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To the moon
June 22, 2006 12:52 PM   Subscribe

Help me with the hard science behind a sci fi idea.

The idea is this: To save fuel, a tiny (I'm figuring small enough for a single inhabitant) spacecraft is launched from space (orbiting the earth) at the moon. It's not landing on the moon, simply passing it for observational purposes. To return home, the ship slingshots around the moon, so the only rocket fuel required would be the fuel to initialy get the ship started and a little for navigational needs.

Is this feasable? If so, how long do you reckon the trip would reasonably take?
posted by Astro Zombie to Travel & Transportation (19 answers total) 1 user marked this as a favorite
 
This is what the Apollo missions did, use the Moon's gravity.

Free return trajectory
posted by sonofsamiam at 12:58 PM on June 22, 2006


Oops, when I said missions, I should have said mission. Apollo 13 did this after their damage, and had to change their course to do so.
posted by sonofsamiam at 12:59 PM on June 22, 2006


Yes. 80 days? From my understanding, escaping the Earth's gravity well is the big fuel cost. How to figure a proper loop round the moon, howver, is best left to someone far more educated than myself. I would intuit that there are a large number of possible paths to follow. The key being entering the Moon's gravity well at the correct velocity and angle of incidence. Getting between Earth orbit and there could be an arbitrarily long period of time. Little fuel, I think, would be necessary for maneuvering into the slingshot vector once in the neighborhood. IANAAP. YAUMV.
posted by bastionofsanity at 1:02 PM on June 22, 2006


It is feasible. How long the trip would take would relate directly to how fast you can get the ship moving during your launch phase, assuming you're comfortable/able to plot an exacting close slingshot maneuver.

Your ship will be moving at some velocity when it swings around the moon. That velocity has to be matched to the gravitational pull on the ship for the duration of the slingshot—if the ship is moving too fast relative to the gravitational pull, it won't pull a tight slingshot, and will thus go flinging off in some direction other than home.

Given that, you have two choices, if a given ship velocity escaped the slignshot to early: fly closer to the moon (increasing the gravitational pull during the slingshot) or slow the ship down.
posted by cortex at 1:07 PM on June 22, 2006


(Oh. It's Earth and the moon. You've got some constants available there, then.)
posted by cortex at 1:07 PM on June 22, 2006


Well, you don't *have to* escape from the Earth's gravity well. The moon is, after all, still in it -- if it weren't, it would fly off instead of orbit. All you need is an orbit that's high and eccentric enough to intersect with the moon's gravity in a useful way.

Realistically, you want as fast a transfer as possible to avoid getting fried by radiation, so you'd want to exceed terrestrial escape velocity. Apollo did it in ~ a week.

If the ship is launched from a space station, you don't strictly need to use a rocket to start it off. You could use a railgun, frinstance, though that would also impart velocity to the station.
posted by ROU_Xenophobe at 1:10 PM on June 22, 2006


I had a point, and it was sort of vague: if you can fly as close to the moon as possible, you can maximize the gravitational pull during the slingshot, and can thus maximize your ship velocity and minimize the total trip length.
posted by cortex at 1:11 PM on June 22, 2006


The real problem, from a fuel conservation standpoint, is the initial boost, which you seem to be dismissing. Movement over long distance in space is cheap, cheap, cheap, but it seems to be what you're most focused on.

To leave the earth, you gotta use gas. The effectiveness of gas is pounds of thrust produced per pound of propellant used per second. So, as my primitive understanding of it goes, you must either (a) increase the burn time or (b) increase the thrust per pound. Chemical rocketry is *right* out, we've pushed that to the limit. Consider also that even for a small launch vehicle (sat sized, say), easily 90% of launch weight is fuel.

The main problem with launch fuel is that no real advances in fueling have been made for decades. Even if you railgun the ship into space (and somehow don't break any of the sensitive equipment), you still gotta fly pilot and supplies up to the station, whatever.

Once you are IN space, you might want to look into solar sails, but a journey to the moon (in system scale) isn't even a stroll to the end of the block - it's a trip next door for sugar. Conventional rocketry is more that up to the task. The Apollo guys did it with stuff less advanced that powers my car, and the last mission left July 30th and got back August 4th. Our current shuttle missions last longer than that usually, I think.

