Using smaller rockets

[Jairo]

I knew a guy that said the Moon manned missions could be made by using a spaceship composed by many modules, launched with rockets smaller than the Saturn 5.

He even said it would be a better idea, and it wasn´t tried because of pork barrel.

I disagree with that, but what you all think abou this?

[JayUtah]

This is the so-called Earth-orbit rendezvous mission mode, and in theory it works. In fact, the Soviets were working on it as a contingency plan if the N-1 failed. It's a terribly fuel-inefficient way of getting to the moon, and it carries with it the necessity to learn skills such as orbital spacecraft assembly, checkout, and fueling. These days that's not so out of the question, but in the 1960s those were considered very difficult tasks.

[Jairo]

Could you indicate sources where I can learn about the early proposals for the moon missions?

Thanks.

[Count Zero]

jairo said:

Could you indicate sources where I can learn about the early proposals for the moon missions?

Chariots of Apollo is a superb & fascinating on-line history of the development of the lunar landing hardware. This chapter has information about the early plans for the Earth-Orbit Rendezvous (EOR) method, but I really highly recommend reading the whole thing (though it does take a while). Enjoy!

[Jairo]

I see that the EOR demanded two Saturn 5 launches because they would use a big ship for direct landing, but why they didn´t consider using smaller rockets for launching an usual CSM and LM?

I also compared some rockets and I see that smaller ones have a bigger payload / total mass ratio. Wasn´t it considered?

[Count Zero]

There are two problems with separate CSM & LM launches. The second is that once you have them docked in Earth orbit, you need to make the translunar injection burn. This requires a mostly-fueled S-IVB stage, which masses ~200,000 lbs. To get that into orbit, you need a Saturn V first & second stage. In principle, you could launch the LM on a separate Saturn IB, but if you've already got a Saturn V headed that way, and it has the payload margin...
The first problem, though, would be docking the combined CSM & S-IVB to the LM. I know that the S-IVB had its own manuevering thrusters, but I don't think they had the finesse needed for a docking maneuver. The alternative would have been to add a remote or automated piloting capability to the LM to make it the active player in the docking, but this would have added a whole new layer of complexity to an already difficult program.

[Jairo]

I understand the second problem. Launch the S-IVB would require, at least, one rocket slightly smaller than the Saturn 5, so it wouldn´t be a good trade.

But I suppose the first problem could be eliminated by launching some crew in the LM, or even launching the CSM as the separated craft. A Saturn 1B can do it, right?

[Jairo]

I just compared the masses of the CSM from Apollo 7 with the CSMs from actual Moon missions. It was much lighter. Ok, a Saturn 1B couldn´t do it...

But what about sending people in the LM?

[Count Zero]

jairo said:

I just compared the masses of the CSM from Apollo 7 with the CSMs from actual Moon missions. It was much lighter. Ok, a Saturn 1B couldn´t do it...

Correct (and well done - most people wouldn't have bothered to check). On the five manned Apollo-Saturn IB missions, the Service Module had roughly 1/4 to 1/3 the full load of SPS propellant. On lunar missions, the extra is used to make the Lunar Orbit Injection and Trans-Earth Injection burns.

But what about sending people in the LM?

Two big problems here: The first is that there is no provision for crew escape in the event of a launch abort. Even if you could attach some sort of escape tower to the LM, the flimsy Ascent Stage would collapse under the g-load. If it could withstand that, it would need to stay in the shroud because the AS is not aerodynamic. The shoud would also have to have a heat shield, in case the spacecraft fails to make orbit. And then there is the little matter of parachutes... Incidentally, all this extra weight means we exceed the max payload ability of the Saturn IB. We could add boosters, in a proposed configuration called the Saturn IB-D
The other problem would be that there's no provision for a reclined seat in the LM to protect the pilot from the 3+ G's at launch or, in case of abort, the impact from splashdown. Even if if we could cram one in there sideways, what do we do with the darn thing once we do make orbit?

