How feasible are Jet/Rocket Hybrid VTOL Lifters?

[The Yellow Dart]

I've seen a few VTOL orbital lifters now that get to orbit using air breathing jet engines for ascent from the surface, supplemented by a little rocket thrust, then switch to rocket only once atmosphere runs out. The most notable are the ones used by Scott Manley in his Reusable Space Program series

, and another one used in his Interstellar Quest Series (his carry pretty formidable payloads as well). It seems like this could be a feasible real world technology to me, and a way to reduce launch costs dramatically thanks to jet engine fuel efficiency. Jet engines could also be ditched (in a solid ring with parachutes maybe?) and recovered (like in the video) since they would stop working well before orbital speed is achieved. They main thing I think KSP doesn't address is the need for cooling jet engines during VTOL. I know Harriers have to carry a large amount of water in order to use their VTOL ability, and are still limited to less than 90 seconds of hovering before they run out of water (sorry True Lies, that thing would have crashed like three times over). This could be solved (it seems to me) by a ditchable water tank or SRBs used to get the ship up to speed so it can be aircooled. Does anyone know if this combination has been explored at all? Anyone else see any problems with this idea?

Here is another one by ihmawrtl

[rpayne88]

Or use high bypass engines. The air is routed around the combustion chamber to cool the engine. I THINK this is how turboshaft engines used in helicopters stay cool.

[Ralathon]

The problem is that jet engines in KSP are absurdly OP. Jet engines in real life lose thrust as a function of velocity, making it extremely difficult to use them at either high speeds or high altitudes. Engines that do work in those conditions (RAM, SCRAM) often have certain minimum velocities before they even start working. Which means that you need another set of engines to start the whole thing moving.

Another thing that's OP about KSP's jet engine model is the TWR. In real life you need a very strong engine to become VTOL worthy. This puts some nasty penalties on your weight, and thus payload capacity. Engines are also mechanically complex, and thus expensive.

That's an awful lot of drawbacks for an engine that's only usable for the first minute of your launch. That's why people use cheap solid boosters instead.

[K^2]

Air-augmented rockets and SABRE are the two practical ways to use the general concept. But as you can see, both have to get rather creative to get around the problems Ralathon talks about.

[Red Iron Crown]

I think the best way to incorporate jet engines into a lifter is SpaceShipOne style, with a jet powered mothership that gets the rest of the rocket to moderately high altitude and speed before dropping the rocket only final ascent vehicle.