AboutDave Landgraf Expertise I will be happy to take on questions about low-power rocketry in general, including rocket design, operating characteristics of blackpowder motors, low-power motor selection, rocket construction, aerodynamics, parachute and streamer duration competition, boost glide (and a little rocket glide), egg loft, altitude competition, and scale competition. I can help you with Barrowman stability calculations, but I am not a rocksim user. I know the basics of mid-power rocketry and small composite motors but I don't fly them much (I'd rather stretch my money by sticking with low-power). I believe I can explain many concepts in a way that just about anyone can follow (and maybe even over-explain?). I absolutely do not mind "newbie" questions one bit; the only bad question is the one you don't ask!
Experience I have flown model rockets for some 40 years, including a few early years of NAR competition. I have been active at the local club level as well as in the NAR. I enjoy designing and developing unconventional designs (the kind of thing you look at and say "it won't fly!").
Organizations National Association of Rocketry
Publications Model Rocketry (magazine)
Education/Credentials I am an aerospace engineer who has worked professionally in the field for over 30 years. I have worked often with local school, church, and scout groups to lead introductory build-n-fly sessions.
Question I have a two engine cluster model in a BT-60 tube that is 34"long. I plan to launch it using a C6-7 and a C6-0 to prevent overpressuring the tube. Do you see any problems with this?
Answer Hi Scott,
You do not have to worry at all about over-pressurizing the BT60. Go right ahead and use a pair of C6-7s, if that delay is correct for your expected flight.
You might need to use a 5 second delay (C6-5 motors) instead of a 7 second delay. Have you checked the performance of your model rocket? There are some online software packages available for free that can run the calculations for you. Apogee Rocketry lets you use a demonstration version of RocSim for free. There is a simple simulation available at http://www.rocket-simulator.com/simulator.php, and the WRASP simulator can be found at http://www.thrustcurve.org/simulators.shtml. If you need help running any of these, jump on The Rocketry Forum at http://www.rocketryforum.com/ and I am sure someone will help you out.
If you did use a booster on one side, you would need to put a plug into the front of that motor mount. (Remember, you can't modify the motor itself!) Otherwise, at the end of the thrust phase, that motor will burst through the last bit of propellant and vent forward through the motor mount, effectively ejecting the recovery system, and way too early.
And if you did use a booster on one side, and somehow ended up with a partial ignition of your cluster, it would be rather bad if the only motor with a delay and ejection charge was the one that failed to ignite. Using two single-stage motors (both with ejection charges) will greatly improve your chances.
Do you remember the early Estes rockets like the Ranger? That design had a three motor cluster, also contained in a BT-60 airframe. You would have used the same type of motor (and delay) in all three motor mounts at once. I don't remember anyone ever complaining that the body tube was burst open.
What you have to consider is that however many motors you have in your cluster, chances are that no two will be ignited at exactly the same instant. Chances are that they will not burn for exactly the same length of time, or delay/coast for exactly the same duration. Chances are that their ejection charges will be staggered. The first motor will deploy the recovery system and any follow-on motors will send their ejection gas through an empty forward body tube.
One other thing...you may want to hold on to any C6-7 motors because they are getting hard to find in many places. They could become collectable!
