What I learned today

[DynaSoar]

Ejection firing electronics have charging capacitors in them to accumulate charge and let it out all at once to fire off the BP. I already knew that capacitors hold a charge for a time after the power is removed.

Today at the Metra launch, someone tested their electronics, then turned it off and started loading it. It discharged and blew the rocket into his leg. He was cut pretty badly. He went to the hospital. By the time he was taken there he was joking and I'm sure he'll be OK, but at first it was a scary scene.

I don't know whether any of these electronic devices have shunts to ground the firing capacitors before laoding, but if they don't, they should.

The Metra people deserve a hand for swift and efficient handling of the situation. For hobbyists, they're pretty much pros.

 

[JDcluster]

I had it happen to me not once but twice,
Luckly I was not injured either time.
The first was with an Adept ALT25 where the Alt wasn't even turned on & the second was with a G-Wiz which fired as I was fitting the payload section.


Both times it was very hot & very high humid.


JD

 

[flying_silverad]

Always believe that it's loaded.

 

[DynaSoar]

Originally posted by JDcluster
I had it happen to me not once but twice,
Luckly I was not injured either time.
The first was with an Adept ALT25 where the Alt wasn't even turned on & the second was with a G-Wiz which fired as I was fitting the payload section.


Both times it was very hot & very high humid.


JD

A charged and undrained capacitor is not a problem with static. Static would be a problem in cold, dry air. Believe me, I was the man with the Texaco star for two different branches of the military, taking care of fuel systems for everything from trucks to hovercraft to Hueys to SR-71s to B-52s to C-5s to Titans to Hound Dogs (they are gone, but the fuel depot wasn't). I was the guy who went inside the fuel storage tanks in a space suit to inspect them. We raised our survival chances by understanding and coping with static.

There's a fuel storage area at the north of of Kinross Airport in northern Michigan. There you'll find about 1/4" high ring of steel 40 feet in diameter embedded in concrete. It used to be the wall of a fuel storage tank when it was Kincheloe AFB. The steel tank burned to the ground because someone on top of it, taking a measurement, didn't pay attention to static. The roof, a steel floating pontoon six feet thick and 40 feet diameter, flew 150 yards. If anyone who's been in the military ever saw the training film about static called "Great Balls of Fire", that was the tank in the movie. I saw the melted residue of the tank every day for 2 years. It helped remind me about static.

 

[wwattles]

Charged capacitors can make your life a lot more exciting, especially when you least expect it. Electronics techs on board some of the ships I've been on loved charging up a high-capacity capacitor and then tossing it to some unsuspecting shipmate. Nothing like an instantaneous discharge running through your body to make you jump!

Whenever I go in to work on electronic gear, I always short out the capacitors first, just so I don't end up getting a stray shot of voltage, either in my work area or through my body.

WW

 

[flying_silverad]

Originally posted by DynaSoar
AThere's a fuel storage area at the north of of Kinross Airport in northern Michigan. There you'll find about 1/4" high ring of steel 40 feet in diameter embedded in concrete. It used to be the wall of a fuel storage tank when it was Kincheloe AFB. The steel tank burned to the ground because someone on top of it, taking a measurement, didn't pay attention to static. The roof, a steel floating pontoon six feet thick and 40 feet diameter, flew 150 yards. If anyone who's been in the military ever saw the training film about static called "Great Balls of Fire", that was the tank in the movie. I saw the melted residue of the tank every day for 2 years. It helped remind me about static.

When I was at Sheppard AFB, I think I saw that film. It was a film on static grounding (Plane to ground, truck or cart to ground, then plane to cart....I think) Anyway, the tank fire sounds familiar.

 

[fireone]

Dyna,

That gentleman is OK. He got a boat load of sutures but was able to drive home that night. I guess we all learned a lesson..... it is always loaded or charged.

 

[DynaSoar]

Originally posted by flying_silverad
When I was at Sheppard AFB, I think I saw that film. It was a film on static grounding (Plane to ground, truck or cart to ground, then plane to cart....I think) Anyway, the tank fire sounds familiar.

Zackly the one. I saw the same movie again when I switched services from USAF 54670 (civil engineering fuel system maintenance) to Army 77F (fuel everything). The tank blew beacuse he didn't ground the brass bob and steel measuring tape to the tank prior to opening the top of the external plumb-bob pipe and dropping it in.

One guy got blown off, landed with his arms out, and broke both his elbows. He climbed the 8 foot cyclone fence topped with barbed wire anyway. The other guy got blown over the fence, his parka got blown/burned off by the blast. They caught him still running through the waist deep snow, 1.5 miles away.

The movie repeatedly makes the point that static is a problem mostly in cold (low water content) air. However, even in the tropics, a hovering Huey builds up such a charge that if you touch it before it grounds, it'll knock you over.

 

[DynaSoar]

Originally posted by fireone
Dyna,

That gentleman is OK. He got a boat load of sutures but was able to drive home that night. I guess we all learned a lesson..... it is always loaded or charged.

Very excellent news, thanks for the telling. It was a bit scary when he went into shock, but that only lasted a couple minutes. When he started making jokes, I knew he was going to be fine. Seriously, laughing regulates breathing very much like the yoga breathing I had him do. Plus it means his mental state is very good, and important thing after a traumatic incident.