So, anyway, I guess I'd say if you're looking for fuel savings, they have to start on the ground (so to speak). Hope this isn't a non-answer for you.
posted by absalom at 2:01 PM on June 22, 2006


(that should be "or whatever." What a difference a word makes.)
posted by absalom at 2:02 PM on June 22, 2006


Oops, when I said missions, I should have said mission. Apollo 13 did this after their damage, and had to change their course to do so.

Actually, Apollo 10 went around the moon without landing as well — only intentionally.
posted by Johnny Assay at 2:13 PM on June 22, 2006


Apollo 10 orbited the moon. It wasn't on a free return trajectory.
posted by Megafly at 2:23 PM on June 22, 2006


I'm actually less concerned with getting off the earth than in figuring out how much time it would reasonably take to sligshot around the moon and get back to earth. Let's say you wanted to make it a relatively short trip, since you've basically gt a guy sitting in a bucket in space, but you also want to use as little fuel as possible. How fast do you figure someone could get going jusy by slingshotting around the moon, and how fast wouldhe get back to earth?
posted by Astro Zombie at 2:36 PM on June 22, 2006


Well, the way I see it, he'd get back as fast as he got there, if you played it aggressively.

If you want to do some napkin math, here's how I see it working:

Figure out what the gravitational pull of the moon is at, say, 1000 meters above the surface. (This is probably silly close for a non-landing, nominally powered flyby, but hey. Hell, what about 500m? 200m? Diminishing returns, though, as gravity won't get much stronger compared to the increased delicacy of the flyby calculations.)

Figure out the corresponding speed the craft would need to maintain orbit at that altitude. Calculate distance to the moon, multiply that by two, and figure out how long it'd take to cover that distance at your slingshot speed. Add another 10-20% to account for fudge factor.
posted by cortex at 3:00 PM on June 22, 2006


Well, like I said, that is essentially what is done now for returnable lunar flybys. It takes about three or four days.

Here's where we run into the problem. So, you're loading down this rocket with fuel to launch someone into space. I imagine her as a little girl, because those are very light in my experience. So, you want to get off the earth. Lots and lots of thrust required, which is all generated by heavy, heavy fuel. (Like the song.) You want to cut down the trip time? That's going to require thrust from space, also requiring fuel, making it even *more* difficult to get off the planet.

Sorry if I'm fixating on this point, but it's a major one. Maybe you are space-elevatoring her and all her burn fuel up. What the hell do I know? Or is it a cannonball run affair, where ALL that matters is speed - cost and scale be damned? If so, something like Project Orion has the theoretical speed limit of up to .1c, but that's after maximum acceleration. In terms of something like this, getting maximum speed takes a long time - earth to moon isn't even a light on the runway.
posted by absalom at 3:12 PM on June 22, 2006


I see what you mean, absalom. I suppose Astro Zombie's scenario is going to determine why and when fuel needs to be conserved.
posted by cortex at 3:29 PM on June 22, 2006


Yeah, I guess my addendum would be "need more info." If the question is simply using as little fuel as possible for a lunar flyby, then the answer is 4 days. It's how it's always been done.

You ever seen the inside of an Apollo capsule? 3 guys, maybe 2/3s the room of a normal size dumpster. Flying bucket, indeed.
posted by absalom at 3:35 PM on June 22, 2006


As mentioned, the orbit you're talking about is the one Apollo 13 followed. But it isn't a tight orbit around the moon; it goes way the hell beyond it before coming back.
posted by Steven C. Den Beste at 4:31 PM on June 22, 2006


Real basic, and possibly wrong thinking here:

The magical internet says that manned moon missions followed a free return trajectory between earth and moon orbit [pdf file] (I have no way of confirming this.) I believe this is the trajectory you want.

Apollo 8 spent roughly 67 hours between TLI burn and LOI burn.*
So double that 67 hours and add an hour or so to get around the moon and I get 135 hours or about 5 1/2 days from the time you leave Earth orbit 'till you get back to where you started.

---
* Apollo 8's return to Earth was only 58 hours but I believe they hauled ass on the way back. This takes more fuel.
posted by Opposite George at 8:36 PM on June 22, 2006


Well, I looked at the second link more carefully and it says ETM naturally takes longer than MTE because of the relative strength of each's gravity (does this make sense?) If that's the case then maybe that 58 hours is okay so you get about 5 days.

Or more likely, on rereading the thread all the folks who said three to four days really know what they're talking about and I'm full of crap. Oh well, at least we're all in the same ballpark.
posted by Opposite George at 8:54 PM on June 22, 2006


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