The current way of accessing space isn’t sustainable if we want to make it as a space fairing species. Whilst the methods we use today are proven and extremely reliable they are amongst the most inefficient ways of lifting payload into orbit around our planet, requiring craft that are orders of magnitude larger than the precious cargo they carry. Unfortunately the alternatives haven’t been too forthcoming, due in part to nuclear technologies being extremely taboo and the others still being highly theoretical. Still even highly theoretical ideas can have a lot of merit especially if they have smaller aspects that can be tested and verified independently, giving the overall theory some legs to stand on.
I’ve talked before about the idea of creating a craft that uses only a single stage to orbit (SSTO), in essence a craft that has only one complete stage and conceivably makes extensive use of traditional aerodynamic principles to do away with a lot of the weight that conventional rockets have. My proposal relied on two tested technologies, the scramjet and aerospike engine, that would form the basis of a craft that would be the Model T equivalent for space travel; in essence opening up space access to anyone who wanted it. In all honesty such a craft seeing reality is a long way off but that doesn’t mean people aren’t investigating the idea of building a SSTO craft using different technologies.
One such company is Reaction Engines, a name that I was only marginally familiar with before. They’ve got a proposal for a SSTO craft called Skylon that uses a very interesting engine design that combines both an air breathing jet engine as well as a traditional rocket motors. The design recently passed its first technical review with flying colours and could see prototypes built within the decade:
They want the next phase of development to include a ground demonstration of its key innovation – its Sabre engine.
This power unit is designed to breathe oxygen from the air in the early phases of flight – just like jet engines – before switching to full rocket mode as the Skylon vehicle climbs out of the atmosphere.
It is the spaceplane’s “single-stage-to-orbit” operation and its re-usability that makes Skylon such an enticing prospect and one that could substantially reduce the cost of space activity, say its proponents.
The engine they’re proposing, called Sabre, has an extremely interesting design. At lower speeds it functions much like a normal jet engine however as speeds approach Mach 5, the point at which my hand waving design would switch to a scramjet, it continues to operate in much the same fashion. They do however employ a very exotic cooling system so that the engine doesn’t melt in the 1000+ degree heat that would be blasting the components and once Skylon is out of the atmosphere it switches to a normal rocket engine to finish off the job.
The issues I see, that face nearly all SSTO designs, is the rule of 6 for getting to orbit. The rule simply states that at Mach 6 at 60,000 feet you have approximately 6% of the total energy required to make it successfully to orbit. Skylon’s engines operate in the jet mode all the way up to Mach 5 to an altitude of 85,000 feet which is no small feet in itself, but it’s still a far cry from the total energy required. It is true though that the first stages of any rocket are the most inefficient and eliminating them by using the atmosphere for both oxidiser and thrust could prove to be a real boon for delivering payloads into orbit. Still whether this will be practical with Skylon and the Sabre engine remains to be seen but there are tests scheduled for the not too distant future.
Walking through unknown territory like this is always fraught with unknowns so it’s no wonder that the team at Reaction Engines has been met with such skepticism over their idea. Personally I’m still on the fence as their technology stack is still mostly unproven but I applaud their vision for wanting to build the first SSTO craft. I’d love to see the Skylon making trips to the International Space Station, effectively replacing the shuttle and extending the ISS’ lifetime but until we see some more proof that their concept works I’m going to be skeptical, but it won’t take much to make into a believer 😉