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Over 45 years ago, we know that USA/Russia were able to send satellites/humans and return them to the Earth. Isn't that supposed to be much more difficult to do than what SpaceX is doing now by returning (landing) the rockets/boosters on the Earth (not the Moon)?

It is more difficult to land on the Moon or the Earth? I would say that it is easier to land stuff on the Earth because it is closer to us, it's easier to exchange signals.

Or is it the mass of the lander/rockets we are landing that makes things harder? The rocket such as Falcon 9 is much heavier than the rovers/landers that were landed on the moon and then returned back to the earth.

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    $\begingroup$ An organization that receives 5% of the federal budget can achieve seemingly impossible things. An organization that receives far, far less the 0.5% of the federal budget can do a whole lot less. $\endgroup$ – David Hammen Jan 25 '16 at 0:11
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Comparing the whole Saturn V mission vs an orbital delivery is totally apples to oranges. There's no doubt that developing Saturn V was vastly harder but they are so different that the comparison is pointless.

There is one point where comparing them makes sense: Vertical landing of a rocket-powered vehicle.

It's still an unfair comparison for one fundamental reason: one was man-controlled, while the other was automatic. There's a human at the heart of the lunar lander controls, where Falcon 9 uses a computer.

In one hand, the tasks before the computer are harder. There's a precise point of landing vs any random semi-flat piece of Lunar surface. There's a vehicle with a much higher point of mass and poorer landing legs configuration. There's a much higher gravity to brake against. In case of barge landing the landing point moves. A computer to land a lunar lander would be much, much easier to build.

But on the other hand you have a human in a terribly unwieldy spacesuit, with only rudimentary instrumentation (heavily depending on observing shadows on the ground!), subjected to all kinds of accelerations, doing something he had never done it outside of simulation, under immense amount of stress, and having only one go at it.

So while theoretically the computer's task is harder than the human's, the number of handicaps the human suffered while performing the landing well balances it out.

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    $\begingroup$ It was certainly possible for an Apollo LM to land entirely under computer control. Much riskier, of course, because it would have been unable to avoid obstacles or jagged terrain, but the US had already landed Surveyors in 1966. $\endgroup$ – Russell Borogove Jan 25 '16 at 16:15
  • $\begingroup$ The lateral control authority of a nearly hovering vehicle that tilts its attitude for side thrust depends on its acceleration capability but also gravity when landing. Ignoring its ability to change its attitude to provide the side thrust, that side acceleration is likely to be lower on earth because the thrust acceleration is likely to be just somewhat above that required to hover. On the moon, the gravity acceleration is low, so tilting it gives little lateral acceleration. In the end, its interesting that Mars might have the best hovering situation! $\endgroup$ – Matt Jessick Jan 26 '16 at 12:52
  • $\begingroup$ To add to the stress & discomfort of the humans landing the LM on the Moon, during the Apollo era, was the human had to do everything you mention while standing. Because of space & weigh limitation there were no seats in the LMs. $\endgroup$ – Fred Oct 21 '18 at 8:18
  • $\begingroup$ @Fred Standing for the 12 minutes or so of powered descent, at a maximum of about 1/3 g, wasn’t a major burden. $\endgroup$ – Russell Borogove Oct 21 '18 at 19:20
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Over 45 years ago, we know that USA/Russia were able to send satellites/humans and return them to the Earth. Isn't that supposed to be much more difficult to do than what SpaceX is doing now by returning (landing) the rockets/boosters on the Earth (not the Moon)?

Landing a rocket containing people on the Moon in the 1960s was orders of magnitude harder than what SpaceX is trying to do now. The Apollo program would have been shut down shortly after Apollo 13 had that mission been preceded by Apollo 11 and 12 punching the barge. The Moon landings had to work, the first time, and every time. Failure was not an option.

There were people on those rockets, and national prestige was at stake. None of the Apollo missions punched the barge. That failure is an option for SpaceX (until they get it right), that people's lives aren't at stake, and that corporate success is at stake compared to national prestige makes SpaceX's task much, much easier.

