One word: Propellant

Some graphs, like pictures, are also worth a thousand words. They do what a beautiful painting does while wandering in a museum, holding your stare like reading from a wall. For rockets and space travel, there is no shortage of figures and numbers and graphs, oh my. One especially telling figure came around in 2010 during a NASA look at refueling rockets in orbit. Suppose you are trying to galivant around the galaxy, but having only reached Earth orbit, you already have less than a full tank of gas. Bummer. The trip only started, and the needle already dropped.

A propellant depot-based architecture, capabilities from refilling a stage. Wilhite, Arney, Jones, Chai, 2012, Georgia Institute of Technology.

Of course, there are options, like making your rocket bigger, so what initial propellant for your trip is placed in orbit is that much more. NASA already did this in the Apollo Moon program decades ago, and it will do so again with its new Space Launch System. You can also break up what you need for the trip, with parts of it launched at different times. You meet up with the truck in your car. This too is part of NASA’s current plans. Yet, there is another alternative. Fill up your tanks before you leave Earth orbit, tanks you might even send up mostly empty, so more easily, on smaller rockets.

“It’s all about delta-v” is a phrase that will always hold true for space travel, any year, any time. Even as technology advances and we come up with new buzz words every year, saying it’s all about 3D printing, or AI, mining, pharma, or SPACs, saying it’s all about delta-v will never get old. How much can you change your velocity in some direction? Your delta-v capability is a number that says how much the gas in your tank can do for you. Where can you go? And always a good idea with a crew, do you have enough to come back?

…if you have seen that series called “The Expanse,” we are getting awful close.

Our NASA community famously enjoys presenting the kind of chart with a few more moving parts than just if this, then that. There is always something else, often buried in a lengthy narrative. But my translation of the beautiful chart above, in simple terms, is – if you have seen that series called “The Expanse,” we are getting awful close. (Excepting the nuclear fusion technology on their most advanced ships.) If we can just get more propellant where we need it, when we need it, cheaply, space opens up in ways that put our outer planets within reach. This is not for the usual robotic probes but much larger and more capable machines freed up from the limitations of watching their weight down to every last ounce. We even open up space beyond Mars to crew. Asteroids anyone?

Consider a spaceship with a propellant tank of a size to fit on a Falcon Heavy rocket. Or perhaps we break this up into two smaller tanks. We launch the smaller tanks on any assortment of affordable launchers to come from United Launch Alliance, Blue Origin, or other new launch providers. Or perhaps we join up small and large tanks in orbit. Mating up is usually a part of NASA’s plans for deep space exploration, some assembly required. Imagine if putting a pound of anything in orbit continues to drop, knowing the one thing most needed in orbit is propellant.

To get to Mars (or anywhere) from Earth, you need more propellant than spaceship. Think of it as your car with a fuel tank bigger than the cabin you sit in, and every other part of the car too. But even as transporting a pound of anything to orbit drops in cost, a pound of propellant and a pound of space systems hardware are two very different things. The former is simple, the latter complex. Different, though, does not mean propellant and hardware are independent. On the contrary, how propellant and space hardware are linked opens possibilities.

…your purpose in being out there is not to just hit and run.

Now imagine a long deep-space voyage, not because your ride is slow, not because you can’t go faster if you wanted to, but because your purpose in being out there is not to just hit and run. The bane of such dreams is dual-fold – space radiation and zero-gravity, each bad for your health. The one being present wants to hit you with particles, or all manner of radiation, many times the doses anyone should safely get in their lifetime. The one being missing, a lack of gravity encourages your body to go haywire starting at the cellular level, and moving on to just about everything else – bones, eyes, liver, and muscle. Yet you can now get anything to Earth orbit much cheaper, and so you can also get much more propellant, to push that mass out of orbit. So, start your trip right, with a full tank.

Artificial gravity schemes, and ever more shielding across more of your spaceship, are suddenly *not* something to discard as too heavy to get to orbit, or to take along. Shielding for crew safety means mass, with passive additional materials, or active schemes deflecting harmful particles. That mass must be propelled as we explore – which brings us back to propellant. Power comes along as well. Having more propellant, you no longer have to be miserly about power. This is propellant translating into electricity because you could add the mass of more solar panels or nuclear generators. Gravity schemes that rotate a spaceship and living quarters also mean more mass. Whereas today such concepts try to make a go of it on a diet, assuming current limitations on launch and the propellant available, tomorrow they may be liberated of these chains. As an added bonus, along comes reusability, because why toss such expensive hardware as if it’s a one and done, when you can just refuel and use it again.

Graphs about your delta-v to get to Jupiter are definitely complex, but the connections are not. More and cheaper propellant in orbit, and refueling before leaving, means being freed of the limits of lightweight thinking. That one word, propellant, is really so many more words – more spaceship, more power, more crew, and longer, yet safer voyages out there. The reusable SpaceX Starship already sees this, with plans to refuel in Earth orbit before a trip to the Moon. The matter ahead is who else sees that one-word propellant, and all the other words that follow.

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