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SpaceX’s path to refueling Starships in space is clearer than it seems

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Perhaps the single biggest mystery of SpaceX’s Starship program is how exactly the company plans to refuel the largest spacecraft ever built after they reach orbit.

First revealed in September 2016 as the Interplanetary Transport System (ITS), SpaceX has radically redesigned its next-generation rocket several times over the last half-decade. Several crucial aspects have nevertheless persisted. Five years later, Starship (formerly ITS and BFR) is still a two-stage rocket powered by Raptor engines that burn a fuel-rich mixture of liquid methane (LCH4) and liquid oxygen (LOx). Despite being significantly scaled back from ITS, Starship will be about the same height (120 m or 390 ft) and is still on track to be the tallest, heaviest, and most powerful rocket ever launched by a large margin.

Building off of years of growing expertise from dozens of Falcon 9 and Falcon Heavy launches, the most important fundamental design goal of Starship is full and rapid reusability – propellant being the only thing intentionally ‘expended’ during launches. However, like BFR and ITS before it, the overarching purpose of Starship is to support SpaceX’s founding goal of making humanity multiplanetary and building a self-sustaining city on Mars. For Starship to have even a chance of accomplishing that monumental feat, SpaceX will not only have to build the most easily and rapidly reusable rocket and spacecraft in history, but it will also have to master orbital refueling.

The reuse/refuel equation

In the context of SpaceX’s goals of expanding humanity to Mars, a mastery of reusability and orbital refueling are mutually inclusive. Without both, neither alone will enable the creation of a sustainable city on Mars. A Starship launch system that can be fully reused on a weekly or even daily basis but can’t be rapidly and easily refueled in space simply doesn’t have the performance needed to affordably build, supply, and populate a city on another planet (or Moon). A Starship launch system that can be easily refueled but is not rapidly and fully reusable could allow for some degree of interplanetary transport and the creation of a minimal human outpost on Mars, but it would probably be one or two magnitudes more difficult, risky, and expensive to operate and would require a huge fleet of ships and boosters from the start.

The question of how SpaceX will make Starship the world’s most rapidly, fully, and cheaply reusable rocket is a hard one, but it’s not all that difficult to extrapolate from where the company is today. Currently, the turnaround record (time between two flights) for Falcon boosters is two launches in less than four weeks (27 days). SpaceX’s orbital-class reuse is also making strides and the company recently flew the same orbital Crew Dragon capsule twice in just 137 days (less than five months) – fast approaching turnarounds similar to NASA’s Space Shuttle average, the only other reusable orbital spacecraft in history.

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SpaceX’s current fleet of four reusable Dragon spacecraft. (NASA/Mike Hopkins/ESA/Thomas Pesquet)
Pictured here during its last launch, Falcon 9 B1060 owns SpaceX’s turnaround record of just 27 days and has completed eight orbital-class launches in 12 months, averaging one flight every ~45 days – an average turnaround time that’s better than the Space Shuttle’s all-time record. (SpaceX)

While Dragon and Falcon 9 are far smaller than Starship and Super Heavy, Dragon is only partially reusable and requires significant refurbishment after recovery and Falcon 9 boosters are fairly complex. Starship, on the other hand, should effectively serve as a fully reusable all-in-one Falcon upper stage, Dragon capsule, Dragon trunk, and fairing, making it far more complex but potentially far more reusable. To an extent, Super Heavy should also be mechanically simpler than Falcon boosters (no deployable legs or fins; no structural composite-metal joints; no dedicated maneuvering thrusters) and its clean-burning Raptor engines should be easier to reuse than Falcon’s Merlins. Put simply, there are precedents set and evidence provided by Falcon rockets and NASA’s Space Shuttle that suggest SpaceX will be able to solve the reusability half of the equation.

What about refueling?

The other half of that equation, however, could not be more different. The sum total of SpaceX’s official discussions of orbital refueling can be summed up in a sentence included verbatim in CEO Elon Musk’s 2017, 2018, and 2019 Starship presentations: “propellant settled by milli G acceleration using control thrusters.”

