Hi Low Orbit. 4 motors sounds good as starting point. Having four specific "points" for the thrust stream (as opposed to more) will mean exhaust
design is that much more crucial to the balance of the platform. It may make it easier to fabricate. All the surfaces and structural bits that aren't
in danger of "bursting into flames" you may wish to consider a lighter, cheaper and more easily worked material like sheet magnesium (source
printing/lithography plates) or aluminum.
For the exhaust stainless sounds like the pragmatic answer although welding it (skillfully) requires skill and experience... when we used to need
stainless bits fabbed-up for race bike pipes we'd go down to a restaurant supply house... their stainless counter top fabricator was the best SS
welder. Good welding costs big bucks (or my favorite, favours)... have your fab guy sniff around.
Engine selection... there are several manufacturers. The one to choose? Uh, your decision not mine! Hey man, "I just work here." LOL. I'd suggest
an airstart (if you have a compressor and/or a suitable air "pig" as it may save some weight compared to an electric start... but do some
(Advanced Micro Turbines Netherlands)... they have American dealers. I'm sure there are other choices, perhaps
even made in America. See if they have stall data and design drawings... my bet is they do.
Oh yeah, ask them if they have "left" and "right" rotation motors... they may not, but I'd feel there was less of a chance of "the whole thing
spinning" from lack of torque cancellation and harmonic couple induced "climb"... Uh, better explain: sit on a BMW motorcycle, rev the motor,
you'll feel the bike try and "rotate" around the crank shaft and press against your right leg... it is this effect we may wish to cancel in the
design stage as opposed to a potential workaround later. My fear is a slow rotation in the same direction as the turbines spin that could increase for
a variety of reasons.
Fuel? Safest stuff would be Kero or Naptha... if you're bucks up I'd go for Jet Fuel Grade A1... set up the ECU and optimize it for whatever fuel
you can get consistantly and have knowledge that the fuel is "fresh" to avoid the "gummy bears" that appear as fuel evaps and screws up fuel
systems and pumps with scummies and particulate matter. Oh yeah, fuel consumption... an engine rated at 88 Newtons uses get this, 290 grams per
minute! Larger motors: a pound and a half per motor per minute. Gonna need significant fuel capacity I think, and depending on layout quite likely a
fuel pump(s) as there may not be sufficient room to get away with a gravity feed delivery.
A central fan "blow down" fan could buy you some lift and a tiny increase in intake pressure allowing more turbine thrust and a safety and stability
factor not to mention this intake side pressure could help avoid compressor stalls from too closed an exhaust (smaller opening equals greater thrust
pressure and back pressure) ... remember that at least some of the downward thrust will always be needed to maintain both altitude and through
flappers, balance, until aerodynamic lift could be generated through "lift" caused by air moving over the disk. Disks (lenticular shaped ones) as a
rule don't generate aero lift. You may wish to give this some thought. The central fan is entirely your call. If you feel you won't require it turf
it. Adding one (or two one spinning each direction) after the fact may present challenges, perhaps expensive.
One of my friends suggested a system of tethers top and bottom to restrain the platform during R&D, this makes sense to me... some on top to prevent
"uncontrolled" landing and some from the bottom to prevent the "bug" from getting too far away... give it two or three feet of vertical "slack"
for the early-on flight enevelope.
Lotsa fun.. get back to me when you can Low Orbit and we'll jam on this some more... lotsa undecided stuff. An engine selection spec seems most
critical, I'd start there.
[edit on 29-5-2006 by V Kaminski]