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Aerospace Mach 6 at low altitude

  1. Nov 22, 2012 #1

    chasrob

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    A cruise missile, small--say 10 feet long, making 4000 mph at 1 mile altitude-

    Would it make a boom loud enough to break windows? The smaller the craft, the less the boom, correct? Probably would be a tough one to answer; no aircraft I'm aware of is that small, making that kind of speed.

    The X-15 could do Mach 6, but at what, 20 miles up. Any ideas about what the skin of a craft built for low altitudes could be made of? Something made of the same material as the shuttles tiles?

    At that speed and low, one mile altitude, would the air be ionized and the skin glow? How would it appear from the ground, in other words?
     
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  3. Nov 23, 2012 #2

    boneh3ad

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    It's hard to say how strong the boom would be in that case, but despite the small size, the strength of the shock is enormous. It depends a lot on the design of the nose, and the most relevant vehicle to what you are asking would be the X-51, but the design data for it to my knowledge is not publicly available. Various assumptions can be made, though. Let's say, just for grins, that the nose is essentially a wedge with a 12-degree half-angle. Under those conditions, the pressure ratio across the shock is 4.538. That's a pretty substantial pressure increase so the intensity of the boom relative to the weight would likely be pretty high.

    As for temperature and the air, the temperature ratio across the shock assumed before is 1.694. At a mile altitude, the average temperature is about 278 K, so the temperature behind the shock for most of the shock would be around 471 K, which is nowhere near the temperature needed to ionize the air. However, right near the nose tip where the shock is quite possibly detached, the shock is essentially a normal shock, and the temperature ratio behind that at Mach 6 would be 7.941, giving a temperature of 2207 K, which certainly could start to ionize the air, though in a very tiny region. In other words, it would likely look like the tip was glowing but the rest of the vehicle would look largely like it does while not moving.
     
  4. Nov 23, 2012 #3

    chasrob

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    Thanks for the info, much appreciated.

    I realize much of the Waverider tech is classified, but do you think a larger vehicle able to carry several passengers is doable with today's tech? The structure... the body of the aircraft, I mean, not the propulsion method to get up to that speed.
     
  5. Nov 23, 2012 #4
    Well, no. An aircraft travelling at Mach 6 at an altitude of 1 mile would have a very short life because of the structural fatigue-induced failure as well as excessive thermal fatigue in the outer skin. And the sonic boom could shatter a single glazed garden shed window :)
     
  6. Nov 23, 2012 #5

    boneh3ad

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    Fatigue wouldn't be noticeably more problematic for a hypersonic vehicle than it is with a standard airliner except in the sense that more exotic materials are less likely to be fully understood in terms of fatigue life. The real issue is having materials which can withstand the temperatures without being extremely heavy or fragile, since most materials that we know of that come close generally are quite dense or else are very brittle. Propulsion aside, I don't think we have the technology right now to produce a large, sustained flight hypersonic vehicle. The materials are just as far if not farther from being ready than the propulsion.
     
  7. Nov 23, 2012 #6

    chasrob

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    How about a structure like the Waverider, no wings, wedge/bullet shaped; say, disregard the small fins/stabilizers too. Still able to carry several passengers; the size of a small civil aircraft. Also, say weight does not matter, so you could have a 3 inch thick titanium alloy skin.
     
    Last edited: Nov 23, 2012
  8. Nov 23, 2012 #7

    Borek

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    This is a question of the "how do the laws of physics look like when there are no laws of physics" type. We don't speculate here.
     
  9. Nov 23, 2012 #8

    boneh3ad

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    Good luck with stability of that craft. You have just removed all control surfaces. I wouldn't ride it. It also doesn't solve the issues of thermal protection either, even in this "ideal" case you posit.
     
  10. Nov 23, 2012 #9

    Astronuc

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    Aerodynamic heating would likely be prohibitive at low altitude for high mach. One can look into the SR-71 for those issues. Some of the highest speed flights of X15 used an ablative covering to remove the heat.

    The waverider concept has already been proposed under the National Aerospace Plane project.

    http://en.wikipedia.org/wiki/Rockwell_X-30

    The outer surface would need have strength and oxidation resistance at high temperature. Not too many materials like that. And thermal fatigue would be an issue.

    http://www.rand.org/pubs/reports/2007/R3878.1.pdf

    The craft needs 'wings' or control surfaces with lift capability at 'low speed', if one wishes to land safely after flight. Otherwise, the craft is a ballistic missile.
     
