The pre-cooler is the key to REL's (Reaction Engines Ltd) amazing SABRE engines, which are themselves perhaps the primary factor permitting a single-stage-to-orbit space plane.The breakthrough for Reaction Engines has been in the development of its pre-cooler system. At Mach 5, SABRE will need to cope with gases entering at temperatures reaching 1,000 degrees celcius. The pre-cooler uses thousands of small-bore thin-wall tubes, each around the width of a human hair, to drop the air temperature to -150degrees celcius in just 30ms. Back when Skylon was still a concept, the required heat exchangers for this type of pre-cooled jet engine were impossible to make, but with improvements in materials and manufacturing techniques, Varvill believes the technology has turned a corner
The company describes the SABRE engine as being a rocket furnished with and additional pre-cooled turbocompressor. Air is taken in at the front of the SABRE and almost instantly cooled down to the point at which it is almost liquid, using terrifically powerful freezer kit employing a liquid-helium loop. The supercold air takes the place of liquid oxygen in the combustion chamber, reacting with liquid-hydrogen fuel to produce thrust in much the same way as the space shuttle main engines. Heat sucked from the intake air is dumped into the fuel.
This use of air instead of liquid oxygen for takeoff and the early part of the flight up to orbit means that the Skylon doesn't need to carry nearly as much liquid oxygen as the Shuttle does, and thus that it can carry a cargo to orbit without needing to throw large parts of itself away. It can also make a runway takeoff, so avoiding the costly and troublesome need to point itself vertically into the sky for launch.
The Register interviewed REL technical director Richard Varvill in 2010, and he said that the company is expecting to test a SABRE on the ground as soon as 2013 or 2014. However, before REL's backers will fund the building of a complete SABRE, it seems that they need to see the pre-cooler working.
REL has a 10-year roadmap to actual Skylon flights, with costs during that time of maybe $15bn. The plan works like a snowball: at first, comparatively small investments lead to comparatively minor tests and trials, which then unlock bigger sums allowing bigger building blocks to be built and validated and so on – until in the final years, big cheques get signed and actual Skylons get built.
The amazing spaceplane is expected to be able to repay those big investment cheques, as it will be able to deliver payloads – admittedly, at first quite small ones of only 10 tonnes or so compared to its own substantial mass of 275+ tonnes – at low cost. The whole Skylon is reusable, and re-using it is a comparatively simple matter of refuelling it, loading it up again and taxiing it back out onto the runway as opposed to strapping on a brand new set of tanks and boosters and standing it on end next to a launch gantry.
The ceramic aeroshell is to be just 0.5mm thick. The undercarriage has had to be lightened too, so that a Skylon won't be able to land on just any runway – it will need a special reinforced one able to cope with heavily loaded wheels moving rather fast. If the craft itself should gain just a few per cent in fuelled-up weight during the development process, this would wipe out its entire payload margin.
There are those who would argue that operations using liquid hydrogen fuel will simply never be economical: the stuff takes up so much room that hydrogen aircraft – including the Skylon – are always made up mainly of fuel tanks.
Private funding is lined up to see it through all stages of development, culminating with the start of commercial operations in 2020. That funding, however, is contingent on Skylon hitting some key milestones along the way, and a big one looms just a few months off.
Nextbigfuture interviewed Richard Varvill at the end of 2009
Crowlspace speculation on eventual Successful Skylon spaceplane costs
If 30 SKYLONs are made and each flies 200 times before replacement, then the development cost divided over all those 6,000 flights is $1.67 million, or $139/kg. If 300 SKYLONs are built then it’s just $13.9/kg. That’s development cost, to which we add per unit cost, per flight costs, and fuel costs, then profit. SKYLON uses 66 tons of hydrogen and 150 tons of LOX for propellant, which costs ~ $250,000 total adding $20.8/kg to the bill. Then there’s vehicle costs incurred due to wear and tear of the components of the cryogenic system, SABRE ejector ramjets, landing gear, RCS and avionics every flight. I’m not sure what the best estimate of those would be, but let’s assume roughly comparable to the fuel costs. Thus the bill is up to $55.5/kg. If a SKYLON costs ~$200 million per unit – expensive jet-fighter value – then that’s $1 million per flight over its lifetime, which adds $83.3/kg. That seems excessive and might go down with large production volumes. Perhaps we can halve it. Thus SKYLON might cost $100/kg to deliver payload to LEO.
How much payload do 300 SKYLONs need to carry to LEO to drive costs down? All up they represent 720,000 tons of payload lofted on 60,000 flights. If a 1 GW SPS masses 3,000 tons, including the GEO delivery system for its subcomponents, then 240 GW of SPS power could be installed. To power the whole world to the tune of 24 TW then 30,000 SKYLONs making 6,000,000 flights will be needed.
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