Detecting burn-out with mercury switch

[Andre]

Hi guys

I would like to know if anyone has ever used a mercury switch to trigger an event at motor burn-out. For example if I wanted to fire a stage separation charge after first stage motor burns out. In theory when the motor burns out, there is a quick deceleration which should throw the mercury drop forward and close the circuit.


Andrej

 

[BLKKROW]

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

Not really anything to do with burnout and staging.

But this might help someone.

 

[CarVac]

Hi guys

I would like to know if anyone has ever used a mercury switch to trigger an event at motor burn-out. For example if I wanted to fire a stage separation charge after first stage motor burns out. In theory when the motor burns out, there is a quick deceleration which should throw the mercury drop forward and close the circuit.
View attachment 119856

Andrej

It's true of most motors in most rockets. Moonburning motors in relatively high drag rockets will begin decelerating faster than gravity (you need at least negative 1 G of acceleration relative to the ground to activate the mercury switch) before the booster motor actually quits.

However, this is fairly (read: extremely) sketchy. I'd personally just go with an accelerometer-based flight computer for staging, because it can perform contingency checks in the case of some sort of failure.

 

[jadebox]

Tripoli does not allow the use of mercury switches to ignite motors. I'm not sure if NAR has the same rule, but an electronic altimeter is a safer and more reliable solution anyway.

-- Roger

 

[Dale Saukerson]

In the 80s I did some mercury switch staging, about 4 flights or so.
It was Estes D to D with thermalite, flash bulb and capacitive discharge for the power source.
The flights worked great but you had to be constantly on your toes with the safety aspect.
That tilt switch is going to close regardless if YOU tilted the rocket (ground handling) or if deceleration closes the switch.
I used a common slide switch to prevent ignition on the ground. I also forgot to open the switch one time and got a nice core sample as a reward.

 

[troj]

Even when not against the rules, mercury switches are frowned upon due to the environmental concerns, should a rocket crash.

-Kevin

 

[jderimig]

Not very safe, a chuff on the pad might trigger the switch.

 

[fyrwrxz]

This is ancient technology. While it mite look cool in your steampunk rocket when you're rocking the paisley Nehru shirt, it's been eclipsed by fire, the wheel and electricity. I watched a two stager separate perfectly within visual range and then the sustainer fell over at arc. The chap next to me was pleading "No! Please No...don't light!" The mercury completed the contact, discharged the cap and the thermalite did what it does best. You could see the little twinkle as the nose cone dipped below horizontal and sure enuff; Power up to full thrust about 300' feet and it disappeared over a rise (still under full thrust) and then a huge cloud of dust and assorted desert vegetation raised it's ugly head. Just like a Roadrunner cartoon. He came trudging back over the hill with yards of film trailing from a shattered camera in his hands and the lifeless carcass of his much shortened rocket. Moral: Please use the wonderful stuff we have now to increase your chance of success and keep us out of the news. And nobody will laff at you.

 

[dixontj93060]

This is ancient technology. While it mite look cool in your steampunk rocket when you're rocking the paisley Nehru shirt, it's been eclipsed by fire, the wheel and electricity. I watched a two stager separate perfectly within visual range and then the sustainer fell over at arc. The chap next to me was pleading "No! Please No...don't light!" You could see the little twinkle as the nose cone dipped below horizontal and sure enuff; mercury completed the contact, discharged the cap and the thermalite did what it does best. Powerup to full thrust about 300' feet and disappeared over a rise and then a huge cloud of dust and assorted desert vegetation raised it's ugly head. Just like a Roadrunner cartoon. He came trudging back over the hill with yards of film trailing from a shattered camera in his hands and the lifeless carcass of his much shortened rocket. Moral: Please use the wonderful stuff we have now to increase your chance of success and keep us out of the news. And nobody will laff at you.

So, SO true!

(Andre) put down the mercury switch and back away slowly (to leave the Stone Age).

 

[Andre]

Thanks for all the replies guys. I already decided beforehand to use the flight computer to detect burn-out. I was just interested with practical experience people had with mercury switches. My main concern is as John D. pointed out that the switch might close if there is a chuff on the pad.

Andrej

 

[fyrwrxz]

It's all good, friend. Keep us posted on what kinda fun you're having over there and remember-these chaps love pictures! Wish I could get some of your motors over here!

 

[JonSivier]

I would like to know if anyone has ever used a mercury switch to trigger an event at motor burn-out. For example if I wanted to fire a stage separation charge after first stage motor burns out. In theory when the motor burns out, there is a quick deceleration which should throw the mercury drop forward and close the circuit.

In theory it should work. In practice, not so much. I've heard plenty of horror stories, but my only direct experience was at one of our launches in the mid 90's. Someone was flying a 2 stage rocket using a mercury switch to detect booster burnout and then ignite the upper stage. We were flying our high power launches on an abandoned air force base in those days. Plenty of room, but also plenty of concrete. The rocket went up very straight, but the switch didn't detect the deceleration due to burnout. It did however detect when the rocket turned over and pointed straight down. The upper stage motor fired and drove the rocket straight into the concrete. After that we made a rule banning mercury switches at our launches.

I'd say use an electronic timer or accelerometer.

Jonathan
-----
Jonathan Sivier
jsivier AT illinois DOT edu
NAR #56437 | Tripoli #1906
Home Page: https://www.sivier.me/