So I read this this morning:
https://arstechnica.com/science/2020/06/the-rocket-motor-of-the-future-breathes-air-like-a-jet-engine/
And wondered how the idea could be applied to KSP... as an alternate to the Rapier.
So I was thinking that you have an air intake that doubles as a convert a tron. You then have IntakeAir converted to Oxidizer 'on the fly' as it were. The intakes that are supposed to be supersonic intakes do this faster as they are collecting more air anyway. But you'd only have the supersonic's do this.
Then use regular rocket engines as per normal. You just don't have to carry as much oxidizer.
Now I totally agree that with as the weight of Oxidizer is so similar to LiquidFuel in the stock game that this might not be practical. It would be better if the rocket burn ratios in stock were different as they are in some mods which changes the .9/1.1 ratio to 1/3. So this would be with mods in mind obviously.
There might also be a caveat... that their is a requirement for high EC to do the conversions. And also extra mass. That could offset with engines that are also alternators.
Happy to hear what people think.