The ‘60s-era dream of refueling a rocket in space may finally get liftoff. The idea, akin to building a gas station for a car, could help boost a rocket’s range and enable ships to send humans to Mars and beyond.
This week, NASA announced plans to work with SpaceX to help develop ways to transfer propellant in orbit, describing it as “an important step in the development” of SpaceX’s Mars-bound Starship rocket. The team will work with both the Glenn Research Center in Cleveland and the Marshall Space Flight Center in Alabama.
“Orbital refilling is vital to humanity’s future in space,” SpaceX CEO Elon Musk declared on Thursday. “More likely spacecraft to spacecraft (as aircraft do aerial refueling), than a dedicated depot, at least at first.”
Imagine you’re planning a cross-country trip. Depending on the model, a full tank can probably hold enough to travel several hundred miles. With an electric car, it’s probably closer to a couple of hundred miles. Fortunately, you tend to assume that there will be places to fill up or recharge along the way.
Now imagine you had to do the same trip, but the car had no means of filling up the fuel after the journey starts. You probably couldn’t do much exploring, and you’d have to get a car big enough to hold all the fuel before setting off. This is exactly how space travel works today.
Experts are only just exploring how to remove this shackle from humanity’s space exploration, but with a breakthrough in refueling, the solar system could suddenly fall within reach of rockets.
How SpaceX Plans to Establish a Planet-Hopping Species
Musk has spoken before about his goal of establishing humanity as a planet-hopping species. The idea would be to establish fueling stations along the way, so the rocket can move out further and further over time. The idea has been praised by Neil deGrasse Tyson, as moving out further to gather more resources could reduce scarcity and “evaporate” a “whole category of war.”
The Starship is crucial to this idea. It’s a stainless steel rocket currently under development at the firm’s Boca Chica facility in Texas. The fully reusable design is powered by the Raptor engine, which uses liquid oxygen and methane instead of regular rocket propellant. The idea is that humans would fly on a manned mission to Mars, establish a land-based propellant depot and use that to return home or move out further.
Work on the Starship has been swift. A miniaturized “hopper” version of the rocket lifted off at the test facility last month, jumping a few meters in the air. Musk has suggested that a full-scale presentation could be just weeks away. From there, the first commercial launch could come as soon as 2021, paving the way for a city on Mars by 2050.
But while land-based depots are cool, the space-bound ones NASA and SpaceX are exploring could unlock even more opportunities.
Refueling in Space: The Next Frontier
Imagine you’re designing a car for your road trip. You can either design the car with enough fuel to complete the whole journey, or you can create a refueling system that enables you to reduce the size of the tank. After all, specialized vehicles filled with gas are already traveling to refueling stations along the trip, so why not make the most of them?
That’s the logic behind refueling rockets in space. Propellant normally takes up around two-thirds of total mass, making it a hefty part of the overall design. Refueling in space could enable a rocket with a smaller fuel tank to leave the Earth, fill up in orbit, and continue on its journey. That would also mean the rocket could launch in favorable weather conditions on Earth, then wait in orbit to continue its journey when conditions were better to reach its destination. Mars and Earth are at their closest around once every two years, but if you knew you could refuel in space to continue on, it could mean launching when conditions are best instead of taking a chance.
The National Space Society notes another benefit of these in-orbit propellant depots. They can keep the fuel cold enough and store it in space for a long time, as cryogenic fuels can boil away in normal conditions. Liquid oxygen, for example, turns to gas at minus 297 degrees Fahrenheit. Picture a floating gas station, but with a space-age freezer to keep the fuel ready.
NASA’s Robotic Refueling Mission 3, which launched to the International Space Station in December 2018, could boost development of this idea. The goal is to store 42 liters of cryogenic fuel for six months without losing any fluid, which would show these space-age freezers could work for missions.
But NASA hasn’t always been so hot on the idea of super-cooled space depots. Ars Technica shared comments Thursday from physicist George Sowers, who claimed NASA’s gargantuan Space Launch System rocket hampered the development of in-orbit refueling. The case for building a giant rocket is somewhat undermined by fueling stations that enable smaller rockets.
In the case of SpaceX, refueling in space is part of the plan to reduce the needed size of the rocket. Musk noted during a September 2016 presentation on interplantary travel that not refueling in orbit would require a vehicle five to 10 times the size and cost. Musk has expressed interest in using other Starships to refuel a Starship on its journey, similar to how aircraft work.
“Mailing Dragon dock with Space Station is much harder than docking with our own ship for refilling,” Musk reassured his 27 million Twitter followers after the announcement, referring to the Dragon capsule that completes regular resupply missions to the International Space Station. “Not a problem.”
Famous last words perhaps, but the dream of a big gas station in the sky may finally reach fruition.