# mercury switch deployment

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#### Bowhunter

##### Well-Known Member
Has anyone done this? Im experimenting with this hopefuly it will eliminate lawn darting even if motor problem occurs even at low alltitude. After it arcs over at apogee POP out comes the chute

#### edwardw

##### Well-Known Member
Rockets rarely go up, flip over and come back down. They have an X and Y velocity component. The idea you have would work great if your rocket had no X component, only Y component. As your rocket is traveling up in the Y and X directions (ideally traveling in a parabola), it exerts some amount of force on the mercury (as does gravity) keeping it there. Now, when your rocket flips over and starts coming down, it is also still moving in the X direction. Usually the speed at which it is traveling in the X direction is fast enough to keep the mercury at the back of the switch and it won't deploy. Think of when you gun your car and you are pressed against your seat a little, this is what is going to happen to the mercury.

Also, there is another issue, burnout. Now when your motor is thrusting it keeps the mercury pressed against the right end. Now when burnout happens your rocket can decelerate quickly, and since the mercury isn't held down it might just 'jump' to the other side and deploy....not a pretty sight.

I thought of a similar concept like this last fall, but I was using a magnetic relay and a marble shaped magnet in a piece of PVC tube. The tube was about 6" long and 1" diameter and the marble was 1/2" diameter. My hope was that when it tipped, the marble would roll and *pop* out comes the chute. Never had that happen. Though a couple times on burnout I had the chute deploy...messy.

Regards,

Edward

#### UhClem

##### Well-Known Member
A mercury switch can be used for staging but NOT for deployment.

Why?

Because when the motor burns out, the rocket will experiance negative acceleration from drag. This will cause the mercury switch to close and out comes the parachute.

Assuming for the moment that the mercury switch doesn't close at motor burnout, it will not close at apogee because the rocket will not see negative acceleration at apogee. While gravity is pulling the rocket back to the ground, it is working on the entire rocket in a uniform manor so nothing happens to the mercury switch.

For example, look at this plot of data from one of my rockets: https://home.earthlink.net/~david.schultz/level3/erint/flightlog/15Marrdas2.gif

Acceleration (in black) before launch is measured as 1G. After motor burnout the acceleration changes to about -1G but then as velocity decreases this decays to zero G. The only reason that the measured acceleration changed from zero G at apogee is because the parachute deployed.

#### solrules

##### Well-Known Member
Originally posted by UhClem
A mercury switch can be used for staging but NOT for deployment.

Why?

Because when the motor burns out, the rocket will experiance negative acceleration from drag. This will cause the mercury switch to close and out comes the parachute.

Assuming for the moment that the mercury switch doesn't close at motor burnout, it will not close at apogee because the rocket will not see negative acceleration at apogee. While gravity is pulling the rocket back to the ground, it is working on the entire rocket in a uniform manor so nothing happens to the mercury switch.

For example, look at this plot of data from one of my rockets: https://home.earthlink.net/~david.schultz/level3/erint/flightlog/15Marrdas2.gif

Acceleration (in black) before launch is measured as 1G. After motor burnout the acceleration changes to about -1G but then as velocity decreases this decays to zero G. The only reason that the measured acceleration changed from zero G at apogee is because the parachute deployed.
yup. You CANNOT use a mercury switch for deployment. It would ideally be used for staging, as the switch should close on burnout of the first stage. However, mercury switches are not very reliable, and hazordous if it breaks. It is best to stick with an altimeter and or accelerometer for apogee deployment and an accelorometer for staging.

#### Ryan S.

##### Well-Known Member
Originally posted by edwardw
Now, when your rocket flips over and starts coming down, it is also still moving in the X direction. Usually the speed at which it is traveling in the X direction is fast enough to keep the mercury at the back of the switch and it won't deploy. Think of when you gun your car and you are pressed against your seat a little, this is what is going to happen to the mercury.
That isnt entirely true, at the begining of the flight the car model works, however, at the end of the flight there is no force exerted on the mercury, their cannot be because in order for there to be a force present the rocket must be going faster than the mercury and "pushing" it forward, and since everything is accelerated towards the earth at the same rate (9.8m/s/s) the mercury still will not move relative to the switch because they will be faliing at the same rate.

