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This would have the obvious benefits:

  • Less runway needed to take-off
  • Fuel savings
  • Faster turn around times for airlines

Could this be implemented in busy airports?

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    $\begingroup$ The first point "less runway" is arguable as the usual runway would be needed for landing, unless you use an arrestor device, which is likely a big challenge when carrying cargo or passengers. Also going from 0 to VR in less time means greater acceleration, what would be your limit? $\endgroup$ – mins Aug 29 '16 at 10:19
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    $\begingroup$ A friendly note: this a perfectly reasonable question. There's no need to vote a question down just because the answer might be no way or because you think it might be naïve - that's not how this system works. If you're in any doubt about the value of the question, look at the quality of the answers below. $\endgroup$ – Daniele Procida Aug 29 '16 at 15:49
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    $\begingroup$ This is, by the way, exactly how aircraft carriers work for takeoffs. $\endgroup$ – David Grinberg Aug 29 '16 at 19:39
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    $\begingroup$ @reirab: Agreed. But one should never say infeasible: Airbus Eco-Climb. "The ultimate, albeit it very extreme, concept is to have a system that not only launches but also captures the aircraft, removing the need for landing gear." $\endgroup$ – mins Aug 29 '16 at 22:39
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    $\begingroup$ @chrisH Delta Air Lines experimented with a variation of this - taxiing on only the left engine. It did indeed save fuel, but maintenance costs on the right engine went through the roof. Turns out the nice, long, low idle "warm up" of a taxi is good for the engine. They went back to dual engine taxi and right engine maintenance levelled back out to match the left. $\endgroup$ – Scott Baker Aug 30 '16 at 21:15
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Let's see what the savings are:

A mid-sized airliner carries maybe 20% of its mass in fuel. This fuel has an energy density of 43 MJ per kg. Of that chemical energy at most 40% is converted into useable work. Heck, let's make this 25% so we are really conservative. Thus, the energy for the whole trip is $$E_{\text{trip}} = 0.2 \cdot 0.25 \cdot 43,000,000 \, \frac{\mathrm{J}}{\mathrm{kg}} \cdot \text{mass} = 2,150,000 \, \frac{\mathrm{J}}{\mathrm{kg}} \cdot \text{mass}$$

Now assume that this airliner saves the energy to accelerate from 0 to 150 knots by using a catapult. This energy is $$E_{\text{accel}} = \frac{v_{\text{takeoff}}^2 - v_0^2}{2} \cdot \text{mass} = 2,977.35 \, \frac{\mathrm{m}^2}{\mathrm{s}^2} \cdot \text{mass}$$

Since I picked metric units, conversion is easy: $1 \, \mathrm{J} = 1 \, \mathrm{Ws} = 1 \, \frac{\mathrm{kg} \cdot \mathrm{m}^2}{\mathrm{s}^2}$. I use $\text{mass}$ for the take-off mass so you don't think it is the unit meter. Now let's put that into proportion: $$\frac{E_{\text{accel}}}{E_{\text{trip}}} = 0.001385$$

Using the catapult saves 0.1385% of the energy needed to fly a typical airliner trip, assuming the same efficiencies during acceleration as during the flight. If we take into account that jet engines are most efficient during cruise, let's double the fuel need for acceleration and make it 0.277%. Granted, it is more for short-range flights, but still insignificant to what is needed to move the aircraft 10 km up into the sky and then for a couple of hundred miles through the air at Mach 0.8. In terms of fuel mass, these 0.277% are taken from 20% of the take-off mass. So the fuel needed to accelerate to v$_0$ is 0,000554 times take-off mass.

To make a catapult launch feasible, you need to add some strength to the nose gear and the forward fuselage. The typical landing gear fraction of the take-off mass is about 3%, and the nose gear is 10% - 15% of that, so $m_{\text{nosegear}} = 0.00375 \cdot \text{mass}$. Relative to the nose gear mass, the fuel saving from using a catapult launch is $\frac{0.000554}{0.00375} = 0.0148$ or 15% of the nose gear mass. Thus, the reinforcements need to add less than 15% to the mass of the nose gear.

