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| Building Blocks to an HOTOL SSTO RLV (was:Re: Boeing jet to break distance record) (74109) | |||
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Building Blocks to an HOTOL SSTO RLV (was:Re: Boeing jet to break distance record) |
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Posted by WillD on Thu Nov 10 02:12:24 2005, in response to Re: Boeing jet to break distance record, posted by RonInBayside on Wed Nov 9 23:31:37 2005. The ramjet was always so limited. It has the same problems a turbojet or turbofan has with respect to supersonic flow through the combustion chamber, but can't even get itself up to that speed by itself. All jet engines to this point have worked on the principle of taking in air, compressing it, putting fuel in it, igniting it and then hanging on as the gasses are expelled, powering the compressor and moving the aircraft forward. This is all fine and well at subsonic speeds, where none of the components are really making enough of a shockwave to knock the flame out, so you can have a massive inlet designed to admit as much air as you can possibly gulp down, and then add to that by using a second turbine set to power a massive fan at the very front, thereby creating a very efficient engine. However, as soon as you start going supersonic you need to play with the incoming air such that you don't have supersonic air through the combustion chamber. In fighters and such this is done with variable geometry inlets and ductwork leading to the engine, where hydraulically operated ramps bend the air until it slows down and increases in pressure. That works just fine until you're doing about Mach 3, where you can't slow the air down fast enough to keep the supersonic airflow from blowing the combustion chamber out. This applies for all engines to this point, be it turbofans, turbojets, or ramjets. Turbofans are in particular very picky about the air they take in, probably because of the massive fan at the front of the assembly. I know Ben Rich, former head of the Skunk Works, claimed that the the J-75 turboramjet inlets he designed for the SR-71 developed most of the power at the edges of it's flight envelope.Here's where the scramjet comes in, and as might be guessed from the acronym it simply means "Supersonic Combustion Ramjet". It's an engine which relies on air pressure alone for compression, atmospheric oxygen for the oxidizer, and is capable of performing combustion of fuels in a supersonic airstream. Up until about 2002 many aerospace engineers didn't believe a scramjet could even achieve measurable thrust. Around that time the Australians launched a tiny cylindrical scramjet on a sounding rocket which they promptly proceeded to lose. It wasn't until it was turned into local authorities that they recovered it and realized that they'd assumed the engine hadn't fired and the data logger onboard had recorded measurable thrust. Then in November of 2004 the X-43 Hyper-X achieved a speed of Mach 10 after riding a Pegasus booster to around Mach 4 and 300,000 ft. This small unmanned aircraft now lying at the bottom of the Pacific is now the fastest non-orbital vehicle in the world. Unfortunately no good deed goes unpunished and the X-43 program, including the X-43B scaled up model, the X-43C hydrocarbon burning model and the even faster X-43D (proposed for flight up to Mach 15), was scrapped as part of NASA's Vision for Space Exploration program. Fortunately there is a bright spot, the X-51 appears to be a restarting of the X-43C hydrocarbon fueled scramjet program. The X-51 is an Air Force run program, an outgrowth of their Scramjet Engine Development program. They of course have a great interest in scramjets for use in hypersonic cruise missiles, hypersonic exo-atmospheric 'skip' bombers, hypersonic recon aircraft (which they already have if you believe the Aurora hype) and perhaps even hypersonic fighters. It's good to see that scramjets aren't being shelved completely just because Dubya wants to go to the moon. However I do fear that if the Air Force team makes a great leap forward in scramjet technology this thing will be locked up at Groom Lake and NASA will never benefit from their research. Then again, it's likely the Air Force has been experimenting with scramjets for years, so perhaps the X-51 is just a convenient way to bring some of their black research into the white, 'non classified' side of aerospace. Either way, the promise of scramjets is great. The promise of a horizontal takeoff and landing single stage to orbit (HOTOL SSTO) is exactly what the shuttle promised us 25 years ago, except that these SSTOs wouldn't have the logistics tail the shuttle incurs on every flight. Being able to dispense with expensive launch facilities, use aerial refuelling to reach orbits of greatly varying inclinations, and a general increasing in flexibility when leaving earth all make these very promising vehicles. Of course an SSTO vehicle has it's downsides, as it's unnecesary to carry wings into space and bringing those wings back through the atmosphere requires a large, expensive and potentially troublesome thermal protection system. Also a scramjet provides no power in a vacuum as there's no atmosphere. A rocket based combined cycle engine or auxilliary rocket system has been explored for the transition from the roughly Mach 15 to Mach 20 (about 11,000 to 15,000mph) cutoff to the 17,000mph orbital velocity. However, the liquid oxygen which would be used by this rocket system, the rockets themselves in the case of an auxilliary rocket system, the wings and their thermal protection system would all be heavy and expensive to carry into space. To this end it might be preferable to use the current Crew Exploration Vehicle in conjunction with the SSTO. The HOTOL SSTO would carry the CEV from the ground up to something like 300,000 to 500,000 feet and between 12,000 and 15,000mph, where the CEV would pop out of the bay, fire up a LOX/Liquid Hydrogen upper stage and fly into orbit. This way the heavy wings and such never have to fly into orbit, the Scramjet doesn't have to be made to cope with the vacuum, and we don't have to squeeze liquid oxygen into what will already be a crowded airframe. |
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