In article , says...
I took a cursory look at this document. There were also liquid fueled
strap-on options studied as well. The Saturn V 23L and Saturn V 24L. The
23L variant used pairs of "standard" F1s in each of four strap-on liquid
"pods" and the 24L used pairs of "uprated" F1s in four strap-ons and an
"advanced" engine for the upper stages. The document mentions aerospike
engines and maybe so, but there were also ideas kicking around at the
time for a higher performance J2 as well.

The 24L didn't make it past phase 1 study but, the 23L got a rating in
this document of 579,000 lbs (289.5 short tons) to orbit (2 stage
configuration) and 220,000 lbs (110 short tons) I presume to the moon (3
stage configuration). Presumably the 24L variant with the higher thrust
F1s could have done even better.* Let's be conservative and estimate
first launch by 1975 (this document was written in 1966 and predicted a
1973 ready date). My date includes assuming mobile launch platform
(known as MLs in the Apollo era) modifications and other pad
modifications also needed. But at the time Wikipedia says NASA had three
MLs, so NASA should have been able to dedicate one to support this
configuration.

Compare this to the also non-extant SLS Block 2 at 280,000 lbs (140
short tons) to LEO.

What a monster this could have been. You're gonna need to move further
back... What other country in the world (besides us) would have so
thoroughly blown this advantage in rocketry? We're talking nineteen
seventy freaking five!

Dave

*There's a tantalizing paragraph on page 9 that ends:

"The liquid pod strap-on concept, with uprated F-1s and advanced engines
in the second stage (SAT-V-24(L)), achieved payloads to 960,000 lbs.
[480 short tons] to 100 nautical mile Earth orbit when stage and total
vehicle length restrictions were relaxed."

I personally preferred the liquid strap-ons, but that's because of the
hindsight of the history of large segmented solid rocket boosters (Titan
and shuttle programs). At the time, solids were still thought to be
cheap to develop and relatively cheap to produce for flights. The
Aerojet facility in Florida could have solved the joint problem by
producing monolithic (single pour) SRBs which could have then been
transported to KSC by barge.

Liquids had the advantage that they were much lighter when transporting
them to the pad. Those concepts using multiple heavy solids would
likely have needed to have been attached at the pad because they would
have overloaded the crawlers.

Jeff
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