That was "The Catwalk" from Season 2 (Ep. #38)
Funny outtake on the DVD blooper reel where Trip and Archer are trying to work thru Archer's "Osmium" alloy line - it's down in engineering when they're in front of a nice graphic on the plasma monitor, trying to come up with a solution to getting thru that neutronic wavefront. I thought it interesting that they used Osmium - it is a real element - and makes sense in the need for shielding as its quite dense.
---a paragraph that falls into the 'too much information' category.