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On aircraft carriers there is a catapult that slingshots aircraft so that they can gain lift on the short carrier deck.

I have a few related questions - the first is what is the force exerted / required by the catapult to sufficiently accelerate the aircraft; and is it adjusted per aircraft type or a uniform value?

I imagine carrier takeoffs/landing cause additional stress on the aircraft; but does it have a dramatic effect on the lifetime of the aircraft vs. their non-carrier configured twins? Or is it compensated for by a more frequent maintenance schedule?

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    $\begingroup$ I do know that they have to adjust the force for each plane as it moves to the head of the queue, but I haven't the foggiest how much it is beyond a lot ™. $\endgroup$ – FreeMan Feb 9 '16 at 17:06
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    $\begingroup$ It is absolutely adjusted for each type of aircraft, just like the arresting gear. The other part of your question about how much force depends on the weight of the aircraft and the speed required at the end of the launch. F=ma after that. $\endgroup$ – Ron Beyer Feb 9 '16 at 17:06
  • $\begingroup$ Total force or impulse force? $\endgroup$ – SMS von der Tann Feb 9 '16 at 17:18
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    $\begingroup$ @SMSvonderTann, force is force. Impulse is something else (force times time, equal to change in momentum). $\endgroup$ – Jan Hudec Feb 9 '16 at 20:31
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    $\begingroup$ From Can carrier-based aircraft takeoff while the carrier is stationary? conventional: 95 MJ, EMALS: 122 MJ. Also China building two aircraft carriers (EMALS): 120 to 140 MJ to launch an aircraft. $\endgroup$ – mins Feb 9 '16 at 22:03
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The force applied by the steam catapult is adjusted according to the aircraft T/O weight (by extension, the type). The catapult has to accelerate the aicraft to some airspeed at the end of the launch procedure. This required force depends on the aircraft mass. From USN T-45 Flight training instruction:

Aircrew should pay particular attention to the A-sheet’s basic weight, fuel and store loads to ensure the gross weight calculation is correct. This is particularly important when launching from the boat because the catapult needs to be set correctly.... Taxiing up to the catapult, a green shirt will hold up the weight board. If the weight on the board matches the weight on the weight chit, acknowledge ...

In case of steam catapults, the force, once set, is fixed; one advantage given for the Electromagnetic Aircraft Launch System (EMALS) is that the force could be adjusted by the system to keep the speed near the requirement. From airspacemag:

The amount of steam needed to launch an airplane depends on the craft’s weight, and once a launch has begun, adjustments cannot be made:... The launch control system for electromagnetic catapults, on the other hand, will know what speed an aircraft should have at any point during the launch sequence, and can make adjustments during the process to ensure that an aircraft will be within 3 mph of the desired takeoff speed.

Carrier launches and recovery is one of the most stressful things the aircraft can undergo; however, the aircraft are designed to handle these, for example, via a strengthened landing gear and fuselage. Due to these, carrier aircraft have lifetimes comparable to that of land based aircraft.

Carrier (in general, naval) environment is hostile to aircraft and as such, the maintenance schedule is different (and more extensive, for example checks for corrosion) in case of aircraft.

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    $\begingroup$ So essentially EMALS can fix incorrect weight setting. $\endgroup$ – Jan Hudec Feb 10 '16 at 7:23
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    $\begingroup$ Huh... I thought the only large boats in the U.S. Navy were submarines. I didn't realize they referred to aircraft carriers as boats. $\endgroup$ – RockPaperLizard Feb 10 '16 at 11:07
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    $\begingroup$ I was in the Navy. Technically, vessels over a certain size are "ships", and under that size are "boats". But in conversation, they're all "boats"... "I'm drunk, ready to head back to the boat?" $\endgroup$ – Steve Feb 10 '16 at 14:30
  • $\begingroup$ @JanHudec, not necessarily. A heavier aircraft needs more speed to get airborne. If the wrong weight value is entered in the computer, the launch will happen at the speed calculated for that weight, which might not be enough speed for the actual weight. Unless it can calculate the acceleration expected based on power input and realize that it's not accelerating fast enough, then increase the power until it is. $\endgroup$ – FreeMan Dec 8 '16 at 13:36
  • $\begingroup$ @FreeMan, for different types, weight and take-off speed are only loosely related as wing loading is what determines the speed. But as long as it has the correct type, it could adjust the speed for the sensed weight, or simply always launch at the speed corresponding to MTOW. Or maybe it could go the other way and abort the launch if it senses the weight too different from what was set—something is wrong, so stop while possible and let the crew fix the problem whatever it is. $\endgroup$ – Jan Hudec Dec 8 '16 at 19:54
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Takeoff

Steam/power settings are adjusted for each a/c type and T/O weight.

The EMALS stores 484 MJ in four 121 MJ alternators spinning at 6400 rpm. It delivers up to 122 MJ over 91 m. That averages out to 300,000 lbf. EMALS more finely controls launch forces (Max Peak-to-Mean Tow Force Ratio = 1.05), allowing it to launch smaller a/c (eg, smaller UAVs) and delivering a smoother ride that reduces airframe fatigue.

