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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 Supercharger Diner food menu gets a sneak peek as construction closes out

What are you ordering at the Tesla Diner?

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Credit: BradGoldbergMD | X

The Tesla Supercharger Diner in Los Angeles is nearing completion as construction appears to be winding down significantly. However, the more minor details, such as what the company will serve at its 50s-style diner for food, are starting to be revealed.

Tesla’s Supercharger Diner is set to open soon, seven years after CEO Elon Musk first drafted the idea in a post on X in 2018. Musk has largely come through on most of what he envisioned for the project: the diner, the massive movie screens, and the intended vibe are all present, thanks to the aerial and ground footage shared on social media.

We already know the Diner will be open 24/7, based on decals placed on the front door of the restaurant that were shared earlier this week. We assume that Tesla Optimus will come into play for these long and uninterrupted hours.

The Tesla Diner is basically finished—here’s what it looks like

As far as the food, Tesla does have an email also printed on the front door of the Diner, but we did not receive any response back (yet) about what cuisine it will be offering. We figured it would be nothing fancy and it would be typical diner staples: burgers, fries, wings, milkshakes, etc.

According to pictures taken by @Tesla_lighting_, which were shared by Not a Tesla App, the food will be just that: quick and affordable meals that diners do well. It’s nothing crazy, just typical staples you’d find at any diner, just with a Tesla twist:

As the food menu is finalized, we will be sure to share any details Tesla provides, including a full list of what will be served and its prices.

Additionally, the entire property appears to be nearing its final construction stages, and it seems it may even be nearing completion. The movie screens are already up and showing videos of things like SpaceX launches.

There are many cars already using the Superchargers at the restaurant, and employees inside the facility look to be putting the finishing touches on the interior.

It’s almost reminiscent of a Tesla version of a Buc-ee’s, a southern staple convenience store that offers much more than a traditional gas station. Of course, Tesla’s version is futuristic and more catered to the company’s image, but the idea is the same.

It’s a one-stop shop for anything you’d need to recharge as a Tesla owner. Los Angeles building permits have not yet revealed the date for the restaurant’s initial operation, but Tesla may have its eye on a target date that will likely be announced during next week’s Earnings Call.

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Tesla’s longer Model Y did not scale back requests for this vehicle type from fans

Tesla fans are happy with the new Model Y, but they’re still vocal about the need for something else.

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Credit: AlwinArt/Twitter

Tesla launched a slightly longer version of the Model Y all-electric crossover in China, and with it being extremely likely that the vehicle will make its way to other markets, including the United States, fans are still looking for something more.

The new Model Y L in China boasts a slightly larger wheelbase than its original version, giving slightly more interior room with a sixth seat, thanks to a third row.

Tesla exec hints at useful and potentially killer Model Y L feature

Tesla has said throughout the past year that it would focus on developing its affordable, compact models, which were set to begin production in the first half of the year. The company has not indicated whether it met that timeline or not, but many are hoping to see unveilings of those designs potentially during the Q3 earnings call.

However, the modifications to the Model Y, which have not yet been officially announced for any markets outside of China, still don’t seem to be what owners and fans are looking forward to. Instead, they are hoping for something larger.

A few months ago, I reported on the overall consensus within the Tesla community that the company needs a full-size SUV, minivan, or even a cargo van that would be ideal for camping or business use.

Tesla is missing one type of vehicle in its lineup and fans want it fast

That mentality still seems very present amongst fans and owners, who state that a full-size SUV with enough seating for a larger family, more capability in terms of cargo space for camping or business operation, and something to compete with gas cars like the Chevrolet Tahoe, Ford Expedition, or electric ones like the Volkswagen ID.BUZZ.

We asked the question on X, and Tesla fans were nearly unanimously in support of a larger SUV or minivan-type vehicle for the company’s lineup:

Here’s what some of the respondents said:

Tesla is certainly aware that many of its owners would like the company to develop something larger that competes with the large SUVs on the market.

However, it has not stated that anything like that is in the current plans for future vehicles, as it has made a concerted effort to develop Robotaxi alongside the affordable, compact models that it claims are in development.

It has already unveiled the Robovan, a people-mover that can seat up to 20 passengers in a lounge-like interior.

The Robovan will be completely driverless, so it’s unlikely we will see it before the release of a fully autonomous Full Self-Driving suite from Tesla.

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Energy

Tesla launches first Virtual Power Plant in UK – get paid to use solar

Tesla has launched its first-ever Virtual Power Plant program in the United Kingdom.

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Credit: Tesla Energy | X

Tesla has launched its first-ever Virtual Power Plant program in the United Kingdom. This feature enables users of solar panels and energy storage systems to sell their excess energy back to the grid.

Tesla is utilizing Octopus Energy, a British renewable energy company that operates in multiple markets, including the UK, France, Germany, Italy, Spain, Australia, Japan, New Zealand, and the United States, as the provider for the VPP launch in the region.

The company states that those who enroll in the program can earn up to £300 per month.

Tesla has operated several VPP programs worldwide, most notably in California, Texas, Connecticut, and the U.S. territory of Puerto Rico. This is not the first time Tesla has operated a VPP outside the United States, as there are programs in Australia, Japan, and New Zealand.

This is its first in the UK:

Tesla is not the only company that is working with Octopus Energy in the UK for the VPP, as it joins SolarEdge, GivEnergy, and Enphase as other companies that utilize the Octopus platform for their project operations.

It has been six years since Tesla launched its first VPP, as it started its first in Australia back in 2019. In 2024, Tesla paid out over $10 million to those participating in the program.

Tesla VPP program in California hits new capacity milestone

Participating in the VPP program that Tesla offers not only provides enrolled individuals with the opportunity to earn money, but it also contributes to grid stabilization by supporting local energy grids.

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