If we assume an acceleration of ½ g = 4.903 m/s², the take-off run to accelerate to 150 kts is 607 m. I expect that even this moderate acceleration (which requires a pulling force of half of the lift at take-off) would translate into much higher mass increases than those 15% of the nose gear mass.

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    $\begingroup$ You forgot to take into account (which would make the concept even less interesting) that a catapult capable aircraft needs to be built much stronger. So heavier wing and fuselage spars, heavier landing gear, etc. etc. End result is an aircraft that, to allow it to need less runway for taking off (runway that's needed anyway for landing) will be much heavier and thus require a lot more fuel while flying. $\endgroup$ – jwenting Aug 29 '16 at 13:26
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    $\begingroup$ You forgot to factor in the fact that 99.9% (rough guestimate) of paying passengers would not like the massive acceleration of being shot off the ground by a catapult. While the lines at Cedar Point for the Top Thrill Dragster ride are long, they are not the longest at the park by any means. $\endgroup$ – FreeMan Aug 29 '16 at 13:42
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    $\begingroup$ @FreeMan: Who says you need to do the acceleration within a few meters? If the launch stretches over 2km, the acceleration would be just as today, and nobody would mind. $\endgroup$ – Peter Kämpf Aug 29 '16 at 15:29
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    $\begingroup$ I made the leap from the "less runway" benefit to "aircraft carrier deck length runway". Maybe I took it to the extreme... ;) $\endgroup$ – FreeMan Aug 29 '16 at 17:12
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    $\begingroup$ Also, in addition to everything else, add in the cost to the airport to install, operate, and maintain the catapult - and also the added fees, surely, to whatever airline wanted to make use of it. $\endgroup$ – J... Aug 29 '16 at 23:10
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It would be possible from an engineering point of view to design some sort of ground-assisted take-off launch mechanism for airliners, even though any of the benefits you describe would be greatly outweighed by new disadvantages, as outlined in other answers.

There's one new point though that I think is worth making. The shorter take-off roll that it would mean is not in itself an advantage, but a disadvantage.

The time spent gathering speed on the runway with the engines at full power is valuable. It's a chance to ensure that they, and the rest of the aircraft's systems, are operating correctly. If there's a failure such as a loss of power or a sudden drop in hydraulic pressure, the take-off roll is a good place for it to happen, because it gives the crew an opportunity to abort it safely.

That opportunity would be lost in an assisted take-off.

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    $\begingroup$ Dunno 'bout that. At least in the past, the captain was supposed to rev up to 105% with the brakes locked to verify all those systems, then rev down, release brakes, and take off. $\endgroup$ – Carl Witthoft Aug 29 '16 at 19:45
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    $\begingroup$ @CarlWitthoft I've never had that happen on any flight I've ever been on. $\endgroup$ – reirab Aug 29 '16 at 21:21
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    $\begingroup$ If given a choice, I would rather do final checks on the engines and systems while the aircraft was still at a stop on the ground, instead of in the takeoff roll. You don't have as much ability to be cognizant of the check due to the increased workload of managing a moving aircraft. It would be simpler to run through the final checks, then do the cat shot, similar to carrier launches. $\endgroup$ – Carlo Felicione Aug 30 '16 at 1:24
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    $\begingroup$ @CarloFelicione without doubt, but we're talking here by definition about problems that become apparent during the take-off roll, rather than ones that would have been discovered during pre-flight checks. Sometimes, all the pre-flight checks don't reveal a problem that does manifest itself during the roll. It's not very often, but it does happen. $\endgroup$ – Daniele Procida Aug 30 '16 at 6:49
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You all are missing the most obvious answer: You could, but aside from a few young adrenaline junkies no one else would want to ride it.

