Originally Posted by NJRick
Actually Mike Robel (who's done some REALLY cool alternative Saturn models (static) and I were discussing just this issue over on YORF. He pointed out much the same thing you just did-- that while the 3 million pound thrust of a pair of F-1's APPEARS at first blush to be "too much" for a Saturn IB replacement vehicle, in actuality it is not, and for the very reasons you pointed out. The Saturn IB's payload limitations basically doomed it. For any type of mission with the Apollo CSM beyond LEO but still in Earth's vacinity (say to geosynchronous Earth orbit or the Lagrange points closer to the moon such as L1 or L2, a fully-fueled capability on the CSM was absolutely required. Being able to launch a substantial payload with the Saturn IB replacement would also have been highly valuable, especially if you decided to do an Earth-orbiting space station project and resupplying that station periodically, unlike Skylab which basically launched "fully stocked" and used the LOX tank of the converted S-IVB stage "workshop" for a "septic tank". A continuously manned, periodically resupplied space station in LEO by the US would have required some sort of resupply/disposal capability, just as the Soviets learned with their Salyut stations when they developed the robotic Progress freighters for exactly that function. If you developed something akin to say the ATV/HTV, or even a much simpler and cheaper "MPLM" type "pressurized tuna can" that could be loaded with supplies, water, boost propellant, etc. that was carried up under the SLA panels on the top of the Saturn IB replacement's S-IVB stage to orbit, and then have it extracted by a transposition and docking manuever identically to how the CSM docked and extracted the LM from the spent stage, and then transported that module to the space station and docked it to the station, or even simply delivered it into station keeping with a space station where it could be grappled by a robot arm and berthed to the station, leaving the capsule free to dock with the station independently of the resupply module, that module could then be unloaded at will and filled with refuse from the station, and jettisoned for reentry, either from it's own thruster system like Progress, or docked to by the CSM and then dropped back into the atmosphere by a partial de-orbit burn by the CSM followed by undocking and then a prograde burn so the Apollo could reenter later, well away from the disposed of resupply module.
In addition, there were plans for small "conical shaped" space stations that would basically fit in the location of the SLA adapter panels of the Saturn IB. Such a design would be an excellent starting point for space station modules which could have been delivered by a Saturn IB to orbit and brought to the space station for assembly by the CSM, then released and grappled by a robot arm which would permanently dock it to the station at a node connector, much as was done with shuttle and ISS. This is most likely how any successor stations will be constructed in the future now that shuttle is retired (or free-flying modules with propulsion systems sufficient to rendezvous and dock with the other modules forming the station, done either by remote control or autonomously as the Mir modules and Progress freighters did/do... this allows for larger modules to be launched unmanned, and this paradigm certainly would have worked for a Saturn IB successor just as well as it would for Proton and the modern EELV's and/or Ariane V, but OTOH it's AWFULLY convenient to launch a manned spacecraft with the module, like shuttle did carrying most of the ISS modules to orbit.
To be able to do it though, you need a "high thrust" first stage capable of getting that much weight off the ground and up and moving. With a fully fueled CSM and a substantial payload module under the SLA panels, the twin F-1 vehicle would have been ideal. Now, as the fuel is expended and the stage nears fuel depletion and burnout, the 2 million pound + thrust is going to be too much, and start building gees quickly. It would be necessary to shut down one of the F-1's early, since the F-1 couldn't be deeply throttled, and that would complicate things a bit. The off-center thrust shouldn't be too much of an issue... after all shuttles flew for 30 years with EXTREMELY off center thrust from the SSME's on the orbiter with the heavy nearly-half-full ET hanging off the side of the orbiter. This might have been a bit "radical" at the time though... roll control might be something of an issue with one F-1 shut down, but owing to the off-center thrust I think it would still be controllable. The remaining F-1 would have had to 'slew' somewhat to thrust through the center of gravity once the other F-1 was shut down, so the vehicle would be flying "slightly sideways" (much as shuttle did) until the first stage burned out.
In thinking about it, the single F-1 teamed up with a pair of H-1's presents some interesting possibilities. I wish I had access to the tools and knowledge to be able to analyze the possibilities properly, but a thought exercise is still possible. Obviously the addition of another 400,000 lbs thrust in addition to the 1.5 million pounds thrust of the standard F-1 would give you just under 2 million pounds thrust, a pretty decent 'bump up' from the standard Saturn IB. Now as the stage gets lighter from propellant burning, acceleration increases (gees) and at some point you're going to have to either deeply throttle or shut something down to reduce thrust. Saturn IB did this by shutting off the center four engines and running on the outer four for 20-30 seconds or so (IIRC, but not exactly) before stage burnout. You have two choices on this particular stage-- shut down the twin H-1's, or shut down the F-1. Shutting down the twin H-1's leaves you without roll control, but this might not be critical. But it also wouldn't drop but about 400,000 pounds of thrust or so-- the F-1 would still be churning out 1.5 million pounds thrust, which is about what the Saturn IB started off with. This would probably throw too many gees on the payload/structures. The other possibility, shutting down the F-1, means you can now burn your twin H-1's quite a bit longer, since they burn propellant FAR slower than the hungry F-1 does. It drops your thrust to about 400,000 pounds, or roughly the equivalent of a PAIR of J-2S's. This makes for some interesting performance. You're still carrying the weight of the probably 80% empty first stage propellant tanks, and you're dragging along tons of dead weight of the F-1, but you're going to burn the first stage quite a while longer on just the pair of H-1's than you would otherwise. The thrust is somewhat low, but it shouldn't be underthrusted to the point gravity losses become objectionable. Essentially the first stage would then be acting somewhat like a twin-J-2S "second stage", only powered by kerosene burning H-1's and using the first stage tankage, and dragging along the dead F-1 (I don't think for a moment it'd be worth it to design a separation system into the stage to drop the F-1 after shutdown). Once the twin H-1's have burned to propellant depletion, the first stage is jettisoned and the single J-2S upper stage takes over to deliver the CSM/cargo to orbit.
Later! OL JR
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