A liquid rocket vehicle would have to be designed to be carried and launched from a carrier vehicle like Strato has, but that's eminently doable. You design in

A liquid rocket vehicle would have to be designed to be carried and launched from a carrier vehicle like Strato has, but that's eminently doable. You design in the load paths suitable for carriage and also for engine-on ascent loads. There will be more structure attached to the cylindrical sections. But this added structural mass further detracts from the delta V advantages of being released from a "Stage 0" air platform at altitude and speed. And yes, you also design for the bending loads associated with acquiring the ascent flight path angle you need. Those bending loads will be more severe than what a well-guided ground-launched system will be designed to endure. Strato's aircraft undoubtedly pitches nose-up pretty hard before dropping the rocket, but it's definitely nowhere near vertical. Solid motor casings are inherently quite stiff, to deal with combustion pressures. There would be less attachment/carriage hardware mass penalty than with liquids. Bottom line, there is nothing inherent to Stratolaunch that makes it incapable of launching liquids, and even with the additional structural mass penalty, it's entirely possible for Strato to have lower cost per unit of mass to orbit via a liquid rocket stack than with solids (likely, in fact).