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  #11  
Old 03-28-2010, 01:52 AM
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Tau Zero Tau Zero is offline
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Thumbs up Retrograde v.3

Quote:
Originally Posted by JRThro
Yeah, definitely use bigger fins rather than adding big hunks of weight just to reduce the deployment velocity.

It's a nice looking design either way, Jay.
John and all,

Thanks so much for your comments! I took all of them under consideration, and decided to enlarge the fins by 137%, but then "thin them back down" to the original 3/32" thickness.

I also substituted many of the original parts with replacements from the "updated" RockSim files, e.g. using fiber instead of cardboard for the centering rings.

I suppose the only suggestion I would make is to use a nylon chute for the B6-4 motor , or just use a B4-4 instead.

Any additional comments before I release this into the wild? I mean, the BARCLONE Stable?


After this, we tackle streamers for the 1.00X Prometheus. (sly look)


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  #12  
Old 03-28-2010, 08:55 AM
stantonjtroy stantonjtroy is offline
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Maybe everyone else is seeing something I'm not but based on the sim data shown, you don't need to change the rocket at all. 18-25 fps decent rate under chute is normal so having these speeds at deployment is ok. 29fps is a little fast ( I wouldn't call it uberfast). Follow Mark's advice; Change up to a B6-6. That should put you dead on the money. Remember the "up" or "down" arrow shown for deployment in Rocsim is realative. On any balistic curve (parabolic) there is only one infintesimaly small data point where vertical velocity is truely zero. A fraction of a second prior or after and your rocket is, in the strictest sence, still moving up or down and Rocsim will show this. The velocities you're getting aren't too bad at all (except the 29fps). For a good visual repersentation, have rocsim plot the data (set it to use Apogee and Deployment as reference lines). The closer those lines are, the softer the deployment. I think you'll find them quite close. My two cents.
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  #13  
Old 03-28-2010, 09:25 AM
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Tau Zero Tau Zero is offline
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Talking "NOT so fast..."

Troy,

Thanks for your comments! I just double-checked Retrograde Version 3 on a B6-6, and it shows deployment velocity at 41.63 f.p.s.

This version doesn't have all the excess weight of Version 2, so there's the current variables that we have to factor in.


Thanks again,
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  #14  
Old 03-28-2010, 01:16 PM
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CPMcGraw CPMcGraw is offline
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Quote:
Originally Posted by stantonjtroy
...18-25 fps decent rate under chute is normal so having these speeds at deployment is ok. 29fps is a little fast...


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...
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  #15  
Old 03-28-2010, 03:19 PM
stantonjtroy stantonjtroy is offline
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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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  #16  
Old 04-24-2010, 10:39 PM
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Ramjet Ramjet is offline
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Default Weight VS drag

I would increase drag for sure. However, have you considered a forward mounted tab or fin vs making the fin larger? Or perhaps just some 3D detailing on the body surface up near Cg.
Enlarging the fin may make the rocket overstable, and thus very prone to weathercocking.
Just my 2 cents.

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