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Apollo already had some hardware for rendezvous on lunar orbit, so why not do the same on earth orbit and use smaller rocket? I suppose designing smaller rocket is simpler and also building double amount of rockets might be even cheaper than bigger rocket that needs humongous machinery. Economy of scale kind of. Was the diameter of the payload important?

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    $\begingroup$ The payload could not be partioned into two or three parts of similar weight. The Command/Service Module CSM had a launch mass of 28,800 kg and the Lunar Module LM of only 4,700 kg. The third stage of the Saturn V was used for acceleration to the Moon too and had a launch mass of 123,000 kg. A part of its fuel was used for Earth orbit insertion. A much smaller rocket may have used for the Moon only, but it would be still heavier than the mass CSM and LM together. $\endgroup$ – Uwe Jun 4 '18 at 17:08
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    $\begingroup$ One big rocket is more efficient than several small ones because of atmosphere. $\endgroup$ – Agent_L Jun 5 '18 at 10:45
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Assembling or fueling the Apollo spacecraft in Earth orbit from multiple smaller launches was considered; it's referred to as the Earth Orbit Rendezvous mission mode.

The original EOR studies were based on the Direct Ascent mission plan, where the Apollo Command/Service Module would land on the moon with the help of an additional descent stage, then return, without a dedicated Lunar Module. For direct ascent, the trans-lunar stage would have massed nearly 200 tons while in Earth orbit, requiring the very large Nova 8L or Saturn C-8 booster (essentially a scaled up Saturn V with 8 engines on the first stage); splitting that into two launches in the Earth Orbit Rendezvous plan would have required two boosters of the Saturn C-4 type -- more or less a Saturn V scaled down to 4 engines. 4-5 launches of a still smaller Saturn C-3 were also considered, but that would have required orbital assembly rather than just fueling, which I don't think would have been practical.

However, the Lunar Orbit Rendezvous plan with its tiny, separate Lunar Module brought the required mass in Earth orbit down to about 125 tons, requiring only a single launcher. This was originally going to be a Saturn C-4, but as the design weight of the Apollo spacecraft grew, the C-4 got upgraded into what eventually became the Saturn V. Lunar Orbit Rendezvous was chosen because it was the fastest and cheapest solution to get to the moon before 1970.

It would have been technically possible to combine Lunar Orbit Rendezvous with Earth Orbit Rendezvous, launching the spacecraft and translunar stage on one booster, and fuel on another, refueling the translunar S-IVB stage in orbit, then going on with the Apollo mission as actually flown. This could be done with one C-4 booster (~99 tons payload) for the spacecraft and one C-3 (~45 tons payload) for the tanker.

However, it would have complicated the mission plan considerably. While waiting for the Earth orbit rendezvous, the hydrogen fuel used by the Saturn upper stages would be beginning to boil off; the sooner you get on your way, the better. Small spacecraft had been docked before -- Gemini with a small Agena target vehicle -- but it would be significantly more difficult to dock a 40-ton tanker with an 80-ton translunar spacecraft. Additional procedures for docking and fuel transfer would mean more training, more technical issues to solve, and more opportunities for things to go wrong. Two independent launches also increases the chances of a failure. There are twice as many guidance systems, staging systems, telemetry systems, and so on, the failure of any of which makes it impossible to continue the mission.

Launching just the Lunar Module on one booster and docking to it in Earth orbit wouldn't have been attractive. The LM massed only 16 tons in its final configuration, so even a C-4 wouldn't be big enough for the rest of the translunar spacecraft, and the docking maneuver would have to be flown by the entire 110-ton S-IVB/CSM stack, not just the CSM -- the actual Apollo TDE maneuver was done after the spacecraft was in translunar coast.

There would have been some economies of scale associated with building twice as many boosters, but the rockets would still have been quite large. The C-3 and C-4 were originally intended to be 8 meters in diameter. The stages would have been individually easier to transport compared to the Saturn V's 10-meter stages, but not enough easier to compensate for the complications in the mission. The payload size wasn't a constraint here; maximum payload diameter was around 6 meters, at the Lunar Module's "hips", and the translunar S-IVB stage (which would have been used in the C-4 as well as the Saturn IB and Saturn V) was 6.6 m in diameter.

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  • $\begingroup$ Looking at the linked Wikipedia article, apparently the Nova concept with 8 engines was considered later for a Mars mission, and the Moon concept was much smaller. $\endgroup$ – kim holder Jun 4 '18 at 14:55
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    $\begingroup$ The "Nova" name got used for a lot of things, but in the middle of the "lunar rockets" section we have this: "Nova was still targeting the direct ascent approach, which required the most lift capacity. The most powerful of the resulting "normal" designs, the 8L, included eight F-1's in the lower stage and placed 68 tons in a translunar trajectory. " I'll make it clear that I'm talking about the 8L. All of these Saturn C-x and Nova designs were little more than lines on paper... $\endgroup$ – Russell Borogove Jun 4 '18 at 15:05
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    $\begingroup$ Good thing too. Nova is an inauspicious name for a launch vehicle, seeing as how novae are collisions followed by tremendous explosions. $\endgroup$ – Mark Adler Jun 4 '18 at 16:32
  • $\begingroup$ Would there have been any practicality to parking an earth re-entry model in terrestrial orbit just before the main mission launch (so that in case of a disaster launching that, the astronauts would be on the ground rather than stranded in space), and then having the lunar mission dock with that? The cost of imparting enough delta-V to carry the terrestrial-reentry heat shield around the moon would seem greater than the cost of imparting enough delta-V to a returning craft to dock with a re-entry vehicle. $\endgroup$ – supercat Jun 4 '18 at 18:09
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    $\begingroup$ @supercat Nope. On return from the moon, the spacecraft has about 3000 m/s of velocity in excess of Earth orbital speed, which it's going to shed by slamming into the atmosphere. The mass of the heat shield, less than 1 ton, is pretty small in comparison to the propellant it would take to cancel that speed. $\endgroup$ – Russell Borogove Jun 4 '18 at 18:31

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