CP and CG

[rocketsonly]

I've heard many recommend a stability caliber of 2 on rockets. Is there a paticular reason for choosing 2 instead of 1 (or any other number)?

Thanks.

 

[Stymye]

the data(from MIT testing) suggests no less than 1 and no more than 2....I think they used a wac corporal for their base model..

some builders go thru great pains to make it exactly 1 caliber, for some reason.but I don't think it's that critical

hope that helps some

 

[powderburner]

If you play around much with rocket stability you will quickly find that there are several factors that influence overall stability and performance (pitch/yaw inertia, vehicle finess ratio, fin design, etc).

For long skinny rockets you probably want a larger margin, closer to 2. But you don't want much more than that because to achieve that, you generally have to add excessive nose weight with hurts your performance. Oh yeah, and it will weathercock badly.

For short squatty rockets (Fat Boy, Big Daddy, etc) you probably want a little smaller margin, like 1 (or even less). And they work just fine that way!

 

[DynaSoar]

Originally posted by rocketsonly
I've heard many recommend a stability caliber of 2 on rockets. Is there a paticular reason for choosing 2 instead of 1 (or any other number)?

Thanks.

A higher caliber will go stable at a lower speed, requiring less rod and so less rod drag. On the other hand it will weathercock more. That's not necessarily a bad thing. The more weathercocking (flies more upwind) the less downwind it'll drift on the chute. But if you're after altitude, less rod is good but weathercocking is bad.

Now toss in the fact that it takes a longer body to get 2 caliber stability, and that'll mean the CG starts higher and so the main lug needs to be higher and you'll probably want two lugs to keep it straight, and that means more rod.....

There are no easy answers in rocketry. Except maybe to the question "What happens if the ejection fails?"

 

[teflonrocketry1]

Theoretically if our CP models were better the rule of thumb might become if the CP is behind the CG in any given flight senario then the rocket will fly stable. But since the calculations typically used don't take into account things like base drag and body lift (boyancy) the rule of thumb of a one caliber stability margin only works for a "typical" rocket with a 10:1 aspect ratio (length to diameter). For a short fat rockets, where base drag is an important and drives the CP further aft than Borrowman or RockSim equations predict; a less than one caliber of stability might suffice. For a long skinny rockets where boyancy is more important and pushes the CP further forward than the typical equations predict; more than one caliber of stability is necessary.

Bruce S. Levison, NAR #69055, Section #535 Tri City Sky Busters