Helios was similar to Orion in that nuclear explosives were to be used for propulsion. However, with Helios the explosions were to be internal, within a spherical chamber. Water would be sprayed into the chamber just before detonation; the explosion would turn it to high temperature steam and it would be expelled through the rather large nozzle to produce thrust.
Helios didn't get even as far as Orion; it suffered from extreme complexity, low specific impulse and uncertainty as to whether it would actually hold together. Several designs were conceived, a few of which are shown here. The color artwork was from Lawrence Livermore National Lab, the B&W ad was from Aviation Week. No further data on these designs appears to be available.