Monday, March 24, 2008

The effect of wind on stability

A thread on TRF just reminded me of an excellent article on rocket stability - Wind Caused Instability, by Bob Dahlquist. This article describes parameters used to ensure a stable flight (velocity, launch rod length, and thrust-to-weight ratio) and relates them both to each other and to the crosswind. My stability post, I may be unstable, but I don't like my rockets that way, touches on rules of thumb such as a launch velocity of 44 fps and a 5:1 thrust-to-weight ratio. Bob's article gives more of the story. Unfortunately, to get the full picture you still need an elusive number, the stability margin in degrees (SMD). SMD is defined as "the number of degrees that the angle of attack can vary to the right or left before the rocket goes unstable." Although you either need a wind tunnel or an unspecified math model to ultimately determine SMD, the article does provide some guidance in making an educated guess based on your Barrowman calibers of stability.

My thanks to Apogee for hosting this useful document!

(PS - I trust RockSim to let me know the required launch guide length. I generally haven't played much with the wind speed parameter. After reading this article, I think I may use it more, especially on complicated designs.)