D-MAN Posted July 5, 2006 Share Posted July 5, 2006 Saw this at the Boeing History of Flight Museum in Seattle over the weekend, and got a kick out of the workmanship. This is an all-wood horn used for wind tunnel testing for jet intakes. It is roughtly about 15" across, I would figure. It is honed down to a very thin edge making it very fragile. The question remains, why wood? Anyway, got a new camera, took some pics that may or may not be of interest, but here goes... DM Quote Link to comment Share on other sites More sharing options...
D-MAN Posted July 5, 2006 Author Share Posted July 5, 2006 Speaking of jet-intake horns, this one is the real winner - the BOAC Concorde jet intakes slow the air down to something the engines can "eat", in 15 feet of pathlength. Reverse horn technology? DM Quote Link to comment Share on other sites More sharing options...
D-MAN Posted July 5, 2006 Author Share Posted July 5, 2006 Sort of the reverse of a horn, this is a wind-tunnel model entirely of machined metal (these are about 6 feet tall and very beautifully made) used for testing the exhaust plume(s) of the space shuttle! Why the exhaust plumes need to be tested who knows? Anyway that seems like an unbounded horn-like application (i.e., phase plugs). DM Quote Link to comment Share on other sites More sharing options...
hwatkins Posted July 5, 2006 Share Posted July 5, 2006 Very cool stuff D-Man. Actually it is all the same physics - just quite different inputs and frequencies. It is all about using the efficiencies created by the shape. Quote Link to comment Share on other sites More sharing options...
djk Posted July 5, 2006 Share Posted July 5, 2006 "The question remains, why wood?" Cheaper to buy and machine than metal, about seven times stronger than steel on a weight basis. Quote Link to comment Share on other sites More sharing options...
WMcD Posted July 6, 2006 Share Posted July 6, 2006 I think our horns and jets are not the same operating conditions. At the intake: The air is entering at about whatever speed the aircraft is flying. In some applications, supersonic. Then it has to be slowed down to near zero velocity (and high pressure) when it enters the combustion area (at the turbines). I've read this is no small issue. At the exhaust the velocity is supersonic. My recall is that a divergent nozzle does weird things when a gas is supersonic and highly compressed. Gil Quote Link to comment Share on other sites More sharing options...
pauln Posted July 6, 2006 Share Posted July 6, 2006 Speaking of which, here is a little rocket science brain teaser: In this example, let the exhaust from a rocket exit the back end at a constant speed relative to the rocket body of 2000 feet per second. In the beginning, the rocket accelerates through the air and goes faster and faster... disregard the drag on the rocket body... which is the case here: Point 1: The rocket will acheive an eventual top speed of 2000 feet per second relative to the air because at that speed, the fuel is being transfered to exhaust in such a way that the exhaust is motionless relative to the air; that is, if the rocket went any faster the vapor trail would have to move forward relative to the air in the direction of the rocket... again, that is; the speed of the exhaust out the back end equals the speed of the rocket through the air - so 2000 ft per second is the rocket's top speed. Point 2: Newton's laws of motion govern... the rocket will continue to accellerate at a constant rate even if the vapor trail of exhaust exits the rocket with a positive velocity in the direction of the rocket's travel. Clue: At what speed is the rocket's exhaust thrust most efficient? Quote Link to comment Share on other sites More sharing options...
boom3 Posted July 6, 2006 Share Posted July 6, 2006 Gil, you're right about the operating regime. These are not horns at all. The intake side is called an inlet diffuser, and the output side is called a thrust cone. http://rcaf-atc.org/misc/bomarc.bomarc8.html These 'normalize' the flow of air (and exhaust). They are closer to what we call phase plugs. Quote Link to comment Share on other sites More sharing options...
ZAKO Posted July 6, 2006 Share Posted July 6, 2006 Pauln.....In case of hypersonic aircraft 7000 mph Quote Link to comment Share on other sites More sharing options...
D-MAN Posted July 6, 2006 Author Share Posted July 6, 2006 I think our horns and jets are not the same operating conditions. Gil Of course not, however, from a static design point of view, the elements appear to be somewhat related in that both audio-frequency horns and jet intake nozzles provide boundaries to channeled airflow, velocity and scale notwithstanding. Definitely different operating conditions and purposes. That goes without saying. But what I find interesting is that the design elements are not entirely unrecognisable by the audio-horn afficianado. The distinguishing feature is the volume of airflow. DM Quote Link to comment Share on other sites More sharing options...
ameen Posted July 6, 2006 Share Posted July 6, 2006 i do belive that in realation to Newton's laws, being that there is no external force the center of mass of the entire system acctually stays in one place. this is including the gasses of the burned fuel which are moving in the opposite direction. So it would theoretically contunie to accelterated. Quote Link to comment Share on other sites More sharing options...
