How can such launching speeds are attained with the referenced system. 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. There are two aspects left out that seem pertinent 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. There is definitely a quick acting launch valve, length of time the valve is open is calculated based on take off velocity and weight of the aircraft.
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. 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. I don't believe water is used as a hydrauli fluid in the arresting gear machinery. It is 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 the water does get very hot, and must be refilled periodically. Don't take fresh water too casually. A lot of land based arresting gear is water-filled - I'd be surprised if the shipborne stuff was significantly different.
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." 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 cantilever 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. 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 heated 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 as 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! 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 noticeable 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 guarantee they're not loosing a hundred gallons of water during launch, there just isn't that much steam coming through the slit, a couple of gallons, maybe ten gallons at the most.
If you were losing that much steam over the length of the catapult, it would lose pressure and stop accelerating towards the end; instead we just see a tiny bit of seepage. Perhaps, it's a difference in carriers.
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