Hybrid beginner questions and some answers

I received an email recently from a modeler who is considering hybrids. It occurred to me that his questions are probably pretty common, so I
asked him if I could post them on some newsgroups, and he kindly consented. Herewith our conversation. I hope people will find it helpful.
On Wed Jan 19 19:39 , Ed sent:

Thanks a lot, Ed! Hybrids have a lot to offer...of course, there are a lot of excellent conventional solid motors you can fly with your Level 1 and no LEUP. I try to design systems that will give you the best value. One problem with hybrids is that they require special ground support equipment. My RTLS Modular system addresses this; the Module 1 Baseblock can be used to fire conventional motors, in fact it's really a Surefire with extra plugs and relays. Add the Module 2 and a tank, and you're filling and firing floating-injector hybrids. Module 3 gives you K- M motor capability, and Module 4 gets you into GOX-based ignition for HyperTEK...and all four modules cost about the same as the HyperTEK ground support system, which only fires HyperTEK motors. The RTLS has been well received, and I'm proud of it.
The Minitanker was inspired by a couple of things. First of all, the RATT Works H70 is the most popular hybrid I've sold. It's 29mm, has enough oomph to fly a lot of common LOC and PML kits, and doesn't cost much. Then Sky Ripper announced their 29mm G and H motors, and I could see that there is strong demand for motors in this size range. All these motors are easily handled by my RTLS-M1 and M2, of course, but I wanted to try to come up with a lower priced way for people to get involved with the smaller motors. The RTLS-M1 and M2 together cost $310....of course, that gets you a relay launcher with built-in battery that will fire clusters, so it remains a good value. But you have to get a tank as well, and it's kind of tough to look at $500 in ground support equipment to fire a motor that costs $80. So I started thinking about alternatives.
Most clubs have a launch system that supplies 12 volts to a number of pads. If that's already there, let's use it. By connecting the Minitanker valve to one set of pad clips and the motor igniter to another set, you can use one channel to fill the motor and another to fire it. The Minitanker doesn't have a dump valve, so if the motor misfires you will have to wait for the N2O to vent, but on G and H motors this should take no more than a couple of minutes. By going to a 5 pound tank, which costs me $40 less than a 10 pounder, I can lower the price some more, and 5 pounds should be good for six or eight fills depending on conditions...not a bad day's flying. So instead of $500 in ground support equipment you're looking at $200 and the use of the club's launch system.
The only thing I'm not happy about is the lack of a dump valve. Adding the dump valve and associated hardware would kick the price up a lot since it would be identical to the RTLS-M2 Valve Manifold, which costs $200 and doesn't include a tank. With the G and H hybrids this isn't a problem because they have less than 1/2 pound of N2O onboard fully loaded, and that won't take long to vent in a worst-case scenario. But I'm concerned that people might try to use the Minitanker on bigger motors. It isn't enough of a concern to keep me from introducing the Minitanker, because I think there's a need for it. I'm trying to address the issue by offering to take the Minitanker valve and hardware in trade to reduce the cost of an RTLS-M2. I can reuse the components, and I hope the trade-in makes the point that people really need to upgrade the system for anything bigger than an H motor.
I hope this helps. Specific answers to your other questions are below.
Doug Pratt www.pratthobbies.com

Nope--see above.

No problem. Sky Ripper preheaters are less than 1/2 ounce. Even the big RATT Works K, L and M motors have preheater grains that are under the 62.5g limit.

Absolutely. Todd Moore, who created the Sky Rippers, won't sell them until they are Tripoli certified. The 39mm motors have been certified for a year, and the 29ss are being tested this month.

We did some tests a year ago and found out that Pyrodex works great in our Ejection Canisters as long as you wrap the canister with a layer of stretchy PVC electrical tape. That provides the containment that the Pyrodex needs to burn completely. We've used Pyrodex instead of BP in most of our hybrid flights over the last year.

There are some very good products available for this. Personally, I like the G-Wiz accelerometer, the cheapest one. It fires at apogee only, but with G and H motors that's all you need, and accelerometers are very good at telling the difference between up and down. We like to use the G-Wiz and an Ejection Canister filled with Pyrodex and taped. A single 9v battery powers the whole business, and the G-Wiz does not need to be in a vented payload compartment since it doesn't measure air pressure. In my faithful old PML Black Brant that I fly with RATT H70s, I wrap the G-Wiz in bubble wrap and tape and stuff it in the nose cone. Works like a champ.
We are working on an altimeter, and also on a very simple (and hopefully cheap) timer. Don't know when they'll be available, but we have made some test flights.
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Doug Your getting very close to a "Beginners How to do it" Book!
Let's see , A small H70 or H124 , Mini Tanker setup and some recovery cheap and easy to use. This G-Wiz acelerometer is available where?? Ken
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I highly recommend Aerocon as a source for the G-Wiz products. He has a nice special going on his website right now. www.aeroconsystems.com.
Funny you should mention the book. I hope to have a book ready for publication by summer. The working title is "Hybrid Rocket Motor User's Guide." I'm setting up a little publishing company called Robin Hill Press to put out this and a revised edition of "Basics of Model Rocketry."
Doug Pratt dad-at-pratthobbies-dot-com
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