How do Aircraft Carrier Steam Catapults work?

For my first post, I want to ask a good question. I’m embarrassed to ask it, working for the US Navy (civilian), and having taken a guest cruise on a carrier, but steam catapults mystify me.

As you know, the damn things throw fully loaded jets off the ship by propelling a piston down a cylinder in the deck which pulls a harness on the fwd landing gear. My problem is that I don’t see how the hook that pulls the harness can stick out of the cylinder without some sort of groove or slot for it to slide through, which of course would let out all the steam pressure. I’ve even asked a few carrier deck sailors and got conflicting answers. Believe me, the answer is nowhere on the web either.

Any mop jockeys out there?

** Dragline**, welcome to the board. That is a good question, if only because I have asked myself the same thing before. Maybe there’s a zipper which is being opened as the hook advances and closed behind it? :wink: Just kidding.

And while we wait for someone who has the answer… why is it called “landing gear” when the airplane is taking off? i would say it is the “taking off” gear. I have a friend who insists on calling an elevator a “descender” when it is going down and in Spanish there are two interchangeable words for screwdriver, one would translate as “screwer” and the other as “unscrewer”. Anyway, let’s see who can tell us the right answer to your question.

There is a groove through the length of the catapult’s track on the flight deck. The slide rides on top of the catapult track and extends through the groove so the piston can move the slide the length of the track. On occasion, you can watch the maintenance on different parts of the catapult: The piston, the slide, the rubber that runs along the length of both sides of the catapult, and even the long “bowling lane” as I called it in which is under the groove.

The pressurized steam powers the catapult. The nosegear of the aircraft is positioned on the slide and the catapult pushes the slide at high speed towards the bow of the carrier.

The landing gear is, IMHO, appropriately called that because it’s more important, again IMHO, to walk away from the aircraft after you land, thus landing gear. Kind of hard to walk away after you take off!

This simplified explanation brought to you by your friendly retired PN1, USN who served aboard two aircraft carriers, the USS CARL VINSON (mighty fine ship!) and the USS INDEPENDENCE (dang, that ship was old!).

And it’s Swabbie, not “mob jockey.”

PN1 Monty

Carrier Steam
Catapult Diagram


Actually, sailor is not far off with his zipper guess. The rubber that seals the catapult cylinder slides open around the shuttle as it passes and then closes behind it.

I, too, have searched long and hard for a good diagram of the shuttle sealing mechanism, but have yet to see a good close-up cross-section. My buddy says he has one in an old copy of Live Steam but if you’ve seen the stacks and stacks of mags around his shop you’d understand my pessimism.

>> Actually, sailor is not far off with his zipper guess

Go figure, I try to say something stupid and end up saying something intelligent. It probably works the other way around too. :wink:

Caveat: IANACarrier sailor.

The catapults have their own steam system, making them independant of any other steam loads on board the carrier. The shuttle (the part that pulls the aircraft down the deck) is attached to a piston, and runs along a precisely-laid track built into the flightdeck. The connection between the shuttle and the piston is sealed by a pair of heavy rubber gaskets that flap closed before and after the connecting arm. Obviously, this isn’t precisely steam-tight (you always see some escaped steam after a cat shot), but it’s close enough.

An aircraft is taxied into place and connected to the shuttle by a bar that projects to the front of the aircraft’s nose gear. In the case of aircraft that don’t have the proper equipment, a ‘bridle’ is used instead (the ramps at the end of the catapult are there to stop the bridles after they release the aircraft and the suttle stops: The bridle whips over and slaps onto the ramp). The aircraft and shuttle are held in place by a restraining bolt of specifically callibrated breaking strain. The exact weight and type of the aircraft are fed into the launching system, allowing the correct amount of launching force to be calculated and set. When all is in readness, the launching trigger is pressed, steam floods the piston, and when enough force is achieved, the restraining bolt breaks, the piston, shuttle, and attached aircraft shoot down the deck. At the end of the run, the piston and shuttle slam to a stop and the aircraft goes flying off into the wild blue yonder.
Flightdeck crew, please correct any over-simplifications and errors…?

Landing is kinda similar, but far harder. The aircraft enters a landing pattern that places the aircraft approaching the stern of the carrier at an angle, on a very precise and narrow flight path. A system of stabilized lights and lenses create visual aiming system (the ‘ball’) to aid the pilot, who is flying one of the toughest routine flight profiles in the world at this point. The pilot’s job is to crash into a moving, pitching, rolling ~100’ by 100’ square at roughly 130 knots. Waiting for the pilot are a series of arresting wires, placed across the deck. These are attached to a variable counterweight system below decks. The arresting systems is also callibrated to the type and weight of each aircraft in turn. When the plane hits the deck, the pilot firewalls the throttles, in case he missed all wires. At the rear of the plane is a hefty hook which grabs (hopefully) an arresting wire. The wire runs out, the energy being absorbed by the counterweights and buffers below deck, slowing the aircraft to zero speed in about 400 feet. The aircraft is unhooked and taxied out of the landing area, the wire is pulled taut back into position, and 30 seconds later, it all starts over again.

