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The performance cost of snap swivels.

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Senior Space Cadet

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I just finished my latest rocket, a pretty small light guy. I used snap swivels to install everything. One for the parachute, one for the nosecone, one for the altimeter. Makes replacing or switching stuff out easy, but I was wondering how much difference in apogee, if at all, snap swivels made vs. a knot.
Turns out that two snap swivels, on this rocket, make a difference of about 36ft in the apogee.
For recreational launching, who cares about 36ft. ? But if I were really serious about going for altitude, which I kind of am with this rocket, I'd probably only use a snap swivel for the chute.
I only ran numbers for an Estes C6-7 motor. Might be more or less for other motors.
 

rklapp

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Eventually the rockets get so small, so high that you’ll never see your altimeter again.

 

Jay Rairigh

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After flying your rocket with altimeter, you can determine a decent drag coefficient number for your model with openrocket. Then you will be able to determine the optimum weight and motor (c5,c6,c12) to get the most altitude using openrocket.

Go FLY it first. Right now that weight doesn't matter. It could end up that you need to add weight. You need data to work with. Use your altimeter and open rocket to get it dialed in.
 

XrayLizard

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That weight data is published.
In graph presentation. Helps for a starting point I would imagine
More importantly, it shows the slope of the curves, so that tells you where you can err +\-
 

Senior Space Cadet

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Eventually the rockets get so small, so high that you’ll never see your altimeter again.

The altimeter won't fit in a body smaller than a BT-50, so I can't go smaller than that.
I think losing it, within half a dozen launches, is possible, maybe even likely. I'm waiting till the club launch before using it again. I'm hoping I can get help tracking it.
 

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After flying your rocket with altimeter, you can determine a decent drag coefficient number for your model with openrocket. Then you will be able to determine the optimum weight and motor (c5,c6,c12) to get the most altitude using openrocket.

Go FLY it first. Right now that weight doesn't matter. It could end up that you need to add weight. You need data to work with. Use your altimeter and open rocket to get it dialed in.
I read somewhere that, sometimes, heavier is better, since the rocket will coast farther after engine shut-down, I'm just going by what Open Rocket said the apogee would be, after I added the weight of two snapswivels. There is a lot I hope to learn from the altimeter, if I don't lose it in the next few launches, and it's obvious I'm just scratching the surface of what Open Rocket can do. Right now, about all I'm doing is making changes to the design and looking at how the projected apogee changes.
 

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Eventually the rockets get so small, so high that you’ll never see your altimeter again.

I watched the video and yeah, the altimeter won't fit a BT-5, it won't even fit a BT-20.
I'm not competing against NAR records. Those records are based on minimum diameter rockets, which isn't how I build and they use piston launchers, which I don't have. After I get data for some of my rockets, I'm hoping to find out how my apogees compare to other similar rockets. For instance, what is a respectable altitude for a BT-50 (24mm) rocket with an 18mm motor, launched from a steel rod?
 

XrayLizard

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I read somewhere that, sometimes, heavier is better, since the rocket will coast farther

No sir it is not “sometimes “
It is physics.
May I suggest downloading the article from the NAR website?

as for the snap swivel, unless you put some honking big one in there the performerance cost is zero, unless you have absolutely made every part , fit and finish, as perfect as possible:):):):)

you mightwell get 36 feet just from air density change !!:oops:

Personally I don’t care for Apogee pop launcher... carrying a tube up with rocket 10 or so feet defeats the purpose imho. At least one of the original designs did not do that as I somewhat recall. ... barely recall haha. There is supposed to be a stop ring in one tube to prevent that.
 

jqavins

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After the measuring the final rocket mass, experimenting to determine drag coefficient, and using that data to run more accurate sims, if it turns out that you do indeed want lower mass...

Why not eliminate all three snap swivels? They're really nice to have for parachutes, but not really necessary.
 

kuririn

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For recreational launching, who cares about 36ft. ? But if I were really serious about going for altitude, which I kind of am with this rocket, I'd probably only use a snap swivel for the chute.
1. Eliminate the snap swivels altogether. Excess weight just for convenience.
2. Use a smaller altimeter than the Estes so you can fit it in a BT-20 tube, like this one:
3. Then you can build a MD 18 mm rocket for max apogee. Use elliptical fins, they are the most efficient. Make it as light as possible. Airfoil the fins.
4. Use a tower or piston launcher and eliminate the drag from a launch lug.
5. Use a balsa nose cone, split it in half and hollow it out. Lighter than a plastic one. Better yet, use a foam one.
6. Replace the chute with a streamer. The rocket should be light enough for this. Or use nose blow and eliminate the streamer altogether if your rocket is light enough.
7. Don't paint your rocket. The weight of paint can make a considerable difference in the max apogee.
I'm not a competitive launcher, the experts can weigh in with their own ideas.
 
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jqavins

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the experts can weigh in with their own ideas.
He-he, weigh in. I see what you did there.

But seriously folks, one can go really nuts on mass reduction if one wants to. Tim van Milligan has a Peak of Flight article about his quest for the lightest rocket, and a method for making composite parts from thin epoxy and a single layer of tissue paper.

It's not something I'm interested in doing.
 

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We did a recent two stage with almost zero steel in the rocket. When we compared the mass of a standard set of eyebolts, d-links, allthread, and other assorted hardware it was almost a full kilogram of mass we saved. Worked out pretty good for our project but your results may vary.
 

rklapp

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I don’t understand how they get their rocket back. My guess is they don’t and measure it from the ground.
 

rharshberger

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I just finished my latest rocket, a pretty small light guy. I used snap swivels to install everything. One for the parachute, one for the nosecone, one for the altimeter. Makes replacing or switching stuff out easy, but I was wondering how much difference in apogee, if at all, snap swivels made vs. a knot.
Turns out that two snap swivels, on this rocket, make a difference of about 36ft in the apogee.
For recreational launching, who cares about 36ft. ? But if I were really serious about going for altitude, which I kind of am with this rocket, I'd probably only use a snap swivel for the chute.
I only ran numbers for an Estes C6-7 motor. Might be more or less for other motors.
Were you flying or simming, if flying a C6-7 tells me the flight was fairly high for an LPR ( seven second coast) and at that altitude 36' is probably within the allowed performance variance of the motor, slightly different wind, slight changes in how the altimeter was reading (two identical make alts may not read the exact altitude on the same flight).

How many flights did you fly with that exact configuration?

Did you optimize the rockets weight for max altitude? Frequently LPR rockets actually need a tiny bit more weight to get best altitude.
 
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