Expert: Dave Landgraf - 6/1/2008

Question
Hi Dave, I would like to build a BIG  D12-3 powered rocket. So here is the question, how big can I go? I would perfer to use a BT-80 2.6"(66mm)airframe, a balsa nose cone and I was thinking maybe 1/8" (3mm) plywood fins for durability, using 3 fins approx 3.5"x4".  I know this will be heavy, but altitude is not my big concern, I would perfer the rocket takes off slow & stays low, so we can see it. If I use two tubes for 36" plus cone for about 40 inches total. Do you think this thing will fly and what altitude would you guess at??
Thanks, for the help - Scott


Answer
Hi Scott,

Your plan sounds pretty good, with just a few safety measures. I really like "low and slow" flights too. I don't think there is much thrill to sending a rocket to a zillion feet---I mean, once it is a teeny dot up there, why go higher?

Anyway, your design idea sounds like it should work OK. You are basically talking about a rocket a few inches longer than an Estes Executioner. That one has the same BT size and is only a bit shorter. The Executioner might be a bit lighter (hollow plastic nose cone, less BT weight), so you don't want to be too much heavier than eight ounces. A solid balsa nose cone with a diameter that large might be a bit heavy, and you may need to hollow out its base to reduce weight.

If you are designing your own rockets, I hope you have found some of the free analysis programs online like wRASP
http://ourworld.compuserve.com/homepages/cgibke/wrasp.htm
Another free one is RockAlt
http://www.mrhq.org/software/freeshar/rockalts.gif
Apogee rocketry sells a powerful software package called RockSim, but they also offer free access to older versions of this software at
http://www.apogeerockets.com/rocksim_demo.asp
If you would rather work with paper charts, you can download your own copy of the old Estes Altitude Prediction series at
http://www.esteseducator.com/Pdf_files/Part_1.pdf and
http://www.esteseducator.com/content/publications.php
(LOTS of good info posted there!)

These performance analysis models can help you estimate the altitudes your rocket is likely to obtain. I have not run any numbers, but if your model is not much heavier than an Estes Executioner you should be attaining something close to 500 feet.

Don't forget to check stability, starting with an estimate of center of pressure. There are free analysis procedures like VCP
http://www.v-serv.com/vcp/
and the Barrowman equations themselves are posted for free use at
http://my.execpc.com/~culp/rockets/Barrowman.html (and other places).
If you want to use a more simple method for estimating stability, try the cardboard cutout test shown in
http://www.apogeerockets.com/Education/downloads/Newsletter18.pdf
The old Estes design manual shows you how to do a swing test
http://www.ninfinger.org/~sven/rockets/nostalgia/alpha11.html

If you use Estes D12 power you should probably use a delay NO LONGER THAN three seconds (as in the D12-3 you were proposing), at least until you have a few actual flights completed with your new design. Remember that if your rocket weathercocks into the wind you will see reduced maximum altitudes and a three second delay might become too long (even though it's the shortest one available!...in other words, BE CAREFUL launching on windy days!) I expect a D12-5 would not do well for your design, and a D12-7 would be very dangerous in ANY weather conditions.

Because you are purposely designing a relatively big rocket to fly "low and slow" you really need to use a big launch lug (like 1/4 inch), a matching 1/4 inch diameter launch rod, and a launch rod length of four or five feet (longer is better). The bigger launch rod will not bend and move nearly as much during launch, and will give your rocket a more vertical path as it reaches the end of the launch rod. (I am going to "yell" at you just a bit here: YOUR ROCKET WILL BE TOO BIG TO SAFELY LAUNCH FROM A 1/8 INCH ROD) Using a four or five (or six) foot launch rod will give a heavy rocket more time to accelerate to minimum safe flying speed before it leaves the end of the rod. And even with all those safety measures, you might still need to wait for a day with low winds...

I would encourage you to think hard before using plywood for fins. Yes, it is stronger. Yes, it is also significantly heavier, and would tend to reduce the stability of your design (extra weight in the back end, and all that). Have you considered 1/8 inch thick basswood? (Basswood is usually stronger than balsa, and is an intermediate step before going all the way to plywood.) Or, use a little thicker balsa; you could easily use 3/16 or 1/4 inch thick balsa with little performance loss if you round the fin leading edges and slightly taper the trailing edges. Or, use 1/8 inch thick balsa and ask the salesman at the hobby shop to help you select a sheet of high density balsa (usually significantly stronger and a little heavier than softer grades of balsa). Last but not least, you can use 1/8 inch thick balsa and laminate it by gluing paper to both faces of the fin; use thinned white or yellow glue, sandwich the whole thing inside waxed paper, and let dry a couple days under a few phone books. Laminated fins can be pretty durned strong! You have lots of options before going straight to plywood, and you may need to keep the weight down on this little monster.

If you want to hedge your bets a bit, you should check out the cluster motor mounts that are available today from many online vendors (like Fliskits). You don't have to fill every hole with a motor for every launch--you can use dummy motor shapes to plug unused motor mounts--but you will still have the option of using more motors later if you go "Tim Allen" (MORE POWER!!)

One more thought: you could play with an initial design that only uses one length of BT. It would be shorter and might not be as impressive, but you could at least test how well it flies for a few launches. Then, if you still want, you could glue in a tube coupler and add on the second length of BT-80.

Whew! I hope some of that is useful. If you have more questions, come on back!

-Dave