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'[OT]Re: Water Rockets'
|Warning: off topic ramble about water rockets ahead...
Sean Breheny wrote:
>I think the accel curve would look like this:
>Initially 1 G downward.
>High acceleration upward at liftoff.
>Gradually decreasing due to air resistance.
>When engine shuts off, it suddenly goes to >1G downward (air resistance +
>Gradually goes to exactly 1G downward (as air resistance decreases)
>then gradually decreases (due ro air resistance in the descent) until
>striking the ground.
Largely right, except for some directions (the acceleration at liftoff is
actually in the same direction as gravity: if you're sitting in the
rocket you feel an *increase* in gravity, not a reversal of it), but
there's one unexpected wrinkle: The thrust is so large and the
acceleration and velocity at burnout so great that there is a SHARP
negative acceleration (slowdown) due to air resistance at the moment the
thrust ends. This is why no one has (so far) come up with a reliable
acceleration-based or tilt-based apogee sensor. The most successful chute
releases for water rockets so far have been a mechanical "airspeed flap"
that senses slowdown (only works on near-vertical launches) and clockwork
timers, which always work :-) This is also why some 2-stage designs
incorporate a slosh baffle in the second stage: the water needs to stay
at the bottom of the rocket to do its work.
(Some fun water rocket facts: for a naked 2 liter bottle 1/3 full of
water pumped to 80 psi, initial acceleration is roughly 111 Gs (!), the
thrust burns out in about 0.7 seconds, and at that point the rocket is
only a couple meters off the ground and traveling well over 70 m/s (over
The lesson: this is lots of fun, but don't get in the way of one of these
Anyway, using acceleration data only for determining apogee, velocity,
etc., is difficult and not very innacurate. Direct barometric pressure
sensing is the way to go for that, I think: all the commercial altimiters
do it, there has to be a reason. But I still want to *record*
OK, enough water rockets on the PIC list, anyone who wants more info
should go to http://www.geocities.com/CapeCanaveral/Lab/5403/ or one of
the many other water rocket sites.
On Sun, 27 Jun 1999 17:25:25 -0700, you wrote:
>Warning: off topic ramble about water rockets ahead...
This is why no one has (so far) come up with a reliable
>acceleration-based or tilt-based apogee sensor. The most successful chute
>releases for water rockets so far have been a mechanical "airspeed flap"
>that senses slowdown (only works on near-vertical launches) and clockwork
>timers, which always work :-) This is also why some 2-stage designs
>incorporate a slosh baffle in the second stage: the water needs to stay
>at the bottom of the rocket to do its work.
you can get some very small GPS receiver modules nowadays - maybe
that's the way to go!
Another interesting site, although Water Rockets are missing, is:
http://www2.csn.net/~bsimon/backyard.html. I recall a link on this site a
few years back that a gentleman was doing velocity experiments using a
SpudGun and a PIC.
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