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Discussion in 'General Chat' started by DIGGS, May 31, 2017.
Sweet. The kinda stuff you probably only get to through the military where I'm from.
This is more complicated than I thought
eh its not rocket science
This is a really bad description. Like, it's designed around explaining the device to people who already know how all the individual components all work, by someone who only just brushed up on it. I hope this guy doesn't have a Patreon / isn't getting paid for this.
what a hater
make your own patreon if its all about the money to you!
Vanilla Ice, why are gas turbine engines with a high exhaust velocity so inefficient at low airspeeds? I undestand the formula
but not the reason why the most efficient engines utilise large mass flows to produce thrust instead of high exhaust velocities.
The short version is that thrust scales with the velocity difference an engine produces, while the power you put into it scales with the square of the velocity difference. So, as the velocity difference grows, the energy you need ends up growing a lot faster than the thrust you get out. To produce the same thrust with a smaller velocity difference, you have to get more mass to pass through the engine. This is why turbofans are more efficient than turbojets.
If you want the long answer, I'm bored and I like writing. So we can start by actually defining efficiency here: what are we putting in, and what are we getting out? We put in fuel - that's easy - and when we burn that fuel we get out some heat or energy. Now, an engine is mechanical, so eventually that chemical energy is converted to kinetic energy at some point. That subscript "P", for propulsive, skips this conversion (from chemical to kinetic energy), which we call the thermal efficiency. The propulsive efficiency just cares about how much of that kinetic energy is actually useful. The kinetic energy that is imparted to the working fluid can be expressed as:
where e is the symbol we use for energy per unit mass, and u is the velocity at the inlet (sub "in") and nozzle (sub "out"). I'm hoping that you would recognize this as pretty similar to that 1/2 m v^2 expression for kinetic energy from high school, we're just missing the mass because its per unit mass. It's easier to express this as a power, so we want to multiply by mass and then take the rate:
where m is mass and the dot above it is a notation for a rate in time, so this would be a mass flow rate (kg/s, say), and this whole expression for power in is proportion to your fuel flow rate (flow rate of fuel times an energy density of the fuel, times a thermal efficiency). So that's what we're putting in. What are we getting out? Of course we want thrust. Engines produce thrust by accelerating a flow, by the change of momentum:
where F is the force our engine is producing, p=mu is momentum (the product of mass and velocity), and that fraction out front is asking how quickly the momentum p is changing in time. Now, this skipped over a lot of steps - because actually showing how the third term becomes the fourth is pretty nasty - but I hope its intuitive that the rate of momentum leaving an engine, minus the rate entering, is the total force. Now a force doesn't actually imply any power or work or effort. I'm applying 160 lbs of force to my ass right now and I'm not expending much energy at all. What we care about is what sort of work that force can do, and in this case that's the work to overcome drag. If work is defined as a force acting over a distance, then a power, or a rate of work, is a force times a velocity. Assuming we're in level flight, thrust equals drag, and this expression is pretty simple:
where we're assuming our inlet velocity is equal to our flight speed. Efficiency is just the ratio of these two powers:
You notice our mass flow rate cancels and we just get a ratio of velocities. This expression here is already totally equivalent to yours, but we can make it exactly the same with a little algebra. First by factoring, and then by dividing through everything by the inlet velocity:
Just saw the aviator last night
what do you guys think about howard hughes' airplanes
Good because I have many questions.
Why can the tips of a ducted fan reach supersonic speeds without completely ruining thrust? How does a ducted fan compare against an unducted propeller in this regard and others; when does it make more sense to use turbofans instead of turboprops as power plants? I'm only talking about fuel efficiency here, disregarding capital costs, maintenance, compatibility issues, etc.
This is an interesting interview with a German test pilot who used to fly the MiG-29 (among other planes). He does reveal some interesting pieces of information: I had no idea that the USSR fielded a helmet-mounted system to fire off-boresight missiles in the late 80's, for example.
went to an airshow
I'm going to go off on a tangent to start here. Wind energy today is developed by a lot of private interests (General Electric, Vestas, Enercon, and others), and by European universities (especially the Technical University of Denmark and TU Delft). But it really got kick-started by the Oil Crisis of the 1970s, and the work that the National Renewable Energy Laboratory, part of the US Department of Energy, did in response. In this era they were using aircraft technology: they were using NACA profiles designed for aircraft, rather than the specialized wind profiles we use today, built from aluminum-lithium by Boeing and Lockheed and Northrop. And they didn't start small. Some of the first experimental turbines were in the megawatt class - some as large as the giant 8 MW offshore machines just coming online in the last decade. If you work out the governing equations of wind energy, you can work out an ideal solidity. Solidity is the fraction of the swept area actually occupied by an airfoil, and in wind energy it turns out to be very small - and decreases with radius. So given a certain amount of real estate (wing area) to work with, how do you divide it up? NREL concluded that the optimal number of blades on a turbine was one, which makes sense: dividing it up to two doubles the number of tip vortices, reduces your aspect ratio, all bad efficiency things - and they actually built them:
The above is not small-scale, either: the NASA/NREL Mod-0 had a rotor diameter of 38 meters - larger than the wingspan of the 737. Of course, when you get into the practical use of a single-bladed turbine you need to add a counter-weight. and if you're adding a counter-weight anyway you might as well make it an airfoil. Two-blade turbines spin pretty fast, and noise scales with the tip speed (to the cube, I believe), so a third blade is added on modern devices just to reduce their operating speed and keep them quiet. Aerodynamically, though, one blade works just fine.
