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Electric catapult

  1. Oct 8, 2007 #1
    Aircraft carriers have been catapulting off aircraft for many years now, so that technology is not strange. At the same time, magnetic propulsion is becoming more and more common for various purposes.

    So why are these two technologies not combined for commercial aircraft? Imagine a rail track a few kilometers long or so with a gentle slope in one end going a few hundred meters into the air. If you used magnets to accelerate aircraft down that rail track, you could have aircraft moving at cruise speeds a few hundred meters above the ground without using any fuel at all, meaning the total weight of the aircraft can be reduced, which in turn saves fuel throughout the entire trip.

    Apart from the fuel saving advantages, you also get noise reduction since aircraft will be high up and far away before increasing engine thrust. This means you could build this ramp very close to cities. If you let it rise into the air earlier, you could even let it go above small cities if space is a really big issue.

    I found this article:
    http://archives.cnn.com/2002/TECH/space/01/03/maglev.launches/index.html

    Five years ago, someone apparently believed we'd have aircraft launching like this now. But I haven't even heard it being suggested in mainstream media since then, despite all the talk about global warming and such.

    So why aren't we seeing this?
     
  2. jcsd
  3. Oct 8, 2007 #2

    FredGarvin

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    I know the next generation of aircraft carriers are going to magnetic rail launches in stead of steam. As far as why civil airlines haven't done this, I think it's another issue of cost per airport and the existing infrastructure. Plus, every aircraft flying would have to be retrofitted. Airlines would have to pick up that tab.
     
  4. Oct 9, 2007 #3
    I don't think it would make much sense from the technical or safety points of view either.

    While it is true that an airplane will use most fuel/second during takeoff, the takeoff lasts about a minute or so, and the rest of the flight goes on for hours. So fuel recovery from this point of view is not significant.

    Significant fuel economy could be achieved by doing away with high lift systems and other compromises with aerodynamics and weight for low speed handling, if the catapult would speed up the airplane far more than the present takeoff velocities. Unfortunately, airplane also has to land :) So no gain there either.

    As for avoiding noise by not spunning the engines to full throtle immediatelly, you really don't want to be at no-abort (or whatever it is called) takeoff velocity and find that an engine is failing.

    In summary, the catapult is rightly used only when there is not enough space to go around for regular takeoff, like on carrier decks. And airplanes made for such operation are less efficient that the land based, because they must have both better high lift systems and strengthened structure (i.e. higher mass) for catapulting/touchdown/arresting.

    --
    Chusslove Illich (Часлав Илић)
     
  5. Oct 9, 2007 #4
    Sure, take off is a small factor in total fuel consumption. But less fuel for take off means the fuel tanks can be smaller and the engines could also possibly be slightly lighter. These things reduce consumption throughout the entire flight.

    I don't think it would be such a big issue if the engines were to fail. Unlike a regular takeoff, you'd have lots of altitude and speed when you leave the rail. If a regular planes engines fail right after takeoff, it'll drop like a rock. In this case, you can glide to a landing strip. That's not perfectly safe, but it's better than dropping like a rock. Besides, the most obvious solution would be that you slowly increase engine thrust along the track. This would both test engines operation and make takeoff smoother without having to start with maximum thrust. I actually think this system would be safer than regular take offs.

    I'd also like to point out that this doesn't have to be anything like catapults on carriers. The stresses there are extreme. The reason for this is the very short runway. This doesn't have to be the case here. You could even make weaker aircraft structures with this system if you just made the track long enough. Since engines could also be made slightly weaker and lighter, they would also put less stress on the aircraft structure.

    I think this system works best almost everywhere. Bringing the fuel along is always inefficient.
     
  6. Oct 9, 2007 #5

    mgb_phys

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    The aircraft uses most fuel in take-off but most of that is used to climb through the thick lower atmosphere. The catapult would only save the amount used to accelrate along the runway. Since you need the engines running at full blast when you leave the runway you don't save much by accelerating the planes. You would probably save a lot more by using tugs to tow the aircraft out to the end of the runway. Using 4 jet engines to push a 747 along at walking pace is very inefficent - this is why Concorde always got priority on departures it couldn't afford the fuel idling on a taxiway because it's turbojets were so inefficent at low speed.

    If you've seen a Harrier jump jet taking off from a British carrier, they use both a catapult and a ski-jump. The last 10m of the flight deck slope upwards at 20degrees and give the plane extra lift, allowing a much shorter and slower catapult and lower stresses on the aircraft.
    Since you have to use many runways in both directions this probably wouldn't be practical.
     
  7. Oct 9, 2007 #6
    Actually, it could save much more than that. In its most simple version, it would only assist the aircraft slightly when going down the runway and that would not save much. But it could be built to launch aircraft just below mach 1 and climb them hundreds of meters above ground by making the jump at the end high.

    If the aircraft was super sonic, it could be accelerated to even higher speeds. As you say, the thick lower atmosphere is what consumes fuel, and by launching at very high speeds, you get part of that climb for free. There are obviously limits to how fast you can go at surface altitude, but because of the thickness of the atmosphere, it is always preferable to go through the lower altitudes as quickly as possible.
     
  8. Oct 9, 2007 #7

    mgb_phys

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    I see you mean to launch the plane with enough KE to reach 10,000ft, I was thinking of just launching it at take-off speed.
    If you could do that it would save a huge amount of fuel because the engines would only need to be rated for cruise not the initial climb.
     
  9. Oct 9, 2007 #8
    There are two problems with using this approach: (1) how many g's can Aunt Minnie tolerate; and (2) how big an installation are you willing to build (look at the size and complexity of a carrier launcher and scale that up to an Airbus. And, of course, it would not save much fuel, if any; it only diverts the energy production from the jet engine to a power plant somewhere else.
     
  10. Oct 9, 2007 #9

    mgb_phys

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    Practicalities aside ( people pay for high G on on theme park rides ;-)

    You would save fuel becuase the take off weight is lower, you can under rate the engines because they only need to handle cruise and they can be made more efficient because they don't have to be designed to handle low speed thick air and high speed thin air. It could also reduce airport noise since you don't have engines running at max power just above the houses.

    A similair thing is done to get heavy transports off from short runways - using rocket assisted takeoff. It should actually be more efficent to under-rate the jet engines and use rockets to boost an Airbus off the runway!
     
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