Gravity Switch Question

A dilligent, intelligent, and responsible young rocketeer wrote: Jimmy: while looking through Richard Nakka's picture gallery I had an idea, this is rather hard to explain but I'll try: A parachute deployment trigger made of a metal ball with a wire attached to it in a short plastic tube just big enough to let the ball slide in it easily with a metal end cap on one end and a plastic cap on the other with a hole in it so the wire could go through it and to the ball. See diagram for more info.  (a right) The tube would be placed plastic end cap down in the payload bay. When the rocket goes up the circuit is broken, at zero gravity or when the rocket starts to fall the ball will (well, should) move and contact the metal end cap which completes the circuit and sets off the low current igniter that sets of the ejection charge and ejects the parachute. Do you think it would work? Do you get how it works? Is it practical? Would it be possible to use this idea for a drogue chute and a regular delay for the main chute? Well, its just an idea

My response is below.  I would appreciate any feedback on its accuracy, as it is based primarily on hearsay and my own "thought experiments" which can get a little wacky at times.


Jimmy writes:

Your idea is not a new one, it has been proposed and tried.  Most have tried using a mercury switch, which would do about the same thing.  The reasoning is mostly sound, but more complicated than it first appears, so this method is not used much, if at all nowdays.

The problem is that when the rocket is in free-flight, all of its components are nearly weightless in relation to each other.  Gravity pulls equally on every part, so there is no impetus for the ball to move in relation to the tube - with exceptions:

Envision the rocket on the launch pad, ball at the bottom of the tube, and the ejection circuit thus "open."  If you picked the rocket up and turned it upside down, the ball would fall, close the circuit, and fire the ejection charge.  But you are holding the rocket up against the force of gravity, so there is a force differential.

Imagine that you do the same thing at the edge of a tall cliff, and instead of turning the rocket upside down, you just toss it over the edge.  It will fall base-first for aways until the airspeed is high enough that the fins cause it to turn around and it starts to fall nose-forward.  The ball also falls, since it has somewhere to go, but the rocket is falling too, and the ball does not fall any faster.  Thus it stays in the bottom of the tube and does not close the connection.

That scenario isn't quite true, of course, and the error in it gives your method a chance to succeed.  The aiframe is also subject to air resistance, and the ball is not.  So the ball will fall perhaps a tiny bit faster than the airframe, possibly moving forward and closing the circuit.  The difference will be small, so the ball must be able to slide with very little friction.  This is why some used mercury switches - the mercury, being liquid, moved with little friction.  When it hit the far end of the tube it connected two electrical contacts to close the circuit.  

This is essentially what would happen to a rocket at apogee.  It stops flying upward, and gradually starts to fall downward, much as if thrown off a cliff.  

So at this level, the tilt switch has a chance.  But there is another problem.

When the rocket motor ignites, the airframe accelerates like crazy for a few hundred feet.  The ball (or mercury) is pressed firmly in the bottom of the tube.  Then the motor burns out, and the rocket decelerates sharply.  The deceleration comes from two forces, gravity and air resistance.  Gravity applies the same force on all components, internal and external, so the rocket and the ball are both in free-flight, and do not move in relation to one another from this cause.   

But at rocket speeds, air resistance is fierce and deceleration can be violent.  Deceleration will be strongest right after the motor burns out and the rocket is moving at its highest speed.  Chances are good that the metal ball will move forward, since the airframe is decelerating by air resistance and the ball is not.  Thus it seems likely that the ejection charge will fire right after burnout.  

Sometimes nose cones are separated from the airframe when the motor burns out.  Others use "drag separation" to get a booster to drop off a 2-stage rocket.  Tape, shear pins, and other means are employed to keep the nose cone on.  

I had an ejection like this at the last NEFAR launch, but using an altimeter.  The rocket went a little crazy on liftoff, zigzagged its way to stability, but fired its ejection charge right after motor burnout.  I still haven't figured out why, but suspect the odd flight at beginning may have affected the altimeter in an odd way.

Page is at:  https://jamesyawn.com/nefar8-13-05/index.html

On the other hand, I have heard rumors of folks that have made this kind of switch work.  Exactly how, I don't know.  But I would be glad to run this by the other rocketeers I converse with to see if anyone has experience with it.  

Sorry about the downer.   I had actually made a switch similar to your idea in a section of PVC pipe and was going to test it.  Then I saw a flurry of messages talking about such switches which alerted me to their dangers, and I abandoned the project.

But I am sure that if you persist with this issue, you will find good answers!

Jimmy Yawn


Recrystallized Rocketry