| How can such launching speeds are attained with the | | | | manual NAEC-MISC-06900 is also a good authority, |
| referenced system. Something along the lines of a | | | | but it only mentions "fluid" in the context |
| steam piston pumped up in pressure until a specially | | | | of the arresting gear. Ditto the LSO manual; |
| designed part breaks and the aircraft is flung into the | | | | "engine fluid" but never |
| air. There are two aspects left out that seem | | | | "water." |
| pertinent to me (as both an engineer and retired | | | | Very little of the steam escapes the catapult- the |
| Navy engineer): | | | | vast majority of it is reclaimed and reheated. It's far |
| 1. the launching valve is a very fast opening-closing | | | | easier to turn 211 degree water into steam than to |
| valve. | | | | boil seawater from scratch! |
| 2. Prior to the launching valve, steam is | | | | The structure supporting the catapult is incredible, as |
| "stored" in a steam accumulator, | | | | is the water brake- ships are designed as a giant |
| essentially a large insulated tank. | | | | cantilever beam with most of the displacement in the |
| The combination of the accumulator and launching | | | | center, and the bow and stern as cantelievers. And I |
| valve is what delivers a large amount of high | | | | tell you what- the entire bow of the ship shakes |
| temperature and high pressure steam to the pistons | | | | when the catapult slams into the water brake! All |
| in a very short time. There is definitely a quick acting | | | | that massive steel and it still moves under the |
| launch valve, length of time the valve is open is | | | | stresses, it's incredible. |
| calculated based on take off velocity and weight of | | | | Pre-heating has other purposes as well. Like most |
| the aircraft. The "holdback device" is the | | | | metallic systems, catapults expand when heated. |
| part that breaks. The aircraft carrier carries a literal | | | | They will expand significantly (I don't recall the |
| truckload of the holdback devices for the duration of | | | | numbers) from ambient to operating. If one were to |
| the voyage, since it's a one-time use device. | | | | try to operate the catapult without the system being |
| For ground-based operations, the aircraft can be held | | | | heated and expanded, I suspect there will be |
| against the brakes until the engine is developing full | | | | significant mechanical problems, including even the |
| thrust. To do that on a carrier, you'd need to find | | | | possibility of the ram hanging up. "Very little |
| some way of coordinating brake release with steam | | | | steam" escapes, this is relative. I believe as the |
| valve opening - and it's probably easier just to have a | | | | equivalent of over a hundred gallons of water is lost |
| weak link in the system instead. | | | | on each cat shot, assuming steam leaks are minimal. |
| I don't believe water is used as a hydrauli fluid in the | | | | From my experience, its not just the bow of the |
| arresting gear machinery. It is a Water-Brake, but this | | | | ship sthat "shakes" on launch, you can |
| is part of the catapult, not the arresting gear. On the | | | | feel it everywhere on the ship! You can't really feel |
| catapult, the pistons that are pushed forward by the | | | | the bow cats outside of the bow, or the waist cats |
| steam need to decellerate from ~160 mph to zero in | | | | much outside of the sponson. I mean, there's a little |
| a matter of feet. This is accomplished by using a | | | | bit of noise but not really any noticeable vibration and |
| "water brake", and the water does get | | | | there's so much noise from elsewhere it's drowned |
| very hot, and must be refilled periodically. | | | | out. If you're down 2nd deck and below aft of about |
| Don't take fresh water too casually. A lot of land | | | | frame 100 or so, you'd never even know they were |
| based arresting gear is water-filled - I'd be surprised if | | | | launching or recovering aircraft. |
| the shipborne stuff was significantly different. | | | | I think they use a lot of steam, but I can guarantee |
| According to the LSO manual, the launching valves | | | | they're not loosing a hundred gallons of water during |
| have adjustable orifices control the pressure of the | | | | launch, there just isn't that much steam coming |
| steam into the pistons. The holdback device is also | | | | through the slit, a couple of gallons, maybe ten |
| called a tension bar and when that breaks, the | | | | gallons at the most. If you were losing that much |
| built-up steam then expands to push the piston and | | | | steam over the length of the catapult, it would lose |
| aircraft forward. Note that the manual discusses the | | | | pressure and stop accelerating towards the end; |
| importance of pre-heating the entire assembly to | | | | instead we just see a tiny bit of seepage. Perhaps, |
| ensure that the steam's energy is not wasted in | | | | it's a difference in carriers. |
| heating up mechanical components. The carrier | | | | |