Mecury tilt switches

[Andrew Leigh]

Hi,

has anyone experimented with mercury tilt switches. These are glass tubes with a small quantity of mercury (I think a ball bearing would also work) and two sets of contacts at each end. When in the upright position the bottom set of contacts are "made" and the top are "open". When turned 180° the "made" contacts are now "open" and the "open" contacts are now "made".

I just thought that these mechanical devices would be perfect to deploy a parachute when the rocket crests through apogee.

I am planning on making one of these level switches to activate an electromechanical device used to deploy the nose cone. The device is energised when in the launch position, when the rocket turns over the ball bearing runs down the short tube and the circuit is broken.

Any opinions?

Cheers
Andrew

[WRA2]

Hi,

has anyone experimented with mercury tilt switches. These are glass tubes with a small quantity of mercury (I think a ball bearing would also work) and two sets of contacts at each end. When in the upright position the bottom set of contacts are "made" and the top are "open". When turned 180° the "made" contacts are now "open" and the "open" contacts are now "made".

I just thought that these mechanical devices would be perfect to deploy a parachute when the rocket crests through apogee.

I am planning on making one of these level switches to activate an electromechanical device used to deploy the nose cone. The device is energised when in the launch position, when the rocket turns over the ball bearing runs down the short tube and the circuit is broken.

Any opinions?

Cheers
Andrew

If you use a "mercury tilt switch" your deploy will always activate immediately after the rocket stops thrusting.

Here is why. As soon as the rocket stops thrusting it will start slowing down due to drag (air resistance). this causes the mercury to move to the top of the switch and triggering it. (think about how it feels when applying brakes in a car and you move forward in your seat). The same thing happens to the mercury when the air resistance "applies the brakes".

Search for "magnetic apogee detectors" which work based on the alignment with Earth's magnetic field to accomplish the same thing.

Welcome to the forum.

[Andrew Leigh]

Thanks for that, was not thinking that the mercury is not "attached" to the rocket and would therefore continue to accelerate at a different rate to the rocket.

[Tim Chen]

Hi,

has anyone experimented with mercury tilt switches. These are glass tubes with a small quantity of mercury (I think a ball bearing would also work) and two sets of contacts at each end. When in the upright position the bottom set of contacts are "made" and the top are "open". When turned 180° the "made" contacts are now "open" and the "open" contacts are now "made".

I just thought that these mechanical devices would be perfect to deploy a parachute when the rocket crests through apogee.

I am planning on making one of these level switches to activate an electromechanical device used to deploy the nose cone. The device is energised when in the launch position, when the rocket turns over the ball bearing runs down the short tube and the circuit is broken.

Any opinions?

Cheers
Andrew

If you use a "mercury tilt switch" your deploy will always activate immediately after the rocket stops thrusting.

Here is why. As soon as the rocket stops thrusting it will start slowing down due to drag (air resistance). this causes the mercury to move to the top of the switch and triggering it. (think about how it feels when applying brakes in a car and you move forward in your seat). The same thing happens to the mercury when the air resistance "applies the brakes".

Search for "magnetic apogee detectors" which work based on the alignment with Earth's magnetic field to accomplish the same thing.

Welcome to the forum.

+1

[Andrew Leigh]

Is +1 rocket speak for ditto?

[air.command]

Hi Andrew,

As was mentioned mercury switches don't behave the way we normally expect them to. They will also not always activate on burnout either, it depends on their orientation and on how fast the rocket is decelerating at that time. Here is a full write up on an experiment we did with mercury switches on water rockets:

http://www.aircommandrockets.com/day85.htm

The video shows you what the mercury does in flight.

- George

[Andrew Leigh]

Brilliant work there, nothing like good research. A last question please.

Hypothetically, if one could have the tube of the mercury switch say 150mm long i.e. contact 150mm apart from each other, would you still expect the same results?

Cheers mate
Andrew

[air.command]

Brilliant work there, nothing like good research. A last question please.

Hypothetically, if one could have the tube of the mercury switch say 150mm long i.e. contact 150mm apart from each other, would you still expect the same results?

Cheers mate
Andrew

I'm not sure I am quite picturing that. Do you mean the tube is 150mm, with the contacts at one end, or one at either end? Or the mercury is at one end and the contacts at the other?

[Andrew Leigh]

Hi,

apologies for the confusion.

You make a new switch. Tube 150mm long with contacts still at both end. The purpose being to lengthen the travel of the mercury significantly in order to remove the chance that the switch will make early contact. A stretched mecury switch.

Cheers
Andrew

[air.command]

Hi,

apologies for the confusion.

You make a new switch. Tube 150mm long with contacts still at both end. The purpose being to lengthen the travel of the mercury significantly in order to remove the chance that the switch will make early contact. A stretched mecury switch.

Cheers
Andrew

Ahh I got it now. So to answer the original question, yes I would expect the behaviour to be the same. The time after burnout to apogee is going to be in the order of perhaps 4 seconds or so for a regular type rocket. The mercury will fly to the other end of the tube more likely in about 0.25 - 0.5 seconds.

- George

[Andrew Leigh]

Thanks,

at least I will not waste hours of time following a hairbrained scheme.

Cheers
Andrew

[Tim Chen]

Thanks,

at least I will not waste hours of time following a hairbrained scheme.

Cheers
Andrew

Are you any good with electronics? If you know how to write software and build a circuit, you can make different mechanical or digital sensors detect apogee with many methods. Air speed sensors or magnetic apogee sensors are some of the most reliable.

[Andrew Leigh]

Hi,

in fact spent some time browsing last night as I am not too bad with electronics cannot design but can create my own PCB's and can "paint by numbers" so to speak.

As far as the software goes my son is a programmer but I would need to find out if he is conversent with whatever language will be used. I suppose one needs to write to an Eprom. Sure I could find someone to write the chip if I give them the software.

Got any decent links for electronics, the one I saw was in fact a flight computer at http://www.geocities.com/rdh82000/rocket_controller.htm

Cheers
Andrew

[U.S. Water Rockets1]

Hi,

in fact spent some time browsing last night as I am not too bad with electronics cannot design but can create my own PCB's and can "paint by numbers" so to speak.

As far as the software goes my son is a programmer but I would need to find out if he is conversent with whatever language will be used. I suppose one needs to write to an Eprom. Sure I could find someone to write the chip if I give them the software.

Got any decent links for electronics, the one I saw was in fact a flight computer at http://www.geocities.com/rdh82000/rocket_controller.htm

Cheers
Andrew

There are dozens of places online who will make a handful of PCBs for you for a decent price. You can google for quick turn or prototype PCB and come up with a lot of suppliers. If you can solder then you should be able to make just about anything you can design.