When a rocket starts a downward plunge it doesnt accelerate quickly, like in a car, it slowly moves forward, as if you were slowly pushing down the accelerater. either way though, you never hit the windsheild (switch? ) and the rocket wont deploy. It might work in that breif second the rocket tips over at apogee, but all of this is moot (sp) because it would have already shredded when the mercury made the connection after burnout

#### shockwaveriderz

##### Well-Known Member
this is why we have altimeters and timers.......

#### Stymye

##### Well-Known Member
Since nobody covered it, I will. Please don't use mercury switches. Mercury is a poison. If your rocket were to crash and the bulb broken, you technically would have an industrial waste issue and possibly be liable for clean-up, fines, etc, not to mention putting the landowner of the field you fly on at risk. If it contaminated the ground water, they may never know the source and then the clean-up activities would be devastating. Just touching it can cause you significant harm, and think if a kid came along and "found" it.

It's just not a good idea.

Just remember...that thermostat on the wall in your living room with the mercury switch in it doesn't fly.

/soapbox off/

#### UhClem

##### Well-Known Member
Originally posted by DynaSoar
The variation in the answers so far really only reflect the reliability of using mercury switches.

Add to the other comments the possibility of vibration during thrust shaking the stuff up, and shaking the switch loose.

For some applications a magnetic reed switch would do the same job more reliably.

For ejection, I've considered trying a short, slightly damped, charged pendulum inside a conductive, grounded tube. Enough angle, and contact.
The variation in answers is because some people do not understand rocket dynamics. There have been a few that say they think it will work, some who say that it has no chance, and no one who says that they have done it and it worked.

Go ahead and try it if you want to. But be prepared for deployment of your recovery system at motor burnout. Also be prepared to have the safety check-in person send you packing when you tell them that you are using a mercury switch for deployment.

Tilt sensors will also fail.

#### ChrisLentz

##### Well-Known Member
I have recently been experimenting with a tilt switch using small pvc conduit and a ball bearing. It is armed by a switch on the side of the rocket just before launch while in the vertical on the pad. When the rocket hits apogee and begins to nose over, the ball bearing makes contact and poof, out comes the chute.

Of course this has yet to be tested in an actual rocket but the prototype I made lights the LED(ignitor replacement) at tip-over every time. I should have a field test soon enough.

#### jderimig

I have recently been experimenting with a tilt switch using small pvc conduit and a ball bearing. It is armed by a switch on the side of the rocket just before launch while in the vertical on the pad. When the rocket hits apogee and begins to nose over, the ball bearing makes contact and poof, out comes the chute.

Of course this has yet to be tested in an actual rocket but the prototype I made lights the LED(ignitor replacement) at tip-over every time. I should have a field test soon enough.
This won't work. The ball is weightless at apogee, there is no gravity force to move the ball one way or the other. See Dave's post just before yours...

#### cerving

##### Owner, Eggtimer Rocketry
TRF Supporter
Suppose you have the ball bearing in a relatively long tube, with some kind of sensor at the "top" so that if the ball gets there it fires the chute. You would think that when the rocket noses over (or possibly tilts slightly downward) that it would detect that, and the chute would neatly come out at or near apogee. The problem with such a system is that at burnout there are NEGATIVE g-forces on the rocket as it decelerates. Depending on the velocity of the rocket and its resultant drag at motor burnout, these forces can be quite large... in some cases, close to the acceleration g-forces. This would lift the ball right up to the top of the tube, and your chute would deploy at a very high velocity, resulting in a very spectacular early deployment and/or shred.

There HAVE been some systems that used this kind of mechanism, but they have some kind of lockout so that they ignore any triggering before a certain time after launch. Of course, you have to be able to detect launch, too... that's another issue.

Keep playing with this Chris... that's part of the fun of rocketry. You're always learning new things, not necessarily what you expected...

