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 am currently working on a model rocket competition for my
honors physics class. The competition is to build the
rocket that will have the longest air time (from launch to
touch down). With this set up would it make more sense to
have a staged rocket that will have a longer total burn
time, or too cluster the engines and fire them
simultaneously, resulting in a greater original thrust???
Answer Hi Jim,
It sounds like you have a pretty fun physics class! I wish we had done some of this sort of thing when I took physics.
Your question is actually a very good one, and involves many aspects of design, build, and launch that might not be obvious to many folks.
First things first: I would suggest that a cluster is flat-out a "no" answer. Why? A rocket with two (or three) motors will have a larger body tube and more drag (more surface area and skin friction drag, more cross-section area and frontal drag and base drag). A rocket with two (or more) motors will have more motor case weight during descent, reducing the duration. A rocket with two (or more) motors will require a more complex ignition system, and will introduce the chance that one motor might not ignite. A clustered rocket design that fires all motors at once will tend to fly at higher velocity (for at least a portion of the trajectory), which will cause higher drag, and you will waste a (slightly) larger portion of the motor impulse.
A staged design will tend to give you better performance. If you can find a good combination of motors that provide a good thrust profile, and still keeps the total impulse within your contest limits, you may be able to design a rocket that cruises to a higher altitude by dropping "dead" motor weight. You will have to be careful to avoid adding excessive weight and drag for the booster stage (extra body parts and weight, extra fins and drag). Don't forget that a staged design adds the (slight) complication and risk of failing to get successful upper stage ignition. You will want to select a big enough booster to get the total rocket moving at a safe flying speed (where the fins are effective enough to provide stability off the tip of the launcher) but you will also want to select motors that have lower thrust and longer burn time.
A one-stage design can often be a very good design choice, especially if the contest sets low limits on total motor impulse. Single stage rockets are a little more simple, and sometimes simple is good. You may be facing a design choice between a "minimum diameter" body tube (just big enough to hold the motor) for low drag and a larger diameter body tube to hold a bigger streamer. You can have a design that gives you the best of both if you use a larger forward body tube and a tapered fairing that necks down to a smaller aft body tube. You may need to do some calculations to check the theoretical performance of these options.
You can use trajectory simulation software to check the performance of your design. A free trial version of RocSim, by Apogee Rocketry, is available online; there are several other software tools also posted on the internet for free use. If you check your design for optimum mass you may be surprised to find that ADDING weight can actually make a rocket travel higher (it's a thrust-drag-momentum thing). Simulation software can also help you find the performance pockets where combinations of motor, weight, and flight time match up well with available motor delay intervals.
A good place to start might be to look over the contest rocket plans posted on the NAR (National Association of Rocketry) website. Even if you don't want to copy these plans directly, they can still give you some good ideas on how to design your own contest rocket.
If you are using NAR contest rules for your class project, you may be stuck with the 10:1 limit on streamer length-to-width ratio. There are further NAR contest rules about streamer attachments and other things, so you may need to be careful with your streamer design. Fan-folding the upper end of the streamer will increase drag and slow the descent. If you use an external anchor point on the rocket (so the streamer and nose cone are tethered to the outside of the rocket body) and select a lengthwise anchor point so the rocket body hangs sideways, you can squeeze out an extra bit of drag during descent.
For any rocket, you can avoid a significant amount of drag by not using launch lugs. Build an inexpensive launch tower (if your rules allow it) by using three lengths of stiff PVC pipe to surround the rocket and provide directional control during launch. Use an old plastic pail or pot, stand the three pipes in the bottom, and pour in something like cement or plaster of paris. There are other, more elegant ways to build launch towers that you can find online, but you will probably not want to turn the launcher into another huge project?
You can also save a significant amount of drag by shaping the fins properly. For contests, the fin airfoil shape will indeed make a significant difference. Sand the leading edge round. Sand the last third of the fin so it tapers down to a thin trailing edge. Spend some time using wood filler to get those fins as smooth as possible (spray-on Kilz works well, as does auto-body primer, and the kind of light-weight wall spackle that has microballoon fillers).
If your contest rules allow, you can also improve overall performance by the use of a piston launcher. However, if you don't have much experience with these, this might not be a good time to try to learn.
Build light, use the least amount of glue possible, keep the outside of the rocket smooth, use a long launcher to get the rocket off to a good straight start, and use bright colors in the streamer so you can see it more easily in the sky and on the ground.
Whew, that was a lot of info, and my fingers are tired. I hope some of that helps you! Good luck on your project.
