Tuesday, August 6, 2013

Swing Test - oops!

Here's one thing that authors never tell you when they suggest a swing test to check your rocket's stability: if your string is wrapped around a section of plain body tube on a longish rocket, don't swing too fast.

I'm currently finishing up work on an Estes Super Neon XL. It strikes me as bottom-heavy, so I was more concerned than usual about the stability. I probably shouldn't worry because it's a proven Estes design, built to spec, with plenty of successful launches listed on RocketReviews.com. (I actually did one small modification - I added an ejection baffle. But this should push the CG forward, if anything.) But I obsess.

I looked online at two posted Rocksim files. I don't have Rocksim (I use OpenRocket, but it doesn't support tube fins), but the pages show the calculated CG/CP values: 18.25"/25.25" and 19.5"/28.8". Quite different, though the former doesn't appear to have included the balsa fins. What then concerned me was that my measured CG value was more like 25" with no engine, and 28.25" with an E9-6, and that seemed a bit too close for comfort even if the CP were truly 28.8. Hence the swing test.

Sure enough, I couldn't get a stable swing, so I began taping lead weight onto the nose. Playing with the Rocksim design in OpenRocket, I figured that I'd need at least a couple ounces at the tip of the nose to get very comfortably stable. Sure enough, once I'd added 2.25 oz I got my first clearly stable swing test. But of course I couldn't stop there, and I had to back off and try again with a little less weight. Again, I had trouble getting it to go stable, so I thought "maybe if I swing just a little faster…" and that's when - FOLD!

Fortunately I had the couplers and spare tube on hand for a quick splice.

But what to do about the stability? I've decided to add just 0.25 oz at the tip of the nose and go for it. Here are my reasons:
  • I've read that the swing test is a very conservative measure, and difficult with larger/longer rockets. 
  • It is a proven design, As stated, I built it to spec, and my finished rocket (minus primer/paint) is 6.25 oz - less than the Estes estimated weight of 7 oz - so it's not like I incorrectly piled weight on the back end.
  • None of the reviews I've read suggest any instability.
  • While the rocket is well under the max lift weight for a D12-5 or E9-6, the idea of piling on a couple of ounces also concerns me. As is now, OpenRocket simulations show a speed of just under 25 mph off the rod with an E9-6 (better with the higher max thrust of a D12-5), which again seems to be cutting things close.

So I'm going to prime it, paint it, and cross my fingers. I'll update this post once it's flown.

Update: Back in September the Super Neon flew beautifully - then landed at the top of a very tall tree, where the bright parachute and nose cone still flutter in the wind five months later. (Oh, and I lost my Little Bucky Jones on its first flight the previous month. Losing streak.