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XB-70 Defensive Missile Concept: The Pye Wacket


Belesarius
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It's a damned interesting idea for sure.

 

I'm curious about the aerodynamics of that shape.  For low supersonic drag, you usually want something long, pointy and skinny.  Does a lenticular shape work well at high speeds?  I don't know, but perhaps it didn't matter too much because it was a short-range defensive system.

 

For a while, the Russians were showing off backwards-mounted missiles on the SU-37, IR and radar both.  The idea was that there would be a radar in the tail stinger that could guide the radar ones.

 

Now LockMart is looking into self-defensive missiles too.

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The page Bele linked gives a speed of Mach 6.5, which is pretty well into the hypersonic region. Once you get to those Mach numbers, you start seeing a lot more blunt looking designs (such as the Apollo capsule), due to thermal issues (with a blunt leading edge, the shock wave is going to detach and form a distance in front of the body, dissipating energy there and reducing thermal loads away from the surface). The lenticular shape could be an effort to do that, although all the pictures I've seen still show a sharp leading edge, which would still have significant thermal loading (albeit less than a point), so I dunno. However, it does look like the L/D ratio would be quite good, which is a point in the shape's favor. (I doubt you could make a fully optimized waverider design with enough maneuverability to serve as a terminal defense missile).

 

The Air Force did quite a bit of wind tunnel tests; http://www.dtic.mil/dtic/tr/fulltext/u2/314095.pdf, it appears that they might actually have been looking at the shape due to its unstable nature.

 

Edit: here's what some waverider designs would look like, for reference;

 

fig24-2.jpg

fig20.jpg

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Look at the Sprint missile.  Mach 10 (+) and it looks like a pointy cone.

 

If the Megafrisbee thing is going around mach 7, it'd hit it's max range (say 80miles) in around .95 minutes.(VERY Approximated math)

 

Considering it's a one way trip, I doubt the designers gave too much concern to heating issues, as by the time it was seeing real damage it'd probably already have delivered its payload in a pretty terminal manner.

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Aerodynamic heating would significantly effect the sensitivity of the IR seeker.

Think about how the thing would work.  We're talking ~7200 FPS, and a max range of ~80 miles.

An AIM9 is only going around Mach 2.5/2700 FPS by comparison.  

 

All that seeker has to do is get it pointed in the right direction, and from there it's going to be on target so fast there likely would not be much in the way of course corrections. 

Better to think of it as a superfast bullet with a skosh of initial guidance, and a VERY brief time to target.

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Assuming a normal shock at the front of the missile at Mach 6.5 (not a perfectly valid assumption, but I'm too buzzed/tired to crunch out some hardcore hypersonic gas dynamics equations), you're going to get a temperature ratio across the shock of roughly 8.2. So, with an outside air temperature of  225K (fairly close to the air temp at 25-30km), you're going to have temperatures at the front of the missile in the neighborhood of 1850K. Not hot enough to start dissociating the nitrogen and oxygen in the air, but definitely enough to introduce a lot of noise into any sort of infrared imaging systems (especially given 1950s/60s signal processing). As a result, I'm assuming that Pye Wacket would have been fitted with some sort of command guidance.

 

Sprint operated in a similar (albeit more extreme) thermal environment, and was also command guided (it helps that the  W66 has a much larger kill radius than 55 kilos of high explosive).

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That's why I'm guessing the IR would just make a minor adjustment right at launch, and the sheer velocity would make subsequent ones unneeded.

 

I'd wonder what the expected range of intercept was, with max being ~80 miles, so the functional average projected range would make guidance much after launch pretty much un-needed.

 

With the thing massing ~500 pounds, the warhead would kind of be just a happy little "fuck you", since the kinetic energy of the megafrisbee itself, would be pretty impressive.

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