R2K turns us on to the work of Damon Moran with 12 Mini-Orions and a detailed diagram of Project Pluto. I borrowed the prior but direct you to the source for the Pluto, 'full-sized' Orions, a ramjet concept, and more. Who's up for an Orion atop a Falcon Heavy? Update: Reader R2K point out that the Falcon heavy couldn't lift one of these. Based on info from Wikipedia and the Space X site, my guess is you'd be lucky to get a thrust to mass ratio of unity. So, the only thing cool about an Orion on top of a Falcon Heavy would be that the Orion would actually exist.