WMcD Posted July 6, 2006 Share Posted July 6, 2006 ameen beat me to it. I don't see how either of those conditions can exist, they both require the center of gravity of the combined rocket and exhaust to be moving (actually to have undergone accelleration without a force applied). A related issue is that the fuel (reaction mass) has already achieved a velocity in the rocket tank. But in Point 1 you have combustion (force) but the exhaust does not achieve an inceased velocity at all (it is the same as the rocket). So there is no reaction force? And Point 2 you have it decrease in velocity (moving in the same direction as the rocket but not as fast). So there is a negative reaction? It seems like you're now using a rocket to pull itself instead of push? So I must be missing something. Gil Quote Link to comment Share on other sites More sharing options...
D-MAN Posted July 6, 2006 Author Share Posted July 6, 2006 Beats me too - I can only guess that there is a drag coefficient associated with the exhaust plume and the "connection" to the exhaust nozzles. It also occurred to me that the exhaust plume columns act as a "platform" at some point to provide additional resistance for the rockets to push against, if that makes any sense. Completely wild and unfounded speculation on my part... but the rapid slowing of the exhaust would cause a temperature change that would be more effective than just undisturbed air and being denser due to the particulates, should act as a "moving platform" to work against? DM Quote Link to comment Share on other sites More sharing options...
DrWho Posted July 6, 2006 Share Posted July 6, 2006 propulsion has nothing to do with "pushing" - it's just conservation of momentum: small mass x high velocity travelling one way results in large mass x low velocity in the other. I never thought about it in terms of center of gravity, but I suppose that's a complicated way of thinking about it (just take into account that the mass of the rocket is changing as the fuel becomes "disconnected" - thus changing the center of gravity of the rocket portion). Quote Link to comment Share on other sites More sharing options...
D-MAN Posted July 6, 2006 Author Share Posted July 6, 2006 The exhaust plumes are clearly acting as a boundary layer close to the vehicle. Like a solid walled "attachment" to the vehicle. It is very apparent that NASA was interested in the air flow around the vehicle including the top-most portion of the exhaust plume columns. So let's review: the vehicle is pushing against what? That's right - air. Otherwise, it is senseless to test the model in an air tunnel, isn't it? Thanks for your input. Quote Link to comment Share on other sites More sharing options...
pauln Posted July 6, 2006 Share Posted July 6, 2006 Newton rules, but the atmosphere is the judge. The heart of it is the clue about efficiency. When the rocket speed is less than exhaust speed, the energy represented by the backwards motion of the gas in air is wasted and not utilized as thrust; likewise but in reverse when the rocket goes faster and the trail is left behind folowing the rocket - wasted potential thrust. The majic speed of a rocket where the exhaust velocity equals the rocket's speed is the most efficient speed because there is least waste - the exhaust is being layed in a trail behind the rocket such that the trail of gas has no relative residual motion against the still air. This rocket speed is called 100% and varies with the particular thrust exhaust speed of the design. With constant thrust from the rocket exhaust, the rocket will continue to increase in speed until the resistance force of the atmosphere levels it off. But this resistance of the air varies with density (elevation) and prevents one from using 100% at all altitudes. The shuttle, for example, does not use a constant throttle for its ascent. If you have ever listened to a shuttle launch, in the early part of the ascent the 'throttle' is backed off for a speed below 100% so that the ascent is slower during the part of the ascent in dense air. As it gets up higher into the thinner air, listen for the mission controllers to say, "Go for throttle up, 110%." This is done because the dynamic load of the lower atmosphere is so strong at these dense levels that a too high speed can be damaging to the craft, but as it gets up into the thinner air - no problem, and they allow speed greater than 100% sacrificing maximum efficiency for a safety margin to get up to orbital speed. Quote Link to comment Share on other sites More sharing options...
WMcD Posted July 6, 2006 Share Posted July 6, 2006 Ah, but what if the fuel tanks are drained through a water sprinkler sitting in the bottom of the tanks. Smile, Gil Quote Link to comment Share on other sites More sharing options...
hwatkins Posted July 6, 2006 Share Posted July 6, 2006 3 cheers from me for Pauln and Doc Who - As I remember and understand it - combined you two hit it on the nose. For the most part I got the others, but they were discussions that looked closer at 'effect' - relevant, but not clearly answering the riddle. As to the horn concept - I stand by my simple comment - in one way or the other it is manupulation of frequencies to get the desired effects relative to the constricted space and shape. That does seem like similar concepts to our audio mis-adventures. Fun stuff folks... Quote Link to comment Share on other sites More sharing options...
pauln Posted July 7, 2006 Share Posted July 7, 2006 Ah, but what if the fuel tanks are drained through a water sprinkler sitting in the bottom of the tanks. Smile, Gil The sprinkler is inside the tank? Quote Link to comment Share on other sites More sharing options...
D-MAN Posted July 7, 2006 Author Share Posted July 7, 2006 Those zany Russians! They built a golfball without any dimples! Here's the question - why doesn't this tumble/rotate/fly all over in re-entry? Check out the burn marks... DM Quote Link to comment Share on other sites More sharing options...
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