The Saturn INT-20 was a proposed intermediate-payload follow-on from the Apollo Saturn V launch vehicle. A conical-form interstage would be fitted on top of the S-IC stage to support the S-IVB stage, so it could be considered either a retrofitted Saturn IB with a more powerful first stage, or a stubby, cut-down Saturn V without the S-II second stage. Three variants were studied, one with three F-1 engines in the first stage, one with four, and one with five.