[cpleppert]

There are a lot of weaknesses in the design that haven't been worked out regardless of the technical merit of the engine. Reusable vehicles are tremendously expensive and Sabre has to peform perfectly not only on the engine but the vehicle as well to get any return on its investment.

Although the sabre is more efficient operating in the atmosphere than a conventional rocket engine the vehicle itself will have to spend the cost of having wings, strengthening the vehicle to operate in several orientations on ascent, operating in the atmosphere for longer than a conventional rocket(which costs fuel and requires the vehicle to be shaped in a way that just isn't as efficient as a conventional rocket), and being able to withstand reentry temperatures(more weight) not to mention not being able to get rid of dead weight on ascent. The reusable components of the engine also weigh a lot more as well. If you go through everything it doesn't look like that great a win; and that is if everything works perfectly.

The space shuttle never came close to its projections and this vehicle is far more technically ambitious in every possible way. I just don't think that their projections of even $10-15 billion in development costs are even remotely realistic. Last time I checked their business plan suggested they could break even if satellite launch costs didn't come down and the volume of launches of large satellites increase.