Thursday, September 18, 2008

Coney Backslider Rocket

I finished working with the "Coney" rocket a week or so ago and found the "sweet spot" for balance. The "Coney" is a rocket designed by Robert Youen. It is a backslider or backglider rocket. You have to be able to calculate the CLA (center of lateral area), CG (center of gravity) and the CP (center of pressure) using the Barrowman equations or better yet the RockSim equations which put the CP even farther aft. Theoretically, a backslider or backglider is a very long skinny rocket with a length to diameter ratio of 30 to 50 and for backsliding the CG should be midway between the CLA and CP according to the research and report done by Bob and Peter Alway. The coney has a ratio of 12:1. After experimentation, I found the "sweet spot" to be farther aft than midway using the Barrowman equations and even a little aft using the better RockSim equations (just using the software and letting it do the caluclating.)

I went through four nosecones using each one through multiple crashes and repairing until they were unusable finding what works. It is a sweet thing to watch a properly balanced backslider return home, just slowly drifting perfectly horizontally to a slow sideways soft landing.

I got a great video of one launch that happened to drift right back to me within 20 feet of the launcher after an estimated 220 foot apogee. I was only using about 70 or 80 psi and could go up to 120 with the one liter bottle I was using with no problem at all. The video is in the gallery page of the website.

The term backslider or backglider is a little misleading because they don't usually glide backwards except right after apogee until they turn over 90 degrees. Theoretically, they could set up a backward glide with a little angle but this is not really even desirable because then they start "flying" in some undetermined direction and at a higher speed so the landing will be harder and farther away. A perfectly flat descent is the best and is usually what you get anyway. The more you set it up for a backglide, the more apt to fishtail on the ascent (and so reduce the apogee) because of the reduced stability.

I estimated from the last frames of the video going down beside my 4' chain link fence that it was traveling at about 15 feet per second (10 mph) compared to a normal lawn dart velocity of 50+mph besides spreading the impact over the whole side of the rocket compared to the nose. No broken fins, no crumpled nosecones or bottles -- just a perfectly soft, no damage landing with no complicated ejection systems or parachutes to get fouled.

Later I will build an easier to build backslider with a tubular extension rather than the very long cone which is harder to make. It should look better, too.

The details are in chapter 5 of the manual.