Orbital service station

If rockets could be refuelled in space, it could open up interplanetary travel

If rockets could be refuelled in space, it could open up interplanetary travel

Following SpaceX’s first successful landing of its Starship test vehicle and the award of a NASA contract for a lunar lander based on Starship, it’s looking increasingly likely that a Starship will reach orbit in 2021. When launched on its booster stage ‘Super Heavy’, it will become the largest launch vehicle yet, capable of placing up to 150 tonnes into orbit. But once it has done that it will have exhausted its propellants, and the driving purpose of Starship, and indeed SpaceX itself, is to provide cost-effective transport to Mars.

The first challenge was to dramatically reduce the cost of reaching space, and SpaceX set about this by developing reusable rockets – initially the Falcon 9, and now Starship. As Musk has commented, if a jet airliner was used for a single transatlantic flight before being thrown away, flights would be impractically expensive. Recovering the expensive stages and using them again was the key to getting into space cheaply enough to ship a lot of cargo and passengers to Mars. Starship will make that cheaper still, but the next challenge is to refill craft in space so they can actually go somewhere else.

Previously giant rockets, like the Saturn V and the N1, were built that way because the missions they supported were taking everything with them. Now SpaceX plans to use the cost-effective launch capability provided by Starship to refuel other Starships headed for the Moon and Mars. But just like spin gravity, orbital refuelling has been long discussed, but never done.

Typical rocket propellants are cryogenic, liquified gases, like liquid oxygen at -186 degrees Celsius (-302 degrees Fahrenheit) and liquid methane at -161 degrees Celsius (-258 degrees Fahrenheit), which are challenging enough to handle on Earth. In orbit spacecraft are exposed to extremes of temperature, and without gravity propellants will float around their tanks in blobs.

To help pioneer this capability, NASA has just given $370 (£261) million to 14 companies working on a range of orbital propellant-transfer technologies. This includes a $53.2 (£38.2) million contract to SpaceX to demonstrate fullscale pumping of liquid oxygen in zero gravity inside a Starship test vehicle. It’s likely that this first test will involve pumping liquid oxygen from Starship’s main tank into the small ‘header tank’ used to run the engines for landing. But to do this Starship will have to accelerate slightly using its manoeuvring thrusters to create a mild gravity effect and settle the liquid oxygen against one end of the tank.

Once fully operational, SpaceX plans to launch up to ten dedicated tanker Starships to refill a single crew transport Starship for trips beyond Earth. This is part of the reason launch has to be cheap. One at a time, these simplified, uncrewed tankers would dock tail to tail with the Starship and prepare it for its long journey. Orbital refuelling is one of many seemingly simple steps that must be solved on the way to Mars, which will hopefully be proven in 2021. For the Moon, Starship can carry all it needs for landing, launch and return to Earth, but when it comes to Mars fuel will be produced from the Martian atmosphere and ice, which is why SpaceX picked those propellants in the first place.

“Once fully operational, SpaceX plans to launch up to ten dedicated tanker Starships to refill a single crew transport”