AboutDave Combs Expertise Can: Nearly all low-power and some high-power rocketry questions.
Can't: Anything that approaches violations of NAR/TRA safety codes; advanced high-power rocketry questions beyond L1 (I'm still learning those myself).
Experience NAR Level 1 high-power certification; BSA Space Exploration Merit Badge counselor; many years of experience in building and flying rockets from mini-motors (A3-4T) up to H impulse.
Organizations National Association of Rocketry; NAR Section 703 TORC in Dayton, OH.
Question I have been flying single stages with my kids recently, and have even designed a couple of multistage rockets.All low power stuff nothing fancy. The thing is I have been drawing a design I want to builds and fly that has two cluster stages and a single burn to apogee. Can I legally do this, I don't want to get into any trouble. The design is still using low power motors but I did not know if the clustering of them would have special rules. Thanks for your help. Signed an Addicted Dad with enabling Kids.
Answer You ask a number of good questions. I'll try to break it into parts.
First, for the pertinent "laws": You are not likely in danger of violating any. You would be concerned with the total weight of the rocket as well as the total impulse of the motors at launch.
In February 2009, the FAA published new regulations for model and high power rocketry (synopsis in the NAR website here: http://www.nar.org/pdf/FARrockets0209 ). Under the new rules, you do not have to notify the FAA if the rocket gross weight including motors is less than 53 ounces (1500 grams) AND if it uses under 4.4 ounces (125 grams) of propellant.
Let's assume you are using Estes C6 engines for the cluster and the upper stage. C6 motors have 0.44 ounces of peopellant, so in theory you could have a total of 10 C6 engines and still fall within the regulations. Further, each engine weighs at most 0.95 ounces, so you have plenty of weight available for your rocket's components.
Next, concerning the cluster/staging itself: I think I understand that you want to use a cluster in the lower stage or stages, and then a single motor in the sustainer (the top stage). You can certainly do this; however, my strong suggestion to you would be to ensure that the staging motors (the ones that light the motor(s) on the next stage above) are in line with the motors they are lighting. What this means is that you should not attempt to have a 2-motor lower stage light a 1-motor upper stage, as you would have to direct the burning particles into the motor above using some kind of ducting. When you make the motor mounts, make sure the motor mount tubes line up directly with the motors above them. Tyring to "duct" the booster motor particles using tubing with angles is unreliable and could result in a failed staging, with the predictable disastrous result.
Finally, the most important part - stability: If you are doing multiple stages and clustering, you are going to have a LOT of mass in the back end of the rocket. Stability dictates that the rocket's center of gravity is ahead of the center of pressure. With that much mass at the back, you will need to shift the CG forward to increase the stability margin. You can do this by either adding nose weight, or make the rocket longer (moving more mass forward of the CP).
What you're doing isn't impossible, but it is very ambitious. Try it with just two stages to start, say for example with a 3-motor inline cluster on bottom, the middle of which will ignite the upper (sustainer) stage. When you do the build, allow yourself enough room to add that third stage to the bottom.
It sounds like a great project; I wish you good luck with it. You should consider joining The Rocketry Forum at http://www.rocketryforum.com and ask more questions of the people who have lots of experience. You can also post a build thread showing how you accomplished it. I also congratulate you on getting your children involved in the hobby; you're paying forward in order to keep the hobby strong into the future.
