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About Dave 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.
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You are here: Experts > Science > Science/Nature for Kids > Model Rocketry > rocket nozzle
Expert: Dave Landgraf - 10/27/2009
Question
I made a rocket nozzle with PVC end cap with pop(plaster of paris) filled in for converging and diverging part. But the erosion is very high, during test fire the 'pop' in the converging part eroded and blocked the throat. Would you suggest any other options. The reason I used 'pop' is it is easy to carve to our convenience during its curing.
Answer
Hello, Veerendra,
It sounds like you have started a very ambitious experiment. I hope you are using good safety practices, and have a teacher or experienced advisor to help build these motors properly.
I think you will continue having trouble using plaster of paris for nozzle materials. It is too soft, too weak, and too combustible to be able to hold a precision nozzle diameter and shape. Without good control of the nozzle geometry (mainly, the diameter) the nozzle can fail gradually by eroding and opening up, by eroding along a crack and steering the thrust vector off into some unplanned direction, or can fail catastrophically by just letting go. You will usually need some sort of clay binder in the mix of nozzle materials, along with finely crunched ceramic granules to stand up to the temperatures and pressures at the nozzle. You will also probably need some precision tools to stamp-form the nozzles with carefully controlled throat sizes and expansion angles.
You can find a lot of the ingredients and tools you will need to make good nozzles at places like Skylighter and Mojo (start googling---look for keywords like rocket, nozzle, amateur, etc). You can find a lot of information online at places like Skylighter ( http://www.skylighter.com/ and http://www.skylighter.com/skylighter_info_pages/articles.asp ). Rocket motors are an important part of fireworks and there is a lot of information already posted on nozzle and motor construction, as well as propellant formulation. Also, check sites like Mojo (http://www.mojopyrotechnics.com/ ). If you have not already found it, you definitely need to study Richard Nakka’s experimental motor website ( http://www.nakka-rocketry.net/ ). You need to join in at places like Rocketry Planet ( http://www.rocketryplanet.com/forums/forumdisplay.php?f=5 ) and Newtons Third ( http://forums.newtons3rdrocketry.com/viewforum.php?f=8 ); most of these forums are free to join, and have many knowledgeable contributors who can help you with specific questions.
You will also find a lot of information by looking for books like "Amateur Rocket Motor Construction" by David Sleeter. This book can sometimes be found online at places like Amazon, or you might find someone willing to sell a copy by posting a note on a rocketry website.
I hope some of this information gives you a direction to find more help and move ahead. Good luck!
Dave Landgraf
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