January 5th, 2010
I never thought I’d share ALL my secrets on altitude, but here they are… in no particular order. Note that some will disagree with some of my stances here, but they have served me (very) well over time. Now, in no particular order:
- Lose the lugs and rail buttons. Tower launch for optimal performance. It doesn’t make a LOT of difference, but it sure does make enough to do it.
- Minimum diameter is a MUST. There is simply no substitute for this one. Remember, the hole you are boring through the sky is calculated at 3.14*r*r. That means any increment is magnified considerably…. Look at it this way – a 16″ pizza is actually 4x the size of an 8″ pizza (trust me). An 8″ pipe carries 4x the water that a 4″ pipe carries. A 98mm rocket has to punch a hole that is 3.29X the size of a 54mm rocket.
- Von Karman nosecones are key - assuming your velocity is above mach for an appreciable amount of time. Conical is typically the next best, followed by ogive. Your actual mileage may vary. Much depends on your flight profile.
- Tailcones help – a LOT. They are also a tough thing to work into a minimum diameter rocket, but trust me – it can be done (and I’ll soon have a kit to prove it!)
- Optimal mass comes into play. This is one that will spark some lively discussion around the campfire – but I firmly believe in it. Lighter is not always better. I like to use the example of throwing a wadded up tissue with and without a rock wrapped inside. Sometimes – more often than you think – heavier is better.
- Little fins are better than oversized fins. It is all about drag…
- Shorter is better. Much of a rocket’s drag is dependent on the rocket’s length. Go as short as possible. I once calculated how short I could make a rocket, and then I cut the tube back two inches to force me to get creative. Believe me, you can get pretty creative.
- Finish must be very smooth. Duh. But it can’t be emphasized enough. I like to paint a rocket with Kilz primer and then wet sand it before and after every flight.
I’m sure there are other things, and I’d love to hear your tricks (now that I’ve spilled my guts on all of mine)!
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December 7th, 2009
As noted in the FAQs, there are basically two kinds of glass tubing – filament and convolute wound. There are a few pros and cons I’ll expand on here. In the end, choose what is best for you-
To quickly recap the FAQ, filament wound tubing is wound at an angle, similar to a barber pole – except that it is wound up and down the mandrel, so it has a criss-cross appearance. Convolute wound tube is wound horizontally, over itself again and again, perpendicular to the mandrel…. sort of like paper towels on a roll.
Filament wound is probably a bit stronger than convolute, and it has a much smoother finish – so smooth, you should really rough it up with sandpaper before painting.
The bad news with filament? If you ever have a less-than-perfect end cut, it is very, very difficult to sand filament wound square. Convolute sands pretty easily. The biggest issue I personally have with filament is drilling a clean hole. Many of my rockets have a dozen or more holes that need drilled (vents, shear pins, static ports, perhaps a rivet or a screw for the ebay, etc.) Filament wound typically splinters, and the fibers can try to re-fill the hole, which is not cool for a static port… or they may cause a chute or a piston to hang up, which is probably disastrous.
Convolute tubing drills like a dream… almost like the material is made out of sugar! A very clean hole, every time. As far as strength? I have pushed convolute quite hard – and I haven’t folded it yet!
Finally, it is worth noting that filament wound tubing is much heavier than convolute. For me? Convolute – every time!
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November 30th, 2009
This is… HUGE. My first blog? Gosh, I’m nearly 50 years old. I guess it is about time.
I’ll wade in here from time to time and discuss rocketry stuff. Knowing me, I may stray off the rocketry path – but mostly I’ll try to keep things to our hobby / industry / passion. I find writing to be very theraputic… I just don’t do enough of it.
Check back soon. My intent is to keep you up to speed on my latest project, a new technique, that sort of thing. Please feel free to drop me a line on the contacts page if there is something you’d like covered here.
Welcome to 3 Dogs Rocketry – JW
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