LV-1000 "Cornet" is a deep space methalox engine. It is unlocked from Deep Space Methalox in Tier 3 of the Tech Tree. It features a telescopic nozzle design. It is efficient in vacuum but significantly ineffective in atmosphere, similar to nuclear engines.
While its thrust and Isp aren't quite as high as the LV-N "NERV", it compensates with a very low rate of fuel consumption and reduced mass making it efficient enough to fulfill a similar role in the formSM factor. Its performance allows it to reasonably function in an interplanetary transfer stage for smaller vessels.
Synopsis
Why would Ionic Symphonic ask Jeb's Junkyard to design their own deep space engine? So their musicians could perform off-world, of course!