If we clean sheet the OP intentions going back farther, to the roll-out period of Ares as a concept back in the mid-2000s, and have NASA thinking much much more along the lines of rational economics of carrying over Shuttle tech on a different concept (no Orbiter, though a reusable human space flight vehicle conceptually on Orion lines might be spun as "Orbiter II") then something much closer to Jupiter/DIRECT might have been conceived as a single launch vehicle. Then as e of pi notes, an actual space program from there probably involves accepting instead of rejecting LEO depot based systems as the main line of development.
"Preserving Shuttle workforce," which
@JEF_300 you yourself say is not really the actual goal, but maybe should be, is a two-headed monster. Do we want to preserve the jobs of the established Shuttle crews, NASA and private contractor, doing their work routinely, hopefully rationalized to reduce or anyway stabilize costs, or do we want to keep the corporate boards of the contractors sweet, and to hell with the actual line workers, they can sink or swim as expedient? In the pork-barrel perspective, you want to do both--the line workers are voters and while total headcount of them does not amount to many CD/Senate votes themselves, they have families, they work in communities dependent on their paychecks to be sustained--there is a multiplier effect at work politically. Meanwhile the corporations holding the contracts are huge hitters, in contributions to campaigns, in general lobbying influence. In a neoliberal age such as the past few decades, the corporations take priority, but ideally, one keeps the whole momentum of the whole mass of them happy. Working people get sloppy seconds, but ideally there is enough to go around so no one is pissed off.
Coming at it from an amateur engineering perspective, STS was only somewhat half-assed; by certain metrics (tonnage to LEO for instance) it was quite respectable. The trick, from the point of view of a space geek who wants to get more
useful tonnage to orbit at a fixed price, Eliminating the dead weight of the Orbiter's 100 tonne return to Earth mass in favor of zero returned to Earth for uncrewed launches or a fraction in the form of an Orion-concept capsule of some kind frees up that tonnage for LEO delivery. Now we are eliminating line workers--the people who refurbished Orbiters are out on the street. But the solid fuel supply line is soldiering on, as are the people who make the tanks at Michoud, and some of the Orbiter refurb people can be kept on to work on the Orions, which we make a small fleet of reusable ones.
The conservative approach then is to go with a version of Shuttle-C, keeping the sidesaddle load philosophy, which limits how much redesign of the tank we have to do and keeps the overall stack height of the system within familiar VAB constraints. Basically this means Yankee Energia--instead of liquid fueled Zenit-type new boosters, we stick with the solids, and frankly admit the damn things are not economical to recover, so eliminate the parachutes and the like, and let them splash and sink for people with a mind to to salvage or let rot as reefs. Someone might suggest it is perhaps worthwhile to recover and reuse the nozzle segments, maybe--those were a bit fancy with gimbaling the thrust and were designed for reuse, but it is just a silly charade to pretend the actual solid fuel segments were reusable. Giving up on shipping those back to Utah to be refilled eliminates some jobs, but if we recover, refurbish and reuse the nozzle sections, there are still recovery crews and reduced shipping and refurb work being done on just them.
Meanwhile most of the value added work done in Utah is still done; brand new segment exteriors are being stamped out, fresh one-shot solid fuel filler is still being churned out, the heavy filled segments are being shipped to Florida (and hey, maybe the Air Force might want to unmothball the Vandenberg facility and launch some loads from California, or perhaps someday Kodiak?)
Continue to use 4 segment boosters, in pairs, on the same tank design, with the now-to-be-discarded RS-25s mounted on a sidesaddle platform preserving much of Orbiter thrust dynamics, but with no intention to recover them.
