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  #1  
Old 08-26-2010, 09:07 PM
soopirV soopirV is offline
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Default "Safe" Deployment Velocities?

Hi again,
I've been asked to shoot some rockets at my sons' school as part of their section on space. Ironically, we don't have a ton of "space" in which to launch, so I'm simulating some of my lesser birds using wRASP (haven't made the commitment to RocSim yet, although not for lack of wanting). For a normal launch I'll just trust "Uncle Estes" recommendations using the "C" will go higher than "A" formula.
I'm seeing now, however, as I work through my list, that many recommended engines have delays that either too short or too long- this is understandable if there's no other choice, but my sims have shown me better choices often exist. Of course there are many assumptions being made- all my rockets have a Cd of 0.600, that the mass is exactly as specified in the rocket file for wRASP, etc (I did customize the launch environment with our altitude and anticipated ambient temperature!).
What I'm wondering is, can anyone tell me a speed at which recovery system failure is beginning to be too great a risk? And again, more assumptions- how did I build the rocket, what type of system is it, what's my tolerance for risk etc. Obviously, something that deploys as close to 0fps is ideal, 20fps is pretty good...what's the line for toxic failure? 100fps?

A total noob question I just haven't given it much thought and would hate to put on a bad show for a group of 3-5 year olds.
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  #2  
Old 08-26-2010, 11:30 PM
Zeus-cat Zeus-cat is offline
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I am not sure what speed is fatal. In my opinio there are numerous factors to consider. Parachute versus streamer would be a big one. I would feel a lot better deploying a streamer at a higher speed than a parachute. How "tough" is the chute? How "tough" is the recovery system of the rocket? How prone to zippering is your rocket? What is the mass of the rocket at deployment?

You could always add weight to the rocket so that the velocity at deployment is slower. I think this might be the easiest solution.
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  #3  
Old 08-27-2010, 12:16 AM
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CPMcGraw CPMcGraw is offline
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Quote:
Originally Posted by soopirV
Hi again,
I've been asked to shoot some rockets at my sons' school as part of their section on space. Ironically, we don't have a ton of "space" in which to launch, so I'm simulating some of my lesser birds using wRASP (haven't made the commitment to RocSim yet, although not for lack of wanting). For a normal launch I'll just trust "Uncle Estes" recommendations using the "C" will go higher than "A" formula.
I'm seeing now, however, as I work through my list, that many recommended engines have delays that either too short or too long- this is understandable if there's no other choice, but my sims have shown me better choices often exist. Of course there are many assumptions being made- all my rockets have a Cd of 0.600, that the mass is exactly as specified in the rocket file for wRASP, etc (I did customize the launch environment with our altitude and anticipated ambient temperature!).
What I'm wondering is, can anyone tell me a speed at which recovery system failure is beginning to be too great a risk? And again, more assumptions- how did I build the rocket, what type of system is it, what's my tolerance for risk etc. Obviously, something that deploys as close to 0fps is ideal, 20fps is pretty good...what's the line for toxic failure? 100fps?

A total noob question I just haven't given it much thought and would hate to put on a bad show for a group of 3-5 year olds.


Somewhere on this forum we've discussed deployment velocities...

The general theme for typical LPR goes something like this:
  • Above 30 FPS = shredder territory; avoid like the plague!...
  • 25-30 FPS = explosive parachute openings, damage likely...
  • 20-25 FPS = high, but manageable; likely shroud breakage...
  • 15-20 FPS = moderate-high, sustainable, still rough on parachutes...
  • 10-15 FPS = moderate, comfortable, shouldn't hurt parachutes...
  • 5-10 FPS = nice, smooth, gentle; a really good range to achieve...
  • 0-5 FPS = SWEET!

The best idea, regardless of the design, is to get the lowest FPS deployment you can achieve. This is one of the really good reasons to use RockSim, as you can try different ways to reach a given goal. Adding mass (ballast) will work to a degree, but so can adjusting the BT length, or the fin area, or some combination of all three.

The list above is not set in stone, but should be a good general rule of thumb. Lower is better under any conditions!
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  #4  
Old 08-27-2010, 08:41 AM
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ghrocketman ghrocketman is offline
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You can get away with just about anything if your recovery system and upper portion of body tube are UBER-Tough though....Kevlar chute, lines, long shock cord, etc...
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  #5  
Old 08-27-2010, 01:39 PM
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Solomoriah Solomoriah is offline
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Quote:
Originally Posted by CPMcGraw
This is one of the really good reasons to use RockSim, as you can try different ways to reach a given goal.

Or OpenRocket, which is FREE.

Quote:
Originally Posted by CPMcGraw
Adding mass (ballast) will work to a degree, but so can adjusting the BT length, or the fin area, or some combination of all three.

Sometimes adding ballast to a very light rocket will make it coast longer, possibly leading to a higher DV.

I have one rocket which just has no peaceful deployments; the best I can hope for is about 25 fps. But it's a BT-70 rocket, so I built a parachute out of a mylar balloon (much tougher than normal) with heavier-than-normal shroud lines and a long, strong shock cord. No problems with it so far.
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  #6  
Old 08-27-2010, 06:15 PM
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rocketguy101 rocketguy101 is offline
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Anybody ever play with "reefing" their chutes? I have read about it on various forums, but have not had a rocket w/ "hot" deployment. A sliding ring (button?) or a loose wrap of masking tape might delay chute opening -- it would act as a streamer initially. It may be an application more suited for HPR, but might be interesting to try.
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  #7  
Old 08-27-2010, 07:32 PM
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gpoehlein gpoehlein is offline
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Quote:
Originally Posted by rocketguy101
Anybody ever play with "reefing" their chutes? I have read about it on various forums, but have not had a rocket w/ "hot" deployment. A sliding ring (button?) or a loose wrap of masking tape might delay chute opening -- it would act as a streamer initially. It may be an application more suited for HPR, but might be interesting to try.


I will occasionally reef my chute for competition - if it is fairly breezy and I need a return in chute duration. I just wrap a small piece of masking tape around the shroud lines a short distance from the chute - it keeps the chute from opening fully and it won't catch as much air/thermal. Result is similar to cutting a spill hole but less permanent.

I don't really think it will help that much tho for dealing with a hot deployment. The canopy is still gonna open and you'll still get that "snap" that you don't want at a high speed.

Greg
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Old 08-27-2010, 08:48 PM
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Bill Bill is offline
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Quote:
Originally Posted by rocketguy101
Anybody ever play with "reefing" their chutes? I have read about it on various forums, but have not had a rocket w/ "hot" deployment. A sliding ring (button?) or a loose wrap of masking tape might delay chute opening -- it would act as a streamer initially. It may be an application more suited for HPR, but might be interesting to try.



Reefing a chute is primarily done to keep it from fully opening so that drift is reduced when flying on a windy day.

But you are on to the idea though. It is a common practice in mid and high power rocketry to fold the shock cord back upon itself and hold the loops together temporarily with masking tape. The tape will give when the laundry is deployed, but the tearing of the tape can absorb an amazing amount of energy.


Bill
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