Driver Killed by Manhole Cover

[Firethorn]

All this math, can we find the cover that blew off the shaft of the nuclear test?

Didn't they estimate that that one was vaporized in the atmosphere, if not it's heading out of the solar system?

For those not familiar:  Shaft buried nuke.  What amounts to super-sized manhole cover on top.  They KNEW that the cover would be heading up very quickly when the nuke blew, so they aimed a high-speed camera at it for the test. 

Problem:  They only got ONE frame of the manhole cover moving, so the best they can calculate is a MINIMUM speed, because they don't know WHEN, exactly, the cover started moving.  Said minimum speed is still a rather high mach number.

[230RN]

But that is exactly my initial point, that calculation involves a perfect frictionless seal in a sealed cylinder does it not? If so, and I am betting it does, there are HUGE losses when you are talking a plate on a poorly "sealed" surface that instantly vents to atmosphere and does not remain perfectly horizontal in flight making such guesstimations several orders of magnitude off at best.

HeroHog, you're missing the point of that initial calculation/estimation.  Surely, 0.28 psia isn't going to blow the cover sky high. It's a numerical guess at the minimum pressure to either just barely move it or just hold it up.

Certainly a real explosive overpressure of 5000 psia would, but as to how high?  You could make some assumptions about the "barrel," meaning the rim of the manhole (call it, say, two inches), how far away from this "barrel" would the gas pulse still accelerate the manhole cover, etc., etc., etc., etc., etc.  But this would only result in estimated parameters for the empirical tests.

And these empirical tests would result in a whole raft of corrections to the original assumptions.  (And probably uncover new, previously unthought-of variables.)

It's like when they first started to develop ballistic tables like the old "Ingall's Tables."  They had to use a standardized projectile, do their measurements, and from this empirical data, they developed parameters as to how to handle the variables of nose ogive, weights, sectional densities, "etc., etc., etc., etc., etc."

And from these tables* we got to the point where we could calculate, in advance, things like the nose force on a 150 grain flat-based cupro-jacketed lead-cored .30-06 bullet at 2743 feet per second.

As well as a 5.56 NATO bullet at 1534 feet per second --which had not even been developed at the time the tables were generated.

So your dysphoria about classroom theory versus engineering reality is unwarranted.  After all, the "Ingall's Tables" for manhole covers have not yet been developed.

If the need for that ever arises, I'm sure I could get a juicy government grant to conduct the investigation for that.

Of course I'd have to hire birdman as a consultant, but I could at least competently manage the research project.

 

Terry, 230RN

* and other derivative tables

[HeroHog]

Am I the only one who can separate the concept of Real World estimation/calculation of variables and givens  VS lab/absolute theoretical calculation? Even moving much less holding up a 30" 150 - 200 lb steel plate set loosely in a recess roughly equal to it's height will take a substantially higher pressure just to "burp" much less hold it up.

Google Image search "manhole cover" and note that the majority are vented and have varying gaps about their circumference while others may be unvented and have dirt and debris tightly packed around their edges. Both conditions will raise the required pressure needed by loss for the first and by increased resistance for the second.

Students need to understand this stuff and consider it and it should at least be pointed out to them at some point in their education. As a Sophomore at Louisiana Tech University College of Engineering I happened to be passing through the machine shop where the Senior engineering class was working on their SAE off-road buggy project (I was a student worker in the Civil/Chemical Engineering departments taking care of their computers and networks). I noticed that they used a version of the Formula V Zero Roll rear suspension design and that alone surprised them that I knew what it was. I looked at it a bit and asked what additional bracing they were going to add to it and they told me that was it. It didn't need any more. I advised them that it would very likely fail and was roundly poo-pooed and laughed at and told how it was N% "over engineered" as it was. I let them know that I was an ASE Master mechanic and Chassis/suspension specialist and designed, built and raced cars with the SCCA and Modified dirt cars but they wouldn't even consider it so I shrugged and went on my way. It didn't finish the 1st lap because the rear suspension failed and folded up where I said it would like I said it would. Their Professor asked me to speak to the next class and give them some Real World 101 and Q&A.

[De Selby]

I remember a problem with exploding manhole covers in Washington back when.  They definitely do rocket up sometimes

[Fly320s]

All you cis-normative cis-males are being cis-sexist!  These objects are not MANhole covers, they are PERSONhole covers.

Now, back to your inane ballistics discussion.

[Ben]

All you cis-normative cis-males are being cis-sexist!  These objects are not MANhole covers, they are PERSONhole covers.

Now, back to your inane ballistics discussion.

Well, you're some kinda hole. 

[birdman]

Am I the only one who can separate the concept of Real World estimation/calculation of variables and givens  VS lab/absolute theoretical calculation? Even moving much less holding up a 30" 150 - 200 lb steel plate set loosely in a recess roughly equal to it's height will take a substantially higher pressure just to "burp" much less hold it up.

Google Image search "manhole cover" and note that the majority are vented and have varying gaps about their circumference while others may be unvented and have dirt and debris tightly packed around their edges. Both conditions will raise the required pressure needed by loss for the first and by increased resistance for the second.

