...Not sure if the Soviet Union will get a spot in the limelight here, but I'm nonetheless curious what their response to a flyback booster would be. Shameless copying does not seem in order here, unless they think that their already-broken N1 project would work better with wings on the first stage. It would be a true spacecraft horrorshow!...
Ah, but there is one feature of the Lifter that might be targeted for improvement in a later iteration--and that is, it is bloody huge. It is going to be most economical, on a per ton to orbit basis, at maximum lift, in the 100+ ton range. Excellent for putting up a really big space station fast, in big modules, or a return to Luna mission cheaper than Apollo. Or launching a big Orbiter, which I do believe has its place. But if the Monday-morning quarterbacking wiseacres are correct, we would rarely need 100+ ton capacity and can get by with any attainable ambition, even say a Mars mission, on 30 tons or so. Maybe less and if OTL experience is a guide, payloads smaller than that will dominate the market. Getting really good economy out of the Saturn V based Lifter will require getting "customers" to agree to bundle their payloads in big batches, and to maintain a launch rate even as high as STS averaged OTL, 8 times a year, would require that the total "market" for tons to orbit average something like 5 times OTL. The payloads themselves are a big cost item, so it means that the ATL Western nations increase their overall spending, from all sectors (US military, foreign allied military, NASA, foreign civil science programs, and commercial both US and abroad) and that also assumes Lifter sucks all the oxygen out of rival launchers--those patronized by the USAF, European and Japanese ventures, everything gets sucked into Lifter from Cape Canaveral or Vandenberg. Or the USA expands its Saturn V capable launch sites. At one time a plan for STS was to develop a Texan coastal launch site near Houston for instance. Could the USA wind up cooperating with Europe to the extent that we lease facilities at Kourou--bearing in mind that Kourou was invested in by the French and others because they could see that STS was not economical and a market niche for Ariane existed? Kourou is a nice site, very near the equator and if I believe the site's web page blessed with a lower incidence of tropical storm related bad weather than Canaveral. But clearly a European space alliance is not going to feel great about relying on an American made booster exclusively, even if they do have more say about Kourou or distant Woomera than they do about US soil sites.
So if it works to justify the price of building and maintaining a Lifter Mark I based system, all the better for us space nuts, but we are looking at an ATL where the total space budget across the Free World has mushroomed tremendously, and yet tolerates a Yankee monopoly on launches. It seems more likely to me that for Lifter to keep itself that busy, efficiently, the market must balloon to even higher levels making room for nationalism to develop rivals overseas with NASA/USAF handling only a portion of the total tonnage.
Otherwise, the market will top out at levels considerably higher than OTL, but either making markedly less efficient use of the Lifter with a comparable number of launches, or very very few launches indeed maxing it out. Which entails higher per launch costs in terms of the fixed annual infrastructure the sites and production factories must maintain year after year thus further undercutting potential economies. The higher the average cost of a ton to orbit, the more competitive feasible foreign or conceivably private developed US rival schemes would be.
Now then look at N-1 again. Post-mortem, the fundamental problem with it is that its development was, in typical Soviet fashion, scanted of pre-launch tests, and to a hard to estimate extent, its design with 24 engines on the first stage was inherently risky. Furthermore, it was being stretched to an unreasonable degree to hit the minimum mass to orbit necessary to sustain a Soyuz L/LK lunar landing mission, which IMHO they should have resolved by going to a two-launch strategy, scaling back the goals for a single launch to something more reasonable and observing that the load lifted by two launches would allow much more generous margins for their moon mission. It may be that the inherent flaws in Soviet development bit them too hard on such a grand scale to be sustained by any approach. And it may be that 24 engines is too many for anyone, although the large numbers of engines used in other systems that have enjoyed good success, such as Saturn 1 or Falcon 9, suggest this was not as awful a hurdle to clear as it seems--admittedly, 24 is a heck of a lot more engines than 8 or 9!
OK, then, let's say the Soviets bitterly agree that N-1 was just too bloody much for them, and that to match the Yankees going head to head with them would merely compound the folly and waste.
Very well--what of a much smaller "Lifterski" that uses just say 4,5, or 6 of the engines developed for N-1's A block? Offhand we can see it would have a lift to orbit comparable to Proton, which they already have. (However, as late as 1972 the dang thing still had not cleared the hurdle of Soviet certification as standard equipment, such was its failure record up to that point). It might work out to be somewhat better than Proton to be sure, and if the Americans have their heads screwed on right, Soviets too ought to enjoy major economies by recovering and reusing the booster many times. Their maximum capacity would remain limited to Proton levels, but the wiseacres say that is good enough, or would be with 10 or so tons increment.
Suppose instead of 24 engines (in some versions, kicked up to 30!) the Soviets were to develop a flyback booster of their own using just say 7 of those same engines?
As it happens, the Soviet engines were designed for somewhat higher ISP (at sea level and in vacuum, designed for sea level optimization of course) than the F-1A attained. I picked 7 engines because that forms a hexagon with a 7th engine packed neatly in the middle. It might be better to forego that engine since it suffers a worse battering by being surrounded by 6 neighbors, the outer ring of them having only 3 at most in such proximity--reduced to just 2 in case a 6-engine hexagon is chosen, though the more distant ones would contribute to problems too. The seventh engine would raise wear and tear on the outer ring and be much more battered by that ring, by a factor of 2 or more. The only J-2 to fail in Apollo was a center engine after all.
