The DAIN-AI App takes two frames and uses a trained neural network to interpolate new frames in-between.
I have been trying this with tons of NASA footage, here are some comparisons:
https://youtu.be/xfC-D6A2QD8
https://youtu.be/NqZOxtNVrzU
https://youtu.be/YtA-i1Guiao
The rendertime of a 720p video is about 1 hour per 1 minute of footage. Enjoy.