Connect with us

News

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

Published

on

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.

Advertisement
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.

Advertisement
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.

Advertisement
Comments

News

Tesla talks Semi ramp, Optimus, Robotaxi rollout, FSD with Wall Street firm

Published

on

Credit: Tesla

Tesla (NASDAQ: TSLA) recently talked about a variety of topics with Wall Street firm Piper Sandler, as the firm released a new note on Friday about their meeting with the company’s Investor Relations team.

According to the note from Piper Sandler, Tesla talked in detail about the Semi program, Optimus, and its potential valuation given its capabilities, the rollout of Robotaxi in Austin, and Full Self-Driving progress in the United States.

Tesla Semi Ramp

The Tesla Semi is set to enter mass production in 2026 at a dedicated factory near the company’s Gigafactory in Reno, Nevada.

The Semi has already been in pilot program testing, as Tesla has partnered with a few companies, like Frito-Lay and PepsiCo., to perform regional logistics. It has been met with excellent reviews from drivers, and it has helped give Tesla a good idea of what to expect when it makes its way to more companies in the coming years.

Piper Sandler said that it is evident Tesla is preparing for a “major ramp,” but it is keeping its expectations low:

“We’ve never expected much from this product, but we’d love to be proven wrong (Tesla is clearly prepping for a major ramp).”

Tesla Optimus and its value internally and externally

Optimus has been working in Tesla factories for some time, but its expectations as a product offering outside of the company internally have major implications.

Its role within Tesla factories, for now, is relatively low, but Optimus is still doing things to assist. By this time next year, Piper Sandler said Optimus should have bigger responsibilities:

“By this time in 2026, Optimus should be moving/staging parts within Tesla’s facilities.”

Outside of Tesla, Optimus could be a major beneficiary for companies as it could be a more affordable way to handle tedious tasks and manual labor. The firm believes that if Optimus can work 18-hour shifts, a cost of $100,000 per unit “would be justified.”

Tesla Robotaxi Expansion

The big focus of the firm with Robotaxi was Tesla’s expansion of the geofence in Austin this week. It was substantial, bringing the Robotaxi’s total service area to around 170 square miles, up from the roughly 90 square miles that rival Waymo is offering in the city.

Tesla Robotaxi geofence expansion enters Plaid Mode and includes a surprise

Tesla has doubled its geofence three times since its launch in late June, and it also revealed that its fleet of vehicles has expanded by 50 percent. It did not give a solid number of how many vehicles are operating in the fleet.

Tesla Full Self-Driving v14 launch

Tesla’s Full Self-Driving suite is set to have a fresh version, v14, rolled out in either September or October, and there are some pretty high expectations for it.

CEO Elon Musk said:

“The FSD release in about 6 weeks will be a dramatic gain with a 10X higher parameter count and many other improvements. It’s going through training & testing now. Once we confirm real-world safety of FSD 14, which we think will be amazing, the car will nag you much less.”

There is also some expectation that v14 could be the public release of what Tesla is running in Austin for Robotaxi. The firm confirmed this in their note by stating it “should enable Tesla owners to use software that is on par with Robotaxis in Austin.”

The only real hold up would be regulator skepticism, but Tesla can alleviate this with strong data.

The firm maintained its ‘Overweight’ rating and the $400 price target it holds on the stock.

Continue Reading

News

Tesla starts Full Self-Driving rollout to owners in Australia

“To have this car drive me around Brisbane for an hour, we’re talking in the city, motorway, spaghetti bowl of on-ramps, it handled it so well. It was mind-blowing.”

Published

on

Credit: Tesla

Tesla has already started rolling out its Full Self-Driving suite to owners in Australia after officially launching the driver assistance suite in the country yesterday.

Earlier this week, Tesla seemed to be moving toward the launch of Full Self-Driving (Supervised) in Australia after numerous media members received early access to test its performance.

Tesla officially launched Full Self-Driving (Supervised) in Australia yesterday. The company told media members who got early access to the suite that the rollout would begin with Hardware 4-equipped Model 3 and Model Y vehicles.

Tesla launches Full Self-Driving in a new region

The release would be slow and gradual, with the process performed in stages.

The first stage of the rollout has already begun in Australia:

Tesla is reminding drivers in Australia who are using the suite for the first time that they must not become complacent while FSD is in operation. It is not fully autonomous and still requires the driver to remain attentive to road conditions and the vehicle’s surroundings.

Currently, the suite is only available to purchase outright, and it comes at a cost of $10,100. A subscription model is in the works, similar to the one in the United States, but Tesla has not yet announced its plans or pricing model for this.

Australia is the sixth country to receive Full Self-Driving (Supervised), or at least some version of it, as the United States, Canada, Mexico, China, and Puerto Rico all have access to the suite currently. In China, it is slightly different and is referred to as “City Autopilot” due to regulatory boundaries.

Early reviews of the suite have been very strong, as local media have also had the opportunity to try it, with one journalist saying, To have this car drive me around Brisbane for an hour, we’re talking in the city, motorway, spaghetti bowl of on-ramps, it handled it so well. It was mind-blowing.”

Continue Reading

News

Tesla expands crazy new lease deal for insane savings on used inventory

Tesla was able to work through the hurdles in three states, expanding the deal to New York, New Jersey, and Florida.

Published

on

Credit: Tesla

Tesla has expanded its new lease deal, offering people insane savings on used inventory.

Last week, we reported on Tesla offering crazy good lease deals on some of its used inventory, as people were able to grab monthly payments for as little as $225 per month with no money down.

There was only one catch: the deal was only available in California and Texas.

Tesla offers new deal on used inventory that you won’t want to pass up

However, Tesla has been working hard to get the lease deal expanded, and it finally has. Last night, Tesla’s VP of Finance, Sendil Palani, confirmed the deal had made it to three additional states.

Palani explained that the deal was only available in California and Texas because the leasing process requires working through regulatory hurdles in each state. He said it “involves a nontrivial amount of work,” which makes things more difficult to iron out.

Tesla was able to work through the hurdles in three states, expanding the deal to New York, New Jersey, and Florida. It is currently active in those states, Palani said:

Tesla is really making a concerted effort to push its inventory out the door, and many areas already are running low on both new and pre-owned inventory. It has cut prices on some new inventory, while offering these new lease deals on used vehicles that remain.

It is beneficial to the consumer for obvious reasons: cheaper payments and the ability to get a great deal on a car for no money down. Tesla is also getting rid of vehicles that were once thought to be intended for the Robotaxi fleet, but it appears these older hardware vehicles are no longer in the company’s plans for that purpose.

This is the first time Tesla has offered lease deals on used inventory, as it has only offered an outright purchase option in the past. In an effort to boost deliveries and rid itself of older cars, these lease deals are truly beneficial for both parties. It is only a wonder how long they will last.

Continue Reading

Trending