That reminds me, earlier this month I saw a great PBS Nova episode about Neil Armstrong's life: http://www.pbs.org/wgbh/nova/space/first-man-on-moon.html It talked a bit about the X-15 and some crazy situations Neil got into flying them, like skipping off the top of the atmosphere at supersonic speeds.
"But something more potent than alcohol was needed for the X-15 rocket-driven supersonic research plane. Hydrazine was the first choice, but it sometimes exploded when used for regenerative cooling, and in 1949, when the program was conceived, there wasn't enough of it around, anyway. Bob Truax of the Navy, along with Winternitz of Reaction Motors, which was to develop the 50,000 pounds thrust motor, settled on ammonia as a reasonably satisfactory second best. The oxygen-ammonia combination had been fired by JPL, but RMI really worked it out in the early 50's. The great stability of the ammonia molecule made it a tough customer to burn and from the beginning they were plagued with rough running and combustion instability. All sorts of additives to the fuel were tried in the hope of alleviating the condition, among them methylamine and acetylene. Twenty-two percent of the latter gave smooth combustion, but was dangerously unstable, and the mixture wasn't used long. The combustion problems were eventually cured by improving the injector design, but it was a long and noisy process. At night, I could hear the motor being fired, ten miles away over two ranges of hills, and could tell how far the injector design had progressed, just by the way the thing sounded. Even when the motor, finally, was running the way it should, and the first of the series was ready to be shipped to the West Coast to be test-flown by Scott Crossfield, everybody had his fingers crossed. Lou Rapp, of RMI, flying across the continent, found himself with a knowledgeable seat mate, obviously in the aerospace business, who asked him his opinion of the motor. Lou blew up, and declared, with gestures, that it was a mechanical monster, an accident looking for a place to happen, and that he, personally, considered that flying with it was merely a somewhat expensive method of suicide. Then, remembering something he turned to his companion and asked. "By the way, I didn't get your name. What is it?". The reply was simple. "Oh, I'm Scott Crossfield."
Vietnam? The belief that low, slow and stealthy would win the day vs high and fast? Look at how many people claim that SAMs won out against the B-70 Valkyrie (basically, a heavy bomber analogue to the SR-71).
Hypersonic flight environments are very, very difficult to replicate in wind tunnels. We can replicate portions of the environment, but due to the highly coupled nature of the responses seen by flight vehicles at these speeds, there's always a tradeoff. To really understand what's going on, you need high enthalpy, high pressure, high Mach number flow for a reasonably long period of time. Experimental scaling is only so helpful depending on what you're trying to investigate. For a fully coupled response, you basically end up with a scaling ratio of 1 since the different physics clash when trying to scale the many parameters you need to achieve similarity. Realistically, flight test is the only way to truly get the conditions and response you need, but that's very expensive and risky and you can't build a huge factor of safety into the vehicle or it won't be able to meet the needed performance. Instead, we use a combination of simulation tools and targeted experiments to try and reduce the risk/cost for flight tests.
NASA published a comprehensive (if a bit dry at times) ebook, X-15: Extending the Frontiers of Flight [1] that covers the development and test program. If you're ever near Dayton, Ohio, the National Museum of the USAF has X-15A-2 [2] and an XLR99 [3] on display.
The X-15 was also a milestone in the development of flight control systems -- in some phases of flight, the aircraft was too unstable to be easily controlled by a human pilot, and on most flights, it had machinery massaging the pilot's control inputs.
"Digital Apollo", by David Mindell, has a chapter on this, before it gets into the space program proper.
Must have been utterly terrifying to pilot that thing... the entire aircraft is basically one gigantic fuel tank with a tiny space for a seat at the front.
I imagine the danger of faster airships is like falling out of successively higher windows. After you reach the 3rd storey it'll kill you, so it quits having any meaning.
I had always though the SR-71 was the fastest airplane, and it turns out I'm right (sorta). The X-15 isn't an airplane, apparently (hence why it's only referred to as an aircraft). I'm not sure why but probably because it's just a rocket and it doesn't actually "breathe air", nor does it seem to be able to take off on it's own?
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[ 2.8 ms ] story [ 73.6 ms ] thread"But something more potent than alcohol was needed for the X-15 rocket-driven supersonic research plane. Hydrazine was the first choice, but it sometimes exploded when used for regenerative cooling, and in 1949, when the program was conceived, there wasn't enough of it around, anyway. Bob Truax of the Navy, along with Winternitz of Reaction Motors, which was to develop the 50,000 pounds thrust motor, settled on ammonia as a reasonably satisfactory second best. The oxygen-ammonia combination had been fired by JPL, but RMI really worked it out in the early 50's. The great stability of the ammonia molecule made it a tough customer to burn and from the beginning they were plagued with rough running and combustion instability. All sorts of additives to the fuel were tried in the hope of alleviating the condition, among them methylamine and acetylene. Twenty-two percent of the latter gave smooth combustion, but was dangerously unstable, and the mixture wasn't used long. The combustion problems were eventually cured by improving the injector design, but it was a long and noisy process. At night, I could hear the motor being fired, ten miles away over two ranges of hills, and could tell how far the injector design had progressed, just by the way the thing sounded. Even when the motor, finally, was running the way it should, and the first of the series was ready to be shipped to the West Coast to be test-flown by Scott Crossfield, everybody had his fingers crossed. Lou Rapp, of RMI, flying across the continent, found himself with a knowledgeable seat mate, obviously in the aerospace business, who asked him his opinion of the motor. Lou blew up, and declared, with gestures, that it was a mechanical monster, an accident looking for a place to happen, and that he, personally, considered that flying with it was merely a somewhat expensive method of suicide. Then, remembering something he turned to his companion and asked. "By the way, I didn't get your name. What is it?". The reply was simple. "Oh, I'm Scott Crossfield."
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*we as a species
[1] http://www.nasa.gov/connect/ebooks/aero_x15_detail.html
[2] http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id...
[3] http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id...
"Digital Apollo", by David Mindell, has a chapter on this, before it gets into the space program proper.
I don't think the X-15 is in it, but it can give you an idea of what the times were like http://www.impdb.org/index.php?title=The_Right_Stuff
It's still worth a watch today.