Copyright 2012 Robert Clark
Ground Ignition Weights.
However, the Saturn V had a quite heavy first stage thrust structure:
SP-4206 Stages to Saturn.
7. The Lower Stages: S-IC and S-II.
Rosen apparently took the lead in pressing for the fifth engine, consistent with his obstinate push for a "big rocket." The MSFC contingent during the meetings included William Mrazek, Hans Maus, and James Bramlet. Rosen argued long and hard with Mrazek, until Mrazek bought the idea, carried the argument to his colleagues, and together they ultimately swayed von Braun. Adding the extra power plant really did not call for extensive design changes; this was Rosen's most convincing argument. Marshall engineers had drawn up the first stage to mount the original four engines at the ends of two heavy crossbeams at the base of the rocket. The innate conservatism of the von Braun design team was fortunate here, because the crossbeams were much heavier than required. Their inherent strength meant no real problems in mounting the fifth powerplant at the junction of the crossbeams, and the Saturn thus gained the added thrust to handle the increasingly heavy payloads of the later Apollo missions. "Conservative design," Rosen declared, "saved Apollo."2http://history.nasa.gov/SP-4206/ch7.htm
Indeed it was heaviest single component of the S-IC stage, and so of the Saturn V:
This online lecture of Dr. David Akin of the University of Maryland gives mass estimating relationships for various rocket components, taken from the reports NASA uses in designing rockets:
Mass Estimating Relations.
• Review of iterative design approach
• Mass Estimating Relations (MERs)
• Sample vehicle design analysis
On page 17 is given a relation between the thrust of the stage in Newtons and the mass of the thrust structure in kilograms:
MThrust structure(kg) = 2.55×10−4T(N)
For 4 RD-171's at 7,900 kN each, this would be 8,000 kg. So we can subtract off 13,000 kg from that S-IC dry mass to get 117,000 kg. We're also using one less engine, so subtract off 8,350 kg for the one less engine. However, the RD-171 weighs about 1,000 kg more than the F-1, so add on 4,000 kg to get about 113,000 kg for the stage, quite close to the 20 to 1 mass ratio estimate. Note this is even without the weight saving alloys and composites now used.