Apogee and water contents

[bob755]

Hello,

Does anyone know how/if apogee is related to the moment at which the bottle becomes empty? Is it reached before, et the same moment or after depending on the mass of the rocket and thus the momentum?
Is it constant for a given rocket?

Thanks

[Azriel]

Apogee is related to the moment at which a bottle becomes empty, yes. Apogee depends on a number of factors, mostly the speed and height at burnout(once the bottles empty). At that point, apogee can be calculated simply to within a few tens of feet for short height rockets, and around 100 feet for high flying rockets (roughly 5-10% error).

[bob755]

Ok thanks. The reason for my question is that I'm thinking of experimenting with an optical parachute deployment system. A system of photodiodes and leds near the neck of the bottle monitor the level of water remaining. When the bottle is empty it triggers parachute deployment after a given delay. It probably requires the water to be colored so as to obscure the light from the leds. The optical detection system would of course be insensitive to daylight.
I'm not sure how the water behaves in the bottle during flight I suspect that the neck remains full until the bottle is almost empty.

Any thoughts on this are welcome.

[air.command]

I'm not sure how the water behaves in the bottle during flight I suspect that the neck remains full until the bottle is almost empty.

Any thoughts on this are welcome.

I think this approach should work. If you put the LEDs and photo transistors in darkened tubes to help exclude natural light it should be able to work. You may also want to look at using an IR LED and a IR receiver module (http://i87.photobucket.com/albums/k121/ ... lecopy.jpg) like is used in remote controls. You strobe the IR led at I think 38Khz or so and the detector looks for a 38Khz signal. This helps exclude pretty much any interference from outside sources. You would have to check how much food colouring is needed to block the signal.

You may also try to set up the emitter and detector offset at an angle so that you can use the refraction of the water through the throat to bend the emitted light to fall on the detector, and when the water runs out the refraction changes and the emitted light no longer hits the detector. ... no food colouring required.

Here is an example of how water behaves in a bottle during flight: http://www.youtube.com/watch?feature=pl ... I1oiRbQQ#! (You can skip to 1:20 and 2:42 for the actual on-board footage). As you can see it can be quite turbulent. Remember also that when the water runs out, fog forms in the bottle which may also partially block any transmitted signals.

Good experiment to try though.

- George

[Blenderite]

That is a interesting system, but also highly complex. And using an optical trigger might not be 100% reliable. The last thing you want is to have your rocket crash into earth with several hours of work being destroyed. I would recommend that if you try this that you have a mechanical back up system just in case. A tomy timer system is good and cheap, only the cost of a tomy timer, which I have gotten for a buck or less. I hope this system works for you!!!

-DogLover

[bob755]

Thanks for the information, tips and comments. I like the refraction idea that avoids coloring the water but the fog may well cause problems. While it is quite complex, I think it should at least be possible to test the system without having to launch the rocket assuming that the water behaves in the same way.
I have some experience building IR transmitters/receivers with these type of detecters so I will salvage some of my old circuits and components.
This idea came to me when I saw the automatic windscreen wipers on my car but I'm indeed using a tomy timer for my first rockets so this is a long term project.

Should be interesting to experiment anyway.

[Blenderite]

I hope that works for you!!!

-DogLover

[U.S. Water Rockets1]

Hello,

Does anyone know how/if apogee is related to the moment at which the bottle becomes empty? Is it reached before, et the same moment or after depending on the mass of the rocket and thus the momentum?
Is it constant for a given rocket?

Thanks

Your concept would certainly work, but uses an indirect method that could be greatly affected by external variables which you may have a very hard time measuring and compensating for. Slight variances in pressure or water volume or rocket weight will throw off the calculations and make it less reliable.

People have had a lot more luck experimenting with a direct measurement of apogee with optical sensors which pick up the differences in light between the sky and the ground. When the rocket tips over the light difference would signal apogee.

[Nick B]

yikes.



My rockets would still have another 500 feet to go after they empty....

[U.S. Water Rockets1]

yikes.



My rockets would still have another 500 feet to go after they empty....

Most water rockets cover majority of their flight after burnout. This is not unusual. You should have no trouble accomplishing this.