I'm less familiar with Kilopower but I spent a couple years working on something very similar that ran off of PU-238. Looks like Kilopower is U-235.
Looking into this, it needs a simple control system. The PU-238 bricks just made heat continuously with no control. But, they were also much less power.
If there's a control system you can launch the reactor cold. With the Plutonium bricks you cannot, meaning your cooling system (engine in this case) has to keep running during launch.
An rtg can be designed to handle cooling failure. It isn't like a water reactor. As the brick heats it expands, decreasing reactivity and heat production. So an rtg can live perfectly well without its cooling system. It won't be making useful power, but it isn't going into meltdown either.
The RTG has to be kept cool continuously on the launch pad with a nitrogen purge. All cooped up in a fairing, it’d cook the spacecraft if the heat wasn’t rejected somehow
Looking into this, it needs a simple control system. The PU-238 bricks just made heat continuously with no control. But, they were also much less power.
If there's a control system you can launch the reactor cold. With the Plutonium bricks you cannot, meaning your cooling system (engine in this case) has to keep running during launch.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/200900...
https://rps.nasa.gov/technology/