Everyday Astronaut has a recent video on "why don't we launch rockets from jets"[0]. The video obviously does not include reference to the engine mentioned here -- but I think most of the points still stand.
In particular
- orbit is mainly about going fast, not so much about getting up high
- your second stage will pay a structural price for allowing air-launch loads, and this structure needs to be carried to orbit
- rockets are *big*
In particular - orbit is mainly about going fast, not so much about getting up high - your second stage will pay a structural price for allowing air-launch loads, and this structure needs to be carried to orbit - rockets are *big*
[0]: https://www.youtube.com/watch?v=AAt9WDQEMoA