How pointy Minuteman III does not burn

[kerbarara]

So, in a googling session I learned the best way to protect from high speed high heat reentry was to have a blunt reentry vehicle, so that a layer is built between the shockwave compressed air and the vehicle structure.

However, the Minuteman III and other ICBMs payloads seem to be pointy and reach speeds in the thousands of km/h and temperatures of times the sun. How does it not blow up and melt before reaching the ground?

The more details, the better!

[A Fuzzy Velociraptor]

I don't know much about the minuteman systems, however for high speed vehicles which do not use a bluntbody, vehicles may be passively or actively cooled. Most ICBM systems will be a passive system which will either use heat resistant materials like tungstun or graphite or may use a ablative material like graphite or cork.

[lobe]

Reentry Vehicles (RVs) in the Minuteman missile is made of close to the same material as the STS's nose cone and leading edges: phenolic carbon-carbon. I think, I did read that somewhere that those parts of the STS where classified. 

[tater]

They don't need to survive terribly long.

[shynung]

They'd have to, else the nukes inside would be useless before long.

Though, the machines inside can be designed to endure higher heat loads than a typical manned capsule, so that's that.

Edited April 27, 2016 by shynung

[Basto]

I think the biggest difference is the fact that a reentry vehicle the point is to slow it down vs a missle payload slowing down would make it possible to intercept and destroy it.  I don't believe they spend nearly as much time in the atmosphere. 

[PB666]

ICBMs are suborbital and you dont have to stop them before the hit the ground. Nose of the icbm is ball shaped to create drag with out creating alot of turbulence, you can create a less speedy rentry by launching an arc leaving the atmospher at 45 degrees or so. you can insulate under the cap to keep the heat on the surface. 

[wumpus]

First: are you sure the "pointy end" goes down?  Unless they put most of the Plutonium or other mass in the pointy end, I'd expect it to be at the top.  Think of a raindrop: if it is remotely of uniform density (unlike a mark1 capsule on top of an empty fueltank...) it will fall pointy end up.

Second, they explode pretty high up, although less than I thought, MMIII "only" has a .17 Megaton warhead instead of the previous 1.2Megaton.  I'd expect it would blast lower than the 1.2M warhead, pretty much an attempt to "even out" ground zero over a wider range.

[sevenperforce]

A blunt re-entry surface is ideal when you want high drag and low heating. An ICBM doesn't need high drag, and it only needs to survive long enough to get to the surface. 

[shynung]

Not only that, an ICBM needs as little drag as possible. Any extra drag means the missile spends more time being vulnerable to interceptors.

[SuperFastJellyfish]

2 hours ago, wumpus said:

First: are you sure the "pointy end" goes down?  Unless they put most of the Plutonium or other mass in the pointy end, I'd expect it to be at the top.  Think of a raindrop: if it is remotely of uniform density (unlike a mark1 capsule on top of an empty fueltank...) it will fall pointy end up.

Just a simple graphic, but this shows them falling pointy end down.

https://en.wikipedia.org/wiki/LGM-30_Minuteman#/media/File:Minuteman_III_MIRV_path.svg

Edited April 27, 2016 by SuperFastJellyfish

[jwenting]

38 minutes ago, SuperFastJellyfish said:

Just a simple graphic, but this shows them falling pointy end down.

https://en.wikipedia.org/wiki/LGM-30_Minuteman#/media/File:Minuteman_III_MIRV_path.svg

it's oversimplified, more a schematic of the systems involved than an actual deployment image.

That said, it's likely the warheads indeed reenter pointy end first for the stated reason: minimizing time in flight so as to minimize the chance of interception as well as reducing the drift from wind. Extensive studies have been done on how to survive the heat shock. I think one involves extending a spike in front of the actual reentry vehicle to break up the shock wave so it doesn't slam into the main body as harshly.
Having the rear of the warhead act as a radiator can also help dissipate heat.
And of course the outer surface of the warhead can act as an ablator.

We simply don't know as those designs are still (and probably always will remain so) top secret.

[sevenperforce]

As long as the warhead survives to a relatively low altitude, the outside of the RV can be red-hot slag. I'm also guessing the warhead is still high hypersonic when it detonates. It certainly doesn't have to slow down to the parachute-safe speeds of a re-entry capsule. 

