Builds Rockets for NASA
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- Aug 27, 2011
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great question. Lots of options, some more practical than others.Question: If a ringtail rocket seems prone to weathercocking, what is the remedy? More weight in the nose? Less? Canards?
adding nose weight will make rocket MORE stable rather than less, with critical cavea: can the motor still get the rocket to sufficient velocity by the end of the rod or rail with that extra mass? You can end up chasing your tail, literally and figuratively, add nose weight, need more thrust, means bigger motor, means more tail weight, so now need MORE nose weight, and so forth. Less nose weight will make the rocket less stable, and while lower weight will INCREASE launch velocity, the FASTER the rocket goes the less weathercocking you get, so if you CAN decrease nose weight and still maintain adequate stability it’s a win,
something often easy to do but often ignored is simply to change the length of the rocket. For CG determinations, the effect of each mass component is multiplied by the distance from the center of mass. So making the rocket LONGER, such as lengthening the body tube and keeping the same mass, moves CG FORWARD,due to both the mass of the nose cone moved forward AND the mass of the extra length of body tube. The price is a heavier rocket (although depending on how heavy your paint is or if you fiberglass your rockets, a length of cardboard tubing doesn’t add much) and a bit more surface drag. Cost is also structural stability, the longer the rocket the more stress on the rocket, hence the SuperRoc competition category, some formula of altitude x length. Super long rockets tend to bend.
The converse is also true, if you find your rockets tend to weathercock too much, consider shortening the rocket as long as you have room for your recovery system and adequate wadding. This will also LIGHTEN your rocket and get it off the rail faster. Really fat bottomed rockets like Big Daddy and Fat Boy had something called “base drag” which I really don’t understand but often let’s them get away with much smaller fins than you would otherwise expect. Dropping your number of fins struts from 4 to 3 will also reduce stability and is usually structurally do-able.
I never like adding weight to the tail of a rocket (unless it is for trimming a boost glide), yes it will reduce stability but also decreases speed off of the rod or the rail, again if your motor is up to it yes it is an option but it may cost you in terms of performance (although in SOME cases heavier rockets fly higher on the same motor.)
I don’t like canards for intentionally reducing stability. I think they can be great for cosmetic effects, but I can’t see putting them in just to adjust stability. Usually I think it’s the other way around, “Hey, I want to add canards cuz they look cool, how much additional nose weight do I need to add or tail fin surface area to add so the rocket REMAINS stable?”
what kind of winds are you usually encountering?
if winds are 5 mph or less, weathercocking may not be much of a problem.
oh, and SPINNING your rocket with a fin tab or an airfoiled fin strut will also reduce weathercocking, although it will cost some altitude. Given you fly small fields, that may be a plus in your case.
good article on that here