Okay kiddo, so do you know what a car engine is? It's what makes a car go vroom vroom!
Well, a fission engine is a type of engine that uses something called nuclear power to make things move. It's kind of like a super powerful version of the engines that power submarines and aircraft carriers.
Now, you might be thinking "wait, isn't that dangerous?" and you're right! Fission can be dangerous if it isn't done properly or if something goes wrong. But we have really smart people who know how to make it safe.
The scientists and engineers who are designing safe affordable fission engines are working on ways to make them smaller and more efficient than the ones we use now. This means they can be used in lots of different ways, like in spacecraft, or even in cars!
And the best part is that because they're more efficient, they use less fuel, which makes them more affordable in the long run.
So, in summary, a safe affordable fission engine is a powerful engine that uses nuclear power to make things move, but it's being made smaller and safer so it can be used in different ways and be more affordable in the long run. Pretty cool, huh?