#### jderimig

Suppose you have the ball bearing in a relatively long tube, with some kind of sensor at the "top" so that if the ball gets there it fires the chute. You would think that when the rocket noses over (or possibly tilts slightly downward) that it would detect that,
Why would the ball go to the end of the tube when the rocket tilts over? The tube and the ball are accelerating at exactly the same rate (-1g), the ball should stay put or with uncertainty is just as likely to move in either direction.

Try this at home. Take a tube and hold a ball next to it in the middle. Drop them at the same time and see if the ball move to the lower end of the tube.....

#### rocket_troy

##### Well-Known Member
The only way I envisage you could do it is to have say 2 conductive cones against each other but electrically isolated from each other with a blob of mercury or metallic ball bearing contained within. So, when the rocket tilts/pitches over the ball/blob closes the circuit between the 2 cones by having contact against both conical surfaces.
However, for this to happen, there&#8217;s needs to be an additional force other than gravity that can differentiate the acceleration of the free falling rocket and the free falling ball/blob. As other contributors have mentioned, the only other force acting on the rocket is aerodynamic.
So for there to be enough differential in the forces acting on the blob/ball opposed to the rocket, the rocket either needs to be travelling with a high enough horizontal component (as it&#8217;s tipped horizontally) for the fins to *aerodynamically* try and maintain pitch OR the rocket need to be aerodynamically &#8220;fluffy&#8221; enough to generate a significant enough drag as it falls in the horizontal (tilted) state.
Either way the falling rate of the horizontal rocket needs to be less than gravity (~9.8m/s/s) and significantly enough less that allows the mechanism chosen to make the contact.
So, in agreement with other contributors, it&#8217;s possible to make such a mechanical mechanism work but it might not be reliable and with the availability of cheap electronics, why would you consider a mechanical approach these days.

Troy

#### ChrisLentz

##### Well-Known Member
Suppose you have the ball bearing in a relatively long tube, with some kind of sensor at the "top" so that if the ball gets there it fires the chute. You would think that when the rocket noses over (or possibly tilts slightly downward) that it would detect that, and the chute would neatly come out at or near apogee. The problem with such a system is that at burnout there are NEGATIVE g-forces on the rocket as it decelerates. Depending on the velocity of the rocket and its resultant drag at motor burnout, these forces can be quite large... in some cases, close to the acceleration g-forces. This would lift the ball right up to the top of the tube, and your chute would deploy at a very high velocity, resulting in a very spectacular early deployment and/or shred.

There HAVE been some systems that used this kind of mechanism, but they have some kind of lockout so that they ignore any triggering before a certain time after launch. Of course, you have to be able to detect launch, too... that's another issue.

Keep playing with this Chris... that's part of the fun of rocketry. You're always learning new things, not necessarily what you expected...
Exactly what I plan to do. Experiment and learn. Thank you for the support in my experiments.

#### cerving

##### Owner, Eggtimer Rocketry
TRF Supporter
Key phrase was "You would think". Of course the ball might not move at all... you and I both know that accelerometers are notoriously fickle at detecting apogee. Like I said, this is something for ChrisLentz to play with.

#### ksaves2

Why would the ball go to the end of the tube when the rocket tilts over? The tube and the ball are accelerating at exactly the same rate (-1g), the ball should stay put or with uncertainty is just as likely to move in either direction.

Try this at home. Take a tube and hold a ball next to it in the middle. Drop them at the same time and see if the ball move to the lower end of the tube.....
It would work but it would have to be an ideal situation which I've only seen once or twice. Rocket goes absolutely straight up, pauses, flops downward before accelerating down. That would do it but it's rare to see.

