Re: PLAN Aircraft Carrier programme..News & Views
You forgot to take in account of major sink of energy, which is the fact that the steam catapult itself leaks as the piston is propelled forward.
In any case, it is known that a steam catapult is only 5~6% efficient. It has also be stated by multiple sources that EMALS is more efficient than a steam catapult. EMALS is more efficient, period, and has nothing to do with other systems on the ship.
No I did not forget to account for the major sink of energy i.e. leakage. that is included in step 2.
But does matter with the other system on the ship, where does the electricity for the EMAL come from? A boiler developing steam to drive a steam turbine which drives a fly wheel that drives a generator which drives the electromagnetic array.
A steam system would only require a boiler to develop steam, desalination is practically distilling, which is heating up water to the boiling point...
My point is to question what energy savings there are, lets have a quick run through some engineering rule of thumbs,
Ecat1,
Boiler, 80% efficient
Steam turbine, 90% efficient
Generator, 90% efficient
linear motor, 50% efficient??? (should be lower, since the linear motor gets the current from a flywheel generator, thus current is fluctuating; and since it is a fully inductive load, efficacy would be low)
mechanical losses - fly wheel yada yada - 95% efficient
Overall system: 30.8% efficient
Ecat2,
Gas turbine generator, 55% efficient (just checked commercial Mitsubishi one)
linear motor, 50% efficient???
mechanical losses - fly wheel yada yada - 95% efficient
Overall system: 26.1% efficient
Steam cat
Boiler, 80% efficient
Steam piston, 6% efficient
mechanical losses - fly wheel yada yada - 95% efficient
Overall system: 4.6% efficient
Where do you get 5-6% efficient?
USA catapult C13-2 used on CVN72 to CVN77 uses 500 KG of water at 450 psig, 459F
Embodied energy including desalination, is 2802 kJ/kg, where 981 kJ/kg is the enthalpy of liquid water at t or 1.4GJ of energy per launch; if all of the steam is lost and none is returned as hot water if all is returned as hot water it will be 0.91GJ.
With a 309 ft runway (94m), it can propel a 80,000lb (39,900kg) plane to 140 knots (72 m/s) (i.e. F111A) full afterburner thrust is 164.6 kN
kinetic energy of the aircraft post launch is 103.4 MJ
Work to accelerate a plane is F*S (assuming force applied is constant which it isn't but you have punch through of the engine and so on) force required to accelerate plane is 1100 kN.
You have some force used to overcome friction and aerodynamic loss and the engine values are bench values meaning they are lower in reality. But lets say 164.6 kN is developed, it means that the catapult supplied 85% of the launching force. lets assume that it account for 85% of the post launch energy, thus, 88MJ of energy, which is between 6% to 10% of the energy required to build steam.
So the 5-6% is reasonable. So yeah, seems like it is likely that Emal would be more efficient overall,
//edit
But this is also still contingent on the fuel source, if it is nuclear powered, it doesn't matter - as the reactor is used to generate steam or the reaction absorbed by boron rods...
