h a l f b a k e r yClearly this is a metaphor for something.
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I thought about this when I found the "linear accelerator to orbit" post.
I read once that taking off and getting to cruising altitude is the most energy-consuming part of an airplane ride, using up 1/3 of its fuel. So why not help them while still on the ground?
Aircraft carriers use steam-powered
catapults to shorten the take-off distance of military planes. I'm thinking of something larger: an electric-powered catapult, which would bring passenger and freight planes to their take-off speed. This would save maybe 5%-10% of the airplane's fuel, plus additional savings from the plane being a few tons lighter. Or, for the same amount of fuel, the aircraft could either go a longer distance, or be heavier.
The runway could also be slightly shorter (though I believe the length of a runway is tuned in to landings, not take-offs), saving real estate.
One downside is that planes would have to have stronger, heavier landing gear to take the stress of the pulling force.
(?) linear accelerator to orbit
linear_20accelerator_20to_20orbit
[piwoslaw, Aug 29 2008]
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The electric catapult is actually baked by the US Navy, though I don't think it is seeing use yet. Catapulting regular planes would take some significant redesign, as they were never designed for thoses stresses. As for fuel savings, I'm not sure because you'd still have to run the engines up to full thrust for the length of the takeoff, just to make sure you can fly after takeoff, and you would be limited by the number of Gs you could put the passengers thru, so I think sadly much of the fuel savings would be lost. |
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// you would be limited by the number of Gs you could put the passengers thru // |
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That would kind of add to the fun. Wouldn't it ? |
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I recently interviewed with the contractor (who shall remain unidentified) who has developed the replacement platform for the steam catapults on aircraft carriers. |
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Damn. After crafting an anno with the details, I have chosen not to post it in the interest of keeping the powers-that-be off my back. |
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Suffice it to say that the bakers will be able to figure out how it should work. |
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Issues: Building a structure that can
withstand the force of the launch
mechanism, and doesn't impair the
efficiency of the plane destroying your
gains. The weight penalty may totally
nix the idea. |
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An inability to cancel the takeoff in case
of engine trouble (prop + turbojet) |
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Most of the fuel is spent gaining
altitude not actually at the point of
taking off |
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Long runways would still be needed for
landings. |
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// Long runways would still be needed for landings. // |
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Not of you follow through on the idea and put arrestor hooks on the rear of civil aircraft. |
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I highly doubt that civil airplanes would be
able to dump fuel prior to landing and
even if the runway isn't in a metropolitan
area the consequences of missing the
ropes would be a pretty tough sell to the
airlines. It would greatly increase the
structural challenge of the thing too. |
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Regarding fuel use in take-off and climb: I'm guessing that pilots push the throttles wide open to take off, and leave them at full power until at cruise altitude. Getting off the runway takes about half a minute, climbing to 32,000 feet takes maybe ten minutes. That's just guessing on times, and throttles may be backed off once in the air to reduce noise or to do a cruise-climb. Still, it seems that a catapult would save maybe one-twentieth of the one-third mentioned, perhaps. |
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Idling on the taxiway takes fuel, too, by the way. There were times when long delays were cutting into fuel before the planes ever took off. |
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As I said in a discussion about boosting planes with wheel motors: You don't have to fire the thing out at neck- and plane-snapping accelerations. Just do a boost that uses all the available runway--the passengers will never notice the difference, and the plane won't be shedding parts. You may need a tougher tow-hook, yes. |
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Like the tow cable on a roller coster only solar powered. Great Idea!! |
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You could press on the rear of the engines
where the plane is designed to take thrust.
Next best place might be where the wings
meet the body on the belly with some
bracing. Still i agree with [bacon] that the
savings would be 1% or less even on very
short flights. Just as in a dragster the
highest G force is at launch but engine
output remains high throughout. |
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For smaller aircraft, such as gliders, similar principles are used. 'Winch' gliders drop the rope once they are up in the air. |
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I used to go gliding at a club that used a reverse tow.
The tow wire was hitched to the back of a pickup truck, taken around a huge pulley mounted on the back of a truck at one end of the runway and then hitched to the nose of the glider.
The pickup driver then drives TOWARDS the glider, and hopefully, before they meet in the middle, the glider will have left the ground.
This could work for larger aircraft, just turbocharge the pickup truck. |
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There is no conflict with cable landings. (catapult launched aircraft can land normally) This includes anything about dumping fuel. This would be helpful: Airplanes do not currently take off at maximum G forces, they are limited by their engines. Also, with the engines at full thrust the take-off becones much shorter, saving fuel. With electric catapults, take-offs are no less cancel-able. Long runways will still be necessary and gaining altitude will still be a problem.(unless you tilt the catapult.... now theres an idea...) |
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At Heathrow a while ago, I thought this might be a better solution to toeing the planes about. Putting them on rail tracks - maybe even using their wheels to sit on something rather than invent something new - could allow them to trundle happily and automatically around the airport and to the runway. Then for take off, knudge the speed up and away they go. |
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Taxiing takes up quite a bit of fuel; perhaps 1 to 2 %. At Heathrow where the planes sit for hours and hours it must use even more. |
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// Putting them on rail tracks // |
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The Wright Flyer took off from a rail. There's nothing new, eh ? |
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still not confident about the numbers here. the basic idea is sound but the benefits might be trivial. there are likely very good reasons to leave the engines idling rather than shutting them down and firing them up over and over and the plane would still need to launch at full output. A system that launched the plane with the belly resting in a channel could use a rail built into the unibody of the plane to apply some real force. |
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Back of envelope calculations, neglecting friction, air resistance, and other difficult matters. Assuming a Jumbo of 370x10**3 Kg , take off speed 80 m/s , cruising altitude 12x10**3 m , then it takes 1.9GJ to accellerate from rest to take off speed, and 43GJ to climb to cruising altitude (from sea level) |
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You want a low C footprint? Go in a balloon. |
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//43GJ to climb to cruising altitude (from sea level) //
But you get them back when you land.
//You want a low C footprint? Go in a balloon.//
But not a hot-air one. |
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You don't get it back when you land. It is all wastefully dissipated into the air during the descent, and ends up warming the atmospehere. |
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You do recover some during landing as distance traveled. That is, descent occurs with less thrust than level flight. |
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Also, 1.9 GJ of jet fuel is about 43 Kg, except that engines aren't really efficient, so call it triple that or 120 Kg. Not all that much, admittedly. |
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However, there is nothing that says a catapult launcher has to stop at take-off speed, say you allow it to accelerate up to three times that before initiating lift (full-length runway, slightly higher G's), it's not linear due to wind resistance, but call it 360 Kg of fuel saved. |
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I'm not saying it's a huge savings, but there might be some. |
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