What is so hard about a proper catapult on aircraft carrier?

Discussion in 'Boat Design' started by Squidly-Diddly, Apr 21, 2018.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I know new USN carriers are linear motor, but thats probably some totally custom rig at Pentagon prices. I'm thinking a system that would be 95% as good could be built, and even retrofitted, for 1/4 of whatever USN spent, using off the shelf motors and mechanical.

    Still not getting why any 1st or even 2nd tier carrier with 1st tier aircraft expense wouldn't have at least a 2nd or 3rd tier cat, so they can at least launch with full load.

    "energy-intensive desalination employed with steam systems" Were they using SEA WATER for cats? Or did they just need to de-salt a lot of water first. I heard they used sea water to cool the blast shields that popped out of deck behind jets just before launch, and those had to be replaced often, and new ones are graphite or something. Never understood how the steam system worked and how they sealed it along the 100yrd long track, but there is a trail of steam for each launch.


    "But according to Navy budget documents, the service plans to spend $579.8 million in FY-18 to put EMALS on the John F. Kennedy (CVN-79)."Navy validates software fix for EMALS https://insidedefense.com/insider/navy-validates-software-fix-emals
    So for 3 cats per carrier that means each EMALS is about $200 million.

    I'm guessing a system as I've described would be under $20 million with all the trimmings.
     
    Last edited: Apr 23, 2018
  2. tom28571
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    tom28571 Senior Member

    The first USN carrier to be fitted with a steam catapult was the Essex class Yorktown not the Midway class. My ship was in the Bremerton, WA yard at the same time as the Yorktown was being fitted with a steam cat in 1953. Midway class may have been the first to have a catapult as part of the design.

    Not sure what Squidly's issue is here but maybe he should offer his services to BUShips so they can get things right. American aircraft has always been a lot bigger and heavier than other countries planes. One thing is surely true though and that is the stupendous amount of money being spent on today's Navy ships. Politicians pork barrel needs often take precedence over fiscal responsibility. In spite of what is now coming out of Washington, military spending is out of hand and takes way too much of the budget. There is an argument as to whether US military spending is greater than the next 12 countries or greater than the next 8 countries. Both positions are ridiculously high and rob needed spending from things that actually offer good service to the people.
     
  3. Squidly-Diddly
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    Squidly-Diddly Senior Member

    That makes two of us. I'm just seeing a lot of money spent on what they call in the software biz "crippleware". I don't understand how AFTER cats were invented and well sorted, both retrofit and new construction, that someone would choose a ramp, given the whole point of an aircraft carrier is to project force at a great distance, which means max fuel. I look at ramps and think "now THATS the sort of thing the USA, with its 20x bigger military budget might waste money inventing and investigating, just for kicks".

    Then I'm seeing what USA spends on new linear motor cats VS what might be done with off shelf parts.

    So....now I'm thinking maybe an off-the-shelf cat of rotational motors and chain/cable might marketed to 2nd tier navies, particularly those with existing helicopter carriers.
     
  4. upchurchmr
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    upchurchmr Senior Member

    Anybody here have some facts to go with the speculation?
    Anybody here even have the requirements for a cat as specified by the Navy?

    Military requirements are often based on historical lessons learned, as well as wishful thinking.
    Off the shelf parts usually don't meet the needs of both requirements.

    Lets see your design Squid.
     
  5. Yellowjacket
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    Yellowjacket Senior Member

    Sorry but it's not that simple.. In fact, it's darn hard to supply that much energy and steam working against a cylinder is a lot easier to provide the kind of forces that are required than it is electrically. For instance, you have to appreciate that if you want to use cables and conventional motors that the motors are going to be really big, and to accelerate those big motors is going to take just that much more power. When you get to the end of the launch you have to stop that big rotor too and the force required to do that are huge. Then there's the efficiency, motors at high speeds are pretty efficient, but at lower speeds they aren't... Electric motors are machines that convert amperage into torque, but as you go faster the back EMF of the motor increases, so you have to increase the voltage as the motor speeds up (and the speed range is big, you go from stall to 120 mph in 3 seconds)... Which leads you to how do you control the voltage or the current... If you don't limit the current it'll fry at low speed. And the amount of electrical power you have to supply is huge, 163 MEGAWATTS! Where are you going to get that much electrical power? If you have a big generator it's going to be sitting there waiting for you to launch. If you use a huge flywheel the power available is going down as the flywheel slows down, just when you need more power at the end of the launch... As they noted if you want to use batteries the amount of power is equal to 3 Tesla's and you'd discharge the in 3 seconds.. That would eat batteries. Super size capacitors would work but if a crew person got across them it would turn them to charcoal in about a half a second. The reason that steam has been used for the last half a century is that it is easy to generate lots of force at low speeds with steam.. Think about those big locomotives that had one double acting piston on each side. A big steam cylinder with a thousand PSI in it can do a tremendous amount of work.
     
