Fission isn't necessarily easy. One problem is that the reactor behavior changes when you add the generators to the reactor. These components can reflect neutro

Fission isn't necessarily easy. One problem is that the reactor behavior changes when you add the generators to the reactor. These components can reflect neutrons back to the reactor, which changes how much power the reactor produces. That being said, 'easy' or 'hard' are really not the best way to think about flight ready hardware. Actually making hardware and guaranteeing that it works involves so much more than just understanding nuclear fission. We can come up with a design on paper, but without testing we don't have assurance that it works. Simply putting the reactor together and testing it are more complicated than they seem. This is because of three things matter can reflect neutrons back, humans are harmed by radiation, and humans are the ones assembling/transporting/testing the reactor. We have to be careful putting the reactor together so that it doesn't spew neutrons and harm the humans assembling it. We need to develop ways to transport the reactor so that it doesn't spew neutrons. When we test the reactor, we have to set up our test so that humans won't have to get anywhere close to the reactor for a couple of months. There's also other issues like training people to do these things and NASA cooperating with the DOE to do the testing. These are 'simple' things, but they still need to be done.