I'd be all for METRA writing it up and having it sent to all rocket electronics makers. If any of them have charge firing equipment that doesnt have grounding shunts, they should add them, both for their protection and the users'. An "interlock" jumper, that runs from the charge cup to the ground side of the battery, which must be inserted before you do anything, would be a good physical boom-preventor.

The volunteer that drove him deserves some recognition too.

 

[Elapid]

i hope it helps raise awareness of some of the less obvious dangers inherent with handling of the compounds we do.

i recently had a static electricity ignited fire.

it makes me wonder about the grounding techniques suggested in the propellant books/online articles/manufacturing practices; specifically, touching the earth manually.

it is my contention that if stainless steel wristband grounding straps would be many times safer due to it being a no-brainer...the charges are never allowed to accumulate.

use of poly buckets might not be such a good idea since they easily build an electrostatic charge.

how about the sprays or wipes https://www.interstateproducts.com/Officetowels.htm on the airframe to keep the fiberglass resins from building a charge?

 

[DynaSoar]

Originally posted by Elapid
i hope it helps raise awareness of some of the less obvious dangers inherent with handling of the compounds we do.

i recently had a static electricity ignited fire.

it makes me wonder about the grounding techniques suggested in the propellant books/online articles/manufacturing practices; specifically, touching the earth manually.

it is my contention that if stainless steel wristband grounding straps would be many times safer due to it being a no-brainer...the charges are never allowed to accumulate.

use of poly buckets might not be such a good idea since they easily build an electrostatic charge.

how about the sprays or wipes https://www.interstateproducts.com/Officetowels.htm on the airframe to keep the fiberglass resins from building a charge?

Grounding to earth is the only real way. And it must be a true earth ground. Even then the grounding rod can't be some piping that happens to go underground, but has a pump on the other end, or even be a grounding rod near such things. Stray currents make the "ground" not zero.

Ground equipment and containers prior to outting anything in them. Keep them grounded during use. All mizers should be permanently grounded, and I'm not talking the third pin on a 3 prong plug.

I don't know much about things intended to reduce static. I do know reductions isn't zero. All it takes to ground anything is the ground (harder to be certain of than you think) and a wire.

A grounded touch pad for you to touch before touching equipment, that too should be standard and common, placed all around.

And two dissimilar materials and friction is all it takes. They can even be insulators.

I don't trust those wrist straps. I'd use a touch pad.

 

[Elapid]

saws...
electrical outlets/plugs...
shouldn't they be constructed and approved for use in explosive environments?

in watching the rocket show on discovery channel, it appeared that they were using standard, shop-grade power tools.

 

[wwattles]

Originally posted by Elapid
saws...
electrical outlets/plugs...
shouldn't they be constructed and approved for use in explosive environments?

in watching the rocket show on discovery channel, it appeared that they were using standard, shop-grade power tools.

It might be a good idea, but then you've got the practicality side to deal with. First of all, who's going to come out to your hobby work site to inspect your safety? OSHA? Somehow I doubt it. Explosion-proof tools are just as good as the regular kind, but since they are a specialty item, they're MUCH more expensive. So who's to stop you from going out to the local hardware store and buying the cheaper version, instead of mailing off to a specialty company for the more expensive version with the brushless motors?

Basically, it comes down to economics and accountability weighed vs the risk of an explosion.

My 2 bits.

WW

 

[DynaSoar]

Originally posted by Elapid
saws...
electrical outlets/plugs...
shouldn't they be constructed and approved for use in explosive environments?

in watching the rocket show on discovery channel, it appeared that they were using standard, shop-grade power tools.

That can be designed for that. What they do is contain the explosion within their own casing, so it doesn't leak out and set off a big explosion. Or so the theory goes.

 

[bobkrech]

I don't know whether any of these electronic devices have shunts to ground the firing capacitors before loading, but if they don't, they should.

Dynasoar

In my opinion, it's the e-match/charge package that should have a shunt wire across it. Properly shunted, an e-match will not fire even if there is full power in the pyro terminals or with static electricity.

All altimeters use some kind of solid-state switch to initiate the pyrotechnic. It could be a standard transistor, a FET, or conceivably an SCR, and it doesn't matter much whether there is a capacitor discharge or simply a high current source. All solid-state switches have some leakage current but it is at most a few ma, and this is insufficient current to initiate any pyrotechnic that is safe to use in a rocket, however should the solid-state switch fail, you have a very different and dangerous situation.

If the solid-state switch fails in the off position (a blown transistor), the pyrotechnic will never fire, and you will have a lawn dart if you launch. If the solid-state switch fails in the on position (shorts), then the pyro terminals will always be hot. If you were to hook up an unshunted e-match to a powered (or recently powered) altimeter with shorted solid-state switch - KABOOM!

To prevent an accidental pyro discharge, it's best to hook up the shunted pyros to the altimeter first, put the rocket on the pad, and then cut the shunts. Then when you power up the altimeter and should the charges go off, the rocket is pointing away from everyone.

There's a number of other ways to shunt pyro terminals on an altimeter, but the hard wire shunt in parallel with the e-match is the simplest and most positive. You do have to make sure that you have a checklist and remember to cut the shunts and power up the altimeter before you launch, but this is good practice to do anyway.

Bob Krech