The only real purpose for a catapult assisted takeoff is to provide and aircraft with a quick acceleration to Vr and beyond off of a short airfield. As virtually all airports used for major commercial operations have runways of at least 1 mile in length or more, there is not an infrastructure crisis which would dictate a need for this.

If you did have an airfield which was so small that it required a CATO launch to get the jets airborne you also face the task of landing them in a small space as well. This would require the field be equipped with arresting gear as well.

As pointed out above no existing airliners are designed to launch and recover using these systems so, even with the investment in a CATOBAR infrastructure for an airfield, no for-profit air carrier could use it. And it offers virtually no fuel savings for the airlines.

And let's not forget human factors here: if we take CATOBAR operations from military aircraft carriers as a yardstick for performance, a cat shot imposes a 2-2.5 G acceleration load upon the aircraft during the launch stroke and a 2-2.5 G deceleration during an arrested landing. While I'm sure a twenty-something adrenaline junkie will get a thrill out if it, it will be an unpleasant experience for most people and quite dangerous for the elderly, the infirmed, pregnant women, etc.

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Keep in mind that unlike military aircraft, civilian aircraft are designed for comfort and economy.

Getting any kind of typical airliner retrofitted for catapult use would mean reinforcing the aircraft frame and structure (thus, possibly increasing weight); reinforced or otherwise strengthened wings and engine mounts, possible modifications to the wheels and landing gear -- all this adds weight, which means cost as more fuel would be required (or, less passengers could be carried) thus eating away at any possible savings.

Lets not forget having the runways modified (further costs) and the inevitable delays from runway closures, and the additional delays as the catapult mechanism has to be "reset" after each takeoff.

Not to mention that passengers don't really like the fact that they are jolted about during mild turbulence - just imagine how popular you'll be if you are going to shoot them off like a slingshot.

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  • $\begingroup$ This is a major point: naval aircraft have a much heavier structure than their land-based counterparts. An F-14 weighs 7 tons more (empty) than an F-15, although it has similar size and only slightly greater payload. $\endgroup$ – Hobbes Nov 3 '17 at 10:46
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We can't make it since there is 0 commercial aircraft designed with catapults in mind.

Your assumption that it would lead to cost saving is wrong, on many levels. The main one would be that:

Takeoff roll (the part where catapults can act) lasts only a handful of seconds.

Moreover:

  1. You need full power for climb, so you can't step down on the gaz
  2. You can't accelerate much faster due to stress to the airframe and passengers

The catapult would help you accelerating from 0 to V1, but they can't do anything to help you climb to cruise level.

Adding a new system would be very costy, impractical, and only save you a couple of seconds of acceleration.

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    $\begingroup$ Also no way to abort takeoff if something goes wrong with aircraft or cable $\endgroup$ – TomMcW Aug 29 '16 at 10:59
  • $\begingroup$ @TomMcW of course you have the arresting hook at the end of the runaway :p $\endgroup$ – Antzi Aug 29 '16 at 15:07
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    $\begingroup$ @TomMcW You could design a catapult such that you can abort the catapult launch at the same speed in the takeoff roll that you can with a standard takeoff - by commanding a stop to the catapult, which, with active braking, could respond faster and with more retarding force than spooling down the jet engines. $\endgroup$ – Johnny Aug 30 '16 at 5:55
  • $\begingroup$ This answer ignored the weight penalty of making the airliner strong enough to handle this launch over a long period of time. (It is otherwise sensible) $\endgroup$ – KorvinStarmast Nov 4 '17 at 17:15
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If there really was a net economic or safety benefit, they would already be in use. Catapults have been around long enough to be a proven technology where used.

Consider that some airlines have altered their paint schemes because it would shave enough weight from the aircraft to save a meaningful amount of fuel, or add extra payload capacity. If they have looked into and implemented things like that, I'm sure catapults would not have gone overlooked for this long.

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The core of it comes down to this for me.

