Are Sabre Engines More Efficient?

[Jouni]

The example I knocked together for this demo uses 2 XM-G50 intakes, which is comparable to a single ram intake or shock cone and weighs about 13.5 tonnes. It would still work with a single intake or even less, but then real-estate to mount the intakes becomes a problem on my design.

Here's an example with less than 1 standard intake:

http://i52.photobucket.com/albums/g13/GoSlash27/Wheezy_zps0moyuzwk.jpg

Well, that contradicts my experience from 0.23. Back then, my 8-10 tonne planes could fly horizontally until >40 km with a single ram intake, but they lacked the power to accelerate past ~1950 m/s. 7.5-8 tonnes was the critical threshold, below which the plane could accelerete to ridiculous speeds. Apparently Squad has changed something and made spaceplanes even easier than before, and now my experience with them is outdated.

This is true, although as you said upstream the entire thing hinges on the vehicle being properly designed and flown. If it's too heavy, doesn't have enough wings, or is flown like a rocket... then closed cycle thrust becomes non-trivial and the RAPIER can become preferable.

The idea is basically that because a single RAPIER can replace six 48-7S engines, you shouldn't use them in small planes that require less rocket power. If you have a plane with one RAPIER and two (four) turbojets, the plane has 5% (3%) less thrust during the early part of the ascent than with all turbojets. At high altitudes, you shut down the RAPIER and get exactly the same performance as with a turbojet-powered plane. Upon switching to rockets, you get 3% less thrust at a 3% higher Isp than with six 48-7S engines, but you have saved 0.6 tonnes of engine mass.