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Old 03-28-2010, 03:19 PM
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stantonjtroy stantonjtroy is offline
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Join Date: Dec 2006
Location: Maryland
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Quote:
Originally Posted by CPMcGraw
At least for me, 18-25 FPS is on the high side of a safe Dv, given that most of our LPR stuff uses traditional plastic chutes with carpet thread shrouds. I don't like hearing my parachutes "explode open" with an audible pop when they're still 800-1000 feet up - this is a clear indicator the Dv is too high. What I and others here are saying is, why not design these rockets so that the Dv is as LOW as we can get them, instead of just "trying to keep it under the limit"? The lower you can get the Dv, the better the chance of a safe recovery. It puts a lower stress on the recovery system, and anytime you can do that, it's better.

That's what "rocket science" is about - making the rocket do what we want it to...



I agree that the lowest Dv is ideal but keep in mind, at 1G (32fpsps) 18-25 fps velocity is less than 1 second pre or post apogee ejection. Even if the rocket deploys right at apogee, it will be .5-1 second into the down side by the time the chute unforls and fills. This puts the velocity at around 13-21fps. Not so bad. I'm mot sugesting that these highter velocities are aideal target; Rather, if you have a design that sims to it, a change in ejection delay time might be better than a physical change to fin area or mass. Especially if the info is sim only. Though sims are fairly accurate, the descrepencies could more than cover a 6-9 fps difference in Dv experienced in actual hardware flight. In the end I agree, Slower is better.
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