Is it harder to make SSTOs with the new model?
Air breating engines seem to have been nerfed hard. Previously the Turbojet could go 2200m/s at 60km pretty easily but now it seems to only be able to do 1400m/s at 20km. The Rapier didn't get hit as badly and looks like it can do 1700m/s at 25km.
Throttle now takes into account your intake air so you can no longer flame out from having the throttle fully open.
A big part of the problem seems to be that the space plane parts can't stand up to the heat stress of being a space plane. Another is that whether or not something burns up makes no sense. I keep having a Mk2 Cockpit burn up while accelerating but the Mk2 Drone Core behind it is fine despite the plane moving faster than before and it being a flat surface and having the same Max Temperature.
EDIT: Pre-Cooler intake can survive going 1660m/s under 10km. but Ram intake cannot. Ram intake has its max temperature listed as 100 degrees higher than the Pre-Cooler. Looks like space plane construction has gotten very trial and error.