  11. Nov 23, 2012 #10
    The thermal stress is not much worse at low altitude than at high altitude. It's more a question of mechanical loading, but if one is willing to build sturdy, no reason.

    If all leading surface are swept they'll keep bearable temperatures. We're no more in 1950. Only the nose needs better materials, but just use the proper one, it's not that hot.

    The limits I see are not of pure feasibility:
    - Why fly low when you can fly high? You're pulling our leg with passenger transport.
    - How to make money with a thick heavy plane?
    - How to convince airframe designers to do it? It will not be built like traditional planes.
     
  12. Nov 24, 2012 #11

    chasrob

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    Great links, thanks.

    Those temperatures in your quote--wouldn't the craft hold its speed down until it reached altitude, to limit stress and thermal issues? The big problem with my scenario is the low altitude, thicker atmosphere, to my thinking. I was also thinking about multi-hour 'flights'.

    Regarding the passenger vehicle, I simplified it(with no control surfaces, etc.) in order to ask about its very general shape, outer surface composition, but mainly I was wondering if such a dumbed down arrangement could be built with today's tech.
     
  13. Nov 24, 2012 #12

    chasrob

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    The small cruise missile I posited? Other folks I asked were not so sure.

    I read somewhere where boom intensity does not appreciably increase above mach 1.3.
     
  14. Nov 24, 2012 #13

    chasrob

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    You're absolutely right, I wouldn't go anywhere near such a beast!

    So there's no metal alloy or multi-layer ceramic/alloy that can handle the thermal issues, especially an extended flight of a couple hours? You said, above, 2200 Ks--wow.
     
  15. Nov 24, 2012 #14

    chasrob

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    You're right its not feasible with those issues.

    I was just wondering if it could be built with recent tech, and simplified it so the many problems about lift, propulsion, could be made manageable for an informed guess about materials that could handle stress and heating issues. And if, say, a general wedge shape would be ideal for the overall structure.

    Weird question, I agree.:grin:
     
  16. Nov 24, 2012 #15

    boneh3ad

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    Not even the superalloys like Incoloy or Inconel come close to the temperature resistances needed for those types of temperatures. Things like tungsten and iridium could withstand it, but all those sorts of metals are either exceedingly heavy or exceedingly rare and expensive or both.

    Like I said, the problem isn't inventing or discovering materials that can withstand the heat. The problem is finding materials that can withstand the heat that are light enough, strong enough and cheap enough to make it economical, and that we just don't have.

    For what it's worth, the difference in temperature at 1 mile vs. 8 miles or 10 miles. The temperature difference would be deadly to a person, but in an absolute sense, it isn't that much.
     
  17. Nov 24, 2012 #16

    chasrob

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    So its possible, but not economically feasible--thanks!

    Wouldn't heat build up over a couple hours through conduction, made worse by the low altitude?
     
  18. Nov 24, 2012 #17

    boneh3ad

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    How is that what you have taken from this? Plain and simple, it is not possible given current technology. Even those exotic metals, while they won't melt, would experience notable expansion and weakening at those temperatures. Oxidation would be a problem. Thermal fatigue. It just wouldn't work.

    And heat will build up over time to a point. Once the surface reaches the temperature of the air around it though, no more heat gets transferred.
     
  19. Nov 24, 2012 #18

    nsaspook

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    There are videos of a Sprint missle launch going from 0 to mach 10 in a few seconds. It's white hot after only a few seconds at that speed. It had a range of about 25 miles and a typical intercept time was expected to be about 15 seconds.



    There is information about the heat shield here:
    http://srmsc.org/pdf/004431p0.pdf
     
    Last edited by a moderator: Sep 25, 2014
  20. Nov 24, 2012 #19

    chasrob

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    Oh, OK, it's impossible, I misunderstood your tungsten and iridium remarks. I'm still under the Saturday slows. Gotta get me that second cup of java.
     
  21. Nov 24, 2012 #20

    chasrob

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    Awesome, as the kids say. 130 g's acceleration! Mucho thanks for the pdf, I've downloaded it and will read later.
     
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