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I do a lot of scratch design/building of model rockets, but one of the things that I've struggled with is testing the models I come up with.

Some of them are purely intended to be static display models, but others I want to fly. I've had nose dives, looping, and various other destructive (to the model) flights on some of my designs.

Is there a way to test the flight characteristics before I just slap an engine in and yell fire?

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  • $\begingroup$ It seems you try to get flight stability without attitude control. If the attitude control of a large orbital rocket fails the mission is lost. See Wikipedia for Pendulum rocket fallacy. $\endgroup$ – Uwe Mar 5 '18 at 17:55
  • $\begingroup$ Is computer simulation an answer to this? $\endgroup$ – Steve Linton Mar 5 '18 at 18:39
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    $\begingroup$ 40 years ago when I was building them the instructions were to tie a thread around the center-of-gravity and swing them around your head to see if they would fly stably. $\endgroup$ – antlersoft Mar 5 '18 at 18:50
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    $\begingroup$ @antlersoft - Agreed, and I have done that. However, it is not without its flaws (Aside from losing a week of work when the knot unravels...don't ask. :p ) $\endgroup$ – JohnP Mar 5 '18 at 19:59
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    $\begingroup$ Such a simulator exists: openrocket.info $\endgroup$ – Organic Marble Mar 5 '18 at 22:41
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For simple stability, you need the center of gravity to be in front of the center of pressure.

You can find the center of gravity by hanging the rocket from a string with a loop that slips along the body. The balance point is the center of gravity. You should measure it twice, once with a fresh engine installed, parachute packed, wadding, etc, (launch config) and again with a burned-out engine casing installed (burnout config).

For simple configurations, stability improves during powered flight, as the engine mass decreasing during burn tends to move the c.g. forward. This explains the phenomenon you might have seen where a poorly designed rocket launches unstably, flies around in circles, then straightens out and flies directly at the most vulnerable object around.

Measuring the center of pressure is more complicated, but Estes has published a Technical Report on how to estimate it here. For a very approximate method, best for axisymmetric models, you can cut out a scale silhouette of the model and balance it to get the c.p., as described in this excerpt from Estes Technical Report 1:

enter image description here

If you really want to get fancy, you could try to rig up a wind tunnel, a mount to hold them out of a car window, etc, etc. The model should be held at the c.g.

You might also find Estes Technical Report 9 "Designing Stable Rockets" of interest.

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    $\begingroup$ Back in the day, you would do a pseudo-wind-tunnel test by tying a string to the center of gravity and spinning the rocket around you. $\endgroup$ – Mark Mar 5 '18 at 23:26
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    $\begingroup$ Using an area cutout is a really neat trick! $\endgroup$ – z0r Mar 6 '18 at 0:53
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    $\begingroup$ @Mark That's how I learned it back in the stone ages. If you can whirl it it should fly. $\endgroup$ – Loren Pechtel Mar 6 '18 at 4:20
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    $\begingroup$ Pro tip to find the center of gravity: place the rocket on your hands with one hand at each end and then move your hands together. The rocket will keep balanced on your hands and when they meet the center of gravity will be exactly above your hands. This can be done surprisingly fast. $\endgroup$ – Christoph May 2 at 7:01

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