Space travel is on the rough end of the stick when it comes to physics. To get ourselves out of the massive gravity well that keeps us from travelling to the stars we have to expend vast amounts of energy, usually in the form of a chemical rocket. It’s a tried and true system however with chemical rockets powering every single mission that has left the confines of earth. There has been talk of many other forms of propulsion that could potentially perform a lot better than our trusty chemical companions but thanks to their fuel being of the nuclear variety they’ve never made it past the theoretical stage. Still for all their successes chemical rockets still have their draw backs, not least of which is the ungodly amount of fuel they use.
Take a look at any rocket and you’ll notice that the vast majority of it is taken up by a single component, the fuel tank. Whilst the actual cost of the fuel is a rounding error when compared to the cost of developing the rocket itself the fuel still makes up the vast majority of the wet mass of the craft, usually 85% or more. To put in in perspective the biggest rocket ever built, the Saturn V, weighed in at a massive 3 million kg when it was on the launch pad but only delivered 120,000 kg to low earth orbit (with 45,000kg eventually reaching the moon). A mere 4% of the total launch weight made it out of earth’s gravity, a truly staggering figure. This is more commonly referred to as the mass ratio.
It should come as no surprise then that the limiting factor for many space missions is weight related. As payloads get bigger so does the rockets and the amount of fuel required to lift them into orbit. This puts an upper limit on how big rockets can get before the amount of fuel required becomes unmanageable and instead many missions will favor multiple, smaller launches in order to get the required payload launched. The International Space Station is a good example of this as its current mass, some 420,000 kg, would have required a rocket of unimaginable size to launch all once. Instead it has been assembled in numerous smaller flights each adding around 20,000 kg each time. Most missions do not have this kind of luxury however and their designs represent a trade off between capabilities and the maximum launch weight they can have.
Most notably this affects missions that want to reach further than earth orbits, such as missions to other planets. Since they have to carry all the fuel required to get into orbit and to get them started towards their destination the payloads they can deliver are far smaller than they could be. Whilst we’ve still been able to do an amazing amount of science and exploration with such vehicles it’s still one of the most limiting factors that keeps more ambitious missions (read: ones with us humans in them) from being realized. There is however one ingenious solution to this problem, and that’s refueling in orbit.
Whilst the notion of flying just fuel up into orbit might seem like a strange idea it’s one that will enable subsequent missions to be far more capable. Indeed the cost of carrying several tons of fuel for pushing out past earth’s orbit adds many times that in launch mass. Thus craft that can refuel once in orbit can be significantly heavier at launch (since they’re not carrying the fuel) and can then fuel up for their trip beyond earth. The idea originally started to get traction back when Obama announced his plan for space exploration back in early 2010 and it seems that it’s finally going to become a reality:
Space explorers who need to top off the fuel tanks on the way to the moon or Mars may soon get their orbital refueling stations. NASA has put out the call for a $200 million mission to show how to store and transfer rocket propellants in space.
NASA wants to look specifically at liquid oxygen and liquid hydrogen, which have powered the main engines of the space shuttle and several commercial rockets. Its proposal calls for “zero boil-off storage” of liquid oxygen, and at least “minimal boil-off storage” of liquid hydrogen.
The proposal comes with the promise of $200 million for the company who wins the opportunity to build the station with an additional $100 million should they be able to demonstrate significant benefits for the additional investment. Whilst there are already companies working on these sorts of ideas NASA’s proposal goes far beyond what they’re currently capable of and is being built with the vision for larger missions beyond earth rather than refueling satellite’s station keeping fuel reserves. The proposal could also have flow on benefits to companies like the United Launch Alliance and SpaceX who could design future crafts around the idea of being able to refuel on orbit.
If we want to get serious about extending our presence beyond our home world (and we’re too afraid to use nuclear rockets) then orbital refueling stations are the key to realizing that vision. We’ve started to make the first steps to commoditizing space travel and the next logical step is to start unlocking access to other parts of our solar system, both for science and the simple prospect of exploring the unknown. Whilst this idea might not be realized tomorrow its a helluva lot more real today than it was yesterday and humanity is one step closer to taking our rightful place amongst the stars.