This phrase first appeared in 2017 (PDF; page 16). (SpaceX)

On the face of it, that simple phrase doesn’t reveal much. However, with a few grains of salt, hints from what the company’s CEO has and hasn’t said, and context from the history of research into orbital propellant transfer, it’s possible to paint a fairly detailed picture of the exact mechanisms SpaceX will likely use to refill Starships in space. The cornerstone, somewhat ironically, is a 2006 paper – written by seven Lockheed Martin employees and a NASA engineer – titled “Settled Cryogenic Propellant Transfer.” Aside from the obvious corollaries just from the title alone, the paper focuses on what the authors argue is the simplest possible route to large-scale orbital propellant transfer.

In orbit, under microgravity conditions, the propellant inside a spacecraft’s tanks is effectively detached from the structure. If a spacecraft applies thrust, that propellant will stay still until it splashes against its tank walls – the most basic Newtonian principle that objects at rest tend to stay at rest. If, say, a spacecraft thrusts in one direction and opens a hatch or valve on the tank in the opposite direction of that thrust, the propellant inside it – attempting to stay at rest – will naturally escape out of that opening. Thus, if a spacecraft in need of fuel docks with a tanker, their tanks are connected and opened, and the tanker attempts to accelerate away from the receiving ship, the propellant in the tanker’s tanks will effectively be pushed into the second ship as it tries to stay at rest.

The principles behind such a ‘settled propellant transfer’ are fairly simple and intuitive. The crucial question is how much acceleration the process requires and how expensive that continuous acceleration ends up being. According to Kutter et al’s 2006 paper, the answer is surprising: assuming a 100 metric ton (~220,000 lb) spacecraft pair accelerates at 0.0001G (one ten-thousandth of Earth gravity) to transfer propellant, they would need to consume just 45 kg (100 lb) of hydrogen and oxygen propellant per hour to maintain that acceleration.

Two possible Starship orientations for propellant transfer. (SpaceX)

In the most extreme hypothetical refueling scenario (i.e. a completely full tanker refueling a ship with a full cargo bay), two docked Starships would weigh closer to 1600 tons (~3.5M lb) and the “Milli G” acceleration SpaceX has repeatedly mentioned in presentation slides would be ten times greater than the maximum acceleration analyzed by Kutter et al. Still, according to their paper, that propellant cost scales linearly both with the required acceleration and with the mass of the system. Roughly speaking, using the same assumptions, that means that the thrusting Starship would theoretically consume just over 7 tons (half a percent) of its methane and oxygen propellant per hour to maintain milli-G acceleration.

With large enough pipes (on the order of 20-50 cm or 8-20 in) connecting each Starship’s tanks, SpaceX should have no trouble transferring 1000+ tons of propellant in a handful of hours. Ultimately, that means that settled propellant transfer even at the scale of Starship should incur a performance ‘tax’ of no more than 20-50 tons of propellant per refueling. All transfers leading up to the worst-case 1600-ton scenario should also be substantially more efficient. Overall, that means that fully refueling an orbiting Starship or depot with ~1200 tons of propellant – requiring anywhere from 8 to 14+ tanker launches – should be surprisingly efficient, with perhaps 80% or more of the propellant launched remaining usable by the end of the process.

On Super Heavy B4, SpaceX has installed what amount to nozzles over the booster’s main oxygen tank vents to vector and maximize the thrust they produce. (NASASpaceflight – bocachicagal)

A step further, Kutter et al note the amount of acceleration required is so small that a hypothetical spacecraft could potentially use ullage gas vents to achieve it, meaning that custom-designed settling thrusters might not even be needed. Coincidentally or not, SpaceX (or CEO Elon Musk) has recently decided to use strategically located ullage vents to replace purpose-built maneuvering thrusters on Starship’s Super Heavy booster. If SpaceX adds similar capabilities to Starship, it’s quite possible that the combination of cryogenic propellant naturally boiling into gas as it warms and the ullage vents used to relieve that added pressure could produce enough thrust to transfer large volumes of propellant.

