Fuel fraction

Fuel fraction is how much of a spacecraft's weight is made up of fuel. It's like thinking about how much of your backpack is snacks for a long trip versus how much is your school work.

So when you're planning a trip to space, you need to figure out how much fuel you'll need to get where you're going and how much you'll need to get back. This is important because the more fuel you have to carry, the heavier your spacecraft will be, and the more powerful your rocket engines will need to be to lift off the ground.

Imagine trying to carry a full backpack with lots of snacks and heavy textbooks, it's going to be harder to lift and carry compared to a backpack with just a few snacks and light books. The same goes for a spacecraft, if it’s carrying a lot of fuel, it will need more power to take off and fly through space.

To make things more complicated, you'll also need to figure out how much fuel you need for any mid-flight adjustments or emergencies. This can be difficult because you don't want to carry too little fuel and run out mid-flight, but you also don't want to carry too much fuel because that extra weight will make it harder to move around in space.

So, when scientists talk about fuel fraction, they're trying to find the best balance between carrying enough fuel to get where they need to go and staying light enough to be easy to move around in space. Just like how you would pack your backpack for your trip, scientists need to efficiently pack fuel in order to make their space missions successful.