I will report I had a motor ejection fail on a 4 inch diameter LOC tubed rocket. The rocket did a death dive from 3600 feet. The recovery laundry was freely movable inside the tube and while still a couple of hundred feet in the air, it did indeed
move down and push the nosecone off. The nosecone loosely fit too. The Covert Parachute (remember them?) "exploded", harness zippered the tube, the rocket was knocked sideways and landed flat in soft dirt. I did a coupler repair and turned it
into a dual deploy. It happened in a past Midwest Power so I had witnesses. Flew the DD version a lot but it too wore out and ended up doing this:[video=youtube;hMvT9BKzbUg]https://www.youtube.com/watch?v=hMvT9BKzbUg&t=4s[/video]
Unbelievably, I have a coupler on the end ready to do another restoration into DD mode again!! The camera fluttered down on the zippered tube that separated from the ebay. Kurt

#### jderimig

It would work but it would have to be an ideal situation which I've only seen once or twice. Rocket goes absolutely straight up, pauses, flops downward before accelerating down.
Wait you're right! Centripital[sic] will move the ball if the tube is away from the center of rotation. But that ball is going to start moving whenever the rocket has any rotation, which can happen anytime before apogee.

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#### ksaves2

Wait you're right! Centripital[sic] will move the ball if the tube is away from the center of rotation. But that ball is going to start moving whenever the rocket has any rotation, which can happen anytime before apogee.
Yeah but I think there are too many variables involved to get the tube thing to work consistently. My only advice if going to try, is build a robust glass rocket that can hack all the mishaps that are bound to occur during testing. Kurt

#### OverTheTop

##### Well-Known Member
TRF Supporter
As others' have said:
rocket under boost, mercury to rear of switch.
rocket coasting, mercury to front of switch due to drag (negative thrust) on airframe (but not on Hg).
rocket at standstill (apogee), mercury position indeterminate.

There is a video on YouTube somewhere that someone took in a payload bay with a mercury switch IIRC. You might find it illuminating.

This is a bit like WiFi networking. If you think it will be of assistance to you, think about it. Think about it some more. Then think a bit harder until the idea goes away.

Apologies for being a bit blunt.

Years ago, before there was the electronics revolution that produced electronic model rocket altimeters, there was a write-up in Brinley's book "The Amateur Rocketry Handbook" that described mercury switches for amateur rockets. I remember back in that time-frame there was a model rocketry company that sold a simple mercury switch. I bought one or two, but I never have been able to find them since, either in my inventory or on-line. Anyways, I tried (I think twice) to ignite a flash bulb. It worked once shortly after burn out, which is what one would expect. For some reason the second time in a different rocket that had a very crowded payload compartment it did not work (maybe a short somewhere).

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#### fyrwrxz

##### latest photo
>RANT ON< Mercury switches are dinosaurs. 'has anybody done this'-really? Shoot - we did this 45 years ago and it was a sketchy idea then and it's even crappier than that now. So-your little experiment catos or core samples. For 1/1000th of the cost of toxic waste cleanup and possible fines-get a real deployment altimeter. Or go water your lawn with used motor oil-it's organic, isn't it? I don't care about 'thought experiments' and that's where they should stay-in your head-not in the drinking water, not all over your hands and certainly not where I might fly someday. Remember-if it worked so good back then- we'd still be using it. I'll support most all my brother rocketeers trying to push the limits or find solutions to problems, but I draw the line here. BAD Idea. try again-without the mercury. >RANT OFF<

#### timbucktoo

##### Well-Known Member
Staff member
TRF Supporter
Global Mod
As others' have said:
rocket under boost, mercury to rear of switch.
rocket coasting, mercury to front of switch due to drag (negative thrust) on airframe (but not on Hg).
rocket at standstill (apogee), mercury position indeterminate.

There is a video on YouTube somewhere that someone took in a payload bay with a mercury switch IIRC. You might find it illuminating.

This is a bit like WiFi networking. If you think it will be of assistance to you, think about it. Think about it some more. Then think a bit harder until the idea goes away.

Apologies for being a bit blunt.
WAs it this video?

#### FredA

##### Well-Known Member
Weird video....
How do the three ball of mercury not behave the same?
How does the mercury on the right switch hit the top at burnout (as expected) but the other balls of mercury don't move until apogee?
I would expect all the balls to move together with very slight delays due to mass and surface tension.

#### tab28682

##### Well-Known Member
TRF Supporter
Weird video....
How do the three ball of mercury not behave the same?
How does the mercury on the right switch hit the top at burnout (as expected) but the other balls of mercury don't move until apogee?
I would expect all the balls to move together with very slight delays due to mass and surface tension.