Last but not least, writing more than a decade and a half ago, the only technological barrier Kutter et al could foresee to large-scale settled propellant transfer wasn’t even related to refueling but, rather, to the ability to autonomously rendezvous and dock in orbit. In 2006, while Russia was already routinely using autonomous docking and rendezvous technology on its Soyuz and Progress spacecraft, the US had never demonstrated the technology on its own. Jump to today and SpaceX Dragon spacecraft have autonomously rendezvoused with the International Space Station twenty seven times in nine years and completed ten autonomous dockings – all without issue – since 2019.

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SpaceX has already developed and thoroughly tested hot-gas Raptor-derived maneuvering thrusters that could be fairly easily added to Starship to boost the efficiency of settled propellant transfer at the cost of added weight and complexity. (NASASpaceflight – bocachicagal)

Even though SpaceX and its executives have never detailed their approach to refueling (or refilling, per Musk’s preferred term) Starships in space, there is a clear path established by decades of NASA and industry research. What little evidence is available suggests that that path is the same one SpaceX has chosen to travel. Ultimately, the key takeaway from that research and SpaceX’s apparent use of it should be this: while a relatively inefficient process, SpaceX has effectively already solved the last remaining technical hurdle for settled propellant transfer and should be able to easily refuel Starships in orbit with little to no major development required.

There’s a good chance that minor to moderate problems will be discovered and need to be solved once SpaceX begins to test refueling in orbit but crucially, there are no obvious showstoppers standing between SpaceX and the start of those flight tests. Aside from the obvious (preparing a new rocket for its first flight tests), the only major refueling problem SpaceX arguably needs to solve is the umbilical ports and docking mechanisms that will enable propellant transfer. SpaceX will also need to settle on a location for those ports/mechanisms and decide whether to implement ullage vent ‘thrusters’, cold gas thrusters like those on Falcon and current Starship prototypes, or more efficient hot-gas thrusters derived from Raptors. At the end of the day, though, those are all solved problems and just a matter of complex but routine systems engineering that SpaceX is an expert at.

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 Model 3 filings in China show interesting hardware addition

The addition of a front bumper camera to the Tesla Model 3 is a big upgrade from a hardware perspective.

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Credit: Tesla Newswire via China's MIIT

Tesla Model 3 filings in China are showing the vehicle could get a very interesting hardware addition, one that was not included on the “Highland” update when it rolled out to customers a year and a half ago.

The Model 3 Highland is Tesla’s updated version of the all-electric sedan, and was launched across the world in early 2024. It featured a variety of updates, including new exterior and interior designs.

However, there were a few things missing from the update that surprised Tesla fans because they were included on other cars.

One of them was the lack of a front bumper camera, a hardware piece that was included on other vehicles within the company’s lineup, including the Model Y Juniper, an updated version of the all-electric crossover that launched earlier this year.

Now, it seems Tesla is preparing to implement that front camera on the Model 3, as new filings with China’s Ministry of Industry and Information Technology (MIIT) showed the car with the addition:

The front bumper camera is a small but powerful addition to Tesla vehicles. It not only enhances visibility for simple tasks like parking, helping avoid things like curbs, but it also helps provide a wider field of view directly in front of the car.

It is also a crucial part of the Full Self-Driving and Autopilot suites, helping provide yet another angle of vision for the vehicle as Tesla makes its suite more robust. It is already improving through software upgrades and data collection, but it could always use additional hardware to enhance accuracy.

A Model 3 Highland test mule was spotted near Boston, Massachusetts, back in May with a variety of additional cameras equipped. Some believed this was a vehicle that was assisting with collecting training data.

Tesla is testing a Model 3 with some mysterious cameras in the U.S.

However, it could be a sign of Tesla planning to add this piece of hardware to a slightly updated version of the new Model 3 that could come to production in various markets in the near future.

