More Rocket, Less Model

If you have been to the club flights, lately, you know some of my creations haven't been flying well.  My Juno II hasn't had a good flight yet, and I don't expect much more from my Thor-Agena.  In an effort to keep the scale at 1:32, I used body tubes meant for HPR, thick and heavy.  So I've wound up with these monsters that few engines can touch.  Maybe an E30T could handle them, but doing a stability test flight on an E30T scares the crap out of me.  From now on I'm limiting my tube size to BT-80 and what ever scale that gives me, that will be the scale I build at.  I am going to rebuild Juno II and Thor-Agena with BT-80's but, right now, I'm working on a Titan III (or 34D) with canted engines in the boosters.  BT-80's take me to 1:46 scale.

Nobody likes it when one of our rockets doesn't work as hoped. I know I don't. But rocketry is nothing of not a continuous learning curve. We have to be willing to fail sometimes. Do you think every one of Dr. Goddard's launches was successful?
Now you're doing what I do. Figure out what went wrong, come up with a possible solution, and try again.
You only really lose.if you quit.
I'm hoping to see more dialogue on these pages and the Facebook page as well. Judging by the variety of rockets that we fly, there has to be a vast reservoir of knowledge among us. "If only we knew what we all knew." I'm willing to risk a possible blow to my ego by putting my ideas out there. If someone can prove me wrong and show me why I'm wrong, then I've learned something.
But if you don't bet, you can't win.
R.

Tim,

If I recall correctly, you may have mentioned you use rocksim?  If one has rocksim (or openrocket which is what I use...  if you don't have rocksim get openrocket as it is free!) then a simulation can give you a pretty good guess as to what delay time, motor etc to  use.   Usually these sims will tell you if you are in the right ballpark (at least in low wind situations).  The modelling tools are decent as far as calculating weight is concerned, but not exact.  For example, openrocket has an extensive catalog of mfg. parts etc... but it's not always an exact calculation.  You can then weigh your rocket, and add a generic mass (I put any mass discrepancy into such an object at the calculated CG of the rocket) and your performance envelope will be a bit more precise.

Some shapes can be challenging to model (the dual cone for your thor-agena is one example) , but not impossible.  As long as you have the dimensions, and weights and fin sizes right, you can be more certain of success.

I highly recommend one of these tools (esp. OpenRocket as it's free)

John

Yes, I use both Rocksim and OpenRocket. As far as Juno II and the sim's, they had me adding at least 2 oz. to the nose cone. Bad enough the things heavy, but the extra nose weight really pushes it past any "D" and out of the range of most "E's". At this point, I start to get a little scared about any first flight with that much power, regardless of the sim's.

The Thor-Agena was a little more stable, only needed 1 oz. in the nose.

With a lighter tube on my next Juno II, and the same size fins (was able to salvage), I should need less weight in nose to achieve same position of CG as on the heavy monster. It will be going from 1:32 scale to 1:36 scale, a couple inches shorter and half the weight. They were pushing 500 grams with no engine and I'm hoping to stay under 250.