Aircraft carrier launching catapult

[woodchips]

Seen an article about China's new aircraft carrier, which makes a thing about the launch catapult being electromagnetic rather than steam.

From a long time ago I have books by Eric Laithwaite who was well into electromagnetism, including linear motion including guns and catapults. It seemed to me that whilst a good idea in theory, in practice didn't work so well. Seems that the extreme magnetic flux was really extreme, remember a photo of a gun and the result from the tearing apart of the air due to the magnetic flux, but can't lay my hands on it at the moment.

So, does it work? Has there been improvements in magnetic materials in the 60 years or so since my books were published? Anyone more up to date?

[Andy Chee]

USS Gerald Ford has an electromagnetic catapault. 

It's been getting a bit of press lately, courtesy of Trump's comments re: military overspending and, "magnets don't work in water".

[soldar]

https://en.wikipedia.org/wiki/Electromagnetic_Aircraft_Launch_System

[tom66]

If it's something like a linear launch motor they are common.  See launched rollercoasters for instance.

[T3sl4co1l]

Yup, not uncommon in special applications.

Flux density most likely doesn't exceed 1.5T or so due to use of iron core materials; any external field is wasted effort that could've gone into the launch, and for something like this, it's a lot easier to just make a longer armature (thus more cross-sectional area A_e) than to increase field strength and cost massively on efficiency.  (Flux density over 2T would leak out of the then-well-saturated core, and thus be wasted at least in part.)

Not that efficiency is necessarily a problem, in and of itself, but when it takes some GJ to launch a fighter jet, and you have only so many 100s of MW available onboard the ship -- there is a certain economic argument to be made for keeping it at least modestly efficient!

It's when the armature itself is a critical component -- like the bullet in a gun -- that flux density needs to go way up. Which is why railguns have been such a problem to develop, and why coilguns will never be more than a toy -- to replicate the pressure of a real (chemical propellant) gun requires 10s of T peak, and needless to say, some fuckoff big capacitors.  (To be technical: a metric fuckload.)

Tim