What’s old is new again – more on refueling in space

On my shelves sits a childhood book “Planets and Spaceflight” published in 1957 by General Mills. The front cover is “Planets” and the rear “Spaceflight”, full of vivid descriptions and beautiful artwork of so many places to go and how we will get there. The publisher being best known for Cheerios leaves me sure the book was picked up by my parents at the supermarket as part of a deal. Was this like those old encyclopedia sets, “A thru Amer” for 25 cents with $10 in groceries? Just days ago, I happened on the book only to see it included refueling in space. This was that proverbial moment in an uncanny valley, having worked on in-space refueling in NASA, a lifetime later that is. NASA teams (2011), later teams (2015) and others outside NASA (2008, 2012) have reduced refueling in space to its numbers and graphs, minus the inspiring artwork and the musty but somehow comforting smell of an old book.

Pictures of the cover and an open page from "The Planets" and "Spaceflight" from General Mills, 1957.
General Mills “Planets and Spaceflight”, 1957, by Otto Binder, Illustrated by George Solonevich

___Today, NASA is investing heavily in refueling, an idea that was once verboten but is now in vogue. This may be thanks to a massive Starship passing as just a lander. Starship “refilling”, the wording better reflecting that both fuel and oxidizer are topped-off, plans to trade the propellant that is not in one ship for the payload that is not placed on another. The imagery of what’s planned goes all too well alongside the artwork of days past, reminiscent, tank after tank all in a row. As bookends on a journey, refueling in space has come a long way. Yet there are images of real hardware that can tell a lot about in-space refueling. Between the art of Solonevich and the plans of SpaceX there was a stage called the Saturn S-IVB.

Two Starships end to end, mated to transfer propellant.
SpaceX Starship refilling. Image: SpaceX (apx. 35 min. mark).

___ The Saturn S-IVB, really a 3rd stage not a 4th, can been seen afloat against a cloudy sky, lunar lander attached as if just waiting for action. I can hear “Mr. Sulu, assume standard orbit” in the background. At first glance the picture would appear to be a stage awaiting the Apollo crew spacecraft which is turning around to mate to the lunar lander, access port to access port. This is the case, just not for a lunar mission but rather for the Apollo 9 test flight of everything, from stage to lander to spacecraft with even some extra-vehicular activity. The scale is telling, a stage large enough to get landers and spacecraft (and adapters) along the way to the Moon, had it fired up its engine that is. Notably, when the lunar missions did leave Earth’s orbit the stage was not entirely full. A good amount of propellant had been burned just getting into its proper orbit. Imagine if such a stage had been refilled. Now imagine again how an empty or near empty stage of nearly the same scale and a lander attached could be launched without a massive Saturn rocket.

 The Saturn S-IVB stage and lander during the Apollo 9 test flight. Image: NASA.
The Saturn S-IVB stage and lander during the Apollo 9 test flight. Credit: NASA.
A table showing the original amount of propellant in the Saturn S-IVB (pp.108), on the far right, 87,315kg of liquid oxygen and 19,780kg of liquid hydrogen. The amount at ignition to leave Earth orbit hours later was less, 61,300kg and 14,395kg.
The original amount of propellant in the Saturn S-IVB (pp.108) is on the far right, 87,315kg of liquid oxygen and 19,780kg of liquid hydrogen. The amount at ignition to leave Earth orbit hours later was less, 61,300kg and 14,395kg.

___ Starships may set the stage for refueling, but refueling can excel at much smaller scales. While we have a very commercial US launch capability today in the SpaceX Falcon 9, we also have a very competitive launch capability on the smaller end, in Rocket Lab’s Electron rocket. Others will follow on both ends. For deep space, a single scale to meet all needs would also seem unlikely. Might refilling also find a useful scale below the Starships? It turns out 100,000kg of propellant, about at the scale of the Saturn S-IVB, is a minimal scale for crewed lunar missions. Between the time of the childhood artwork and the recent plans for massive rockets, there was real hardware that points to these possibilities.

The numbers, 10 tons here, 100 tons there…

___ To say refilling a stage in Earth orbit has a lot of moving parts would be an understatement. All the parts begin their story by scaling a stage. It’s no coincidence the other current development, the NASA Exploration Upper Stage, is close in scale to the Saturn S-IVB. Orbital mechanics and physics have not changed since Apollo. If the smallish Apollo lunar lander was about 17,000kg fully loaded, and an empty stage to be refilled later is around 12,000kg, there is now an extremely affordable launch capability to place that tonnage in low Earth orbit. Better yet, in a growing economic ecosystem there will be other capable launchers to follow. Say a Falcon Heavy can place 63,800kg into low Earth orbit, then add the caveats. If we wish to use it in fully reusable mode, it’s more like a 34,000kg capability, and if we want to place the ship to await tankers up higher, at 400km, and say at 51.6 degrees inclination (like the International Space Station), then we have to work within about a 30,000kg launch capability. That empty stage and a small lander add up – but with little margin.

___ However, there are plenty more puts and takes. A stage doesn’t have to be quite as large as the Saturn S-IVB, it’s already been placed about where it should be. More so, expending a center Falcon booster and reusing only the side boosters, still a great cost advantage, quickly jumps total capability for that initial mass put at 400km/51.6° to around 51,300kg. Now that stage can get bigger again, and the lander too, and other elements like the spacecraft service module can scale up too. Some initial propellant might be added, just for starters. Adapters, maneuvering, losses and other factors all regain ample margin to spare.

An image of the Saturn S-IVB (bottom), 13,500kg dry mass and the NASA Exploration Upper Stage (top) now in development, 12,000 kg dry mass. Dry mass being before loading up propellant. They are very similar in size/scale.
The Saturn S-IVB (bottom) was 13,500kg dry mass and the NASA Exploration Upper Stage (top) now in development is 12,000 kg dry mass. Dry mass being before loading up propellant. Credit: The EUS wiki, by ThePrimalEarth – Own work, CC BY-SA 4.0.

Beyond the numbers

___ Aside from the masses and the physics, the real possibilities are in the cost, safety and reliability improvements from NASA partnering and buying launches and propellant as competitive commercial services. This is so even when NASA is the only customer in the near term. It takes a while to get to Inspiration4 using commercial crew spacecraft developed for NASA, but well before and regardless there is every indication NASA reaps enormous benefits. Time will tell how refilling will play a role in the exploration of our solar system – but it appears the stage is set so a ship in space filling up on propellant before its long journey is no longer just the stuff of childhood stories.

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