  6. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Plan F: drop a big Sea Anchor doing at least 20knots and run the cable through about 6:1 gearing.

    But seriously, I agree that Tesla Motors is a good starting point to scale up from, since their hardware does something pretty similar: from 0-120mph+ in a few seconds, using a system that weighs maybe 1/3 (motors and batts alone) of weight being accelerated. Just spitballing- Tesla S weighs over 2 tons and equip is about $100,000, and max takeoff weight for Super Hornet is 33 tons, so Tesla based hardware to launch the heaviest current carrier born aircraft (heavier than Eye-in-Sky Hawkeye) is $1,650,000 (not counting mods and extra assembly). USN currently spending close to $200M per cat, about 100x more.

    For a Cat maybe a system of 1/2 batts and 1/2 capacitors would be better, since unlike a car a Cat doesn't "cruise" most of the time. But having a large battery powered source of electricity on a ship always handy for emergency.
     
    Last edited: Apr 28, 2018
  7. jgb
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    jgb New Member

    the steam catapult was a british design and with the cat came the angle deck to allow launch and land at the same time.the russians just stuck more powerful engines in their aircraft and eliminated the cat
     
  8. PAR
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    PAR Yacht Designer/Builder

    My understanding was the first steam catapult was off of ships, not aircraft carriers. Some battle ships, but usually a much smaller naval vessel, launched from platforms, often atop a gun carriage, with no on board recovery possible. These aircraft were usually light, worn out and mono wing conversions. These "CAM" upgrades where possibly British in origin, but also quickly adopted by other navies. I think most CAM's launched with rockets, not steam. HMS Perseus was the first steam cat and quickly followed by others. In the 40's hydraulic catapults were tried (Midway class), but steam was much easier to make work.
     
  9. tom28571
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    tom28571 Senior Member

    My experience is quite old and may not be well applicable to the modern aircraft carrier. Typical aircraft launching speed of fleet carriers in early 1950's was 29kts without a CAT. That speed is not arbitrary but is the max speed attainable by destroyers of that period using the normal half power of two boilers that they operated with. The job of escorting destroyers was to pick up those in the drink that did not make it into the air. Even with a good headwind added to 29kts, a TBM looked like a real effort to get airborne. No CAT also meant that fewer planes could be parked on the flight deck.

    Most of the inertia problems of electric or other rotating machinery are eliminated with steam which was in a high state of development then. How to achieve the enormous (accelerate and stop) pulse of power needed for a launch with other than steam is not very easy. Still, its clear that there is effort going on to launch without steam and it may prove electric or other is best in the end. I still have no idea what Squiddly is talking about.

    Ramps can be of help is launching but they cannot violate the basic physical requirement of attaining enough energy in a short time and gain flying speed for whatever aircraft is taking off. The ramp itself actually takes energy away from the aircraft and translates it into altitude. With lightweight aircraft and a strong headwind, the ramp may be the best choice as altitude might then be the critical factor. I may be off base but I don't think that a ramp can allow take off with the same load of fuel and ordinance in the same aircraft as a CAT. Those who order the mission will decide which launching method is best for achieving the goals of their missions. Ramps seem to make sense for some missions and not for others.
     
  10. PAR
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    PAR Yacht Designer/Builder

    I agree, in that simple physics still apply and a ramp isn't launching anything, though may help get enough altitude, so the aircraft can approuch a stall, but have enough height to nose over and recover, before the carrier runs it down. Even extreme STOL, that may benefit from a ramp, likely more fuel savings than anything else. I'd be surprised if a full up FA-18 at nearly 50,000 pounds could get up a ramp, without stuffing it's nose gear at takeoff speed. An F-35 may benefit, but why would you crawl into a semi vertical takeoff, wasting that much fuel, if a catapult was available, saving a lot of fuel.
     