Students need to understand this stuff and consider it and it should at least be pointed out to them at some point in their education. As a Sophomore at Louisiana Tech University College of Engineering I happened to be passing through the machine shop where the Senior engineering class was working on their SAE off-road buggy project (I was a student worker in the Civil/Chemical Engineering departments taking care of their computers and networks). I noticed that they used a version of the Formula V Zero Roll rear suspension design and that alone surprised them that I knew what it was. I looked at it a bit and asked what additional bracing they were going to add to it and they told me that was it. It didn't need any more. I advised them that it would very likely fail and was roundly poo-pooed and laughed at and told how it was N% "over engineered" as it was. I let them know that I was an ASE Master mechanic and Chassis/suspension specialist and designed, built and raced cars with the SCCA and Modified dirt cars but they wouldn't even consider it so I shrugged and went on my way. It didn't finish the 1st lap because the rear suspension failed and folded up where I said it would like I said it would. Their Professor asked me to speak to the next class and give them some Real World 101 and Q&A.

I used the drag coefficient of a circular plate in a free stream for the calc, so it assumes INFINITE gap, hence the most conservative calculation possible.  The reson I did that is real world experience....given that I have almost 20 years of real world aerospace experience -after- my degree in the topic.  So I still don't understand why you feel it's not real-world.

Rarely will i use theoretical values if I have empirical, and even more rarely will I not make extremely conservative assumptions in modeling...and that's one of the reasons that at work they keep track of how many times I'm wrong in a whiteboard...I don't put forward numbers or a statement in engineering unless I'm really damn sure im right. 

[birdman]

Didn't they estimate that that one was vaporized in the atmosphere, if not it's heading out of the solar system?

For those not familiar:  Shaft buried nuke.  What amounts to super-sized manhole cover on top.  They KNEW that the cover would be heading up very quickly when the nuke blew, so they aimed a high-speed camera at it for the test. 

Problem:  They only got ONE frame of the manhole cover moving, so the best they can calculate is a MINIMUM speed, because they don't know WHEN, exactly, the cover started moving.  Said minimum speed is still a rather high mach number.

Brownlee's satellite.

it wasn't just a high Mach number...it was past escape velocity. 

However, I (and others) have done a bunchmath on the most conservative case...it likely went pretty high, was bent into a self-forging fragment type shape, and could have landed anywhere in a relatively large area.

Unfortunately, the shape also means it would be aero-stable on the way down, but have a relatively low ballistic coefficient, meaning it would hit at mid/high sunsonic speeds.  The problem with that, isn't that it could have landed anywhere within a 10+ mile radius, was that it wasn't fast enough to make a really big crater, but fast enoug to somewhat re-bury itself...hence why it's never been found.

[Fly320s]

Well, you're some kinda hole. 

Touché

[KD5NRH]

Unfortunately, the shape also means it would be aero-stable on the way down, but have a relatively low ballistic coefficient, meaning it would hit at mid/high sunsonic speeds.  The problem with that, isn't that it could have landed anywhere within a 10+ mile radius, was that it wasn't fast enough to make a really big crater, but fast enoug to somewhat re-bury itself...hence why it's never been found.

So the driver was killed by a pretty pathetic underground nuclear test that only got its manhole cover a couple feet in the air?

[Scout26]

I could see it laying on the pavement and being struck by the edge of a truck tire and being launched.  Much like rocks and pebbles are launched when hit by car tires.

I know there was a lady killed here a few years back, when a truck struck one of those embedded lane markers and sent it through the windshield of her car going in the opposite direction. 

[HeroHog]

I used the drag coefficient of a circular plate in a free stream for the calc, so it assumes INFINITE gap, hence the most conservative calculation possible.  The reson I did that is real world experience....given that I have almost 20 years of real world aerospace experience -after- my degree in the topic.  So I still don't understand why you feel it's not real-world.

Rarely will i use theoretical values if I have empirical, and even more rarely will I not make extremely conservative assumptions in modeling...and that's one of the reasons that at work they keep track of how many times I'm wrong in a whiteboard...I don't put forward numbers or a statement in engineering unless I'm really damn sure im right

I stand corrected. I will admit it's not my area of expertise, it just sounds so low given experiences I have had around the shop with various objects and high pressure air. I suppose Volume may be the part that is tripping me up or actual pressure at the object VS my estimate of the pressure at the object based on line pressure.

Ignorance on my part. I didn't go on to be come an Engineer but a computer geek. All my "engineering" was learned in the shop and at the tracks. After enough experience your eyes and experience will tell you things that raw numbers don't take into account.

[dogmush]

I stand corrected. I will admit it's not my area of expertise, it just sounds so low given experiences I have had around the shop with various objects and high pressure air. I suppose Volume may be the part that is tripping me up or actual pressure at the object VS my estimate of the pressure at the object based on line pressure.

Ignorance on my part. I didn't go on to be come an Engineer but a computer geek. All my "engineering" was learned in the shop and at the tracks. After enough experience your eyes and experience will tell you things that raw numbers don't take into account.

