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Airliner engines are sized to continue the takeoff if one engine fails after it has become too late to brake. This means that the oversized engine has to be lugged around the whole flight, increasing weight and drag. Creating traditional brakes that could withstand the heating of a rejected takeoff just before rotation would also add significant weight.

The brakes on the SKYLON spaceplane boil water during abort, and it appears that the steam is dumped. The water is fed by pyrotechnic blowdown. Following a successful takeoff, the water is dumped overboard. This saves weight for the remainder of the flight.

My question is, can this approach be used to save weight on airliners?

SKYLON info. https://web.archive.org/web/20151129034506/http://www.reactionengines.co.uk/tech_docs/SKYLON_Users_Manual_Rev_2.1.pdf https://web.archive.org/web/20110615104534/http://www.reactionengines.co.uk/tech_docs/JBIS_v57_22-32.pdf https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=36826.0;attach=1073534

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    $\begingroup$ How does water cooling solve the primary braking problem, friction (or lack thereof) between tires and runway? $\endgroup$
    – jamesqf
    Apr 28 '20 at 15:57
  • $\begingroup$ it's about temperature $\endgroup$
    – Abdullah
    Apr 28 '20 at 16:11
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    $\begingroup$ @jamesqf have you ever seen footage of an emergency brake test on an airliner? Typically, by the time the plane stops the brakes are on fire and the tires have exploded. $\endgroup$
    – Hobbes
    Apr 28 '20 at 17:11
  • $\begingroup$ I wonder what this does to the brakes. If it does more damage than overheating the brakes, we might as well let the brakes overheat. I'd say it's an OK deal to let the brake burn its entire service life by one RTO. $\endgroup$ Apr 28 '20 at 18:01
  • $\begingroup$ But a fire.........? $\endgroup$
    – Abdullah
    Apr 28 '20 at 18:19
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This solves the problem of rejected takeoff, which is certainly a major factor in brake design. However, dumping the water after takeoff means that benefit is unavailable for landing. Events such as flap issues can require an aircraft to land at a higher speed than usual, requiring more braking capacity. The difference between this and rejected takeoff may not be enough to justify the weight, complexity, and expense of the water cooling system.

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  • $\begingroup$ A flap issue could surely allow the full runway to be used, as opposed to a high-speed rejected takeoff surely? $\endgroup$
    – Abdullah
    Apr 28 '20 at 16:17
  • $\begingroup$ @ABJX you have the full runway, but the speed is higher, and kinetic energy increases with the square of the speed. $\endgroup$
    – fooot
    Apr 28 '20 at 16:21
  • $\begingroup$ So the brakes heat up to rejected-takeoff levels anyway? $\endgroup$
    – Abdullah
    Apr 28 '20 at 16:23
  • $\begingroup$ @ABJX it all depends on the specifics. Often it's at the end of the flight where much of the fuel has been burned, which helps, but it could also happen right after takeoff where the aircraft is still heavier. $\endgroup$
    – fooot
    Apr 28 '20 at 16:39
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I'm sure it could be used, the question is what problem it would solve that would make it worth doing. Dumping water for each take-off means more stuff that could break on the aircraft itself, and more stuff that requires maintenance every now and then.

Having to refill the airliner with water prior to each takeoff would require extra infrastructure on the ground; if water-cooled brakes are critical for operating the aircraft, you probably wouldn't be allowed to use just any water. Distilled water refilled using a documented process would probably be a must, adding time and making the whole thing more expensive to operate.

Airliners routinely operate in cold weather, so the ground supply and aircraft mechanisms have to account for that. No point having efficient brakes if they are frozen solid as a result of having to wait 10-15 minutes extra prior to takeoff. You can of course install heaters, but that adds additional costs and complexity.

All this stuff would mean nothing for anyone traveling with the airline. Compared to airlines using conventionally braked aircraft tickets are likely to be more expensive, driving people to use the cheaper non-water-cooled airlines.

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  • $\begingroup$ From what I hear, airliner engines are sized to continue takeoff if one engine fails when the plane is moving too fast. This is especially bad for today's twinjets. And that is why I was thinking of a lightweight high-power braking solution to allow braking instead of needing overpowered engines. $\endgroup$
    – Abdullah
    Apr 28 '20 at 16:15
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    $\begingroup$ More powerful brakes don't allow you to get rid of engine-out redundancy. If the engine fails just one foot off the ground, you can have the most powerful brakes in the world, they won't help you one bit. $\endgroup$ Apr 29 '20 at 9:06
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Of course, they could technically use water-cooled brakes, but there are downsides. For example, airports don't want water spilt all over the ground just because one airplane had some hot brakes. It's just more efficient to taxi around and wait for the brakes to cool down. Plus, it would be too expensive to install a whole water brake-cooling system, for something that just doesn't happen that often.

So to sum it up,

  1. It interrupts airport operations

  2. It's too expensive

BONUS: The airline manufacturer would have to get permission for the technology

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  • $\begingroup$ Would it be more expensive then needing additional engine power to continue the takeoff? $\endgroup$
    – Abdullah
    Apr 28 '20 at 16:18
  • $\begingroup$ @ABJX it sounds like this comment (which you've made on 2 answers) is really the crux of your question. Possibly this is what you should have asked in the first place. $\endgroup$
    – FreeMan
    Apr 30 '20 at 16:58
  • $\begingroup$ All right, will add to the question. $\endgroup$
    – Abdullah
    Apr 30 '20 at 17:17

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