Current steam catapults deliver up to 95 MJ over 94 m. Each shot consumes up to 614 kg of steam piped from the reactor (NB: not the primary coolant loop). That averages out to 230,000 lbf.

Accelerations average around 3 g's, peak around 4 g's.

Landing

Landings are stressful (notice the fuselage skin wrinkling beneath the radome on this Hawkeye):

enter image description here (Source: DoD photo by: PHAN KRISTOPHER WILSON, USN Date Shot: 11 Jan 2005.)

An F/A-18 touches down around 720 fpm (12 ft/s). It's rated to twice that. CTOL fighters typically do about half that. I believe airliners average under 200 fpm (3 ft/s).

Here an F/A-18 is dropped from 20ft (36 ft/s, 2200 fpm):

Service life

NATO fighters are typically designed for 30 year service lives (6,000 to 8,000 flight hours). Historically, Soviet/Russian fighters fly much less. The Su-27SK is rated for 2,000 flight hours over 20 years. [See table for cost and service life comparison.] F/A-18's were built for 20 year service lives, 30 years with service life extensions.

The fuselage and landing gear are much stronger to withstand carrier landings and launches. The nose gear regularly transfers 4x the takeoff weight of the a/c into the rest of the fuselage.

Compare the F-35A's CTOL nose gear: enter image description here (Source: USAF)

...to the F-35C's CATOBAR nose gear: enter image description here (Source: USN)


Here's an excerpt from a Naval Air Warfare Center paper on EMALS:

Other drawbacks to the steam catapult include a high volume of 1133 m3 , and a weight of 486 metric tons. Most of this is top-side weight that adversely impacts the ship's stability and righting moment. The large volume allocated to the steam catapult occupies "prime" real estate on the carrier. The steam catapults are also highly maintenance intensive, inefficient (4-6%), and their availability is low. Another major disadvantage is the present operational energy limit of the steam catapult, approximately 95 MJ. The need for higher payload energies will push the steam catapult to be a bigger, bulkier, and more complex system.

Endspeed:      28-103 m/s [54-200 knots]
Cycle time:    45 seconds
Weight       < 225,000 kg
Volume       < 425 m 3

At max speed, the output of one of the disk alternators would be 81.6 MW into a matched load... These magnets have a residual induction of 1.05 T at 40 oC and create an average working air gap flux density of 0.976 T, with tooth flux densities approaching 1.7 T... Maximum output voltage is 1700 V (L-L) peak and current is 6400 A peak per phase. The disk alternator's overall efficiency is 89.3%, with total losses of 127 KW per alternator. This heat transfers out of the disk alternator through a cold plate on the outside of each stator. The coolant is a WEG mixture with a flow rate of 151 liters/minute. The average temperature of the copper is 84oC, while the back iron temperature is 61oC.


Sources:

  1. EMALS/ AAG: Electro-Magnetic Launch & Recovery for Carriers
  2. Electromagnetic Aircraft Launch System - EMALS
  3. Steam catapult types
  4. Electromagnetic Aircraft Launch System - EMALS - NAWC
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Catapults powered by steam from the carrier's engines, the amount of pressure is set per aircraft according to its weight and how fast it needs to be at the end of the deck. If too little steam power is set the airplane will not get up to speed and will end up in the ocean, too much and the airplane can be damaged by the excess force.

Carrier aircraft have to be much tougher than their non-carrier cousins, and have to be specially designed for those kinds of operations. The whole structure has to be strong enough for repeated carrier landings and takeoffs, and also it has to be able to fit on the elevator, so have folding wings of some sort. They are generally so well built that they last as long as their non-carrier counterparts.

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  • $\begingroup$ Newer carrier aircrafts use catapults powered by electro-magnets. However, I don't know if they are already in use or still getting developed and tested. $\endgroup$ – jklingler Feb 9 '16 at 18:58
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    $\begingroup$ The F-35 is a good example: a family of three airplanes built around a common core design. Why three airplanes instead of two (normal take-off / vertical take-off)? Because the naval version not only needs an arresting hook but also a strengthened fuselage precisely because of the catapult and arresting stresses. $\endgroup$ – Jörg W Mittag Feb 9 '16 at 20:13
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    $\begingroup$ @jklingler: the USS Gerald R. Ford, the first of the newest class of US aircraft carriers will be the first aircraft carrier to use the Electromagnetic Aircraft Launch System (EMALS) which generates 484MJ within 2–3 seconds, enough to accelerate a 45000kg aircraft to 130kt in just 91m. $\endgroup$ – Jörg W Mittag Feb 9 '16 at 20:19
  • $\begingroup$ @JörgWMittag Do you know how many MJ the steam powered catapult generates? $\endgroup$ – jklingler Feb 9 '16 at 20:28
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    $\begingroup$ @jklingler: I guess the most powerful one would be USS George H.W. Bush's, but the article doesn't list any numbers. The best I could find was 80,000 pounds (36,000 kg) @ 140 knots over 94m. $\endgroup$ – Jörg W Mittag Feb 9 '16 at 20:56

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