Stability concerns

[Keseimron]

Hi guys, I go by the rule that for a standard length MPR the stability should be: 1<stability<2. So I designed a rocket with 1,4cal stability, but the stability at the moment of leaving the launch rail is around 0,78cal. This raises a concern for me, as I don't want the rocket to be unstable at launch, but also not weathercock through the flight.
Should I build bigger fins and increase the stability, even though it would exceed 2cal during the flight?
Every opinion is greatly appreciated.

Thanks for the help.

 

[rocket_troy]

So long as the CG is forward of the CP, the rocket should be aerodynamically stable. The x1 or x1.5 or x2 calibre margins are just rules of thumb for good safe operating practice. I've flown many (like >50) rockets that have been significantly over stable without weathercocking but for the scenario of slow acceleration take off in windy conditions, then significantly over stable rockets aren't ideal.

TP

 

[Sandy H.]

Also double check the length of the launch rail in the sim. I think OpenRocket defaults to 39.37" (i.e. 1 meter) but if you're using an 8 foot rail, that is a totally different scenario.

Sandy.

 

[lakeroadster]

Hi guys, I go by the rule that for a standard length MPR the stability should be: 1<stability<2. So I designed a rocket with 1,4cal stability, but the stability at the moment of leaving the launch rail is around 0,78cal. This raises a concern for me, as I don't want the rocket to be unstable at launch, but also not weathercock through the flight.
Should I build bigger fins and increase the stability, even though it would exceed 2cal during the flight?
Every opinion is greatly appreciated.

Thanks for the help.

From what you've described, my mindsim confirms you should be good to go. But wait.. Have you done a swing test? I'd give that a go before launching, IMO that's always a must for a new LPR/MPR design.

 

[Keseimron]

Thanks a lot for all the info, here is a photo of the fins on the rocket for further clarification.