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
No matter which way you cut it space is still the playground of governments, large corporations and the worlds wealthy. The reasons behind this are obvious, the amount of effort required to get someone or something into space is enormous and past applications that result in either scientific or monetary gain there’s little interest to take the everyman up there. That has rapidly changed over the past few years with several companies now making serious investment in the private spaceflight sector. Now nearly anyone who wishes to make the journey out of Earth’s atmosphere can very well do so, a privilege that until today has been reserved for mere hundreds of people. Still we’re far off from space being just another part of everyday life like flying has become but that doesn’t mean the seeds of such things aren’t already taking hold. In fact I believe with the right investment we could well see the Model T Ford equivalent of space within the next few decades.
Right now all commercial and governmental space endeavours use some form of chemical rocket. They generate thrust by throwing their fuel out the back of them at extremely high speeds and whilst they’re by far the most energy efficient jet engines you can create they’re also one of the most fuel hungry and also require that the craft being propelled by them carry their oxidiser¹ with them. Putting this into perspective the Space Shuttle’s external tank (the giant rust coloured cylinder) carries around 6 times more oxidiser than it does fuel with it, to the tune of 630 tonnes. That’s about 30% of the total launch mass of a completed Space Shuttle launch system and this has caused many to look for alternatives that draw their oxidiser directly from the atmosphere, much like the engine in your car does today.
Most solutions I’ve seen that use the atmosphere to achieve orbital speeds rely on a technology called scramjets. From a design standpoint they look a lot simpler than it’s turbojet/turbofan predecessors as there’s no moving parts used to compress the air. Scramjets rely on extremely high speeds to do the compression for them, meaning that they can’t be operated at lower speeds, somewhere in the realm of Mach 6 for a pure scramjet design. This means that they need some kind of supplementary thrust for them to be able to function.
One such solution is a that of an aeropsike engine. Apart from looking like something straight out of science fiction aerospike engines differ from regular rocket engines in that they don’t use the traditional bell shaped exhaust nozzles that adorn nearly every rocket today. Instead they use a concave spike shape that in essence forms a bell with outside air pressure. This has the effect of levelling off the performance of the engine at all altitudes although they suffer at lower mach numbers due to the reduced pressure. Still they compliment scramjets quite well in that they can be used in both situations where the scramjet can’t function (vacuum and low speed) whilst still remaining more efficient than current rocket designs.
Both of these ideas have been proposed as base technologies that would be used in a single stage to orbit (SSTO) launch system. All orbital capable launch systems today are done in stages whereby part of the rocket is discarded when it is no longer required. The Space Shuttle for example is a two stage rocket shedding the SRBs whilst it is still within earth’s atmosphere. A SSTO solution would not shed any weight as it climbed its way into space and the main driver for doing so would be to make the craft fully reusable. As it stands right now there are no true reusable launch systems available as the only one that’s close (the Space Shuttle) requires a new tank and complete refurbishment between flights. A fully reusable craft has the potential to drastically reduce the cost and turnaround time of putting payloads into orbit, a kind of holy grail for space flight.
SSTO isn’t without its share of problems however. Due to the lack of staging any dead weight (like empty fuel tanks) are carried with you for the full duration of the flight. Nearly every SSTO design carries with it some form of traditional chemical rocket and that means that the oxidiser tanks can’t be elminated, even though they’re not required for the full flight. Additionally much of the technology that a SSTO solution relies on is either still highly experimental or has not yet entered into commercial use. This means anyone attempting to develop such a solution faces huge unknown risks and not many are willing to make that jump.
Despite all this there are those who are working on including these principals into up and coming designs. NASA recently announced a plan to develop a horizontal launcher that would use maglev based track to accelerate a scramjet plane up to the required mach number before launching it, after which it could launch small payloads into space:
As NASA studies possibilities for the next launcher to the stars, a team of engineers from Kennedy Space Center and several other field centers are looking for a system that turns a host of existing cutting-edge technologies into the next giant leap spaceward.
An early proposal has emerged that calls for a wedge-shaped aircraft with scramjets to be launched horizontally on an electrified track or gas-powered sled. The aircraft would fly up to Mach 10, using the scramjets and wings to lift it to the upper reaches of the atmosphere where a small payload canister or capsule similar to a rocket’s second stage would fire off the back of the aircraft and into orbit. The aircraft would come back and land on a runway by the launch site.
Such a system would significantly reduce the costs of getting payloads into orbit and would pave the way for larger vehicles for bigger payloads, like us humans. Whilst a fully working system is still a decade or so away it does show that there’s being work done to bring the cost of orbital transport down to more reasonable levels.
A SSTO system would be the beginnings of every sci-fi geek’s dream of being able to fly their own spaceship into space. The idea of making our spacecraft reusable is what will bring the costs down to levels that will make them commercially viable. After that point it’s a race to the bottom as to who can provide the spacecrafts for the cheapest and with several companies already competing in the sub-orbital space I know that competition would be fierce. We’re still a long way from seeing the first mass produced space craft but it no longer feels like a whimsical dream, more like an inevitability that will come to pass in our lifetimes. Doesn’t that just excite you?
¹As any boy scout will tell you a fire needs 3 things to burn: fuel, oxygen and a spark. Rockets are basically giant flames and require oxygen to burn. Thus oxidiser just means oxygen which also lets rocket engines operate in a vacuum.