Skylon Spaceplane Design Passes Key Review

gbjbaanb writes "A revolutionary UK spaceplane concept has been boosted by the conclusions of an important technical review. Skylon is a design for a spaceplane that uses engines that work as normal jets near the ground and switch to rocket propulsion in the upper atmosphere. The concept means the plane will not have to carry as much fuel and so will not need disposable stages. It is estimated (by its developers) that the Skylon will drop the cost of delivering payloads to orbit from $15,000 per kilo to $1000."

Comments

Re:PopSci != Tech Breakthrough

[rufty_tufty]

"The 10 or 20 kilometers that you can save by using this kind of design is really a small fraction of the distance to cross"
Distance isn't the problem for getting to orbit, velocity is.
By running as a plane you don't have to burn thrust to support the weight of the craft and fuel, you can accelerate up to Mach 5 (as they plan to) using the atmosphere to support you. That's a truly massive gain, for reference the first stage of the Saturn V got you up to just over mach 6. Now I don't know what percentage of their fuel they burn to get to that speed but to not have to support that weight with thrust for such a long period is a huge gain. Remembering as well that during this phase they are air breathing too which is another massive gain.
Fine you say mach 5 is 1/5th of the way to mach 25. so at best they've saved 20%, better but still not amazing.
Not quite because you get a weird multiplier effect, because (when you are at say mach 5) you have accelerated the fuel you carry to mach 5 so it effectively has more energy that when it was at rest on the ground. If you run the numbers for a multistage rocker you'll find that they can't reach orbit unless you take this effect into account. Trying to find a good source for this, will hopefully reply to this later with said source...