  11. upchurchmr
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    upchurchmr Senior Member

    The major issue with launching a Navy aircraft is to gain enough angle of attack on the wing to start flying.
    You can do one of two things. Pitch the nose of the aircraft up, providing angle of attack to the wing so it develops enough lift, or, launch at a higher speed, which translates into more lift.
    A cat provides the speed, since the pitch on the aircraft is defined during the initial design. Historically, the F-8 had a wing which was hinged at the trailing edge, and jacked up at the front of the wing. This provided the increased angle of attack for takeoff only, enabling it to be launched at a lower speed.
    A Jump literally pitches the nose of the aircraft up allowing more lift to be developed at a lower speed.

    A cat has the advantage that the amount of force and speed can be varied for launching different aircraft and different launch weights.

    The ramp is not variable, requiring all aircraft to be designed for the increased nose and main gear loads. Of course, landing on a carrier is probably such a higher load that there would really be little impact - but it has to be taken into account. Different types of aircraft can still be launched at different ship and aircraft speeds, making it adaptable in a different way.

    If you watch carrier launches in WW2, the aircraft will sink below the deck height until they get enough speed and lift.
    Modern carriers have enough power to launch aircraft straight off the deck (no dipping below the flight deck), at a significantly higher loaded weight. Of course the aircraft engines have significantly higher thrust, doing some of the work of the cat. I.E., the aircraft is already at flying speed. Safer
     
  12. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Super Hornet is 66,000lbs max but not sure if that is carrier launch, but I'm pretty sure the nose gear is able to take a pretty big slap when they land and arresting hook rotates the plane nose down.

    I'd rather go for max speed straight off, and use some Ground Effect if needed and waves aren't too high, instead of approaching stall and hoping to nose over and recover.

    Just take into account that rear gear is gonna be still sprung and will try to rotate you nose down after nose gear clears.
     
  13. upchurchmr
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    upchurchmr Senior Member

    The difficulty for landing is to approach slow enough for the hook to catch the cable. If you hit the cable with the rear tires, it slaps down onto the deck poping back up. Hopefully the hook is there to catch the wire. The whole thing is a timing issue.
    You have to approach the deck slow enough to catch the wire - to do that you have to be nose high to keep the speed down (with full throttle). The trick is to be able to see over the nose to see the deck. You can always go slower with more pitch, but then you can't see the deck.

    Not a lot of leeway to just "go for max speed and use some ground effect". Where do you get this stuff?
    They are really not "at stall", although you can always screw up anything.
     
  14. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Forgot to mention but thx for bringing it up. IMO all that white-knuckle Top Gun carrier landing stuff has been strictly for show since no later than 1970. One thing computers do very well and better than any human is fly aircraft, and in particular react to changes.

    For my design, my first proposal would be to add about 7ft (walking headroom) extra deck on top of one of these France has 3 options for those Mistral warships it was going to sell to Russia http://www.businessinsider.com/r-sink-or-sell-russia-spat-leaves-france-with-warships-to-spare-2015-6

    7ft of space across the whole deck should be more than plenty of space for Small Format (Tesla based, maybe Tesla semi-truck) cats, batts and caps, as well as for arresting cable.

    The front would be the cat equip, and rear would be arresting equip, and the deck would be flat from bow to stern. Sure, it would hurt stability but doable. The deck only needs to be strong where big jets are gonna land. Have plans for adding bulges on the sides if its an issue. Lots of warships got extra bulges for anti-torpedo and stability.

    Main lift is at the stern so bigger-than-helcopter aircraft could ride the lift with tails hanging over. There would need to be a "lift on the lift" of sorts to lift them up the extra 7ft and also allow them to roll off into the hanger level.

    IMO the whole conversion wouldn't cost more than $100 million for new modular deck, two cats and set of arresting cables, and extra helper rear and side lifts.

    PS-
    "Not a lot of leeway to just "go for max speed and use some ground effect". Where do you get this stuff?
    They are really not "at stall", although you can always screw up anything."

    I'm just thinking I'd want to be going faster, with Ground Effect help, to be as far from stalling as possible, rather than be near stall, with only at most a couple hundred feet to recover. It bugs me that you want airspeed, but a ramp is going to decrease your airspeed. If you have airspeed for flight who cares about being angled up? Its not like there is a mountain you need to clear out in front of a carrier at sea. Some jets (Rafale?) have an extendable nose gear to increase angle for launch. I'd consider a jack on the cat "shoe" to give similar effect.
     

  15. upchurchmr
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    upchurchmr Senior Member

    This is the biggest waste of conversation I've seen in a long time.
    All talk and no real data to show what you are talking about.

    Done, gone.
     
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