Over two percent of the entire United States federal budget went to NASA from 1963 to 1969 (peaking at over 4% in 1965 and 1966), and over half of NASA's budget from 1964 to 1970 went to the Apollo program. A sizable fraction of those monies went to the problem of landing a rocket on the Moon, first time and every time.

In comparison, NASA currently receives less than half a percent of the federal budget, and it only sends a tiny fraction of that tiny fraction to SpaceX. SpaceX in turn only dedicates a small fraction of its incomes to the problem of landing a rocket on a barge.

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Landing on the Moon is simultaneously easier and more difficult than SpaceX's landing of a first stage:

  • Easier: there's no atmosphere, so you don't have to worry about aerodynamic heating and pressure.
  • Easier: because the Moon has a much lower gravity, you need far less energy to ascend from the Moon to an orbit around the Moon than you need for a launch from Earth. This also makes the landing on the Moon easier.
  • More difficult: The hard part about the Moon landing is getting off the surface of the Earth and into Moon orbit. We needed a gigantic 2000-ton Saturn V to get that far, but we could land and ascend again in a 10-ton LM.
  • More difficult: In the 1960, computers were barely up to the task of helping a pilot land safely. These days, we have enough computer power that we can allow a rocket to land autonomously without human intervention.
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    $\begingroup$ So… on an absolute scale the Moon landing itself was simpler, but viewed as the entire project relative to the technology available at the time, it was harder. $\endgroup$ – radex Jan 24 '16 at 22:22
  • $\begingroup$ Landing on the moon entails shedding at least low lunar orbital velocity, ~1.6 km/s. For the SpaceX booster, most of the velocity is shed by aerobraking and only terminal velocity has to be killed. $\endgroup$ – HopDavid Jan 25 '16 at 0:27
  • $\begingroup$ True, but shedding more velocity is just a matter of adding more fuel or scaling up the engine. On the F9, you have to build the stage to withstand atmospheric heating: the engines get red-hot during reentry, and that's before the center engine is ignited. $\endgroup$ – Hobbes Jan 25 '16 at 9:01
  • $\begingroup$ 1960s computers were absolutely up to the task of landing a rocket on the moon autonomously; Surveyor did it in 1966. Landing under computer control with a pilot in the loop (as Apollo did) is actually much harder. If the Apollo LM was guaranteed a sufficiently large, flat landing site, it could have landed "hands-off" easily. $\endgroup$ – Russell Borogove Aug 6 '16 at 18:15
  • $\begingroup$ Easier(?) because the lunar descent engine could be throttled to hover/translate to a gentle touchdown at the desired site, whereas limits on how low the Falcon 9 engine could be throttled requires a precision hoverslam. More difficult because the final LM touchdown required human skill, whereas the Falcon 9 touchdown is handled by computer. $\endgroup$ – Anthony X Oct 21 '18 at 22:02
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It depends on how you want to define "difficulty". Using cost as a proxy for difficulty, the Apollo program cost more than the equivalent of US\$100 billion. SpaceX has probably spent under ten billion to date, maybe closer to 5.

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  • $\begingroup$ Any sources to back up (or at least give a good clue) as for the $5B number? $\endgroup$ – radex Jan 24 '16 at 22:23
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    $\begingroup$ Wild-ass guess. $\endgroup$ – Russell Borogove Jan 25 '16 at 4:17
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    $\begingroup$ Via wikipedia, they operated on total funding of \$1B up to 2012, and at that time had another \$4B of future contract revenue on the books. en.wikipedia.org/wiki/SpaceX#Funding $\endgroup$ – Russell Borogove Jan 25 '16 at 4:27
  • $\begingroup$ Have you included inflation? $\endgroup$ – Ian Ringrose Jan 26 '16 at 10:15
  • $\begingroup$ The Apollo figure is inflation adjusted, yes. $\endgroup$ – Russell Borogove Jan 26 '16 at 16:43

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