The SSM-N-6 Rigel was a proposed United States Navy submarine-launched, nuclear-capable ramjet-powered cruise missile.
[1] In 1946 the US Navy sanctioned development of the Rigel missile as a sub-launched supersonic weapon to attack enemy shores, in parallel with development of the subsonic SSM-N-8 Regulus.
[2] The SSM-N-6 was to be launched by means of 4 rocket boosters and a catapult, with two ramjets for the cruise mode of the flight.
[2] Subsequently, scaled-down Flight Test Vehicles (FTVs) were built with a configuration similar to the full-scale missile, and the first FTV launch occurred in May 1950.
Unfortunately, plans to build the SSM-N-6 missiles were cancelled because the failure of FTV flight tests, but also due to the fact that Rigel posed a problem for submariners by requiring a longer launch rail on submarines than the SSM-N-8 Regulus.