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tekochip
User Rank
Platinum
Even on the Ground (small pun)
tekochip   5/23/2012 1:00:20 PM
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I used to design scale systems and many of our application were used on agricultural equipment that would be towed on the highway from field to field. The load cells in these systems were the actual wagon axles with strain gauges glued to the axle surface. One of our customers began to experience field failures and I found that all of the failed load cells had gauges shorted to the axle. I opened up the axles to examine the strain gauges, and using a microscope, I could see a burn mark on all of the failed gauges. I made a trip to the factory and confirmed that nobody was performing a highpot on the gauges, or that maybe the test stand had an isolation issue. No, there were no problems at the factory and only this particular model sold to this customer was failing.
I called the customer to see if he was performing any unusual welding or highpot testing. He said no, and that he thought it was odd that the all of the failures came from their new wagon which had greatly improved suspension. I asked what was different about the suspension and he said that all of the wagon mounts were now floating on rubber. I asked the customer to perform a continuity test from the axle to the frame, and sure enough, it was wide open. As the wagon was towed down the road the static would build up on the tires until it discharged from the body of the axle, through the strain gauge, through the instrument and then to ground. I had the customer attach a lead from the axle mounts to the frame and the failures ceased.
As a side note, although the problem had been solved, the customer didn't believe that something as benign as static electricity could cause the failure. The customer believed that there was a design flaw on our end that we were covering up the flaw with a lot of mumbo jumbo, so the customer then switched to our competitor. A case of winning the battle but losing the war.


corey872
User Rank
Iron
Re: Why worry about damaging something which is useless anyway?
corey872   5/23/2012 12:17:55 PM
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I'm with you, wb9ddf . . .that was the same thought I had when first reading the article.  I also thought, "well at least the plating would get you 8,000 feet total"  Reasoning the first 3,000 feet of cloud you're safe behind the plating, and you can go an additional 5,000 on the raw insulation... 3,000 + 5,000 = 8,000... no?


 

notarboca
User Rank
Gold
Re: Why worry about damaging something which is useless anyway?
notarboca   4/13/2012 12:58:51 AM
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Perhaps static wicks, like those used on aircraft?

wb9ddf
User Rank
Silver
Re: Why worry about damaging something which is useless anyway?
wb9ddf   4/12/2012 11:28:28 AM
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This would seem too obvious to an old Ham - Why not put the insulation on the inside of the metal shell? Let the metal take the aerodynamic abrasion and bleed off the static charge. 

 

Wayne Eleazer
User Rank
Gold
Re: Why worry about damaging something which is useless anyway?
Wayne Eleazer   4/11/2012 11:03:48 AM
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The 5000 ft cloud layer with freezing layer limitation would stand anyway, since more than the fairing has to be protected from the effects of static charge buildup.  For example, the command destruct receiver antennas might conduct the charge inside and destroy the ability of the vehicle to receive the destruct command if it went off course.

But things are rarely so neat or simple when it comes to weather.  A more likely condition is a lower cloud layer 3000 ft thick, or perhaps multiple lower layers adding up to 3000 ft of cloud, with a cloud layer at a higher altitude that contains the freezing layer but is less than 5000 ft thick.  So if you are concered about static charge buildup on the fairing due to, say, the impact on the payload, you still won't be able to launch under conditions that otherwise would be acceptable from the range safety aspect.  You have just built in a feature that carrys a limitation beyond that which would exist anyway.

And the thickness layer limitation for all vehicles later was changed from 5000 ft to 3000 ft with a freezing level in the cloud because an Atlas was launched into a thunderstorm from Cape Canaveral and was promptly shot down by lightning.  They got more conservative after that.      

   

Dave Palmer
User Rank
Platinum
Re: Why worry about damaging something which is useless anyway?
Dave Palmer   4/11/2012 10:19:26 AM
NO RATINGS
@naperlou: Nah, that would make too much sense.

As they say, "You don't have to be a rocket scientist..."

naperlou
User Rank
Blogger
Re: Why worry about damaging something which is useless anyway?
naperlou   4/11/2012 9:18:27 AM
NO RATINGS
Dave, good question.  A related question would be why not redesign the layer to work in the conditions acutally encountered.  Alternatively, why not try another approach that would work in the indicated conditions?

Dave Palmer
User Rank
Platinum
Why worry about damaging something which is useless anyway?
Dave Palmer   4/11/2012 7:19:43 AM
NO RATINGS
Wouldn't the limitation still be 5000 feet? As you've described it, it doesn't sound like things would actually be any worse with the conductive layer, they just wouldn't be better (because the conductive layer would be gone in any situation where you actually needed it). Given that the conductive layer, as you've described it, is useless, why worry about whether or not you damage it?



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