CATAPULT ON AN AIRCRAFT CARRIERl

[chicopee]

How can such launching speeds be attained with the referenced system . I have always been fascinated by such system yet I have no idea about its mechanics. An explanation is appreciated

 

[IRstuff]

Something along the lines of a steam piston pumped up in pressure until a specially designed part breaks and the aircraft is flung into the air.

I've got the USN carrier manual at work. I'll read up next week and see if there's a better answer

TTFN

 

[IRstuff]

close:

http://www.fas.org/man/dod-101/sys/ship/cv.htm

TTFN

 

[mshimko]

Not a bad description. There are two aspects left out that seem pertenent to me (as both an engineer and retired Navy engineer):

1. The launching valve is a VERY FAST opening-closing valve.
2. Prior to the launching valve, steam is "stored" in a steam accumulator, essentially a large insulated tank.

The combination of the accumulator and launching valve is what delivers a large amount of high temperature and high pressure steam to the pistons in a very short time.

 

[IRstuff]

Is there an actual valve opening during the launch? My understanding was that the valve was opened prior to launch and that the only that holds things in place is the doodad that breaks once the engine is revved up and pressure plus thrust exceeds the breaking strength of the doodad.

TTFN

 

[mshimko]

I'm not sure of the "breaking part" that you refer to, but there is definitely a quick acting launch valve. Read the link above, as it describes how the length of time the valve is open is calculated based on take off velocity and weight of the aircraft.

 

[IRstuff]

The "holdback device" is the part that breaks. The aircraft carrier carries a literal truckload of the holdback devices for the duration of the voyage, since it's a one-time use device.

TTFN

 

[zeusfaber]

My guess (and it is a guess - I was still in short trousers when the UK last had a proper carrier capability) is that you'll have both - I imagine the holdback will allow the engine to spool up (a slowish process) and the quick acting valve to open fully before the aircraft starts drifting down the deck wasting catapult stroke.

For ground-based operations, the aircraft can be held against the brakes until the engine is developing full thrust. To do that on a carrier, you'd need to find some way of coordinating brake release with steam valve opening - and it's probably easier just to have a weak link in the system instead.

A.

 

[chicopee]

IRstuff, thanks for the site that you brought up on your October 28 reply. While reading the section of the damping action of the perforated cylinders while air planes are landing, it mentions water as the hydraulic fluid which surprises me for several reasons one of which is temperature control of the fluid. It seems to me that boiling could eventually take place. Do you have any thought on this?

 

[zeusfaber]

The nice thing about using water on a ship is that you don't have to cool it - you just pump it over the side and put some more in.

Just a matter of trying to prevent the system from rotting away (but that's ships for you).

A.

 

[IRstuff]

Agreed. Much of the cooling is often done by seawater, since there's so much of it available.

TTFN

 

[mshimko]

For chicopee - I think you mis-read. I don't believe water is used as a hydrauli fluid in the arresting gear machinery.

What the link refered to was a Water-Brake, but this is part of the catapult, not the arresting gear. On the catapult, the pistons that are pushed forward by the steam need to decellerate from ~160 mph to zero in a matter of feet. This is accomplished by using a "water brake", and yes, the water does get very hot, and must be refilled periodically.

Last - don't take fresh water too casually. My engineers were the ones that had to make it!

 

[zeusfaber]

A lot of land based arresting gear is water-filled - I'd be surprised if the shipborne stuff was significantly different.

A.

 

[IRstuff]

According to the LSO manual, the launching valves have adjustable orifices control the pressure of the steam into the pistons. The holdback device is also called a tension bar and when that breaks, the built-up steam then expands to push the piston and aircraft forward. Note that the manual discusses the importance of pre-heating the entire assembly to ensure that the steam's energy is not wasted in heating up mechanical components.

The carrier manual NAEC-MISC-06900 is also a good authority, but it only mentions "fluid" in the context of the arresting gear. Ditto the LSO manual; "engine fluid" but never "water."

TTFN

 

[SteveFehr]

Very little of the steam escapes the catapult- the vast majority of it is reclaimed and reheated. It's far easier to turn 211 degree water into steam than to boil seawater from scratch!

The structure supporting the catapult is incredible, as is the water brake- ships are designed as a giant canteliever beam with most of the displacement in the center, and the bow and stern as cantelievers. And I tell you what- the entire bow of the ship shakes when the catapult slams into the water brake! All that massive steel and it still moves under the stresses, it's incredible.

 

[mshimko]

Pre-heating has other purposes as well.

Like most metallic systems, catapults expand when heated. They will expand significantly (I don't recall the numbers) from ambient to operating. IF one were to try to operate the catapult without the system being haeted and expanded, I suspect there will be significant mechanical problems, including even the possibility of the "ram" hanging up.

"Very little steam " escapes......this is relative. I believe a the equivalent of over a hundred gallons of water is lost on each cat shot, assuming steam leaks are minimal.

From my experience, its not just the bow of the ship sthat "shakes" on launch, you can feel it everywhere on the ship!

 

[SteveFehr]

You can't really feel the bow cats outside of the bow, or the waist cats much outside of the sponson. I mean, there's a little bit of noise but not really any noticible vibration and there's so much noise from elsewhere it's drowned out. If you're down 2nd deck and below aft of about frame 100 or so, you'd never even know they were launching or recovering aircraft.

I think they USE a lot of steam, but I can garuntee they're not loosing a hundred gallons of water during launch, there just isn't that much steam coming through the slit. A couple gallons, maybe tens of gallons at the most. If you WERE losing that much steam over the length of the catapult, it would lose pressure and stop accellerating towards the end; instead we just see a tiny bit of seepage.

 

[mshimko]

Well, steve, this is an example of where we'll just agree to disagree. I clearly recall feeling the shots way down in engineering on the 7th deck (where I typically worked), and in my bunk on the 3rtd deck. I also recall from the daily water reports how much feed water we (the engineers) were assumed was lost each shot.

Of course, its been so many years that I may have forgotten more than I ever knew. Of well - anchors away!

 

[IRstuff]

Perhaps, it's a difference in carriers. Don't forget that there have a total conversion to Nimitz-class carriers since about 2000. None of the non-Nimitz carriers are operational at this time. Note also, the first 3 of this class are structurally different than the remaining 7 as well.

As far as I know, at least for the Carl Vinson, which is in the first group, reports are that that from aft of the hangar deck, there's no indication of the launch noise or vibration/shock, at least, that's what I was told by the electronics maintenance personel working with the CASS test systems.

TTFN

 

[IRstuff]

Sorry, my bad. There are at least two non-Nimitz carriers still operational.

TTFN