wolit@mhuxd.UUCP (Jan Wolitzky) (10/04/85)
Could one of the technically knowledgeable people on this net enlighten the rest of us about pulsejets? I seem to recall that the V-1 "buzz bomb" used one, and that they work by having a set of venetian-blind type shutters at the front open and close at a high rate (hence the "buzz"), with the fuel charge being ignited when the shutters are closed (high-speed exhaust goes out the back, providing thrust) and the new fuel-air mix introduced when they're open. I always thought, though, that you needed a rocket-assisted take-off in order to get the airflow going, which contradicts what's been said here about starting them on the ground at zero speed (in models). Is there any compression of the charge in pulse-jets? If so, how is that done? Without compression, they'd seem to be pretty inefficient: just a gasoline fire with the hot stuff coming out one side only. How durable are they? It would seem that the shutters take a beating, slamming back and forth like they do. On the other hand, pulsejets ought to be pretty cheap to manufacture -- or am I forgetting some component with tight tolerances? How much did the model airplane engine cost, and how did its cost/thrust ratio compare with the ubiquitous glow-head 2-stroke piston engines everyone uses? -- Jan Wolitzky, AT&T Bell Labs, Murray Hill, NJ; 201 582-2998; mhuxd!wolit (Affiliation given for identification purposes only)
irwin@uiucdcs.CS.UIUC.EDU (10/08/85)
>"which contradicts what's been said here about starting them on the > ground at zero speed (in models)." I do not see a contradiction here, the man said you started them with a tire pump, and he is correct. There was a metal "valve stem" like a tire has, mounted on the top front. You connected the pump and then pumped like he**. The combustion chamber had the "reed valve" for air inlet in the front, much like the little Cox model engines. Slots in a steel plate, with reeds behind that bend back and allow air to pass, but will close with pressure behind, forcing the reed against the slot. The chamber had a reduced diameter at the rear, ahead of the tail pipe, which formed a venturi, so you had a pressure chamber where the combustion took place. The venturi provided the compression, the tire pump provided it during starting, pump faster than could escape through the venturi. After it had started, combustion would force the hot gasses through the venturi and those leaving the tail pipe would cause a negative pressure (vacuum) in the chamber, so the reeds would open as new air rushed in to fill the low pressure area. Really wasn't much to it, seems that I remember them costing somewhere around $140 mid 1940s dollars.
todd@sphinx.UChicago.UUCP (Todd Nugent) (10/09/85)
>Could one of the technically knowledgeable people on this net >enlighten the rest of us about pulsejets? The pulse jets I have played with were rather simple: you somehow get a combustion chamber red-hot and then drip some sort of liquid fuel into it while you control the fuel flow so that the expanding vaporized fuel mixture detonates at a reasonable distance down an exaust tube. The "compression" is provided by a slight constriction just before the exaust tube. It is the same principle as a gas backpackers stove. (If you have ever used an MSR/XGK backpacking stove you have probably noticed that it sounds like a pulse jet!). The drops of vaporizing/detonating fuel provide the distinctive "pulse". Maintenance isn't so much a problem as is the fuel flow: too much fuel and the expanding gas drives the detonation front down and out of the exaust port causing a "flame-out". To little and the detonation moves up into the evaporation area leaving you with an expensive hand warmer. I have used one powered with liquid propane and one powered with avgas. In both cases the fuel flow was controlled with a needle valve (the only moving part!) and in both cases they were more than just "touchy": I would not like to try and adjust 8 of those things at once! Presumably, a computer controlled fuel meetering system would provided greater thrust, at least according to NASA Lewis it will provide about 40% more thrust for a typical gas turbine aviation engine--then again who wants their engine to flame out when the contoll system rolls-over? Pulse jets are relatively expensive--considering the single moving part--in materials and welding owing to the requirement that the combustion chamber be able to glow cherry-red hot for long periods of time. Todd. P.S. Is anyone else out there working on a homebuilt airplane? I am currently in the early stages of a KR2 project--NOT pulse jet powered--and would be interested in talking to others who prefere to build their own. (other than a few times a year at EAA events!)