Testing flight characteristics before actual flight in model rockets?

Question

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?

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. — Uwe, Mar 05 '18 at 17:55
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. — antlersoft, Mar 05 '18 at 18:50
@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 ) — JohnP, Mar 05 '18 at 19:59
@SteveLinton - Realized I forgot to tag you in response. If the computer sim allows for accurate representation of the model, sure. That would actually be a benefit, as you could enter theoretical designs and test before building. That would be ideal. — JohnP, Mar 05 '18 at 20:00

Organic Marble · Accepted Answer · 2019-05-01T20:49:40.447

15

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:

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.

edited May 01 '19 at 20:49

answered Mar 05 '18 at 18:50

Organic Marble

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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. – Mark Mar 05 '18 at 23:26
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Using an area cutout is a really neat trick! – z0r Mar 06 '18 at 00:53
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@Mark That's how I learned it back in the stone ages. If you can whirl it it should fly. – Loren Pechtel Mar 06 '18 at 04:20
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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. – Christoph May 02 '19 at 07:01
For end-burning solid rockets, the CG will tend to move forward during a burn, but for liquids, it will tend to move backward. – Russell Borogove Oct 14 '20 at 23:21
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@RussellBorogove context here is model rockets. – Organic Marble Oct 14 '20 at 23:22
I knew I should have mentioned center-burning. – Russell Borogove Oct 14 '20 at 23:24

Testing flight characteristics before actual flight in model rockets?

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