Saturn INT-20
The Saturn INT-20 was a proposed intermediate payload follow-on from the Apollo Saturn V launch vehicle. An interstage would be fitted on top of the S-IC stage to support the S-IVB stage, so it could be considered either a souped-up 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. Currently we only simulate the four-engine variant, though the three and five engine variants can be enabled in the config file (there's no guarantee that they will work).
References
http://www.astronautix.com/lvs/satint20.htm
