[benl]

Are they really more efficient, for real world uses?

The way I understand it, the argument goes something like this. To reach orbit you need to accelerate to very high speeds (Mach 25). To avoid crazy drag losses you therefore want to get out of the atmosphere as quickly as possible. So being able to use oxygen from the air on your way up is a marginal gain at best, and almost certainly not worth the extra complexity.

[olex]

The main advantage of the Sabre concept is not only using outboard air as substitute for onboard oxidizer, but also as reaction mass. This allows to accelerate to over Mach 5 while still flying in airbreathing mode, using a fraction of the amount of fuel that would be required to reach the same speed on pure rocket power, and no onboard oxidizer. Once that speed is reached, the air becomes too thin for the air-breathing mode to continue functioning, and the engines switch to rocket mode, burning onboard oxidizer instead of air; but because the ship is nearly out of the atmosphere at this point, it doesn't take much to finish accelerating to orbital speed. Think of it basically as replacing the whole first stage of a conventional rocket with some jet engines and a sip of fuel for them.

[nemof]

The primary benefit is weight reduction:

http://www.reactionengines.co.uk/sabre_howworks.html

This approach enables SABRE-powered vehicles to save carrying over 250 tons of on-board oxidant on their way to orbit, and removes the necessity for massive throw-away first stages that are jettisoned once the oxidant they contain has been used up, allowing the development of the first fully re-usable space access vehicles such as SKYLON.

[Retric]

The end product from a hydrogen fueled rocket is H2O. Of which oxygen represents 88.8% of the weight so there is a lot of weight savings available to make up for the increased drag and a more complex engine design. Also of note you need to accelerate not just the ship and cargo but also all that unspent fuel so weight savings tend to multiply.

An air breathing rocket that hit's Mach 5+ means on the order of 30% less fuel and less structure to support that fuel. Which not only allows for more cargo but significantly more redundancy and a much higher structural safety margins.

[willvarfar]

Maybe they make most sense for military missiles?

[moocowduckquack]

I think you may have it back to front. To avoid crazy drag losses you cruise the lower atmosphere, then kick to high speed once into much thinner air.

[ianstallings]

My understanding is that a lot of the energy used to get to space these days is to get through the lower atmosphere. The idea of a space plane is to fly higher to the thinner atmosphere and then begin the ascent to space, avoiding the energy loss used burning through the entire troposphere. If you can fly to 5-6km you just took off a significant amount of total energy cost. Then you crank these babies up and fly to mach 5 and launch your payload or maybe the craft itself using rockets. Because you already got 1700 m/s already to start with.

And to answer the obvious question yes I have been playing entirely too much Kerbal Space Program.

[speeder]

You need to avoid drag losses if you are not using air in first place, because then any drag losses is wasted fuel and reaction mass.

But if you are using a air turbine, you can attempt to make it more economical than the losses from drag and extra machinery weight.

This is more for some type of missions though, if you wanted to make a rocket to pluto, then probably the extra weight is bad, but for a orbital space shuttle this is awesome.

I see the optimal path for long term exploration is use such turbine/rocket hybrids to build orbital shipyards, and then use those shipyards already in the orbit to build pure-rocket ships for missions that don't need to start on the planet (like a voyager-like exploration mission, or launching a hovercraft scout to gas giants...)