|| Larry Cottrill |
| Date :
|| 2002-01-24 12:09:24
| Subject :
|| Re:Mini Myers Initial Tests comment and digression
There is certainly nothing wrong with seemingly 'crude' construction materials and methods for experimental purposes. If I had no welding skills and just wanted to make noise, threaded pipe would be an ideal material -- for the mini spark plugs I use, you wouldn't even need to build up a mount, just drill a 7/32 inch hole and thread it with the tap! Couldn't be simpler. I think the conduit is perfect for my skill set (you do have to watch those fumes while welding, of course!). What would really be nice would be to have a good collection of scrap lengths of chrome-moly aircraft steel in various sizes. Sometimes, the ideal thing would be double-butted Reynolds bicycle tubing (also basically chrome-moly). Chrome-moly is the most weldable material on earth, hands down. I like using pieces of spent 12.5 gram CO2 cylinders (also high-strength steel) - the 'neck end' is a wonderful, smooth nozzle that you can trim back to any area ratio you want, though of course you don't always get the 'ideal' shape you're after, in every case.
I don't think I've ever lost anything meaningful by building small in terms of learning from the experiments. My only continuing gripe is not getting anything to run without constantly piping air in -- if you want pressure-driven constant-burn liquid-fuel combustors, I'm your man! Yeah, things would probably be more successful on a larger scale, but on the other hand, if I do get something workable, scaling up will be a shoo-in. Building small means minimal tools (I have no shop of my own -- my little bench vise isn't even bolted down to anything!) and minimum lineal inches of weld to get something built. But, those aren't the only reasons to think small, and that relates to your DynaJet tale of woe:
I almost totally disagree with the DynaJet guys about their commercialization problem. I love my old Djet -- I'm probably one of the very few on the forum that not only own and run one, but have actually FLOWN it! But let me tell you something -- the whole process is an awful lot of effort for the enjoyment you get out of it. I don't think of these things as Science Fair projects or lease-breakers; I think what you ought to do with an engine is PUSH SOMETHING WITH IT. That's what most purchasers of the DynaJet were interested in -- the dream of beating the world speed record, or whatever. And that's where the designers of the engine lost it. The problem with their engine was only partly high cost -- the real killer is that the fun-to-effort ratio was just too low! They must have sold thousands of units, and I'll bet 90 % of them were only flown two or three times. Look at what you need to build your model: a huge fuel tank; insulation; heat shield cone; non-flammable glue for construction (this is no big deal, today); heavy duty control system components; a chunk of ballast in the nose to balance the engine; oversize intake ducts for cooling; etc. Now look at what you need to fly it: a field where the noise doesn't cause complaints and where you can have a minimum 160 ft diameter clear circle; a 'starter' guy to start the engine and pull off the hose and wires as quickly as possible; a 'launcher' guy to hold the plane while it's started, and let it go just as soon as the starter has everything disconnected; a good fire extinguisher; a lot of fuel; a tank design that totally prevents fuel sloshing during the launch; a pair of at least 80 ft heavy gauge control lines; a heavy-duty control handle; a place in the middle of the circle where you can really dig in your heels; and so on. Man, you only have to do this a few times before everybody's pretty sick of it. When the Tiger Jet came out, it hit the DynaJet market pretty hard because it offered similar quality, but in a lighter, slimmer package that was easier to build for and less stressful to fly (although most of the problems remained). Most successful inventors of products fall short in the areas of usability and marketability, and that's where you've got to excel to be commercially successful. All the problems can be greatly reduced with a smaller, lighter (and yes, cheaper) design, even if the efficiency is fairly low (which means virtually nothing to most hobbyists, anyway). Many hobbyists will pay almost anything for the piece of equipment they really want, but you've to create a short path from the shipping carton to the 'having fun' part!
What would I do if I came up with a design that really works well, but just can't get a sterling thrust/weight ratio? Let's say the best I could do was a valveless design that weighs four ounces and develops six ounce of thrust. As a model aircraft designer, I would feel downright dishonest to claim that this is a flight engine. So what do I do? Answer: Start building them anyway, and at the same time I'm stocking up on engines, figure out how to produce a simple R/C jet hydroplane speedboat kit -- market them as an easy-to-build kit package, with simple radio gear included. Market a pre-built version for the truly lazy, at a premium price. Give a few away to well-known enthusiasts to demonstrate all over the country. Once everybody's seen somebody HAVING FUN WITH IT, I could market and sell thousands in a year or two (IF I could gear up to meet production and fulfillment demand -- fail there, and I'd be in Bruce's shoes, only far worse). And of course, I'd have to market simple, easily portable starting rigs as well.
Maximum user fun/effort ratio is the key. Get right to the fun with minimum hassle.