Friday, August 18, 2017

Power output of a lightsaber

It's been a long week and lots of seriously bad things have been in the news.   I intend to briefly distract myself by looking at that long-standing question, what is the power output of a lightsaber?  I'm talking about the power output when the lightsaber is actually slicing through something, not just looking cool.  We can get a very rough, conservative estimate from the documentary video evidence before us.  I choose not to use the prequels, on general principle, though the scene in The Phantom Menace when Gui-Gon Jinn cuts through the blast door would be a good place to start.  Instead, let's look at The Force Awakens, where Kylo Ren throws a tantrum and slices up an instrument panel, leaving behind dripping molten metal.  

With each low-effort swing of his arm, Ren's lightsaber, with a diameter of around 2 cm moves at something more than 2 m/s, slicing metal to a depth of, say, 3 cm (actually probably deeper than that).  That is, the cutting part of the blade is sweeping out a volume of around 1200 cc/sec.  It is heating that volume of console material up to well above its melting point, so we need to worry about the energy it takes to heat the solid from room temperature (300 K) up to its melting point, and then the heat of fusion required to melt the material.  At a rough guess, suppose imperial construction is aluminum.  Aluminum has a specific heat of 0.9 J/g-K, and a density of 2.7 g/cc when solid, a melting point of 933 K, and a heat of fusion of 10.7 kJ/mol.  In terms of volume, that's (10.7 kJ/mol)(1 mol/27 g)(2.7 g/cc) = 1070 J/cc.  So, the total power is around (933K-300K)*(0.9J/g-K)*(2.7g/cc)*(1200 cc/sec) + (1070 J/cc)(1200 cc/sec) = 3.1e6 J/s = 3.1 MW.  

Hot stuff.


Anonymous said...

"Only" 3 MW! Another laser engineering question is this Death Ray that blasts unlucky planets into smithereens.

Here is what I found so far:


Dima said...