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Date:  10th January 2010
Location: Doonside, NSW, Australia
Conditions:  Hot (35C) with clear skies and light breeze. 5km/h early increasing to ~20km/h later.
Team Members at Event: PK, GK, Paul K, John K and Jordan K.

This week we did a couple of different water rocket experiments. One involved the first MicroLab payload flights and the other a flight test of a new lighter parachute deployment mechanism. We also flew Paul's 2-stage pyro rocket successfully after the last crash. Since it's a bit of a long write-up I'll only cover the mercury switch experiment here, and do the deploy mechanism and full flight day report as a separate page.

MicroLab - Mercury switch experiment

These MicroLab flights were designed to demonstrate what happens on board a rocket during flight. As many people start out designing their first parachute deployment mechanisms they sometimes base them on incorrect assumptions about a rockets' flight characteristics.

A very common question we get asked all the time: "Why not put a mercury switch on the rocket that will be able to detect when the rocket tips over at apogee?"

This is a very reasonable question since a system designed on this principle can work very well on the ground. This MicroLab experiment shows what actually happens to mercury switches in flight.

Experiment Setup

A small digital video camera is arranged to look through a lens at a set of three vertically oriented mercury switches. Two are mounted normally as would be expected and the other is mounted upside down. Switches #1 and #3 are identical (5mm diameter), while #2 is larger. (6mm diameter).

They are each wired to a separate LED so it's possible to see when in flight they actually activate. There is a white LED light source so the mercury can be clearly seen. There is also a barometric logging altimeter (Z-log) mounted on the side to correlate the event timing vs. altitude and speed. The payload is powered by a lithium 6V battery (2 x CR123A). The camera has it's own built-in power source.

The launch is video taped from the ground in order to get a reference of the rocket's attitude in relation to the flight time line.

Parachute deployment is controlled independently by an electronic timer that is triggered at the time of launch. The deploy delay is set to initiate the parachute deployment at apogee so that the rocket would travel a certain distance past apogee as it takes 2-3 seconds for the parachute to fully open. This allowed us to observe what happens to the mercury prior, during and just after apogee.

The MicroLab is attached to the top of the Polaron VIIIx rocket with the following parameters:

	The experiment consists of 3 mercury switches wired to separate LEDs.
	Back view of the experiment showing the various components
	Front view of the experiment
	MicroLab mounted inside the payload bay. Note the addition of a light diffuser for the light source LED as well as a light shield for the activation LEDs
	Payload ready to be attached to the rocket.
	Rocket configuration for the experimental flights. The recovery system is mounted on top of the payload and is powered independently from a 9V battery.

Results

The experiment was flown two times on the day within 30 minutes of each other. The event timelines below were reconstructed from the onboard and ground videos as well as the altimeter data. The annotated altimeter plots show when events took place.

Flight #1 Timeline

Flight #1 - Annotated Altimeter Plot

	T+ 0.24s - As the rocket clears the launch tube there is a small jolt on the rocket causing the mercury to jump briefly.
	As the rocket continues to accelerate the mercury bead is flattened and pushed into the little pocket at the bottom of the switch.
	Upon burnout the mercury either flies up (#3) or shows a significant bulge on the top surface (#1 & #2) The mercury pops out of the little pocket again.
	T+ 6.64s As the parachute is ejected it starts to exert a drag force on the rocket and switch #2 activates.
	T+ 8.04s The parachute is now fully open and with the increased drag all the switches are activated.

Flight #2 Timeline

Flight #2 - Timeline Events

Flight #2 - Annotated Altimeter Plot

	Rocket at rest on the launch pad. Notice how the mercury's surface tension keeps it from the small pocket.
	During acceleration the mercury is pushed into the pocket. Here the rocket has just cleared the launch tube and the jolt again causes the mercury to splash.
	Deceleration after burnout causes a bigger bulge on the surface. (#1 & #2). Switch #3 flies up again.
	Mercury splashing around as the parachute opens.

In the video below we have combined the ground and on-board videos. They are both shown in real time as well as slow motion. An animated altimeter plot is also included that shows the rocket's altitude during the flight.

Video showing the mercury switch behaviour  in relation to the rocket trajectory.

(Best viewed in HD - click the HD button so it turns red)

Conclusions / Analysis

• The video from both flights showed very similar mercury switch behaviour.
• Switch #3 showed the kind of behaviour one would typically expect during flight. It activated soon after burnout and peak velocity. This happens because air drag on the rocket causes it to undergo -ve acceleration. There is no air drag on the mercury bead and so it's inertia carries it forward.
• However, Switch #1 and #2 behaved quite differently to what was expected. Why did the mercury not rise like in switch #3? I can only assume that it's either one of two things or both. a) The adhesion forces/friction between the mercury and the glass surface were higher than the deceleration force. b) There may be a small vacuum produced behind the mercury.
  
  The simulator predicted a deceleration of around -0.3G due to drag at burnout, but this would have reduced to near zero as the air speed dropped near apogee and the rocket experienced less drag.
• It appears that switch #3 behaved as expected because of the electrical contacts were inside the mercury bead. The mercury's surface tension would have helped to push the contacts out and get the bead moving away from the contacts when the small deceleration force was applied.
• No switch on both flights showed any sign of activation as the rocket passed through apogee. Switch #2 on flight #1 activated somewhat earlier than switch #1 but this was due to the drag of the not yet fully open parachute. Switch #3 remained deactivated until the parachute fully opened.
• A mercury switch cannot be reliably used to detect apogee. If a mercury switch is used to detect burn out, careful attention must be paid to its orientation and ensuring the deceleration forces are sufficient to activate it.

Other Observations

• If you look carefully there is a little jump in the mercury just as the rocket clears the launch tube. This could potentially cause a false trigger.
• During acceleration, the mercury fills the little pocket at the bottom of the switch. (see switch #1 and #2) When acceleration stops the mercury pops out again but the mercury does not float upwards. The mercury does not fill this pocket normally due to its high surface tension.