One thing I hate about Ares/SLS that I suppose you like is the potlatch involved in taking engines designed to be used many times and discarding them. That mentality was pretty predominant in the mid-2000s though and hate it though I might I suppose it prevails--of course with the promise that an allegedly cheaper disposable design is in the pipeline, so burning up the SSMEs is OK as a stopgap, or potlatch in my more acerbic view--a lot of irrational atonement for the supposed sin of STS hubris in seeking a reusable design. Now rationally of course the SSMEs were not so economical, since refurbishment costs were high, but it is not clear to me that one could rationally hope to save a lot of money in cheaper manufacture and still maintain reliability levels. The sin, if there was one, of SSME design was seeking heroic high levels of chamber pressure to justify them as sea level lit engines in booster mode, and thus justify ignoring the J-2S design which was quite good if air-lit, and could be air lit of course. Given the high performance needed to deliver mediocre Isp and thus fairly good thrust at SL, I doubt a disposable design would be a lot cheaper really--though the fact of one use and disposal would mean ongoing line production, which surely would lower unit costs if the overall program could enable launches frequent enough to justify keeping the production line going steadily! For the moment, the "interim" early program would require using pretty standard SSMEs at full cost of production, somewhat diminished if delays in producing the disposable version required new ones to be made after the old stock is exhausted. But NASA is the home of Success Oriented Management and the assumption would be that the new allegedly cheaper engines are going to be available in time.
Speaking as you do as someone who says Ares 1 was a good design, why are 5 segments better than 8? Going with Shuttle C/Yankee Energia means wasting those RS-25s of course, but perhaps not three at a time. STS was able to operate with one of the three SSMEs out during later portions of a launch burn--the reason there were three was mainly I think for this engine-out capability since the Orbiter crews would not have a prayer of survival if one of two went out, but Shuttle C versions proposed often went down to just 2 SSMEs for their cargo launches, the hit in performance was something like 20-30 tonnes reduced LEO payload, versus the savings of destroying just 2 main engines instead of 3.
As for crewed launches in this ATL Ares, they go up in an Orion of sorts, an alternate payload on the sidesaddle booster. Side mount complicates launch escape but not so tremendously I'd think; a rapid capsule escape straight up should still be feasible I'd think, or perhaps angled over 5-15 degrees away from the tank. So your basic Mercury-Apollo style escape system, perhaps fancied up a bit, covers the crew escape a hell of a lot better than on Shuttle, and there are tricks we can consider doing that salvage the dead weight delta-V penalty of having an escape system that God willing we never actually use, or using a light enough crew capsule, we just eat the cost as part of the price of a crewed system.
So regarding development of 5-segment boosters, NASA should have told ATK "look, you want this for DoD market purposes, go partner with the Air Force for a separate contract, NASA does not need 5 segment boosters." To do it more diplomatically, and keeping options open, NASA can offer to partner with the Air Force on a limited basis for long term development of the option of a 5 or more segment system, as a possible upgrade path of Ares, keeping the booster contractor happy with the pork of this speculative development by brokering the deal with Defense and at some modest cost staying in the loop of development. But not risking holding up their successor launch system on this bet!
A pair of 4 segment solids, for a total of 8 segments and at a cost of having to use 2 fancy nozzle segments (instead of one undeveloped one resized for 25 percent more mass flow; the two known nozzles are a cost but also birds in the hand) clearly have more oomph than one paper 5 segment deal, and at the cost of using up at least two, or perhaps for more modest payloads one, SSME/speculative future disposable hydrogen engine, and perhaps for conservatism and higher payloads three, maybe four or more, sustainer engines, the overall payload of even the most barebones Shuttle C type approach should quite exceed that of OTL proposed Ares 1. In terms of economically competing in the existing launch market, that is not good, but Ares 1 clearly would not be a commercial launcher! The idea is to have workhorses for a NASA BEO program, and for that we definitely want more capacity not less.
So--conservative seems the way to go--stick to the 2 4 segment solids, tried and true and with an established supply line maintained; this does not entirely please ATK but they have DoD paying for the 5 segment development and meanwhile their established line workers keep churning out segments. We can envision upgrading from the baseline by replacing the 2 side boosters with 2 5 segments, for a 25 percent increase, or alternatively an even bigger redesigned tank can take 3 boosters, 4 segment for a 50 percent increase, 5 segment for a near doubling to 15/8, which given the overcapacity of 3 SSMEs in STS might not require using more than 4 hydrogen burning sustainers--maybe 5, but not 6! I suspect this is already exceeding volume capabilities at Michoud for the tank, so there is no point in going crazy with 6 segment boosters or trying to strap on 4 4 segment ones.
