Could airliners use a steam or electric powered catapult for take-off?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

Yes there would be fuel savings, from multiple sources:

• Efficiency gain from the ground based launcher.
• Weight saving from smaller engine size.

Efficiency gain. 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.

So in an aeroplane, air is accelerated by a thermodynamic process with 35% efficiency. When launched by a catapult, the aircraft is accelerated from a thermodynamic process with over 60% efficiency. Total efficiency gain is the factor 2 reported above, times 60/35 = 3.4 times higher. Leading to 5,700/3.4 = 1,700 lbs required for catapult launch take-off. A potential saving of 4,000 lbs at every take-off of a B747. Of course, a lot of this is negated by having the engines running at RPMs that allow for a climb right after take-off, but even a saving of 1,000 lbs/heavy aeroplane would provide an incredible yearly saving at a busy airport like O'Hare.

Weight savings. Airbus is making a case for this (now behind an authorisation wall). 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?

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.

Answer 6

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.

Answer 7

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.

Answer 8

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.

Answer 9

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).