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Tesla CEO Elon Musk details massive FSD update set for September release

“This will substantially reduce the need for driver attention, but some complex intersections, heavy weather or unusual events will still require attention.”

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Credit: Tesla Asia | X

Tesla CEO Elon Musk detailed the changes that are expected to come with a massive Full Self-Driving (FSD) update, which is set to roll out sometime in September, he revealed earlier this week.

Tesla has been refining its FSD suite for years, but it has never been as good as it is now. The focus is to get the suite to a point where interventions are no longer needed and drivers simply become passengers, as they will not be responsible for paying attention to the road.

Elon Musk teases crazy new Tesla FSD model: here’s when it’s coming

That version of FSD will come eventually, but not next month. However, there are dramatic improvements that will come with next month’s FSD update that will roll out to the public, Musk said:

“The FSD software update next month will be a major step-change improvement for rare conditions.”

Additionally, he provided specific details on what would change, hinting that the need for a driver to pay attention will be “substantially reduced,” but there are some “complex intersections, heavy weather, or unusual events” that will still require drivers to assume responsibility for the car:

“This will substantially reduce the need for driver attention, but some complex intersections, heavy weather or unusual events will still require attention.”

We have been teased about these types of updates before, but usually they involve some kind of mention of FSD being ready for unsupervised driving “by the end of the year.” Musk did not mention that here.

There is also the fact that Tesla has another FSD build in Austin for the Robotaxi suite that is more advanced than what is available to the public. It has performed well, Musk says, making claims that there are times when it feels “eerily human.”

Tesla Q2 2025 vehicle safety report proves FSD makes driving almost 10X safer

The improvements in FSD capabilities in subsequent releases are usually very evident. As Tesla continues to refine the suite for the public, it gains more confidence and becomes smarter through the collection of data and the use of neural networks.

The only thing left to wait for is the release itself, and we are hopeful it will roll out to the public in September, as Musk says.

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Tesla Model Y L’s impressive specs surface in China’s recent MIIT filing

The Tesla Model Y L is expected to launch later this year.

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Credit: Tesla

The specs of the upcoming Tesla Model Y L has appeared in new Chinese regulatory filings, revealing key specifications including a six-seat layout and an extended range of up to 751 kilometers. The variant is expected to launch later this year alongside a new long-range Model 3 variant rated at 830 kilometers.

The updates were listed on the China Ministry of Industry and Information Technology’s (MIIT) latest batch of new energy vehicle models that are eligible for vehicle purchase tax exemptions.

Model Y L to debut with larger battery, six-seat layout

Listed under the model code TSL6500BEVBA0, the Model Y L will feature dual motors producing 142 kW at the front and 198 kW at the rear. It will be powered by a 465-kilogram 82.0-kWh lithium-ion battery from LG Energy Solution, with a pack energy density of 176 Wh/kg, as noted in a CNEV Post report. The long-range crossover achieves 751 km on the lenient CLTC cycle, making it Tesla’s highest-range Model Y to date in China despite its curb weight of 2,088 kg.

The “L” designation is believed to refer to the vehicle’s larger size and seating configuration, as the new variant is listed with six seats. It builds on Tesla’s strategy to diversify offerings in the Model Y lineup, which currently includes both RWD and AWD five-seat versions.

Model 3+ breaks record with 830 km CLTC range

Alongside the Model Y L, Tesla China also registered a new rear-wheel-drive Model 3, which was designated with the model code TSL7000BEVBR1. The vehicle boasts either 800 or 830 km of range on the CLTC cycle, depending on its trim. This marks the highest range yet for any Tesla vehicle in China.

The variant will use a 448-kilogram, 78.4-kWh LG-supplied battery with an energy density of 175 Wh/kg and a peak motor output of 225 kW. The vehicle’s curb weight is listed at 1,760 kg. The model was previously identified in filings as “Model 3+,” hinting at a possible tier above the existing long-range variant, which tops out at 753 km CLTC.

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