I'm fairly sure Jupiter/Direct considered all these variations. Sort of; Jupiter as I understand it did not give consideration to side saddle loads since part of the whole irrational fashionable head trip was to abjure the sins of the Shuttle and repenting, return to in-line as well as expendable philosophies. Well, I actually like in-line too, but I suspect that overall, the tank redesign involved would be at any rate heavier if perhaps simpler, with the compressive load of full thrust having to be carried full length through the entire tank, and as mentioned, there would be concerns about VAB height issues too sidesaddle largely sidesteps.
Let's look at extremes here. The basic minimum cost no frills simplification of basic Shuttle tech carried over is:
2 side boosters, simplified by ripping out the recovery stuff, which lightens them a bit for better performance at the cost of having to manufacture new nozzle segments (the cost of new fuel sections is actually a savings versus the costs of recovery and refurbishment). One tank, restressed just a little bit because we are going to use just one SSME type engine, that is what is minimal here. It might also not be feasible, though I suspect it is--the most critical difference is pathetic low thrust after the solids burn out. We can clearly go straight to a two SSME design and have something clearly viable anyway, and only use up SSMEs at 2/3 the rate of Orbiter sorties. But we don't have 100 tonnes of Orbiter return to Earth mass burdening it either, so I suspect one engine might work, at some prodigal waste of tank propellant of course, but overall match or exceed on the shelf expendable alternatives perhaps while frugally using up just one SSME. The fuel tank ought to be a bit lighter too.
With 2 SSMEs, we have well explored Shuttle C type designs, which IIRC ought to put 80 tonnes or more into LEO after discarding the tank. With three SSME, we have the bog standard Shuttle tank, slightly better performance due to tossing the recovery gear from the solids, and thus a good 125 or more tonnes in LEO, including of course the 3 SSMEs. This approximates closely what Saturn V put into LEO, so a reprise Lunar Apollo is possible already, or pretty near.
If Michoud can accommodate a 50 percent volume increased tank, we can go on to design one to take 3 solids instead of 2, thus a 4 SSME sidesaddle stage should orbit with about 180 or more tonnes, so with modest improvements on J-2S Isp and thrust, we should then be able to go on to the Moon with considerably more mass than Lunar Apollo. If it is possible to manage hydrogen boiloff during a LLO loiter, with lunar orbit rendezvous, the greater Isp of hydrogen burning engines should further multiply the mass available for the lunar lander/ascent vehicle while still enabling a somewhat heavier thing than an Apollo CM to return to Earth.
I believe part of the Ares/Orion package of concepts was that the lunar lander should shoulder the burden of insertion into LLO, to enable the Orion capsule system to be sized for LEO missions by itself. The people who wanted Constellation also wanted to ditch ISS, but I think it should be clear that the politics of continuing ISS would prevail (and I am glad it did, though I resent that we did not use the decade or so "going round and round Earth" to better explore free fall/low gravity medicine and biology with an eye toward long lunar stays and missions to Mars.
Top of the line I suspect this version of "Direct" might enable in terms of capacity to LEO would be using three five segment solids for a near doubling of Orbiter overall mass to LEO which brings us to over 200 tonnes to LEO. Note that this is Musk's target for Starship to routinely boost into LEO. The cost of this version of Ares doing that launch will be tremendously higher than SpaceX is aiming for since nothing is being reused, but it ought to be a hell of a lot lower than ten Shuttle sorties! And it is a huge load orbited all at once. Again since the 3 SSMEs of Orbiter numbered that many in part to enable engine out capability, whereas crewed Ares launches would involve a spacecraft with a launch emergency escape capsule instead of crossing fingers and hoping nothing goes wrong, we don't need to worry about that with Orion, and so I suspect no more than 5 SSME type engines are needed for even this maximal version, and we might do well with just 4; going to 6 would maximize payload I suppose, and by the time we can do this, with both triple booster and 5-segment booster feasibility checked out and debugged, we really ought to have the allegedly cheaper one-shot hydrogen engines on hand.
At some point we might also go over to putting some or all these engines on the bottom of the tank instead of sidesaddle on the payload. Indeed the main reason to put the hydrogen engines sidesaddle on the payload, once we abandon the idea of reusing the engines, is to lift the weight of the payload during launch so it is not burdening the tank, and for that we might never want more than two there, and any engines in excess of those doing just that ought to go on the tank to lift the propellant most efficiently.
I obviously consider Ares 1 to have been quite a turkey, and see no reason for it at all.
