Propellant Depots the Real Disruptor

Gateway to the Stars

SpaceX plan to deploy a network of orbital propellant depots and currently readying a propellant depot prototype called Starship 26 (S26) for launch. Orbital depots promise to disrupt the paradigm for space exploration by changing what we think possible with chemical rockets. The ability to refuel in orbit is equivalent to adding an extra stage to conventional rockets, allowing them to compete on performance with much larger launch vehicles. Normally second stages expend all their propellant to reach orbit but if they can be refilled at a propellant depot that should allow them to boost payloads Beyond Earth Orbit (BEO).

This orbital refueling concept isn’t new, in fact it was first proposed for the Apollo moon landing program: -

“Wernher von Braun and others at NASA’s Marshall Space Flight Center had favored the Earth Orbit Rendezvous concept, in which two spacecraft would have been launched by two [Saturn I] rockets, one carrying the Apollo astronauts and the other carrying fuel as a tanker, for rendezvous and assembly in low-Earth orbit. Ultimately, von Braun and NASA decided that the Lunar Orbit Rendezvous approach [using a single Saturn V] was necessary to make Kennedy’s famous deadline.” ~ Aerospace America

The Saturn V was an enormously powerful rocket, capable of delivering 16 tonnes to the lunar surface, i.e. the Lunar Excursion Module (LEM), in one launch. However Starship, with the help of a propellant depot, could land 200 tonnes of useful payload on the moon, consisting of 100t of passengers and/or cargo plus the 100t Starship itself, which can be used as a surface habitat.

Starship variants used for Artemis moon landings ~ credit NASA

Once realized, Starship’s capability would render current Super Heavy Lift (SHL) vehicles, like the Space Launch System (SLS), obsolete because its most powerful iteration (SLS Block 2) can only send 46 tonnes towards the moon, in what’s called a Trans Lunar Injection. Unfortunately the sheer potency of the orbital depot has long been its downfall, because it threatens to disrupt big ticket projects like SLS: -

"Senator Shelby called NASA and said if he hears one more word about propellant depots he’s going to cancel the space technology program." ~ Eric Berger

However, these depots are quintessential to SpaceX’s mission, so they bravely decided to build this groundbreaking technology, come what may.

Propellant Depot Network

The military have a saying: “two is one, and one is none.” They operate in difficult environments, where equipment commonly fails on the battlefield hence a backup becomes indispensable – a situation not dissimilar to the space environment. This implies SpaceX would have to maintain two or more orbital depots for redundancy, as a contingency against operational issues. For example, when a passenger Starship arrives in orbit, it will need to be refilled promptly – their oxygen supply is limited and holding your breath really isn’t an option…

SpaceX intend to leverage this massive increase in payload capacity to populate space with Starships journeying to the moon, Mars and beyond. However, for every ship sent afar they might have to build two depots, one to use the other for spare. Eventually SpaceX plans to send 1,000 Starships to Mars during the relatively short launch window, which occurs once every 26 months. Hence they might need upwards of 2,000 propellant depots deployed along a common orbit, like a string of pearls. This common orbit should allow a maintenance vehicle to service and repair multiple depots on a single flight, with minimal use of propellant. Given the numbers involved it seems unlikely these depots would be tended, except perhaps by small maintenance robots, which suggests only limited repairs would be possible without a service visit. Note, the number of Tanker Starships (used to refill these depots) could be relatively low because they are designed to be rapidly reusable, and should have ~25 months to refill these long term storage depots between Mars launch windows.

Given the current model where NASA is lucky to launch one SLS every two years, the projected launch cadence for Starship appears almost unbelievable. However, it only takes a month to build a new Starship and SpaceX aim to produce one per day, so they could build this fleet in less than a decade if they can reuse returning vehicles.

Of course there are many other applications for orbital propellant depots besides Moon and Mars missions. They should also enable a thriving space economy in LEO, acting as gas stations for space semis, tugs and passenger transports.

Big Horizon

“SpaceX so far has been evolutionary, Starship is revolutionary. It’s a fully reusable vehicle that’s designed to operate daily like an airliner; it’s gonna change access to space. People like say why do you need to take 150 tonnes to space, you know for $5m… What are you going to do with this thing, we don’t even know yet. We do know some things: you’re gonna exploit resources on the moon, water and minerals on the moon, mine asteroids. Use that water to make propellant. Use those minerals and metals to build structures in orbit and in the inner solar system. Build gas stations throughout the inner solar system so you can transport. Put giant solar PV (Photovoltaic) arrays up there so you can have massive energy and move manufacturing into space.” ~ Tom Mueller, former Propulsion CTO at SpaceX

As Tom suggests, SpaceX’s ambitious plan for propellant depots stretch far beyond Earth. These remote depots would need to be supplied from the world below, using propellant produced in situ from raw materials (something feasible on both the moon and Mars). This should massively increase the payload shuttled from other worlds (similar in effect to Earth depots), in addition to improving overall safety. Alternately, these depots could be used as way-stations, a place to re-provision and refuel, enroute to the asteroid belt or outer solar system. This is true disruption, space is vast so too must be our vision.

Once these new worlds are established, new space technology with propagate exponentially as they find solutions to problems we’ve barely considered. SpaceX specialize in vertical integration and every reason to translate this approach to new worlds. Frontiersmen need to be resourceful because they can’t wait on the next supply train to come through. The final evolution of this exponential growth is the jump to new stars in our stellar neighborhood, a giant leap from where we are now.

In conclusion

S26, while rudimentary, carries enormous promise; the promise of freedom from the status quo. When propellant depots are married to fully reusable spacecraft we can fulfill the dream of colonizing new worlds and normal people living and working in space. While the technical challenge would make Atlas wilt, SpaceX appear comfortable with the load, at least judging by the advances made by Starship. The next Starship launch will be key as it will prove whether they can safely attain orbit. Given success, the launch after that should be S26 which takes things to a whole new level.

Chris Prophet