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Take-off in a craft with rockets allows a gentle acceleration that doesn't kill the occupants.

Given that the Moon has no atmosphere: Is there any current technology that would allow safe takeoff without using rocket propulsion? A giant spring for example?

You can assume that technology is today's (to avoid FTL, wormholes etc.) but that the moon has all the necessary resources to construct a device that isn't a rocket.

For the purposes of this question I define a rocket to be any device that expels matter in order to make use of reaction. I define 'safe' as meaning that neither the crew nor their craft are physically damaged and are able to reach escape velocity in a good state of health.

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    $\begingroup$ Related: worldbuilding.stackexchange.com/q/128601/809 ; Suggested read: Moon is a harsh mistress. $\endgroup$
    – Mołot
    Commented Nov 17, 2018 at 11:20
  • $\begingroup$ @ Mołot - Looks good. Wouldn't the curvature of the Moon simply throw them back into orbit or lift the train off the tracks? $\endgroup$
    – chasly - supports Monica
    Commented Nov 17, 2018 at 11:28
  • $\begingroup$ There were entire books on train / tube / railgun based lainch systems. Both popular science ones and hard sci-fi. Also, for current tech level Space Exploration Stack accepts such questions and have many of them answered. See, for example, space.stackexchange.com/q/19574/11639 $\endgroup$
    – Mołot
    Commented Nov 17, 2018 at 11:40
  • $\begingroup$ @ Mołot - Thanks for that, however it's talking about a non-manned missile. I'm never quite sure what to do when questions are borderline between world-building and science. $\endgroup$
    – chasly - supports Monica
    Commented Nov 17, 2018 at 12:06
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    $\begingroup$ @chasly that's why I'm only pointing out to sources os information, and not answering. It may be useful to you or other users, I hope, but I don't claim it answers your question fully. $\endgroup$
    – Mołot
    Commented Nov 17, 2018 at 13:08

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The problem is that you will always come through the same point on your next trajectory if you do not change your orbit in the same time. This means that anything like:

  • a catapult
  • a slingshot
  • a railgun up a mountain

Will not work. You have to go up, and out. As you pointed out there is no real atmosphere on the moon, so aircraft won't be able to generate lift. Rockets would be a good solution, but those are explicitly removed from the answer.

The only thing I could think of is increasing the railgun up a mountain technique to rididulous proportions. Simply speed up your craft launch it into space and continue propelling it with a huge magnetic field once it leaves the ground. This way you can adjust its orbit after it leaves the ground, preventing it from crashing on its next orbit.

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    $\begingroup$ This is only a problem as long as you actually want to enter an orbit around the moon. If, however, you want to get somewhere, it would be great if you could accelerate directly into the required transfer orbit from the moon surface. If you want to reach earth, all your spaceship would need would be a decent heat shield for reentry. $\endgroup$
    – cmaster - reinstate monica
    Commented Nov 17, 2018 at 20:35
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You could build something like maglev train, with no drag from air it would not find too hard to hit needed 2.48km/s to leave moon.

No giant springs or such, you would die from not-so-small acceleration. You would need around 1.5 minute of acceleration at ~3G and 95km and 48s and 47km at ~5G, so that human can survive.

For sure, we are sane people and wont be sending trains to Earth only delivery vehicle. Something close to what we use now for atmospheric entry: capsule, shuttle.

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  • $\begingroup$ These maglev catapults would be built a) retrograde to moons orbit at the point nearest to earth for perfectly rocket-less transfer to earth (using aerobraking for deceleration), and b) prograde on the opposite side to launch deep space missions. The later would need to launch ships with rockets, though, to allow the ship to maneuver and to decelerate at its destination. $\endgroup$
    – cmaster - reinstate monica
    Commented Nov 17, 2018 at 20:41
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The advantage of a rocket is that you adjust your speed as needed. And they don't have to give you a rollercoaster G force to lift you up; In the Moon, a leisure 1G or less will get you in a good altitude for an orbit in little time, and then the orbital arjustment can be done gently. Think of the Appolo mission lunar modules and how they did their ascent.

Anything that does not allow you for an in loco speed adjustment (i.e.: non-magical, non-scifi engines that are not rockets) will have two disadvantages:

  • They can only give you an impulse in a very short time that will give you either a suborbital path (i.e.: you will crash down even if it takes hours) or an escape trajectory (which is not an orbit);

  • Due to the very small time window for the impulse, even for the suborbital paths you would be exposing the astronauts to G forces that could kill them before they even knew what hit them.

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Thanks to all

A comment by Mołot has set me off in the right direction with a link.

Further contributions on here are welcome. In addition I'm going to post a related but different question on the Space Exploration site.

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