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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Some thoughts from Maidenhead

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Dave Woodman
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Joined: Fri May 20, 2016 6:27 am

Some thoughts from Maidenhead

Post by Dave Woodman » Fri May 20, 2016 8:38 am

OK, so here's a large first post.

I note with interest other posts regarding altimeters, and the desire to be able to save cost. To that end, I have a some of observations, and a would like to fly a possible solution.

The problem, as I see it, is that any home-made altimeter is essentially un-calibrated. Now, Uncle Sam spent a great deal of money letting anyone on the planet find out where they are using GPS - but the accuracy out of the box is not up to the task that we would need for an altimeter.

There are several well known methods of dramatically improving the accuracy to the order of 10cm or so.

- The first, most obvious, and useless to us is that of long term averaging - removing the error by collecting readings over a long period.

- The second is Differential GPS (DGPS) - this involves the reception of error corrections from a fixed base station. It has the advantage of being real-time but requires another receiver to work - prohibitive from cost and weight viewpoints

- The third, and most interesting is post-processing. This involves the use of two GPS units, one fixed and one mobile, and both in the same locality. Since the GPS errors would apply equally to both units the difference between the readings is highly accurate. This is not a real-time system, as the term "post processing" implies, but the comparison of two simultaneous captures is easily performed.

So, what about sample speed? There are now cheap GPS modules available that provide 10 fixes per second. The use of something like a Pi-Zero to log the data to a memory card seems feasible, and two such installations would still be relatively cheap.

Since the Pi-Zero is now camera ready, another bird can be slain with the same projectile.

So, would such a solution provide data acceptable to the WRA2?

This all assumes, of course, that GPS lock is maintained - something that will need to be tested. If, however, it is not then all is still not lost.

There are also cheap i2c barometric sensors - un-calibrated, of course - but using the same method as above their position, including altitude, can be determined. Taking them for a few hikes up hills or even driving them up hill and down dale should yield enough data to calibrate them.

If I were to use a Pi-zero as a flight computer, I'd also include a barometric sensor in any case, since I'd want a second data source to indicate apogee and deploy a parachute.

So, enough ramblings. Would either of these two ideas be considered sound enough to use in a record attempt? What extra burden of proof would be required (barometric calibration data etc) to make this acceptable, if at all.

If the idea has legs, I might be persuaded to develop the idea, and locate and/or write the necessary software.

Enough idle musings!


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