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Question: Why doesn't NASA or SpaceX use the command module as their lander, instead of having another spacecraft as the lander?

I was playing Kerbal Space Program the other day, and noticed how many people used the command module as their lander. Which is something NASA doesn't do at all.

NASA has another spacecraft that serves as the lander, and another spacecraft as the command module. The lander lands on the Moon, and the command module orbits around. After the astronauts are done collecting samples on the Moon, they rendezvous with the command module and return back to Earth.

But what if NASA had instead of adding an extra spacecraft, they used the command module as both a lander and the command module. So instead of having two spacecraft, you can have the command module land on the Moon, and then blast off back to Earth.

Is there something wrong with my thinking, because I'm sure NASA would've thought of this idea a long time ago.

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  • $\begingroup$ SpaceX is planning to use its spacecraft ITS as both spaceship and lander, integrating the functions of Apollo CM, SM and LM. $\endgroup$
    – Hobbes
    Commented Oct 22, 2016 at 18:09
  • $\begingroup$ How do you see below you? Even in KSP you can see the problem if you try flying from inside the cockpit rather than external view. $\endgroup$
    – Joshua
    Commented May 25, 2017 at 2:37
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    $\begingroup$ @Joshua - On a modern spacecraft, you'd have down-facing cameras which would give you a far better view than the small windows of the Apollo LM. $\endgroup$
    – Russell Borogove
    Commented May 26, 2017 at 2:27

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What you are describing was termed the "Direct Ascent" mission mode. As the Wikipedia article says, it was considered but rejected because of the large launch mass requirement.

If you think about it, there are a few good reasons in favor of the architecture they went with. Some that come immediately to mind:

  1. It included a service module which represented a lot of mass that did not have to be round-tripped to the lunar surface. Although the SM engine was designed around the direct ascent mode, it was done before the final choice of Lunar orbit rendezvous was made.

  2. The command module (the only portion of the vehicle to go all the way to lunar orbit AND soft-land back on Earth) was designed for re-entry (cone with heat shield on the bottom). This would have been problematic for lunar landing (no downward view as afforded by the lunar lander).

  3. As demonstrated by Apollo 13, the separate lander offered a degree of redundancy (albeit imperfectly so). If an Apollo 13-like disaster befell a "direct ascent" type vehicle, it might not have been survivable.

  4. Launch from Earth was (and remains) a complex operation requiring considerable ground support. Lunar liftoff had to be accomplished without any external support, favoring a small, simple ascent vehicle.

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  • $\begingroup$ Excellent answer. Note that the Apollo C/SM's basic design was settled with the assumption that it would be the ascent-and-return-to-Earth stage in the direct-ascent mode. The thrust required to lift off from the moon determined the size of the service module's engine, which is quite a bit bigger than would be needed just for orbital maneuvering. $\endgroup$
    – Russell Borogove
    Commented Oct 22, 2016 at 4:13
  • $\begingroup$ Another thing you might want to mention, but which doesn't warrant an answer of its own, is this: While the comparison isn't perfectly fair, the Saturn V needed a huge launch complex, tons of infrastructure and hundreds or thousands of people directly involved in its preparations for launch, to get reasonably safely off the Earth. Combining the landing and Earth-return vehicle would have required doing something very similar with no ground support, in a potentially unknown environment, on the Moon, 400Mm away. Maybe doable (the LM did parts of it), but definitely increases complexity and risk. $\endgroup$
    – user
    Commented Oct 22, 2016 at 11:20
  • $\begingroup$ Indeed point #2 is rather vital. It's hard to land safely when you can't see where you are landing. $\endgroup$
    – Joshua
    Commented Oct 22, 2016 at 15:20
  • $\begingroup$ @MichaelKjörling -- the LM did it, and I don't see that the process would have been much different for the direct-ascent lander than it would have for the LM. $\endgroup$
    – Russell Borogove
    Commented Oct 22, 2016 at 17:39
  • $\begingroup$ @RussellBorogove The take-off mass for the full CSM stack (maybe minus some landing-specific stage similar to the LM's descent stage, but not minus the descent engine because the SPS would have been used for both descent and ascent) from the surface would certainly have been far larger than the take-off mass for the LM ascent stage. I pretty much ran out of space in my comment, and I did concede that it probably would have been doable (very few things are outright impossible), but it would have raised a number of questions and difficulties that the separate CSM/LM conveniently avoided. $\endgroup$
    – user
    Commented Oct 23, 2016 at 13:24
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The "direct ascent" version of Apollo would have been about twice as heavy as the actual Apollo spacecraft, and would have required a launcher more than 50% larger than the Saturn V.

Once en route to the moon, a lunar landing mission needs to make four significant "burns": one to enter lunar orbit ("lunar orbit insertion" or LOI), one to kill orbital velocity and find a safe place to land (descent), one to lift off and return to orbit (ascent), one to leave lunar orbit and return to Earth ("trans-Earth injection" or TEI).

The velocity change for the LOI and TEI burns are about 700 m/s each. The velocity change for the ascent burn is about 1750 m/s; the descent burn is theoretically similar but in practice extra fuel needs to be carried to allow the pilot time to find a safe landing site, so the Apollo LM's descent stage carried about 2500 m/s worth of fuel.

Since the descent and ascent burns are much larger than the LOI and TEI, a great deal of propellant mass can be saved by doing the descent and ascent with the smallest practical spacecraft. In particular, the Apollo LM doesn't have to carry the fuel for the return to Earth all the way to the moon's surface and back.

The Apollo CSM, if used in direct ascent mode, has just enough fuel to leave the moon and return to Earth, so it would need a larger landing stage to execute the LOI and descent burns. The combined CSM and Descent stage would mass about 89 tons, as compared to 47 tons for the CSM+LM.

This spacecraft mass increase would in turn necessitate a much larger launcher than Saturn V, such as the proposed Saturn C-8 configuration.

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The first plan was the direct ascent. It avoided any rendezvous maneuvers in earth or moon orbit. But one very clever engineer found out that this would require a very heavy launcher, much larger than Saturn V. But his plan with the special moon lander and the moon orbit rendezvous maneuver made it possible with Saturn V.

This man was John Houbolt: https://en.wikipedia.org/wiki/John_Houbolt http://www.nasa.gov/content/john-c-houbolt-unsung-hero-of-the-apollo-program-dies-at-age-95/#.WBHa0_SGPFQ http://www.nasa.gov/centers/langley/news/factsheets/Rendezvous.html

Without his plan a successful moon landing and return to earth before the end of the decade would not have been possible.

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