# Moon-earth slingshot

## Main Question or Discussion Point

lets assume that a base was constructed on the moon for the purpose of launching spacecraft.

if spacecraft were dropped off the moon toward the earth, with a horizontal velocity component such that they would then slingshot around the earth, could the initial velocity provided by a mechanical catapault on the moon be sufficient to reach other solar objects in a timeframe realistic for manned missions?

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tony873004
Gold Member
Unlike a rocket which burns its engines over a period of minutes, the mechanical method would have to produce all its delta v in just a few seconds. A mechanical catapault would produce a G force too high for astronauts.

In theory, if you weren't concerned about G forces, and you built a large enough catapault, there's no reason that you couldn't reach other solar system objects in a reasonable timeframe, with or without passing by Earth.

But your spacecraft would still need rockets for mid course correction burns, including plane transfers. And if you wanted to orbit your target planet, rather than do a flyby, you'd need rockets as well. If you were planning on this being a return mission, you'd need a lot of fuel since there'd be no catapault on your target planet.

i was considering the catapault mainly to have enough escape velocity in the vertical component to get off of the moon, and also to have a horizontal component to avoid hitting the earth (or the atmosphere). i wasn't really thinking about the catapault actually providing the energy, but rather the gravity of the earth...so would the gravity well of the earth (that is, the delta v being provided solely be something falling from the moon toward the earth) be enough to reach, say, Mars in a decent amount of time (or even at all)?

i had tried doing the calculations myself, and got a bit lost.

and i was hoping that this scenario would allow for a manned crew (so accelerations are a concern, as well as length of travel). i also realize that the orbital periods of the earth and mars mean that astronauts would not be able to leave mars (back for earth) for at least a year. im also not concerned with them returning at the moment.

having rockets to correct the trajectory periodically would be fine, but would additional rockets be needed to enter an orbit around mars?

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Janus
Staff Emeritus
Gold Member
quetzalcoatl9 said:
lets assume that a base was constructed on the moon for the purpose of launching spacecraft.

if spacecraft were dropped off the moon toward the earth, with a horizontal velocity component such that they would then slingshot around the earth, could the initial velocity provided by a mechanical catapault on the moon be sufficient to reach other solar objects in a timeframe realistic for manned missions?
You gain no advantage doing this. The gravitational Slingshot works because the probe has a different oribtal velocity around the Sun than the Planet does.

Also, at the moon's orbital distance escape velocity is about 1400 m/s. the Moon's orbital velocity is 1000 m/s. Which means you only need to add 400 m/s from the moon's orbit to escape the Earth. It would take a velocity change of over 800 m/s to drop an object to low Earth orbit distance from moon orbit. It would take more effort for no gain.

pervect
Staff Emeritus
I'm not sure what's "reasonable" to get from a mechanical catapult.

As far as the orbital dynamics go, what you want to do is to be able to reach the L1 point. This puts you on the "Interplanentary Superhighway". (The "Interplanetary Superhighway is actually a serious concept, in spite of the hokey-sounding name).

http://math.ucr.edu/home/baez/lagrange.html

I've just recently been reading about the "Interplanetary Superhighway" (IPS). Have you heard of it? If not: It turns out that the L1 and L2 points act as "gateways" for very low-energy transfer orbits - better than Hohmann transfers. The IPS works by transferring between unstable and stable manifolds winding off and on unstable orbits like at L1 and L2.

The recent Genesis probe, the one that crashed, used such an orbit.
You'll need very slightly under the lunar escape velocity of 2380 meters/second

http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html

to reach L1 from the moon. I'm getting 2320, unless I made an error, only 60 m/s different from escape velocity.

Of course, you'll need something to stop you when you reach wherever it is you're going.

tony873004
Gold Member
quetzalcoatl9 said:
would be fine, but would additional rockets be needed to enter an orbit around mars?
You could do an aerocapture and eliminate the need for a lot of rocket power. And continued passes through the Martian atmosphere could help circularize your orbit. But you'll still need some rocket power to make fine adjustments during this procedure. And at the end of the procedure, you need to raise your perapsis out of the upper atmosphere. You'll need rockets for that.

Janus said:
Also, at the moon's orbital distance escape velocity is about 1400 m/s. the Moon's orbital velocity is 1000 m/s. Which means you only need to add 400 m/s from the moon's orbit to escape the Earth. It would take a velocity change of over 800 m/s to drop an object to low Earth orbit distance from moon orbit. It would take more effort for no gain.
ok, i follow, so a slingshot doesn't make sense.

pervect said:
As far as the orbital dynamics go, what you want to do is to be able to reach the L1 point. This puts you on the "Interplanentary Superhighway". (The "Interplanetary Superhighway is actually a serious concept, in spite of the hokey-sounding name).
this is a really cool concept. i understand that sending spacecraft this way is also very slow though, isn't it? for unmanned satellites that's ok, but wouldn't it take too long to get a manned crew to mars this way?

tony873004 said:
You could do an aerocapture and eliminate the need for a lot of rocket power. And continued passes through the Martian atmosphere could help circularize your orbit. But you'll still need some rocket power to make fine adjustments during this procedure. And at the end of the procedure, you need to raise your perapsis out of the upper atmosphere. You'll need rockets for that.
so disappating the energy as heat in the atmosphere, like how the mars rovers were landed?

tony873004 said:
Unlike a rocket which burns its engines over a period of minutes, the mechanical method would have to produce all its delta v in just a few seconds. A mechanical catapault would produce a G force too high for astronauts.
yeah, getting 2320 m/s at an acceleration of 10g would require a very, very large catapault.

pervect
Staff Emeritus
quetzalcoatl9 said:
this is a really cool concept. i understand that sending spacecraft this way is also very slow though, isn't it? for unmanned satellites that's ok, but wouldn't it take too long to get a manned crew to mars this way?
Yes, while very low energy transfers are possible with the IPS concept, they take a very long time.

i think it would be more efficient to drop one towards the sun

tony873004
Gold Member
OSalcido said:
i think it would be more efficient to drop one towards the sun
The Sun is not easy to get to. Look at the all the slingshots required to get the Mercury Messanger spacecraft to Mercury.

this had come up in a sociology class i was in a long time ago, where someone was adamantly arguing for launching our garbage into the sun.

my counter-argument was:

1) we would wind up spending a billion $to dispose of$10 worth of garbage
2) the amount of velocity needed to kill off the velocity inherited from the earth is far beyond the means of our chemical rockets (30 km/s - ~3 times escape velocity), that things do not just "drop to the sun" on their own.

needless to say, i was not liked by the professor very much