Even if all the hardware was ready, and the entire Apollo infrastructure was up and running, it would have taken more than 3 months to add another mission on the end of the Apollo programme. Basically if, in 1972, you needed to get to the moon real fast, it would have taken at least 3 months. And that would have been using the hardware from the cancelled Apollo 18 and 19 missions that were already built.
The insane level of difficulty seems to have slipped out of people’s understanding in the years since Apollo. This was a mission on the absolute edge of what could be achieved, with the concerted application of tens of thousands of people, over nearly a decade.
The simple reality is that we have not advanced all that greatly since then. Basic rocket engineering has not changed much. Much like jet airliners. The basic principles were worked out in the 50s, and since then it has only been incremental improvements.
The idea we could restart production of the Saturn V comes around every now and again. And the answer is basically no. A Ranger Jeff says, first you have to create the tooling, and then you try to build a rocket. And then you have to test it to see if you built it right. The time needed to construct a Saturn V, even with the entire production system up and running was years. Look at a passenger aircraft, the assembly time - not the production of the parts - just the assembly, of a large jetliner - 747 - 5 months, A380 - 9 months. And this is a 24x7 production line.a There are limits to how many people you can throw at the problem at the same time as well. There are lots of highly complex systems that get installed in tiny spaces. You simply physically can’t get more human beings working on the job.
A Saturn V is significantly more complex, and much higher stressed, and has zero room for failure. About five years to build one.
Use existing old Apollo hardware? No chance. It is approaching 50 years old. There is nothing that could be expected to work properly. The Saturns have spent decades outside. The LEMs have been on public display. Nothing has been kept in controlled conditions. Even it they had, none of the seals, electronics, even wiring would be useful. It would all have to be stripped out and rebuilt. Essentially disassemble and reassemble the entire craft. Call it a year. Even if you had one.
Since we don’t have any useful hardware directly useable, what do we have? We don’t have a crew module that could go to the moon. Nobody does. We have capsules that can buzz about in LEO, but not manage the week plus trip to the moon and back, and certainly not survive re-entry at trans lunar speeds. Fastest thing to do would be to use Orion. Things could be compressed. Orion isn’t exacty being well funded. Maybe get it going in five years and not 10. Complicating things is that the service module isn’t designed and will be built by the Europeans.
We don’t have a man rated launch vehicle that could loft it. There are suggestions to man rate the Delta 4. A few Delta 4 Heavy launches might get enough stuff launched. At least a year to man rate. And you would want a few test flights with the manned capsule before trying for the moon.
And you don’t have lander. Maybe the fastest thing here would be to try to build something based on the old LEM. But that thing was on the absolute ragged edge of things. A lot of it had to be hand made, because machine fabrication wasn’t able to machine such delicate parts. A huge amount of its fabrication was human, and the skills took a long time to build up. And they are gone now. Maybe three years, probably longer.
In all, five years would be my guess too.