I would think that a minimum of historical skewing can justify Multibody, perhaps earlier than it was adopted in the original TL. Retaining some use for the magnificent F-1A engine has got to have some attractions for somebody, hasn't it?
Maybe not. Other discussions elsewhere suggest Rocketdyne was profoundly uninterested in capitalizing on the equally impressive and crucial J-2S OTL, an engine much easier to find justifications for using on upper stages. I suppose management was excited by the challenge of following through on SSME, an evolution from an alternate hydrogen burning engine of modestly higher thrust than J-2 and considerably higher ISP whose title eludes my memory at the moment; I'm sure it falls trippingly off the tongues of some others. I suppose the other engine had higher chamber pressures and perhaps went partway down the path of staged combustion followed by SSME, and therefore also had somewhat better sea level performance than even the J-2S, which seems to have perhaps eliminated the special problem the J-2 had with its low pressure gas generator at SL--offset by superior expansion from a narrower throat. The other engine was I suppose meant for possible ground firing from the beginning as SSME would be. Apparently this was the lineage Rockedyne, looking forward, preferred to develop instead of simply standing pat with minor tinkering on the J-2S.
This is a general problem in the astronautics biz, I guess, perhaps greatly exacerbated in the USA by the money-no-object gung-ho Apollo moon landing to meet a self-imposed deadline. This doubtless encouraged not only NASA big wigs but the entire chain of their private contractors to look ahead to ever more nifty future goals, and be generally disinclined to pursue maximizing utility of what they had in hand. And of course Apollo tech, being a series of kludges with designs frozen in the early 60s, would tend to appear quaint and out of date to such future-oriented designers. The entire premise of ETS is a major reversal of that mentality, to instead take what they have in hand at a given moment (the time of the OTL Shuttle Decision, specifically) and see how much utility they could wring out a fixed kit. Early in the TL's publication I did suppose there would be continual incremental innovations, and I think we saw some, but the pace seemed glacial to me compared to expectations.
And this is of course because even modest, incremental variations in important technologies involve a need to go back to square one as far as confidence in reliability of the new system thus modified goes. If Rocketdyne were to come out with a significantly modified F engine meriting a designation change, that engine would have to undergo a new battery of tests comparable to those carried out on the original F-1, then when integrated into a Multibody stage tested again as part of re-certifying that new model, and so on. I thought a virtue of throwing away each rocket set (and even manned craft) with each launch would be the opportunity for steady incremental improvement, but this did not allow for the testing factor.
So the TL assumes that with NASA under other management, this one man (tacking to be sure, on alternate winds that were present OTL, so this one man is leveraging a different mix of OTL pressures) can first of all bring NASA management around to a new mindset, and then the formation of alternate market pressures presumably redirects the thinking of dozens of major corporate contractors to fall in line, and this new way of thinking stabilizes pretty quickly. Then it is possible to work out the potentials of a fairly fixed kit of equipment and show that it compares well to the dreams of the most highly advanced tech boosters of modern times. Especially if one is of the view that the most exciting real world developments of OTL past decades do not involve quantum leaps to new regimes of performance requiring highly developed new materials and so forth, but rather holding firm at current levels in terms of engine pressures and temperatures and so forth and focusing on lightening and cheapening the fabrication of modern engines that do as much, or in absolute terms somewhat more poorly, but at a drastically lowered cost.
As many subsequent threads have explored, this is quite an about face from the mentality of the industry of the early 70s, that assumed that problems like high costs per launch would be addressed with radical new paradigms rather than incremental cost reductions, and furthermore forced a paradigm based on analogy with aeronautics rather than take a fresh new look at rockets for what they are, rather than Buck Rogers evolutions of airplanes. That's two different psychological hurdles to overcome right there, and in retrospect it may be far too much to ask of one man to accomplish.
Then again this one man was responding to strong pressures that existed OTL, to the hard reality that first of all Apollo era NASA budgets were inflated well beyond politically sustainable levels, and secondly the USA and world as a whole was entering a "hangover" period in which even perfectly sustainable levels of spending seemed possibly extravagant. Nixon and Agnew had no personal animus against the astronautics business, but certainly were under pressure, from majority Democrats and from a couple different wings of Republicans with different reasons but overlapping agendas, to rein costs in.
In that context, making lemonade in the form of a national launch system with standardized parts from the lemon of excellent but highly specialized systems developed for Apollo certainly would have some weight. It would annoy most everyone, but if the overall projected budgets included room for firm planning of new space stations, these could appear as the glamor product for their wild-eyed designers to focus on.
Rationally speaking, I don't think the case for ending Saturn 1B production and regarding the multi-tube first stage as inefficient and quaint was nearly as strong as it looked--but I have to admit, it looked strong in the day. Knowing it was a kludge, that it perpetuated 1950s technology (and a contractor who was very much out of the mainstream in the aerospace biz, however strong they may have been in the field of making tanks and other field vehicles for the Army) all worked against noticing that it was a highly effective kludge. Knowing what I know now, replacing the stage structure with an extrapolation of the S-1C monolithic two-tank was no guarantee of improvement and risked steps backward in terms of actual performance, and in any case called for a lengthy series of tests delaying introduction of Saturn 1C. But at the time of publication this seemed like a no-brainer requirement to me, and doubtless would look like that to legions of highly qualified and experienced astronauticists as well.
Given that we could expect Boeing, getting the message that economy and simplicity were at a premium and if they played their cards right they could look forward to long decades of serial production on a large scale, could come in on time and under budget with a solid design for both 1C and then Multibody, the Multibody Decision seems sweetly reasonable and painless to me.
Therefore--if the TL falls, it falls on the assumption that it would be possible to redirect the mass mentality of both NASA and her accustomed contractors so deeply and do it so quickly. It depends I guess on which is the tail and which is the dog, a government (the bulk of which is effectively the military) capable of rational self-assessment, or contractor interests shrewd at short term manipulation of the state. The modern debacle of SLS would seem to cast a cloud over the government's ability to examine its own soul and aim at thought-out, rational objectives, but this may be a product of a couple generations of inertia given a salutary re-direction in the ATL.