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spool rocket descent rate

Is there a formula or "rule of thumb" for estimating the descent rate for rockets without parachutes? I'm working on some spool rockets and I figure that as I build larger ones at some point they will need a recovery system with a parachute and everything. I'm trying to figure out where that point is. I figure there should be some way to estimate the descent rate, say as a function of the surface area of the convex hull versus the weight of the rocket. With small spools (and other rockets) tumble recovery works fine, but at some point they become too heavy to tumble safely. I figure this is may be a result of the fact that as the rocket is scaled up the weight increases faster than the surface area that produces drag.

Any insights on this issue would be appreciated.

Jonathan

----- Jonathan Sivier Secretary, Central Illinois Aerospace jsivier AT uiuc.edu NAR #56437 Tripoli #1906 CIA Web Site:

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"Remember to always keep the pointy end up."

Reply to
Jonathan Sivier
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I suggest conducting an R&D experiment.

Then document and share or enter in competition !

Reply to
AlMax

When I did my "L1" Super Sized Happy Meal, I considered it equivalent to a parachute of the same diameter, and used the weight with RocketCalc (comes with pRASP) on my PDA and it gave me a 20 fps descent rate. Thats within the accepted rate.

Now the spool rockets I've built (CD and mini-CD sized) tumble rapidly at burnout, further slowing their descent.

YMMV.

Reply to
Bob Kaplow

So perhaps a starting point might be to calculate the area of the elipse that is the cross-section of the volume the spool would occupy as it spins. Translate that into the diameter of a parachute and figure the descent rate based on that and the weight. Drag caused by the vortices as it tumbles may slow it further, but it shouldn't fall any faster than that. Then if it is much greater than 20 fps I may need a recovery system.

Jonathan

----- Jonathan Sivier Secretary, Central Illinois Aerospace jsivier AT uiuc.edu NAR #56437 Tripoli #1906 CIA Web Site:

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"Remember to always keep the pointy end up."

Reply to
Jonathan Sivier

...

Reply to
Will Marchant

I spoke to him about how the decent rate was calculated & I suspect that he just pulled it out of his ass.

Reply to
Phil Stein

With mine, once I got bigger than A-B-C motors, they fell fast enough to break the end plates of the spool. I figured that meant they were falling fast enough to damage something, so I started adding streamers and chutes.

Not exactly the true scientific method, but it saves lots of repairs that way.

Bob

Reply to
Bob n Robin

Could you post the size (diameter of end plates, diameter and length of center tube) and weight of some of these? That will give me some data points to compare mine with.

Thanks.

Jonathan

----- Jonathan Sivier Secretary, Central Illinois Aerospace jsivier AT uiuc.edu NAR #56437 Tripoli #1906 CIA Web Site:

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Page:
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"Remember to always keep the pointy end up."

Reply to
Jonathan Sivier

Raw stats and notes:

13 mm Red Chain spool, 3.5" dia end plates, 4" high, 34 grams, 5 flites w/A3 and A10 motors, no damage.

18 mm White Chain spool(s), 3.5" dia end plates, 4" high, 40 grams, 6 flites w/C6 motors, no damage. 1 flite w/D reload toasted BT, retired.

24 mm Balloon Ribbon Spool(s), 3.35" dia end plates, 6" high, 65 grams, 15 flites total, B4 to F39 motors, 3 broken after no or bad deployment w/E and F reloads, none broken after bad deployment w/B and C single use motors, no failed deployments w/D reloads. Very small BT cavity makes for really energetic deployments. Love these Balloon Ribbon Spools w/F39 reloads!

24 mm Plywood and LOC Motor Mount tube, 5" dia end plates, 7" high, 40 grams, 4 flites w/D12 motors, damage both first two flites w/no chute, no damage last two flites w/chute.

29 mm Wire Spool(s), 6.5" dia end plates, 7.5" high, 250 grams, 7 flites total, F and G motors, 2 broken after bad deployments, 1 no damage after chute separated.

Bob

Reply to
Bob n Robin

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