Some further questions:

  • Why use steam instead of just compressed air?
  • What is the pressure used? It seems the rubber flap system would require relatively low pressure to be effective.
  • How is the shuttle moved back to the initial position?
  • " Why use steam instead of just compressed air? "*

Doesn’t answer your compressed air question but steam units are apparently more adaptable than other types.

  1. Two pistons not one - They travel in parallel tubes and are connected with something that looks like a “yoke” (no idea what the tech term is.

  2. Rubber is ONLY used to cover the slots in the deck to keep FOD out of the tubes when the cats aren’t in use. The slots in the tube are actually “sealed” (and that’s a very loose term) with very long lengths of rectangular steel strips that flex to allow the “yoke” to pass. It’s pretty difficult to explain it unless you’ve seen the damned thing in action; pretty ingenious set-up.

  3. Steam expands a lot more that compressed air; more efficient use of energy after all is said and done.

  4. A rather simple cable system retracts the pistons to the aft position.

  5. Stopping the runout of the arresting cable a landing aircraft has caught is controlled by a single, rather huge hydraulic piston (approx 20" diameter X 40’ in length) mounted on the deck just below the flight deck. Huge multiple-purchase sheaves at both ends of the piston contain several hundred feet of cable that are available to compress the piston as the flight deck wire slows the aircraft. A valve controls hydraulic fluid outflow rate depending on the weight and speed of the aircraft that’s landing. All four wires have to be set for every bird that’s landing. A “wire” is retracted by pumping the piston to its full length again. Then reset all outflow valves for the next aircraft in the pattern.

(At least that’s the way they were set up on CV59 and CV60)

Thanks, Monty - I meant “mop jockey” in a GOOD way, no disrespect, especially during these times. Somebody told me about the ‘zipper seal’ but I did’nt buy it at the time…now I do and owe somebody out there an apology.

The first time I heard the term MOP JOCKEY was in Korea. I was Army, but there was a smallish sailor sitting at a bar when some Marines came in. A big one grabbed the little guy by the collar and said “Step aside, mop jockey, this bar is taken by the US Marine Corps!” The sailor sat a table, steaming, until he jumped up, ran across the table kicked the big guy in the teeth and kept on going out the door. Not sure if it’s a true story, but sounds good.

Nice post there 'Uigi. Now my search for some really diagrams is started again.

The reason for the use of high-pressure steam is… well there is just a great deal of it available on board a steam driven ship. Both oil-fired and nuclear carriers make steam to power themselves. Using that power for air compressors or hydraulic systems would be an unnecessary step.

Bear in mind that the steam system for the cats is seperate from main propulsion steam. There’s still a need for a seperate system (and cosequently, extra steps), else your available steam pressure for flight ops would change with every change of bell. Can’t have a change in the throttles affect steam pressure just as the trigger is pulled… That would be bad.

I have realized as I grow older that I am wrong sometimes. Actually, knowing that makes me like this board so much more. But Tranquilis, could you give me a link or some info on this? I ain’t saying you’re wrong, I just like to learn, my man.

CV (oil fired) 8 boilers feed propulsion and cats. Higher speeds require more boilers on-line to drive more shafts (1-4) faster. Launches further tax the system. Steam from the main boilers is fed to huge steel accumulators (?) IIRC they’re 15’ diameter and twice that in height. The steam is held there with a valving system until a huge butterfly valve (controlled by the deck edge launch guys) releases that stored steam into the catapult piston tubes.

CVN (nuke teakettles) 2 huge teakettles (Westinghouse A4W’s) perform the same basic function with a lot less fuss.

One of the problems with steam cats is you can use just any old water to make the steam; it’s gotta be ultra-pure to reduce mineral build up in pipes, etc. Seems I recall it takes something like 500 gallons of this clean water to launch each aircraft from a cat. That’s why the older CV carrier crews ended up on water hours (i.e. no showers, or laundry) more frequently than their CVN sisters.

US Navy pictures:
Steam rises from the number one catapult aboard USS Enterprise (CVN 65), as an F/A-18C Hornet launches from the bow catapult.

A member of the flight deck crew aboard USS George Washington (CVN 73) ensures the launch bar on an F-14 Tomcat is properly seated when the aircraft is in tension before it is launched off the catapult

I find that I’m partially incorrect… It’s not a seperate system, it’s a system that is isolable from main steam. Diagrams here (bottom of the page). The accumualtor is charged from main steam and accumulator pressure is isolated from main steam pressure fluctuations.

The system that I recalled used secondary boilers, but that seems to be obsolete now.

Both of which (the USS Forrestal and USS Saratoga) happen to be mothballed at Naval Station Newport’s Pier 1 in Newport, Rhode Island.

I drove right by them this morning. :cool:

I am slightly abashed to do this, but


Some trivia: missing all 4 arresting cables and having to try again is called “boltering,” IIRC.