2 things that would dovetail well with each other (that I'd like to see)

[StevieC]

Pure ramjet engines (by which I mean ones with no turbojet mode, thus they can't start unless first accelerated to an adequate airspeed) and,

a MagLev launch-sled (aka a "Mass-Driver") to accelerate a winged, ramjet-propelled launch-plane to an airspeed at which its ramjets can be started.

This would allow a purely rocket-propelled orbiter to be hoisted aloft by the launch-plane to a higher altitude for its launch, and then the winged launch-plane could fly back and land, followed later by the re-entering orbiter which would also be designed to glide back for landing.

[ShadixDarkkon]

I don't know if you've seen it, but Instell Inc. mod adds two SCRAMjets which operate similar to what you're describing. Only start up once you reach 900m/s.

[StevieC]

Scramjets are kind-of a bit TOO dependent on speed. By the time one reaches a speed of 900 m/s, one is often at an altitude so high, and climbing so fast, that the scramjets aren't going to be usable for much more than ten seconds. Ramjets are what I'd prefer, for that particular reason. A MagLev launch-sled, simply-put, a horizontal mass-driver, could accelerate the winged, air-breathing lower-atmospheric stage to a high enough airspeed that the ramjets could be started, propelling the plane to a high-enough speed and altitude for a scramjet/rocket hybrid to get an orbiter up into orbit, while allowing the ramjet-powered lower-atmospheric stage to fly back down and land on a runway for reuse.