Still, let's look at 7 for a moment anyway. The first stage engines developed for N-1 were designated NK-15, with modified versions for the upper stages but set that aside for the moment. I get somewhat conflicting data from this source, Brügges's site
versus Encyclopedia Astronautica
which he also cites but criticizes. Combining their inputs conservatively, what we have is an engine that can produce at least 140.6 tons of thrust at sea level, with a vacuum ISP and thrust of 318 sec and 1544 KN respectively. Each masses 1250 kg, so taking 7 of them gets us 984.2 tons of lift on the pad. They are designed to burn for at least 113 seconds, and consume 495 kg/sec each, thus seven of them will consume at least 391 tons, more if they can burn longer. Assume that the Soviets decide to go for a moderately more conservative burn out speed (at max payload; they too will be using propellant ballasting) such that delta-V at that point is 3600 m/sec, say burn is extended to 120 seconds for total propellant mass of 416 tons, about 1/6 the tonnage I guessed for Lifter (2500 tons). This implies a maximum lift-off mass of 610 tons, about (1/5.6) times the American system. Say the post-braking mass of the stage is 60 tons all up, which might be an unfairly high estimate. This leaves merely 134 tons for the upper stack, which must attain orbit from a lower speed than the American version. With the burnout speed being less than 90 percent the American (delta-V is less by that amount, but both systems must lose real velocity to gravity loss and air drag so the 400 m/sec difference will count for more) the aerodynamic drag will be far less on the Soviet version, so 1/5 Lifter braked mass is probably a gross overestimate, we ought to be able to transfer 10 tons or more to the upper stack. But let's roll with 60 for the moment.
Soviet launchers have a tendency to go for higher G loads than American, which cuts gravity loss significantly, such that perhaps the Soviet upper stage has less mission delta V than the American, not more--the 400 m/sec deficit already made up to an extent by lower gravity loss in the booster burn (some 30 seconds less burn time) and heavy upper stage thrusts can close the gap further or surpass it. Let us conservatively guess that the Soviet upper stack has 6 km/sec delta-V to provide, and must use developed Soviet engines and tankage, either derived from N-1 using more ker-lox or from Proton using hypergolic propellant. On this page
, look at the N-1 Block G, the fourth stage. Brugges gives the all up stage mass as 59 tons, 56 of which are propellant, with a single NK-19 engine of thrust 451.1 KN and ISP of 345 sec (!). It does seem unlikely this would be adequate for 74 tons of payload (what we need to match up the mass) but the 3rd stage, V block, is clearly much too large. I'd have to iterate around a lot in Silverbird Calculator (which is probably valid to use since I am estimating a maximum payload version here).
Indeed Silverbird gives just 13 tons here which is not terrible compared to R-7 but does not even match Proton. But it also falls short of the upper stack maximum by some 62 tons! Let us assume a comparable distribution of stage to payload mass for the maximum--82 percent booster--so we need a 110 ton booster which is nearly double the G block estimate--raise the net mass by that amount, doubling the thrust as well.
I took this approach some iterations and also considered substituting a single NK-15V engine, which has the advantage that it is essentially the same as the first stage engines but vacuum optimized, so its construction cost will be more economical than using a more divergent engine or set of them, offsetting somewhat inferior performance. At most we get around 16 tons payload, which is inferior to Proton of course, and about half that magic 30 ton goal. Perhaps going back to the drawing board with 9 engines instead of 7? This is no worse than Falcon 9 after all and about the same as Saturn 1 so why not? That would close the gap with Proton.
Speaking of Proton, what happens if we scale down the Mark 2 second stage, data for which can also be found at Brügges's site? At 2/3 the standard stage mass, we would have 7.8 tons dry, 104 tons of propellant, and an engine set delivering 2352 KN at ISP 327 sec. That only delivers 14.5 tons to orbit.
Well then, let us look at what 10 engines can do!
Raising propellant and dry mass in proportion, and with it the first stage burnout mass, we would of course raise the upper stack mass to 191 tons. Using a 156 ton stage derived from the G stage of N-1, with a 9 ton dry mass and thus 147 tons of propellant and 3 engines...I iterated a few times, and with upper stage mass being 10 tons dry holding 158.5 tons propellant, the payload comes in just over 22 tons, more than matching Proton payloads to LEO.
Considering the very large thrust margin, we can probably put bigger stages on it too. Note that the gross mass of the upper stack is larger than Apollo's lunar stack atop a full S-IVB, so if the Soviets develop hydrogen engines they will be able to multiply their capacity further still.
Meanwhile with propellant ballasting, this booster can also launch smaller payloads. But being much smaller than the American version (post-braking mass 86 tons and holding 594 tons of propellant) it will be cheaper to develop and maintain. Thus even allowing for Soviet inefficiencies and a late start, they might well have it operational in the same time frame the vaster American version is, or perhaps even sooner. It would be called upon to launch lighter loads only rarely compared to the American version, and its economics would more aggressively cut Soviet launch costs until Americans developed a smaller version themselves, if optimism about filling 100 ton payloads for frequent launches turns out not to be warranted. Meanwhile the Soviets can contemplate going even bigger, or multiplying their payloads by developing high altitude hydrogen engines to replace the ker-lox interim second stage with. Simply swapping in the ISP of a J-2 and throwing in a ton of dry mass penalty, I suddenly get over 37 tons to orbit! I suppose I should have put more tons of load into the stage dry mass, but clearly by using hydrogen there there is very much to extend the capability--even bearing in mind the need to scale down the propellant load.
So that is something Ivan can do. And note that if a ten-engine booster is too risky, a 5 engine booster that begins with a mere 8 tons to LEO capability can still replace the R-7 system, and developing hydrogen engines for its upper stage can put it back in the Proton-rivaling game.
Six engines might be about right.