Couldn't Apollo be made cheeper without duplicating parts??

Hey there.

I'm wondering why the designers of Apollo made two crew capsules rather than one. The same living space that was used for the journey to the Moon could also be used on the surface and it would save weight.
In addition the main engine (for mid-course corrections, entering lunar orbit, exiting lunar orbit) could be the same used for descent and ascent to and from the Moon.
In that case the only thing left in Moon orbit during descent would be service module with fuel for the return. Don't say that one man was needed in command module for docking, because gemini docked with unmanned agina a few years before.
I thing such spacecraft would be much lighter and consequently cheeper.

Does anyone know why the designers didn't have such thoughts in mind??

greets

reply to post by sovietman


Apollo 13 would have ended terribly without all these parts, probably why they keep 'em.

Originally posted by sovietman
Hey there.

I'm wondering why the designers of Apollo made two crew capsules rather than one. The same living space that was used for the journey to the Moon could also be used on the surface and it would save weight.
In addition the main engine (for mid-course corrections, entering lunar orbit, exiting lunar orbit) could be the same used for descent and ascent to and from the Moon.
In that case the only thing left in Moon orbit during descent would be service module with fuel for the return.

The main engine, called the SPS, was an integral part of the service module, so I don't see how that could have been used for ascent or descent on the moon if you intended to leave the service module in orbit. If you're talking about the LEM's main engine, it wasn't designed to handle the heavier mass of the command module and you would still need a separate ascent engine since the LEM used a separate ascent engine. Such an engine could not be integrated into the command module because the heat shield is in the way. Perhaps I'm misinterpreting what you meant by "main engine" though.

As for the overall concept, here's why it didn't work that way. It takes a lot of fuel to land and launch, even from the moon, for a given unit of mass. The ascent module for the LEM was very light, especially compared to the well-insulated and shielded command module (2,000kg compared to 5,400kg). The ratio of the living space mass to fuel mass will not change much - the LEM had about 10,750kg of fuel mass total in descent and ascent stages all to get a 2000kg compartment to the surface and back. That means the command module (assuming somehow you do leave the SPS behind) would have needed about 29,000kg of fuel to get it to the surface and back. That's 29,000kg of fuel that would have had to be brought all the way from earth and it's 5000kg more than the mass of the service module itself - in other words, it's as massive as a second fully loaded command module stacked on top of a service module. Instead of saving the 2000kg of the LEM's living space, it will require an extra 16,250kg (compared to the total mass of the complete Apollo stack), and that's assuming that the total mass of the engines that bring the command module down and back up from the lunar surface is the same as the mass of the LEM's descent engine alone.

*Incidently, the Saturn V's maximum payload to the moon was about 47,000kg. The regular Apollo stack had a grand total of about 45,000kg. Being over-budget by 14,250kg would mean the Saturn V would have to be significantly larger, more powerful, and consequently more expensive than it already was.

[edit on 22-10-2008 by ngchunter]

Well, Thebudweiserstuntman, I agree Apollo 13 would end in disaster, but we are talking about designing the craft and the engineers didn't think of disasters like Apollo 13 when they were designing the capsule. It was pure luck that it ended the way it did.

ngchunter, thanks for your time, but i think we didn't understand each other very well. For better understanding I made a (really) quick sketch of my idea. It is located on this link. It's true that the heat shield and larger (=heavier) living spaces would have to be brought to Moon surface and back to orbit, but isn't it still more fuel efficient than launching from Earth with additional living spaces and two additional engines?

Maybe it isn't, I thought it was. And another question . Why would you take additional crewman and all life support system for him with you if that man does nothing but sitting in CSM when the other two are playing on the Moon? Again docking with an unmanned module was done before the first Apollo missions.

Greets

Originally posted by sovietman
ngchunter, thanks for your time, but i think we didn't understand each other very well. For better understanding I made a (really) quick sketch of my idea. It is located on this link. It's true that the heat shield and larger (=heavier) living spaces would have to be brought to Moon surface and back to orbit, but isn't it still more fuel efficient than launching from Earth with additional living spaces and two additional engines?

From your sketch I'm guessing that you're going for a design that basically brings a LEM minus the ascent stage with living area. That's kind of what I assumed into my calculations later in my post. More on that in a second.

Maybe it isn't, I thought it was.

It isn't. Reason why - the command module weighs more than twice what the LEM's entire ascent stage (minus ascent fuel) weighed. For a trip to the surface and back using apollo's engines you need a certain ratio of fuel to payload mass. 10,750kg fuel/2,000kg living space = 5.375. In other words, you need more than 5 times as much mass in fuel than the mass of living space for a trip to the surface and back. This still assumes that you leave your excess descent equipment behind on the lunar surface the way the real LEM did. It also assumes your ascent engine is significantly lighter than your descent engine since you don't need as much thrust because you're not carrying descent fuel anymore - not the case in your example, so we'll just be generous and pretend the engine size isn't a factor. Just for the sake of showing how generous this is, here's an image of a LEM ascent engine model:
www277.pair.com...
About the size of a watercooler. For comparison, here is the descent engine:
www.ninfinger.org...
Ok, enough about that, let's just pretend for simplicity sake that that's not an issue and for some reason your universal descent/ascent engine is lighter and smaller than the LEM's descent engine. We know that we need to bring at least 5 times as much fuel for the landing/takeoff as the mass of our living space assuming we leave stuff behind on the lunar surface and discard the rest when we reach orbit again. 5 x 5,400kg = 27,000kg vs 5 x 2000kg = 10,000kg. That's 17,000kg extra mass you need to bring to lug that heavy command module to the surface and back - this is why they made an entirely separate landing module - it needed to be optimized for mass in every possible way. Payload mass is an absolute premium in space travel, every single kg counts big time. You'd save only 2,000kg if you dropped the redundant living area of the LEM, but it would cost you a whopping 17,000kg in extra fuel you would need. 17,000 minus the 2,000 you saved is 15,000kg. That would require a radical redesign of everything from the Saturn V to the descent stage itself.

And another question

. Why would you take additional crewman and all life support system for him with you if that man does nothing but sitting in CSM when the other two are playing on the Moon? Again docking with an unmanned module was done before the first Apollo missions.

Good question. It is true that Gemini docked with an unmanned Agena as you wisely pointed out. In fact, as you know, that was done to practice for Apollo. Apollo was somewhat of a unique case though. For apollo, and in particular the later missions, the slow rotation of the moon moved the launch site of the LEM out of the plane of the Command Module's orbit during the time the astronauts were on the moon. On earth you can just wait another day for a launch window, on the moon it takes a month to complete a single rotation and the astronauts didn't have nearly that long to wait. That meant the command module (and service module with it of course) must complete a plane alignment to put it back in a place where the LEM's ascent stage could reach it because the LEM didn't have the fuel for an alignment of that magnitude. Even in the earlier missions when the rendezvous procedure was much more conservative I think it was important to have the redundant ability to use the command module pilot to finish the docking should the LEM underperform during the proceedure (assuming it could still safely reach orbit close enough to be rescued).

[edit on 22-10-2008 by ngchunter]

ufff... Thanks! I finally got answers to questions which bodhered me for quite a long time. I really didn't think command module is that heavier than LEM. Also that about rotation of the Moon and stuff is interesting. Didn't think of it.