Why weren't Saturn V and the Soviet N-1 Moon rockets made larger in order to simplify Lunar missions?

Question

During the Apollo missions the Lunar lander was separated, turned around and docked during flight. And the command module was left in Lunar orbit during the surface mission. The Soviets planned to have the single cosmonaut to land on the surface do a space walk between the Soyuz-like orbital module and the lander.

Is it correct to say that both these mission designs were compromises made because the launchers were slightly too small for a direct launch from Earth to the Lunar surface? Or was it perhaps a deliberate choice after all, maybe in order to win time in the competitive space race?

Why weren't the launchers designed to be larger so that the Lunar missions could be simplified? How much larger would they have had to be, 10%? 20%? Were they maybe as large as it was feasible to build launchers back in the 1960s, or did they during the development process fail to achieve the initial specifications? Is SLS Block II designed to perform a direct launch of astronauts to the Moon without dockings and EVA's?

Image from SpaceIsThePlace. A model of the two cosmonauts transferring a box with Moon rocks from the Lunar lander to Soyuz.

Answer 1

In order to use the direct ascent method of landing on the moon, which is where the entire vehicle descends and leaves the moon, you would need a rocket an order of magnitude bigger than the Saturn V, not just a bit bigger.

Here's an early comparison NASA made back before they decided to use Lunar Orbit Rendezvous:

The C1 became the Saturn I, the C-5 became the Saturn V, the Nova never got off the drawing board because it was going to be very expensive and hard to build considering that with LOR you could do it for much less.

Here's a good article on the history of the Nova rocket. Here's an excerpt:

Nova was NASA's ultimate launch vehicle, studied intently from 1959 to 1962. Originally conceived to allow a direct manned landing on the moon, in its final iteration it was to put a million-pound payload into low earth orbit to support manned Mars expeditions. It was abandoned in NASA advanced mission planning thereafter in favor of growth versions of the Saturn V.

As time went on the rocket grew and grew and grew until it was a true monster! See some of the concepts in the image below, they make the Saturn V puny in comparison.

Basically, the program died because it wasn't needed, there was no plan for a manned mission to Mars, which was the only use case for it.

Answer 2

During the early part of the Apollo program, the "direct ascent" mode was favored and Lunar-orbit rendezvous (LOR) was considered far too complex. In fact, the specifications of the Apollo service module were set by the direct ascent plan: the SPS engine is sized to lift off from the moon, and the fuel tankage is sufficient for lunar ascent and return to Earth.

The Nova rocket needed for direct ascent would have been about 60% heavier than the Saturn V (~4750 tons versus ~2970 tons). Development of that large rocket was going to take a very long time. Kennedy's "before the decade was out" speech set a deadline for the program, and Nova would take too long. The project took another look at LOR, concluded it was the fastest and cheapest approach, and decided to commit to it. This shaped the Gemini program; if the lunar mission required rendezvous, NASA needed to learn how to do it.

Kennedy's deadline also effectively set a deadline for the Soviet space program, so it's likely that the same logic drove them to a rendezvous strategy. The great advantage of LOR is that the smaller the lander, the less fuel needs to be carried for lunar descent and ascent, and the Soviet LK was indeed tiny and compromised, carrying one man instead of two, requiring the crewman to EVA to board, and so on.

Answer 3

On the Soviet side, Vladimir Chelomey's UR-700/LK-700 project would have used a direct ascent mission profile. The UR-700 was 76 meters tall and 17.5 meters wide at the base; it would have looked something like this:

(left-to-right: N-1, Saturn V, UR-700)

The UR-700 didn't have much design work done on it, but there were problems developing the high-thrust RD-270 engines powering the whole thing. It ended up overlapping too much with the already-existing N-1 project, and never received further development.

Answer 4

Its not rocket science, well actually, it is rocket science. Rockets are not very efficient at all and the worst bit is that to carry anything useful, they have to carry the fuel to push the useful bit, then they have to carry the fuel to carry the fuel to push the useful bit and so on. Saturn V was about three thousand tons at takeoff, and was able to push 45 tons towards the moon, so only about 1.5% of the take-off weight left the earth towards the moon. To increase the moon lander to 50 tons, Saturn V would have had to increase to almost 4,000 tons. The Apollo 17 space capsule they picked up out of the pacific probable weighed not much more then three tons, giving a total efficiency of 0.1%.

Answer 5

There was another argument against the direct ascent mode. A very large and complex rocket should be landed and started on and from the moon surface. But rockets of this size would require on earth a launch pad, a start tower, connections to a control room, a count down with many steps and a lot of personnel. But the rocket motors of the service module and the lunar module used for lunar rendezvous mode could be built very simple and reliable. No turbo pumps, no gas generators necessary, just only compressed nitrogen from tanks for the pressurization of propellants. These motors could be ignited in zero gravity. The hypergolic fuel made unlimited reignition possible.

Answer 6

Compare the extra work to build a bigger rocket (and don't forget that costs have the ugly tendency to be exponentialised, thanks to our good friend, the Tsiolkovski equation) with a few hours for the docking. Rockets aren't cheap.

The main thing to consider is that you need to dock anyway - to get the lunar crew back from the Moon. So you're not saving on anything but those few hours; all the required docking equipment is still required with a bigger rocket. Unless you're considering having the landing module powerful enough to land, ascend and fly back to Earth and land there. In which case, refer to paragraph one - absurdly expensive.

The Soviets didn't use docking. But this wasn't because they thought docking is undesirable - it was because they simply couldn't do it. They didn't have the capability. So instead, their astronaut was expected to make a space walk, and they couldn't exploit the (remainder of the) landing module for the trip there and back again.

Note that even with all the modern technology we have now, we still use the same basic approach. The Space Shuttle was still a staged rocket, though the bit returning back to Earth was considerably larger than any earlier attempt. Modern private rockets experiment a lot with recovering the spent stages, but they still use the same basic staging approaches. This is because our engines are horribly inefficient, which in addition to having to carry fuel to carry fuel means that the costs of additional weight don't scale very well - we tend to use the smallest possible rocket for the task at hand. You use a big rocket to get out of atmospheric drag, a smaller rocket to make the rest of the trip (and velocity) to orbit, a smaller rocket yet to circularise your orbit or start an injection, even smaller rocket for the landing and even smaller for taking off again and going back home (do you think Mars is within our easy reach nowadays? For a single trip, kind of; add a return trip, and we're back to "oh boy").

Answer 7

During the Apollo missions the Lunar lander was separated, turned around and docked during flight.

This point hasn't really been addressed by any of the other answers.

Unlike separation to land the lander, then subsequent docking to get the crew back, there is a fairly simple reason for this manoeuvrer.

The command pod needs to be the top of the stack on launch, as there is no other way to have a working abort system. This means that you cannot pre-attach the lander on the top of the rocket. Likewise, it also cannot be dragged along behind, because the final stage rocket is behind the command capsule, and

1. you can't transfer crew through the rocket
2. firing the rocket engine at the lander is undesirable

That leaves the only viable option being to have the lander under (behind) the command capsule at launch, but move it to be in front once the abort system is no longer needed, which is the operation described in the Question.

A larger rocket would not allow this step to be removed.