The problem with space is that there's nothing up there. A normal car engine, diesel engine, or even jet engine mixes a small amount of fuel with the outside air to burn it. You need that air in order to be able to burn the fuel—and as such, normal engines don't quite work in space.

The way we work around this problem is by using rockets. Rockets, unlike normal engines, take the "air" with them to space. In order to burn fuel with air we have to bring concentrations of the chemicals in air with us to space. While this works, it's inefficient because it makes our spacecraft very, very heavy. Whereas a plane's fuel may only be 10-30% of its total weight, a rocket's fuel is upwards of 90% of its total weight.

Just look at it all. The amount of fuel attached to that thing is ridiculous!

Since it's so ridiculous, this makes rocket launches very expensive and inefficient. If you want to go to space, you're going to need a lot of fuel. A lot of fuel costs a lot of money. This is why we've been trying to develop alternative methods of getting to space.

One such alternative is a SSTO, or single stage to orbit craft. It's a space plane—you could think of it as a plane that can fly high enough and fast enough to go to space. This is a dream that has existed in people's minds since the whole space race started, but only now is it coming close to reality.

Meet the Skylon, by Reaction Engines Ltd.

It can take off and land like a plane, and it has air-breathing jet engines like a plane does. This means its first and only stage is lightweight—like a normal airliner, it doesn't carry too much fuel because it uses the fuel that's already in the air.

Unlike a normal airliner, the Skylon's jet engines are a little different. The Skylon is the first plane to be designed with the SABRE (Synergistic Air-Breathing Rocket Engine). This means the engines function as jet engines would in the atmosphere, but they also have the ability to switch to a closed-cycle mode. When the plane is high enough and the air in the atmosphere is too thin to sustain a burn, the engine can switch to its internal fuel source and burn that.

Unlike normal jet engines, the SABRE functions a little differently. It doesn't burn the air at as high of a temperature, so it can be made with lighter, less heat-resistant materials. As a result, it also has a thrust-to-weight ratio much higher. Unlike ramjets or scram jets, which operate at high speeds, the SABRE runs at lower speeds in the atmosphere, and thus needs less heat shielding. Again, this means less weight, and better efficiency.

Just how much better are these engines, you ask? The peak thrust is at 1,350 kN for each engine in air-breathing mode. The F-22 Raptor, a modern American jet fighter, has a thrust of 116 kN for each engine without afterburner. Skylon is serious business.

The engine has already been tested on the ground, and Skylon's first launch is expected to occur in 2019.

All I can think is, "this probably wouldn't work in Kerbal Space Program," though.