the paper say explorer 6 must be around year 1959
in that time were a lot "Cluster last stand" proposals
Convair Juno 5 => Saturn I
Martin Arcturus build from Titan 1 first stage tanks and engine
first stage 7 x tanks with 2 x F-1 engine, second stage 4 x Tanks 4 x LR-91 ?, third stage 1 x Tank with 1 x LR-91
Aerojet General, Thiokol proposed soild rocket cluster
new to me is that the Boeing Cluster was Solid fueled