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Comment by paulsutter | Hacker News Reader

You're misquoting the article:

> Dr. Brownlee believes that the plate did not leave the atmosphere, as it may even have been vaporized by compression heating of the atmosphere due to its high speed.

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Well it sounds like the the only reasonable, responsible course of action is to replicate the original test but use modern high speed recording equipment to determine what would happen to that steel plate.

How critical is the nuclear explosion in this experiment? Could you use an equivalent traditional explosive? 1KT explosion can be achieved without going nuclear.

According to the wiki page the Pascal-B test was the equivalent of 300 tons of TNT.

hold my beer

I'm not a physicist but as far as I know a nuclear explosion releases its energy much more quickly than e.g. TNT, so just releasing the same number of joules wouldn't exactly duplicate the test unless you were able to release them in the same number of nanoseconds.

This video https://www.youtube.com/watch?v=T2orn6T2QFM is a good DIY experiment along those lines and is interesting if not necessarily correct at the nuclear scale.

I'm curious, what speed is necessary for compression heating to be a significant factor? Obviously a regular rocket doesn't melt on its way into orbit, while it does need special protection for the trip back.

Rockets do actually heat in flight. But their launch profile is carefully chosen to keep the heating to managable levels. Liquid fuel rockets typically throttle down somewhere around a minute after launch, at a point called "max-Q", which is the point of maximum aerodynamic pressure. They throttle back up again once they have gotten above enough of the atmosphere.

As a point of reference, the SR-71 was one of the first major aircraft programs to use titanium extensively, the reason being that at the speeds the SR-71 flew, aluminum couldn't maintain structural stability due to the heat. And the SR-71 was "only" a Mach 3 airplane, flying at "only" 60,000 to 80,000 feet. It's fuselage would reach nearly 600 degrees.

An orbital launch vehicle typically reaches Mach 3 very roughly that same altitude, or somewhat higher (the figure I can find is Mach 4.5 at 28 nautical miles, which by interpolation I'm guessing is Mach 3 at between ten to twenty nautical miles; ten nautical miles is roughly 60,000 feet).

Another bit about the SR-71 (and supposedly concordes) is that their structures are designed with tolerances based around the forces they experience at speed. The effect of which is that when sitting on the ground not going anywhere there are gaps between some parts, and others are tightly compressed together (and then separate while traveling at speed). I don't remember if it was the last concorde, or one of the SR-71s, but there's a story out there about the pilot on its final flight sticking his hat into a gap in the cockpit, which on landing promptly closed up permanently sealing his hat in place inside of the cockpit (he did it on purpose to commemorate the last flight). Similarly there are stories out there that claim the SR-71 leaked fuel like a sieve while sitting on the ground because it needed the atmospheric heating from going supersonic to seal up all the various fittings in its fuel system.

I do recall seeing an image of an SR-71 on the ground with pans under it in various places, supposedly to catch leaking fluids.

Ironically there's a series of Russian dive watches that does approximately the same thing.

Instead of attempting to overengineer a high-depth tolerant system that's still sealed in open air, it's sealed more tightly as pressure increases.

An elegant weapon from a less civilized age.

https://en.m.wikipedia.org/wiki/Amfibia#Design

Here's the reference to the pilot's cap:

http://www.seattlepi.com/business/article/Concorde-s-arrival...

Any SR-71 story usually comes with the obligatory story about the Cessna v navy ground speed check. I'm replacing that story with the story of the time an SR-71 went very slowly.

https://www.google.co.nz/amp/foxtrotalpha.jalopnik.com/the-s...

Thanks! Tyler Rogoway can certainly relate a story.

IIRC they would refuel mid-air shortly after take off for long range missions.

Basically a rocket tries not to accelerate too much until it's above most of the atmosphere. (Which isn't as far as you might think - just a few tens of miles up.)

i.e. it climbs, but does not try to gain much horizontal velocity (i.e. the kind necessary to orbit).

On the way back the heat is just a way of shedding energy. So they shed energy "slowly", to keep within the thermal limits, they do that by shedding energy while still relatively high up.

This simulation of a Falcon 9 launch shows the heating rate peaking at 11 kilowatt (!) per square meter around maximum dynamic pressure.

https://youtu.be/oxtAa7QCbxw

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