It takes x energy to get from 0 to climb speed. If that energy comes from "the ground" via a catapult or from the engines, there is no real savings. You still have to spend the energy. The only savings may be in cost of fuel to generate that energy. However, even if the savings in fuel costs are extreme, the total energy spent getting to a cruising altitude is minor compared to the energy spent keeping an aircraft up there. The cost of maintaining the catapult, would likely out weigh the cost of the "extra" fuel needed to have the engines produce the climbing energy.

Remember catapults on air craft carriers are not used because the cost savings, there used because there is no other way to get an aircraft up to speed on that short of runway. As VTOL becomes more popular, the catapults are used less and less.

Many Navies have gone to STOBAR or STOVL systems entirely. The current trends seem to be "find me something besides a catapult to get this thing in the air", even when that is at the cost of flexibility.

So to run down your points:

  • Less Runway : Nope, still gotta land, and even if this were true, very few locations are so tight that runways can't be extended. It may be expensive to do so, but hell, Japan (I think) build a whole new island to hold their airport.

  • Fuel Savings : Maybe. If you use a steam catapult and make the steam with coal, and the cost difference per unit of work between coal and jet fuel was enough, then yes, there could be a fuel savings. However it would almost certainly get offset by maintenance costs.

  • Faster turn around : No! It takes time to charge that catapult. It's not instant. You can't just launch a second aircraft as soon as the first one clears the runway. The catapult has to be adjusted, charged, and then fired. In military operations, you can only launch X number of craft. So it's possible that a military catapult could be charged for the entire launch. An airport, however is continuous. So there will be time when charging is needed. There are ways around this, like "side loading" from two sources, so that as one is depleted, the other is charging. But this would increase cost and complexity even more.

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  • $\begingroup$ Just for the record you can actually launch as many aircraft as the runway is long. One right after the other. I am ignoring all the rules about minimum spacing and such. $\endgroup$ – coteyr Aug 30 '16 at 13:25
  • $\begingroup$ VTOL looks nice, but it's not necessarily all that. The F-35B is still not 100% working. VTOL also has the disadvantage that the jet exhaust is hitting the deck directly, and is hot enough to melt steel (and runways, which is an issue for the USMC who intend to operate them ashore). The UK currently has two very expensive aircraft carriers but no aircraft for them, and the USMC have limited air support, because they chose to retire their Harriers before the F-35B was working. $\endgroup$ – Graham Aug 30 '16 at 17:07
  • $\begingroup$ @Graham, yep, that's all true, but the trend is still away from the catapult. So much so, as you point out that the British navy doesn't seem to have anything to use on it's newest carrier. Though I'm pretty sure navies have STOBAR working. $\endgroup$ – coteyr Aug 30 '16 at 17:27
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    $\begingroup$ "It takes x energy to get from 0 to climb speed. If that energy comes from "the ground" via a catapult or from the engines, there is no real savings." As a ground propulsion system jet engines are horrifically inefficient. Far more energy goes into the air pushed backwards than into the plane pushed forwards. $\endgroup$ – Peter Green Aug 30 '16 at 20:55
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    $\begingroup$ Don't forget VTOL requires a great deal more thrust than traditional takeoff. Horizontal takeoff allows the craft to accelerate along the runway even if thrust is less than gravity, since it is directed perpendicular to gravity. Therefore it can accelerate until wing lift overcomes gravity. VTOL on the other hand must have enough thrust to overcome gravity. Bottom line, horizontal takeoff only requires enough thrust to overcome rolling/air resistance(and enough runway to reach takeoff speed), but VTOL requires enough thrust to overcome gravity which is much greater. $\endgroup$ – AaronLS Aug 31 '16 at 7:44
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So far all the answers seem to have focused on a short launch, flat catapult, much like you would see on the deck of an aircraft carrier. This does have flaws as listed by many answers. However, what if it were a ramp? Could be a standard, slightly elevated flat surface or it could be a curved ramp that changes angle exponentially. Let's say we are retro-fitting an existing airport. That gives us up to one mile of horizontal we could use for our catapult run.