Since we're playing around, perhaps an elegant solution would be to use a moderately enhanced IB (either a Saturn IB-B or a Saturn IB-C) and launch a LM with a manned Gemini capsule. This would take care of the crew concerns. The Apollo could launch with one man, atop a Saturn IV ( I haven't run the numbers, but I'm pretty sure the narrower S-IVB on the Saturn IV would have been able to make TLI on a CSM-LM stack, if it didn't have to lift the LM to orbit). The mission would look like this:
- Gemini-LM launches on a Saturn IB-B. Once in orbit, they remain stacked.
- Apollo launches on a Saturn IV and rendezvous with Gemini-LM. They orbit ~100 yards apart, with one in front of the other.
- Meanwhile, the Gemini crew spacewalk to the LM and check it out. If all is well, and the Apollo makes orbit & rendezvous, they jettison the Gemini. Otherwise, they reboard the Gemini and return to Earth.
- The LM docks with the Apollo, and the crew takes their seats for TLI.

Note that the S-IVB has only a narrow window from two to six hours after launch to make the TLI burn, because of its cryogenic propellants.

OK, so this mission is more complex than the historical Saturn V route, but it does get points for coolness. If bean-counters had gotten penny-wise & pound foolish and not bought the Saturn V, who knows?

[Jairo]

Impressive.

Well, I was going to ask again about launching the CSM alone with something bigger than the Saturn 1B.

Until I imagine it smashing the LM during the TLI.

So, this rised a question about any method that includes a docking before leaving LEO: would the LM or CSM resist its own weight aplied in the docking hatch during the TLI?

[Count Zero]

Good question. IIRC, TLI was good for ~1-G accelleration. The LM was could take some vertical thrust on the docking port, since this was the configuration for LOI. The question is, how much could it take?

[Bob B.]

countzero said:

Good question. IIRC, TLI was good for ~1-G accelleration. The LM was could take some vertical thrust on the docking port, since this was the configuration for LOI. The question is, how much could it take?

I just checked some numbers and it looks like TLI produced accelerations ranging from about 0.65 g at the start of the burn to about 1.4 g at engine cut off. LOI was much gentler with accelerations of, if my numbers are correct, approximately 0.20-0.28 g. I don't know if the LM could handle the higher accelerations.

[Count Zero]

I got about the same. The Lunar Orbit Circularization (LOC) maneuver after LOI put the highest load on the docking apparatus at .3 G.

The Apollo 13 burns (using the LM Descent Propulsion System to propel the CSM-LM stack) averaged ~.1 G.

If we assume that the LM was built to take twice its anticipated load, that still would not be half-way good enough to support it for TLI. We would have to beef it up substantially. A Saturn IB-B or -D could still lift it to Earth orbit, but now we have to wonder if the S-IVB/C-4 has enough fuel to make TLI with the extra mass. This drives us back to the wider S-IVB designed for the Saturn V.

Incidentally, the decision to build the Saturn V drove the size of the S-IVB. If we had gone with the Saturn IV, the smaller S-IVB would have affected the payload on all Saturn IB variants.

[Count Zero]

Although all of the above arguements lead towards the conclusion that the historical Apollo-Saturn V profile was the most sensible, I still like to have fun by imagining alternatives.

To continue the above discussion, perhaps we can launch a standard LM on a Saturn IB, but configure it with a docking adapter fitted to the underside of the descent stage. The adapter would use the same attach points that would otherwise be used to secure the LM to the S-IVB. After launch, the adapter/LM separates from the S-IVB. Automated systems to stabilize the LM as a passive target would not be difficult to integrate.
The CSM/S-IVB would have to have maneuvering capability to allow it to dock to the adapter, and it would have to dock in its first two or three orbits, or the mission's off. Once docked, They make TLI (we know the LM can take Gs in that direction). After TLI, the CSM undocks, the adapter is jettisoned, and the CSM redocks with the LM itself at its top docking port. The mission proceeds from there.

[PeterB]

I asked similar questions on the old board as part of my research for a possible short story. I haven't written the story, but my premise ended up reasonably close to Count Zero's. Except I was going to use a cut-down open-platform LM, and the S-IVB which launched that would be the one to do the TLI, as it would have more fuel left over.