On paper the Space Shuttle was the signal of the new space age where access to the final frontier would be cheap and reliable, ushering in the next wave of human prosperity. It would do this through two innovative (at the time) ideas: make the craft reusable and reduce the turn around time on launches to a mere 2 weeks, enabling 26 flights per year at a drastically lower cost than any other launch system. Unfortunately due to the requirements placed on it by the numerous different agencies that had their hand in designing it the final incarnation could not meet the latter goal and thus failed to provide the cheap access to space that it dreamed of. Of course it also taught us a lot about spacecraft design most notably that giant space planes aren’t particularly efficient ways of getting payloads into orbit.
That doesn’t seem to stop people from designing more of them, however.
DARPA recently announced that it was seeking designs for a revolutionary space vehicle, dubbed the XS-1, with the intention of drastically lowering the cost per kg to orbit for small sized payloads (up to about 2,000KG). The design requirements are fairly open with the only stipulations being that the main craft is a reusable, hypersonic vehicle with the payload achieving the desired orbit using a traditional rocket. This means that whilst the potential craft detailed in the artist’s impression above is a good indicator of what the XS-1 hopes to achieve the actual craft could end up being radically different, especially if any of the other companies currently playing in this field having anything to do with it.
The main goal of this program is to drastically reduce the cost to orbit for smaller payloads, almost by an order of magnitude if you compare it to traditional launch systems. This, in turn, would lead to a lot of missions that were otherwise infeasible to become a reality and whilst the initial applications are more than likely to be military in nature I’m sure any private contractor would ensure a dual use agreement for the bulk of the technology. The crux of the XS-1, at least in my opinion, is whether or not this is achievable in the time frames that have set out for the project, considering that the first launch is scheduled for 2017.
Taking the rule of 6 into account (Mach 6 at 60,000 feet is 6% of the energy required for orbital velocity) a craft with such a flight profile would need to make several strong technological advances in order to be able to fly. The only engines capable of achieving speeds above that (at the required price) are scramjets and the fastest we’ve ever managed to get one to fly was Mach 5.1 last year. That means there’s still a long way to go to get sustained flight out of a hypersonic, air-breathing engine and it’s questionable that anyone would be able to achieve it in that time frame. Indeed even Lockheed Martin, who recently announced the hypersonic SR-72, doesn’t believe they’ll get a prototype flying before 2023.
I’m a fan of the idea, and indeed if anyone can pull it off I’ll be wildly impressed, however the technology to support it is still in its infancy with the cutting edge being far away from viability. There are other ways of tackling it of course but I can’t really see any of them being done for the price that DARPA is asking. Indeed the cheapest fully rocket solution goes to SpaceX but it’s still double the asking price for less payload than what DARPA requires. In any case the designs will hopefully show some ingenuity and, if we’re lucky, 2017 will bring us another baby brother to the retired Space Shuttle.