Advantages:

  • Can accelerate more slowly but over a longer period, allowing a smoother ride for both the aircraft and the passengers.
  • Depending on whether it is a flat or curved ramp, can launch up to a few hundred feet into the sky with a decent AOA.
  • Launch speeds can be near the aircraft's maximum speed rather than just over what is needed to get it into the air, saving on fuel as it is easier to maintain a speed than to accelerate up to one.
  • Landings can only be in one direction but with gravity to assist breaking, allows for greater deceleration and hopefully decreases time from runway to terminal.
  • Less likely to clip the tail as you land as the ramp's angle will make the tail proportionately higher.

Disadvantages:

  • Your aircraft might fall off/get blown off the side of the ramp during landing/taking off.
  • Non-catapult converted aircraft might not be able to take off from your airfield once they had landed (depends on ramp and aircraft type as well as prevailing winds).
  • Mentioned earlier but landings pretty much end up as a mono-directional affair, so if the wind is against you, you will have to redirect.

Mitigation:

  • Build the ramp near the existing runway but not on top of it. As the end of the ramp is pretty high, other structures such as hangars could be incorporated under it. This would save on space and allow regular/unassisted aircraft take-offs and landings on the regular runway but might have serious repercussions should an aircraft fall off the side.
  • Given the length we have for the ramp, we could use some kind of high speed tractor or pushback instead of a dedicated catapult which would reduce reload/calibration times - same vehicle tows the aircraft to the runway then drives very quickly with it (ok, some handwaving going on here but multiple roving "catapult units" is an idea only needing a solution).
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According to this site, a B747 uses 5,700 lbs for the take-off, out of 422,000 lbs max. fuel. That is 1.35% of fuel for a long range airliner, percentage for a shorter range airliner would be higher. The catapult or electric tow line would now have to deliver the take-off energy - if powered by electricity, the efficiency is much higher. A factor of over 2 is gained from not accelerating air but the aircraft itself, and combined cycle power is much more efficient than a single gas turbine. From wikipedia:

By combining these multiple streams of work upon a single mechanical shaft turning an electric generator, the overall net efficiency of the system may be increased by 50–60%. That is, from an overall efficiency of say 34% (in a single cycle) to possibly an overall efficiency of 62.22% (in a mechanical combination of two cycles) in net Carnot thermodynamic efficiency. This can be done because heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%). In an ordinary (non combined cycle) heat engine the remaining heat (e.g., hot exhaust fumes) from combustion is generally wasted.

Also, Airbus is making a case for this. Maximum thrust is only used at take-off, an assisted take-off would mean lighter engines with associated lower fuel burn. From the article:

Listen to the changing sound of engines during flight and it’s obvious: an aircraft draws on its power reserves more during takeoff than at any other time. The power needed to take off is determined based on a number of factors - including runway length, wind speed, temperature, and the weight of the aircraft itself.

However, this takeoff power only is required for a very brief portion of the total flight. Once cruising in the sky overhead, an aircraft doesn’t need as much to maintain altitude. So why not source the energy required at takeoff from an innovation installed on the ground? Can the burden (and weight) be removed from the aircraft itself?

enter image description here

An assisted takeoff – using some form of propelled acceleration – would mean aircraft could be lighter, with smaller engines consuming less fuel.

So there would be saving in fuel, additional to the fuel saved for take-off.

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    $\begingroup$ For a "go around" or a "wave off" are you sure you want a weaker engine? As your (prospective) passenger, I surely don't. $\endgroup$ – KorvinStarmast Nov 4 '17 at 17:17
  • $\begingroup$ @KorvinStarmast It's the manufacturer of one of the most succesful airliners publishing this. They consider all factors, including the two you mention, in selecting the power for the engine. $\endgroup$ – Koyovis Nov 4 '17 at 23:34

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