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


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.”
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.
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.
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.

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.
News
Tesla posts Optimus’ most impressive video demonstration yet
The humanoid robot was able to complete all the tasks through a single neural network.

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).
“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.
News
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.

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.
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.
News
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.

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.
“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.
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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