Now, we have the opposite problem. We don't want to extract momentum from the flow, we want to add momentum to it. Is the solution much different? Think about our last post: the ideal propulsor adds an infinitesimal amount of momentum to an infinite amount of mass. What better way to do that than with an infinitely slender single blade of infinite aspect ratio? The ideal lifting surface has an infinite aspect ratio, and that's all a propulsor is - a lifting surface that is rotated in order to force "lift" to be forward. Now, there's a couple practical problems in building an infinitely-long blade. Not material cost, of course, it still has a finite area. And not power, because even though it takes an infinite torque to rotate it, we'll just use a gearbox with an infinite gear ratio so it spins at a infantesimal speed, that way we still only consume a finite power. You know, my background is fluid mechanics, not solid mechanics, but I'm pretty sure if you put an infinite torque through any material, it's going to break. Another big issue is space: we only have so much room for a propeller that's mounted on a wing next to a fuselage. Is the aircraft high-wing or low-wing? Is the axis flush with the wing, higher, or lower? How far from the fuselage are we mounting the engines - or is it on the nose?
Once we have this constrained real-estate, it's no longer about what's best, it's about what's the best we can do. Part of that is blade speed - the more blades we have, the lower the tip speed we need to get our required thrust for a given blade chord and blade length. Part of that is blade loading. So we decide that we only have so many square meters of useful area to use for engines, and our aircraft requires a certain amount of thrust - what does that mean for the force on our working area? What's the pressure? Now we start getting into functions of flight speed. If you want a slow aircraft - half the speed of sound - maybe you only need a ton of thrust (approximate value for the Bombardier Q400). The Q400 has high wings, with the engines roughly level-mounted, so we have a lot of real-estate - the props are four meters in diameter; something like 30 kilos per square meter. So how many blades do we need to carry that load, at a reasonable blade thickness? A single blade would have to be super thick, not very efficient. Two? Four? The Dowty propellers on the Q400 settled on six - and the solidity is still a modest 10% or so. And this gives us enough blades that our tip speed is below supersonic. So we're okay. What about the 737? Its low-wing, with the engines mounted underneath to shield the cabin from engine noise, so it only budgets a quarter the fan area as the Q400. And it needs twenty to thirty times the thrust - so we need on the order of a hundred times the blade loading - 4000 kg per square meter? Six blades might not cut it, structurally. And if they could, they would have to spin super fast, killing our efficiency. Turbofans run 20 or more blades, and often have solidities exceeding one - there's more blade area than there is fan area, with fan blades overlapping each other. And to get to a nice, efficient thin thickness, they have really big chord lengths.
So we have a huge number of blades (20 or so) with really low aspect ratios (three or so). How do we deal with that? If left in the open, each one would have its own tip vortex, producing all sorts of induced drag. One option is to stick a cowling on it. If you have a really close clearance with the blades, you interrupt the tip vortices, giving them a more two-dimensional behavior. You don't want to do this unless you have to, though, because cowlings are heavy, and they eat into your engine real-estate that we've just established is so precious. But we can use it to its best effect: we can shape it such that our inlet speed is almost constant. This is huge - an open prop has to deal with a huge range of inlet speeds (that's why they can pitch), so the blade profile is usually designed for sub-sonic regimes. You can design a prop to operate super-sonic (really thin profiles on the outer edge, maybe a fancy sweep and tip device), and we have done so (we know all about how to combat drag divergence, wave drag, and other factors), but then you're sacrificing your low-speed performance. But if you design the thing to operate at one inlet speed, you can have a constant shaft speed, a constant tip speed, and therefore design your blade to operate at any tip speed you like - even supersonic, if that's what your design point mandates.
video I made many years ago
dummies like vanilla ice dont get that a video is worth a million words
**** YES supersonic
The max. payload of a 373-800 is about 2,5 times the payload of the Q400. But it needs significantly more thrust. What I gather from this that from a pure fuel economy pespective, everyone would be better off hauling people and cargo in slow turboprops?
