mercury switch for deployment

No. What happens is that as the motor thrust starts to tail off and drops below the drag force, the rocket begins to decelerate and the Hg blob's inetia carries it forward making contact and ejecting the chute, right after motor burnout, while the rocket is at max velocity. Not good.

That's why you need an integrating accelerometer. It measures acceleration (and deceleration) and calculates when the velocity is at 0 (apogee), then pops the chute.

However, a mercury switch may be suitable for staging, but even then, you run the risk that a slight jarring of the rocket could prematurely ignite the upper stage, maybe while you're holding it :)

Doug

I've tried it, both for staging and for recovery. Wasn't very reliable for either, though it did usually work.

The secret seemed to be using it in a relatively high drag airframe. Otherwise, the drop of mercury might just float and not touch the contacts at the far end of the tube.

After boost, the rocket IS technically in freefall, after all.

HEY, I just thought of this - does anyone know how much thrust the tracking smoke/delay charge might be providing? I used to use a standard motor in the first stage, so its eject charge could recover the booster.

Won't work for deployment. The mercury gets thrown forward as soon as thrust ceases, not when the rocket "tips over". It has been used for staging (for the very same reason) though. Another consideration is the toxicity, if the glass tube fractures.

The best 'cheap' way to go for deployment, if you have an aversion to paying for an altimeter, is either a MAD (Magnetic Apogee Detector) or a simple timer circuit activated by a breakwire or liftoff switch.

Aerotech is just now ramping up production again, some stuff is available in spurts now and then, but from what I hear things should be getting better real soon.

"TomNavyRet" wrote:

its too dangerous, what if you tip the rocket over while loading them and it shoots nosecone at YOU or someone else? maybe the force isnt that much but still enough to hurt and if hit in the eye could cause perm. damage. You would however have to develop some kind of fail safe system like those arming switch or G-switch. But the trouble of all that and even then when motor burns out the murcury could prematurely trigger the ejection causing early deployment destroying your rocket. Or another scenario is you armed the thing and then somehow the wind tips the rocket over just enough to cause the murcury switch to turn on, then you just have a failed launch...

Assume that your Hg switch is aligned with the principal acceleration axis, this switch will trigger at burnout (because the vehicle will be undergoing a negative acceleration at that point).

mercury is toxic, and a CATO or a failed deployment may release it

Aerocon has a apogee detector that works by sending the change in the rcckets attitude in relation to the Earth's magnetic field

- iz

TomNavyRet wrote:

Oh, bosh. There are a lot of reasons not to use a mercury switch, but poisoning isn't one of them.

The toxicity of mercury is actually very low, for occasional slight exposure. It's this kind of thinking that obsoleted thousands of perfectly good cameras, when their required mercury batteries were forced out of production.

Now we DO need to keep it out of the ocean (tuna) and I wouldn't choose a career making felt hats - but I'm tired of losing the use of perfectly good engineering materials because they can occasionally be harmful if carelessly used in other ways..

Alas, you've forgot you physics. You're in a micro gravity environment during the coast phase of flight. Mercury switch will trigger at burnout (actually when thrust drops below drag, which on some motors can be well before burnout), not at apogee.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

And in the metallic form, as would be found in a switch, it's not very absorbable, so incidental contact with the liquid metal will typically result in only a very slight exposure.

Vapors, soluble salts, and organic complexes can be quite another matter - I heard of a case where someone picked a severe exposure from skin contact with a drop of dimethyl mercury - through a rubber glove! - and was dead within a few months. (That's actually a pretty violent case - as it occurred in miners, hat-makers, etc., mercury poisoning was generally more of a slow, chronic affair.)

-dave w

Some forms are decidedly nasty. Here's the case from Dartmouth that I'm familiar with.

Wetterhahn died of dimethylmercury poisoning as the result of the accidental spillage of a few drops of the chemical on her latex glove- covered hand. The accident occurred on August 14, 1996, and the poisoning produced progressive destruction of Wetterhahn's nervous system, until she finally died 10 months later.

ScottE

I think that's the same case I heard of previously.

-dave w

On Thu, 13 Nov 2003 22:15:42 GMT, In the heat of the moment, Ismaeel Abdur-Rasheed posted: .

If pure, you can safely drink murcury. Although I don't know why a person would want to.

-- Bill

I am a member of the rabble in good standing. -- Westbrook Pegler --

my concern was it's poisoning the soil or water

apparently mercury is both more and less dangerous than I had imagined

- iz

Bill Schowengerdt wrote:

According to Ismaeel Abdur-Rasheed :

Correct. Metallic mercury is actually pretty innocuous. The well-publicized hazards of mercury are NOT about metallic mercury, but about its compounds.

On Fri, 14 Nov 2003 18:37:54 GMT, In the heat of the moment, Ismaeel Abdur-Rasheed posted: .

IIRC and my math is right.... The murcury would eventually vaporize. Once in the atmosphere, it would join the MUCH larger amount that it already being emmited by poluters.

In fact, if it joined JUST THE INCREASE Bush is allowing in his friend's polution, it would comprise less than 1/33600 of that total

It is truly interesting stuff. You can drink it, but it will disolve gold.

-- Bill

I am a member of the rabble in good standing. -- Westbrook Pegler --