evening all, Ive got (yet another) project in planning. It will involve some small jet nozzles (about 1/4" or less) . They will be subjected to centrifugal force (disk of about 12 -18" dia doing a few thousand rpm), and supplied with compressed air from a seperate source. I think probably an inconel combustor/nozzle would do, but which inconel, and where would I get it? Would stainless be any better? There are people here with more rocket experience than me (zero) so Im asking for opinions, and also sources?
Hi Peter, hoped you'd see this one. (remembered your rocketry background) Yes, thats the question. This is a new field for me. Do you know of the Fairey Rotodyne? If not think helicopter with rocket/jet mounted to the rotor tips. This is the kind of thing Im thinking of. Its just an 'in my head' design at the moment, not even sure if its feasible, but basically a motor/engine powers a small compressor. This air is fed up a tube where the mainshaft would sit and into an airbearing/thrust assy for the rotor head. The 'spinny' part of the air bearing has 2 (or maybe 4) tubes which form the leading edge of a rotor blade. down here goes the compressed air to exit via a jet at the tip and spin the rotors. Thats stage 1. I suspect the air jets alone will not have enough 'oomph', an so will probably have to have some fuel added and burnt. I think the fuel wil make its way up via a seperate tube and mix in the tip. Glow ignition would be favoured, to avoid having to run wires down there to - heat the plugs pre flight then not have to worry about them. Im not really sure if its a jet ot a rocket (think its an unusual jet
- remote air breathing). anyway Id rather the tips dont melt / fly off / shrapnelise If I can get a rotor that works and will lift its own auxilary equipment then I hope to make an RC heli from it. I think that 1/4" bar with an appropriate hole drilled in it is all thats required, but like I said this is a totally new area for me, so any advice is welcomed
A very small fuel/air rocket will have severe problems with the combustion time or L*, the characteristic length (very roughly the same thing). What fuel do you plan to use? What chamber pressure, flow rate, OF ratio? Isp?
If you have no idea about these, a place to start is Huzel and Huang Modern Engineering for Design of Liquid-Propellant Rocket Engines, though it may be over-technical for the beginner. Also find something on air-breathing design, but I can't recommend anything offhand.
One of the great advantages of rotary rockets is that liquid fuels and oxidisers can be pressurised by centrifugal action, but this doesn't work for gases or air. On the other hand, you don't have to store oxidiser in your design, and the nitrogen in the air is useful reaction mass.
Pulse jets, or even ramjets, are possibly a better idea.
You will almost certainly need Inconel 718 (or X-750, or HX, or Haynes
230) for such an engine, most likely 600-series Inconels won't do.
I don't know offhand of a small-scale source of 1/4" 718 bar, but Wren sell 6mm tube which might be of use, at £30 per meter:
I can let you have 300 mm of 10mm 718 bar for £17 plus postage at cost #if# you can convince me your design might work (I'm not willing to let stuff like this go, except to good causes, as it is extremely hard to get in the first place).
If you can find it cheaper please let me know - I'll buy some myself!
You might consider using 316 stainless for prototypes - cheaper. easier-to-get, and easier to machine.