A capsule must slow from around 8 km/s to less than 1 km/s without melting. An ICBM is probably slowing from around 5 km/s to around 2 km/s.

[cantab]

https://en.wikipedia.org/wiki/Thermonuclear_weapon#Variations

Indeed the "easy" way to sort the aerodynamics out is to have the dense secondary in the nose, and the lighter primary behind it. The opposite arrangement allows a bigger secondary for more boom, but probably requires ballasting and makes designing the primary harder. (The secondary usually has a load of uranium, hence the weight)

[SuperFastJellyfish]

On page 92 of http://ed-thelen.org/Minuteman.pdf, it shows a picture indicating that the reentry vehicle is spin stabilized.  I wouldn't think you'd need to spin stabilize it if it was pointy end up since it should be self stabilizing like manned capsules are(or raindrops ).

Edited April 27, 2016 by SuperFastJellyfish

[kerbiloid]

12 hours ago, wumpus said:

First: are you sure the "pointy end" goes down?

Looks like two different renderings of the same animation.

https://www.youtube.com/watch?v=3wdjgL40wc4

https://www.youtube.com/watch?v=HNlOsko1H7Q

Edited April 28, 2016 by kerbiloid

[radonek]

Early ICBMs used blocks of copper (Thor) or beryllium (Polaris) as heatsinks because ablation technology was not well understood at a time. But they were not that heavy, since warhead goes down fast and detonates before gathering too much heat. The way of Blunt-End-Forward was not even known then.

Fun fact: early Gemini designs followed same path, because bolting on piece of metal was considered as reliable as it could get. Only when they fed in numbers it was realized that astronauts would spend long minutes on top of glowing hot chunk of metal, which hence had to be jettisoned by some mechanism, which in turn made whole thing less reliable.

(source: http://history.nasa.gov/SP-4201/ch5-7.htm )
 

[wumpus]

First, I would ignore all the animations.  Knowing they bothered to spin-stabalize such a thing strongly implies that they would go pointy-end down (but it would probably still require ballast and/or extra empty volume in the non-pointy end).

Second, I expect that pointy-end down actually slows the thing down more.  

16 hours ago, sevenperforce said:

A capsule must slow from around 8 km/s to less than 1 km/s without melting. An ICBM is probably slowing from around 5 km/s to around 2 km/s.

The expected flight was nearly always from some fairly northern US base to somewhere in the USSR.  This would require only 4 km/s or less in flight.  Of course, but the time Minuteman III was designed (80s?), I suspect that such missiles were designed to hit anywhere on the planet, just in case.  Wiki (under sub-orbital flight) lists re-entry to under two minutes (presumably this is the 4 km/s case).

All in all, I suspect that this was a design that was "good enough, and worked".  Non-pointy-end-down return vehicles might have worked better, but wouldn't fit in existing missiles.  New missiles needed to carry existing warheads which required pointy-end-down packaging.  I suspect that you could put a "pointy-head-up" return vehicle into an older missile (although the CoM will change, possibly a no-go), but then you would have to re-calculate.  Then there are all the issues with the explody part.  Presumably weapon designers have all the information on air-detonating an H-bomb pointy-head-down, whereas changing the design (and all the pressures involved) would require additional testing.  No idea if below-ground would be good enough (the bombs were pretty much meant for air-bursts) and above ground has all sorts of political, environmental, and stability (Andrei Sakharov figured out how the US H-bomb worked by learning what isotypes the test gave off.  Later tests could add new members to the "nuclear club").

I would assume that such things are really weird in the eyes of a rocket scientist.  It is entirely possible that the goal was to maximize pressure at a certain height and that the nuclear blast use such pressure to contain the explosion and make it just a little bigger (nuclear bombs tend to blow themselves up before much of the fuel is ever consumed).  While KSP doesn't include any worries about max-Q (not sure if true with Ferram), it seems a key design point in real rockets.  While it feels entirely backwards to increase max-Q (at least during entry), it is entirely possible that is a design goal for ICBMs.  No idea if a non-aerodynamic design (which slows the craft down, but has high pressure low in the atmosphere) has a higher max-Q than a more aerodynamic design with lower pressure at the same speed, but comes in faster.

