There were proposals to use eight F-1 engines on the first stage of the Saturn C-8 and Nova rockets. Numerous proposals have been made from the 1970s and on to develop new expendable boosters based around the F-1 engine design. These include the Saturn-Shuttle, and the Pyrios booster (see below) in 2013. , none have proceeded beyond the initial study phase. The Comet HLLV would have used five F-1A engines on the main core and two on each of the boosters.