Excluding the propulsive and thermal efficiencies of the engine, ideal fuel economy occurs when the rate of energy consumption per unit distance is minimum. Excluding engine efficiency, this happens to be when lift-to-drag ratio is maximized. For any aircraft, this will be faster than the speed of minimum drag. Although this varies by aircraft configuration, I believe the highest achieved for any aircraft has been on the order of L/D=30, and speeds of around 100km/h, for straight-winged, high-aspect ratio aircraft (best L/D will be faster for a swept, high-speed jet, but their L/D will not be this high). At this speed, props have by far the best efficiency, but given your power requirements (tens of horsepower, not thousands), you might not even want a turboprop for maximum fuel efficiency. A Wankel engine or a small piston engine might win out at these power levels (since gas turbine efficiency basically scales with size - the most efficient heat engines of any kind on the market, internal or external combustion, are 600MW Siemens gas turbines).
But then again, on a pure fuel economy perspective, we'd be better off not using aircraft at all. Get everything on a train, pipeline, or ship. And personal cars are right out. The whole conceptualization of of FedEx, when it was originated, was that the coincidence of small, high-value items (in particular, integrated circuits) being introduced in large volumes, and the introduction of lean-manufacturing that depend on minimum inventory levels and high flexibility in supply chains, would increase the value of time in transportation. Putting stuff in the air isn't about fuel efficiency. Its about time efficiency.
This cant be the most efficient way can it?
Which aspect are you asking about, specifically? This is a P-800, closely related to the Brahmos. For most of its flight it uses an air-breathing engine for thrust and uses aerodynamic control surfaces for control authority. This is essentially the most efficient way you can design a missile. However, the specific type of engine it uses is a ramjet, which has a minimum operating speed. Ramjets are super compact and are very reliable, because they have no moving parts, but to get up to that minimum operating speed (normally on the order of mach 3) you need a carrier rocket, and if you have no forward velocity, you have no aerodynamic forces to orient your rocket.
What you're seeing here is a cold-launch of its carrier rocket, and the initial stage of flight. Cold launch (where the missile is ejected from its launch tube by a separate system before its main engine is turned on) is not efficient, but it is safe. It guarantees that in the event of a rocket malfunction, the rocket will be removed from the carrier vehicle before the inevitable brown-pants moment comes. The carrier rocket for a ramjet is designed with minimum complexity to reduce cost, since most of the range and control will come from the aerodynamic payload. Solid fueled, and without a steerable nozzle - more akin to a fin- or spin-stabilized rocket than a guided missile, the carrier is *just* to get the payload up to speed before it's discarded. Without forward speed, the missile's aerodynamic control surfaces cannot control the direction of the missile immediately after ejection from the launch tube, and the carrier rocket does not have any thrust vectoring, so another method of orienting the vehicle needs to be used. Reaction-control jets ("RCS") are used instead, with several nozzles in the nose that can fire to orient the vehicle. But once the vehicle is going, the aerodynamic surfaces can activate, and the RCS is no longer needed. This is why the RCS system, on the nose, is ejected very shortly after launch once the system has some forward speed. This also uncovers the inlet of the ramjet.
(In the third view, you can actually see the RCS system in the nose eject itself shortly before forward flight is established, fall, and splash into the water)
So what I imagine you're thinking was the rocket's *entire* mode of flight is, in reality, just the ejection stage.
I was referring to the method of orienting it, the RCS.
It looks like the main engine/method of propulsion is firing shortly after leaving the tube, but youre saying it is not?
What "kind" of missile is this, and what are other ways of performing a cold launch and quickly orienting it? Most of the ones Ive seen are launched from a rotating pod. I guess that isnt ideal if you want to launch it quickly in any direction?
What about this tho?
The "main engine" you're talking about is the rocket engine that just gets the vehicle up to the sort of speeds where the ramjet can operate.
Vanilla Ice can tell you why. I'm wishing for some dumbed-down versions of Navier-Stokes equations for the mathematically illiterate.
Its possible to just shoot the missile straight up (or nearly straight up) until you have the forward speed required to aerodynamically control the missile, which then point the missile horizontally, yes. In that sense it's not so efficient. However, these are anti-ship missiles. Ships expensive, so they're protected by a lot of radars and radar-guided munitions specifically to thwart anti-ship missiles. As such, these missiles have to maintain a low altitude to remain below the horizon of their targets - getting too high means the enemy knows you're coming, so they have to re-orient the missile without forward motion before igniting the main engines (which come on shortly after leaving the tube, during the re-orientation manoeuvre).
If your missile can vector its thrust (eg: THAAD), you can just re-orient yourself with your main rocket. At low speeds this can be done at super-high accelerations. THAAD missiles do a little loopty-loop shortly after launch, for instance, if they need to burn off excess fuel early in their flight. I believe THAAD is hot-launched, though.
Your video is of a tomahawk. They used to, but no longer, have an anti-ship variant, and so its launch profile doesn't have to be so low-key. Ship-launched anti-ship missiles in general are not as low-key, as ships are (by their nature) pretty juicy radar targets with or without missiles flying off their decks.
What type of rotating pod are you thinking of?
There is no such thing as a dumbed-down version of Navier-Stokes. Only dumbed-up. Maximum dumb.
Do you mean anti-submarine munitions such as these: ?
The launch assembly doesn't rotate, though.