Volume makes a pretty big difference. Most shop air is actually very low volume.

[brimic]

Volume makes a pretty big difference. Most shop air is actually very low volume.

Tell me about it.
Last night I replaced a leaky fitting on my well tank. It took a good 10 minutes to repressurize the tank with a portable air compressor. 

[birdman]

I stand corrected. I will admit it's not my area of expertise, it just sounds so low given experiences I have had around the shop with various objects and high pressure air. I suppose Volume may be the part that is tripping me up or actual pressure at the object VS my estimate of the pressure at the object based on line pressure.

Ignorance on my part. I didn't go on to be come an Engineer but a computer geek. All my "engineering" was learned in the shop and at the tracks. After enough experience your eyes and experience will tell you things that raw numbers don't take into account.

I thought it seemed low as well, for the same reasons...but volume and area matter.
But think of it this way, a 747 has an average wing loading of <2psi...most aircraft even less.
Also, an aircraft carrier is only about 15-18psi (draft is 10-12m)

Another note, stick your hand perpendicular to the wind out the window of a car going 60mph...you can definitely feel the force, and that's only 0.05psi of dynamic pressure, which increases with the square of speed

[HeroHog]

Motorcycle rider/racer here, former anyway, air hitting ya at 100+ mph is substantial. Just popping upright from a tucked position makes for a dandy air brake.

[230RN]

Aw, jeeze.

May I reiterate that 0.28 psia is that amount of pressure that will develop 200 pounds of force against a guessed-at 30" diameter surface area of 706.85834705770347865409476123789 square inches?

If my arithmetic and the Windows calculator is correct.

0.28 X 707 = 197.96 (close enough with rounding errors)

No holes, no escape from the sides, no tipping of the "surface area," no "gun barrel effect of the rim of the actual manhole."  Just that amount of pressure which will exactly balance the supposed 200 lb weight quoted in the original article.

It is a starting point in estimating the pressure required to balance the given weight and guessstimated (by me) diameter of a manhole cover.

At 0.29 psia, it would start to move.  If nobody left a piece of chewing gum between the manhole rim and the manhole cover itself.

<sarcasm tag just in case>
And with a 437 millisecond overpressure (as from a gas blast) of 5,280.84 psia, this manhole cover would fly up 38.394 feet.  In a vacuum.  And assuming no chewing gum.
</sarcasm tag just in case>

And I'll bet the damned thing doesn't even weigh exactly 200 good old American Avoirdupois pounds (90.718474 kilograms).

Jeeze.

Terry, 230.7398389594RN

[Fitz]

So the driver was killed by a pretty pathetic underground nuclear test that only got its manhole cover a couple feet in the air?

Interesting thread, but THIS comment is the one that made me laugh my ass off.


Bravo!

[birdman]

Motorcycle rider/racer here, former anyway, air hitting ya at 100+ mph is substantial. Just popping upright from a tucked position makes for a dandy air brake.

Current both here...that's why I used that analogy for you, figuring by your username :)

[HeroHog]

Aw, jeeze.

May I reiterate that 0.28 psia is that amount of pressure that will develop 200 pounds of force against a guessed-at 30" diameter surface area of 706.85834705770347865409476123789 square inches?

If my arithmetic and the Windows calculator is correct.

0.28 X 707 = 197.96 (close enough with rounding errors)

No holes, no escape from the sides, no tipping of the "surface area," no "gun barrel effect of the rim of the actual manhole."  Just that amount of pressure which will exactly balance the supposed 200 lb weight quoted in the original article.

It is a starting point in estimating the pressure required to balance the given weight and guessstimated (by me) diameter of a manhole cover.

At 0.29 psia, it would start to move.  If nobody left a piece of chewing gum between the manhole rim and the manhole cover itself.

<sarcasm tag just in case>
And with a 437 millisecond overpressure (as from a gas blast) of 5,280.84 psia, this manhole cover would fly up 38.394 feet.  In a vacuum.  And assuming no chewing gum.
</sarcasm tag just in case>

And I'll bet the damned thing doesn't even weigh exactly 200 good old American Avoirdupois pounds (90.718474 kilograms).

Jeeze.

Terry, 230.7398389594RN

Ya may have missed it but I already fell on my sword a few posts back...

[230RN]

No, I saw that, this was for general distribution.

[brimic]

The last time I've seen a manhole cover dislodged, the situation was accompanied by a 4' column of water spraying out of the manhole during a flood.

[HeroHog]

<sarcasm tag just in case>
And with a 437 millisecond overpressure (as from a gas blast) of 5,280.84 psia, this manhole cover would fly up 38.394 feet.  In a vacuum.  And assuming no chewing gum.
</sarcasm tag just in case>

But, does that factor in wind resistance and humidity? (ducks and wheels rapidly away!)

PS: Carp, just noticed calculations were in a vacuum...

[French G.]

European or African manhole cover?

[HeroHog]

European or African manhole cover?

I don't know... [AAAaaaaaaahhhhhhhhhh....]