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