[sevenperforce]

24 minutes ago, wumpus said:

First, I would ignore all the animations.  Knowing they bothered to spin-stabalize such a thing strongly implies that they would go pointy-end down (but it would probably still require ballast and/or extra empty volume in the non-pointy end).

Second, I expect that pointy-end down actually slows the thing down more.  

The expected flight was nearly always from some fairly northern US base to somewhere in the USSR.  This would require only 4 km/s or less in flight.  Of course, but the time Minuteman III was designed (80s?), I suspect that such missiles were designed to hit anywhere on the planet, just in case.  Wiki (under sub-orbital flight) lists re-entry to under two minutes (presumably this is the 4 km/s case).

I can't imagine that going pointy-end-down results in greater drag.

[Kerbart]

16 hours ago, SuperFastJellyfish said:

On page 92 of http://ed-thelen.org/Minuteman.pdf, it shows a picture indicating that the reentry vehicle is spin stabilized.  I wouldn't think you'd need to spin stabilize it if it was pointy end up since it should be self stabilizing like manned capsules are(or raindrops ).

The raindrop is "self-stabilizing" due to its geometry—it's round. If you're a ball shape it doesn't really matter what "side" is pointing up. Or more precise; there is nothing to point up.

[SuperFastJellyfish]

53 minutes ago, Kerbart said:

The raindrop is "self-stabilizing" due to its geometry—it's round. If you're a ball shape it doesn't really matter what "side" is pointing up. Or more precise; there is nothing to point up.

According to http://pmm.nasa.gov/education/videos/anatomy-raindrop, they are shaped like hamburger buns while falling.

[cantab]

Raindrops are "self-stabilizing" because they're fluids and will take whatever shape the balance of forces dictate.

[kerbiloid]

2 hours ago, wumpus said:

The expected flight was nearly always from some fairly northern US base to somewhere in the USSR.  This would require only 4 km/s or less in flight.

About 7 km/s, nearly orbital. 4.5 is just horizontal projection.

2 hours ago, wumpus said:

but the time Minuteman III was designed (80s?), I suspect that such missiles were designed to hit anywhere on the planet, just in case

10000-12000 km, except special ones.

2 hours ago, wumpus said:

Non-pointy-end-down return vehicles might have worked better, but wouldn't fit in existing missiles.  New missiles needed to carry existing warheads which required pointy-end-down packaging.  I suspect that you could put a "pointy-head-up"

Aerodynamics hasn't changed since 1960s. Sharp and flattened cones are used because a supersonic flow lives by its own rules. And CoM doesn't play any role in it.

 

2 hours ago, wumpus said:

 It is entirely possible that the goal was to maximize pressure at a certain height and that the nuclear blast use such pressure to contain the explosion and make it just a little bigger

Air pressure is negleable in comparison with.
Aerodynamics tube decides, And yes, the sharp point is always prograde. Due to this they spin the thing to keep it stable. As you can see in the animation .

Edited April 28, 2016 by kerbiloid

[wumpus]

2 hours ago, Kerbart said:

The raindrop is "self-stabilizing" due to its geometry—it's round. If you're a ball shape it doesn't really matter what "side" is pointing up. Or more precise; there is nothing to point up.

Anybody who has visited a "shot tower" should know this.  And I plain forgot.

[DDE]

On ‎27‎.‎04‎.‎2016 at 8:29 PM, wumpus said:

First: are you sure the "pointy end" goes down?  Unless they put most of the Plutonium or other mass in the pointy end, I'd expect it to be at the top.  Think of a raindrop: if it is remotely of uniform density (unlike a mark1 capsule on top of an empty fueltank...) it will fall pointy end up.

They do, as we know from the W88; it's unconventional but doable.

As to the OP subject, it's a combination of ablative heat shielding, carbon-carbon, and, in some designs, an entire layer of uranium, which is very difficult to melt.

The cavity up front actually forms itself out of a perfectly conical heat shields.

And while it seems that insulation is a very big issue, the g-force tolerance is not. Remember how the steep entries in KSP can easily lead to 12 g or more, while saving ablator? The nuke coming down from the 1800 km apex doesn't care for your g's.