions administered under NHTSA’s Automated Vehicle Exemption Program (AVEP), which have proven effective for ADS.”
This will benefit any company making autonomous vehicles, but it will especially benefit Tesla in the short-term as it is readying for the launch of Robotaxi.
Tesla is trading up 1.89 percent at the time of publication.
Part 555 Letter June 2025 by Joey Klender on Scribd
News
SpaceX produces its 10 millionth Starlink kit
The first 5 million Starlink kits took nearly four years to build.

SpaceX has achieved a major milestone, producing its 10 millionth Starlink kit. The accomplishment was celebrated across the company’s Hawthorne, California, and Bastrop, Texas, facilities.
The milestone was shared in social media by Sujay Soman, Senior Facilities Engineer, in a LinkedIn post, which has since been deleted.
Starlink Production Ramp
Soman noted in his LinkedIn post that the first 5 million Starlink kits took nearly four years to build, but the next 5 million kits were completed in just 11 months. This underscores SpaceX’s intense efforts to ramp up the satellite internet system’s production, and it reflects the private space company’s manufacturing prowess.
The SpaceX Senior Facilities Engineer shared a couple of photos of the Machine Maintenance and Facilities team in Bastrop to commemorate the event.
“Today, Starlink Product teams across our Hawthorne and Bastrop sites produced the 10th Million Starlink Kit! It took almost 4 years to build our first 5 million kits, and we doubled that in about 11 months. Monumental accomplishment!” Soman wrote in his post.

World-Changing Technology
The Starlink kits, featuring dish hardware and supporting equipment, enable users to connect to the company’s growing constellation of low Earth orbit satellites. With over 6,000 satellites launched to date, Starlink now provides fast and reliable internet connectivity to over 6 million customers worldwide. This was a significant increase from the 5 million customers that the company reported in February 2025.
SpaceX has not detailed its next production targets, but the production of Starlink’s 10 millionth kit milestone signals the company’s readiness to scale further. Being an Elon Musk-led company, SpaceX is arguably the best in the business when it comes to efficient and cost-effective manufacturing. It would then be unsurprising if SpaceX announces another Starlink production milestone soon.
News
Tesla retires yoke steering wheel in base Model S and X
Tesla’s controversial steering yoke is now exclusive to the Model S and Model X Plaid.

Tesla has closed a chapter in the saga of the Model S and Model X’s controversial steering yoke. Following the announcement of the new iterations of the flagship vehicles, Tesla promptly removed the steering option for the vehicles’ base variants.
This means that if drivers wish to experience the Model S or Model X with a yoke, they would have to go Plaid.
The new Model S and Model X
The refresh of the Model S and Model X were quite minor, with the two vehicles featuring a new front camera, a new color, and a handful of other small changes like new exterior styling for the Model S Plaid. Tesla also noted on its website that the two vehicles now have a much smoother and quieter ride.
The changes were quite polarizing, with some appreciating the subtle improvements made to the two flagship cars and others arguing that Tesla should have done more. Others, however, noted that the level of improvements implemented on the Model S and Model X would already be considered major refresh for a tech company like Apple.
No More Yoke Unless Plaid
When Tesla refreshed the Model S and Model X in 2021, the vehicles were released with a steering yoke as standard. The yoke was controversial, with critics stating that it was unsafe and fans stating that it made driving the Model S and Model X fun. Tesla later introduced a round steering wheel option for the Model S and Model X, which later became standard on the two flagship vehicles.
This remains true today, with the most recent versions of the Model S and Model X still being released with a round steering wheel as standard. Those who wish to experience the Model S and Model X Plaid as envisioned by the company and its CEO, Elon Musk, however, might find it a good idea to spend the extra $1,000 for the vehicles’ yoke steering wheel.
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