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Date:  23rd December 2009
Location: Workshop
Conditions:  Pleasant and warm
Team Members at Event: PK and GK.

Pressure Switch

We have been developing a new pressure switch for the Acceleron V rocket as the previous ones have been either too leaky or not reliable enough. The latest iteration is the PS3.

The switch is based on a regular push button switch mounted behind a rubber membrane. The pressure switch is simply screwed to the top of the booster. The diagram below details it's construction. The lower bottle cap contains the rubber membrane and has a small hole for the switch arm to pass through. The switch body is mounted in another cap which is held in place by a piece of T8 FTC. Two plastic reinforcing rings were added to the outside of caps to help secure the T8. Only the bottom cap is glued to the FTC. The upper cap is held down by two screws to allow the switch to be removed for servicing if needed. There is a small plastic spacer between the two caps to keep the right distance.

The particular switch we used was an old one we had on hand, but it was chosen because of the force needed to make contact. The spring in it is strong enough to deform the membrane back into the bottle when the pressure drops. One of the problems with a previous pressure switch was that the spring in the micro-switch just wasn't strong enough.

A cover made from a kinder surprise egg prevents moisture entering the switch when the second stage is released.

Testing

Activation pressure

The first tests we put the switch to was to see what it's activation pressure is. A low activation pressure means you could get a slow response and potentially false triggers, and a high activation pressure means that it triggers too early before burn out. We were aiming for a 20-30psi range. The activation pressure we saw on the first tests was 30psi which was at the upper end of the desired range.

When hydro testing the switch to 130psi, we noticed that the activation pressure was closer to 40psi which was a little high. But this may have been skewed by the fact that we were reading the pressure at the tank end, and since the hose has little holes it means that it takes a while for the pressures to equalize at the bottle end. Since we were going all the way up to 130psi, we didn't bother to wait for the pressure to equalize at the lower pressures. ( During later static tests the activation pressure was closer to the 20-25psi mark)

Activation Timing

The main test we wanted to do was to see when it activates in relation to the trust produced by the booster. During Acceleron V's last flight it looked like the old pressure switch activated really late. It was well after the back up system fired. We wanted to see if the new pressure switch would also activate in a similar manner.

For this experiment we made a booster segment out of two of the new spliced-pairs and fitted the pressure switch on top. Since the switch was now sitting on top of the booster we could not directly connect it to the test stand. So we attached half a bottle to to the top of the booster covering the pressure switch and giving us a way to attach the booster segment to the load cell. A small battery and LED were connected to the switch so that we could see when it turned on.

We recorded the thrust curve on the laptop and video-taped the LED with both high-speed and regular video. In a video editing suite we measured the time it took for the LED to turn off after firing the rocket. In hindsight I should have connected the pressure switch to a second channel on the data acquisition unit. We then compared this time against the thrust curve to see when the switch was activating.

I had to turn down the gain on the load cell amplifier, because with the 15mm nozzle simulation predicted the booster segment would generate about 270N  thrust on release. (The load cell can go up to ~700N) We did 5 tests all up to measure the timing.

The capacity of the booster segment was ~6.3L. Each test used 2L of water.

Notes

1. We chose to use slightly lower pressures in these tests in order to reduce the stress on the bottles since these are flight hardware.
2. The 9mm nozzle test was used to stretch out the thrust curve so we could see the timing more clearly.
3. Thrust was only measured on tests 1,2 and 3. Timing was measured on all 5 tests.
4. *The first test showed the LED turning off as soon as the rocket was launched, but the mounting bottle had also buckled which may have caused a false reading.
5. Tests 2-5 showed quite consistent readings.

Thrust curve and switch activation timing for Test #2.

Thrust curve and switch activation timing for Test #3.

Here is a short video of the tests.

Conclusions

The switch seems to be performing quite well and is holding up to the pressure. The timing is also good as it activates just prior to the end of thrust which allows us to set a staging delay via the flight computer. This delay will also depend on other tests such as how long it takes for the staging servo to release the second stage.

When we attached the pressure switch during one of the tests, the rubber membrane did not seat properly and we had a small leak. The main problem is the switch is distorting the membrane. The solution on launch day will be to first mount the rubber membrane and cap and then secure the switch body into it.

Other than the buckling support bottle, we also managed to damage the release head on the end of the hose as it smacked against the concrete tiles on launch. We have a safety line that normally prevents that from happening, but this time the higher force managed to stretch the string more than normal and the head made contact. We did a temporary fix to finish the tests, but it will need to be replaced.