Author Topic: Throwing dumb loads to EML2  (Read 3039 times)

Offline Hop_David

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Throwing dumb loads to EML2
« on: 09/20/2009 02:18 am »
Assume a lunar rail launcher (a science fiction notion, I know).

To get payloads to L2, would the payload need to execute a braking burn to get to its destination?

If so, each payload would need to include a rocket engine, fuel and fuel tanks, guidance navigation and control, communications, command and data handling, etc. Getting lunar material to L2 would be expensive even with rail launch.

I was wondering if lunar material could be thrown from the moon's surface in such a way that it would be retrievable with a tug sent from L2.

With my Launch from Lunar Orbit shotgun I set lunar longitude at 0 degrees (the point on the moon closest to earth), from an equatorial circular orbit. I was able to get payloads to loop about L2 for a long time. When the pellets finally fall through L2's neighborhood, they seem to be moving .12 km/sec.

Using L2's frame, the white circle is the moon moving about an L2 center. To get the pellets to loop about L2 that long seems to require a precision velocity to 7 or 8 decimal places.

Here is a launch with a velocity precision to about 5 decimal places. Here the pellets still seem to be falling through L2's neighborhood at about .12 km/sec., just like the higher precision shotgun blast.


In both of these pics, the rail launcher would be throwing payloads at about 2.3339 km/sec.

Setting up a rail launcher at 180 degrees (the far point on the Moon's equator), I was able to get similar results for launching to L1.

Is it feasible to throw lunar propellant to the L1/2 points without a spacecraft ascending from the lunar surface? I am guessing it would be possible for a tug from an L2 base to retrieve a dumb mass traveling through the neighborhood at .12 km/sec.
 

Offline mheney

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Re: Throwing dumb loads to EML2
« Reply #1 on: 09/20/2009 02:35 am »
The "classic" solution to this was to have a "mass catcher" at L-2 - essentially, a big kevlar (or lunar fiberglass) bag that catch what the mass driver launches.  You adjust the launch of each payload to account for any perturbations casued by the last "catch", and can use the caught mass for station-keeping or departing L-2.

The proposal from the '70s was a mass driver/mass catcher combo.  Check out O'Neill's "The High Frontier" or Heppenheimer's "Colonies in Space".

Offline kfsorensen

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Re: Throwing dumb loads to EML2
« Reply #2 on: 09/20/2009 02:45 am »
On the thread about lunar slings I showed some work I did where a lunar sling on the near side could throw to a lunar elevator stretched from the sub-L2 point on the farside across the L2 point and arrive at the elevator with zero relative velocity.  A whole lot simpler in my opinion than the complicated O'Neill catcher-bag, many of the engineering details of which were pretty sketchy at best.

Offline sdsds

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Re: Throwing dumb loads to EML2
« Reply #3 on: 09/20/2009 03:18 am »
I was wondering if lunar material could be thrown from the moon's surface in such a way that it would be retrievable with a tug sent from L2.

Without need for extreme precision the launcher might be able to send payloads on trajectories that loiter in the vicinity of L5 for quite some time, awaiting the arrival of a tug.

Is the delta-v from L5 to L2 prohibitive?
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Offline Hop_David

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Re: Throwing dumb loads to EML2
« Reply #4 on: 09/20/2009 03:33 am »
On the thread about lunar slings I showed some work I did where a lunar sling on the near side could throw to a lunar elevator stretched from the sub-L2 point on the farside across the L2 point and arrive at the elevator with zero relative velocity.  A whole lot simpler in my opinion than the complicated O'Neill catcher-bag, many of the engineering details of which were pretty sketchy at best.

Interesting. In the thread you started "Sling stuff FROM the Moon"? Am poking around in that thread but can't find that.

Offline kfsorensen

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Re: Throwing dumb loads to EML2
« Reply #5 on: 09/20/2009 03:57 am »
Interesting. In the thread you started "Sling stuff FROM the Moon"? Am poking around in that thread but can't find that.

You're right--I looked for it too but it must have been in a different thread.  I believe it's one of the pre-loaded cases in my lunar trajectory sim.

Offline Hop_David

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Re: Throwing dumb loads to EML2
« Reply #6 on: 09/20/2009 03:59 am »
I was wondering if lunar material could be thrown from the moon's surface in such a way that it would be retrievable with a tug sent from L2.

Without need for extreme precision the launcher might be able to send payloads on trajectories that loiter in the vicinity of L5 for quite some time, awaiting the arrival of a tug.

Given an ideal CR3BP (Circular Restricted 3 Body Problem), I don't think so. The L3, L4 and L5 are in a region known as the Forbidden Realm. Orbits are time reversible. Imagine an object parked in L4 or L5. It ain't gonna spontaneously drift away and escape the L4 or 5 neighborhood. Now play that backwards. An object drifting in from outside the L4 or 5 neighborhood isn't going to come and rest in that neighborhood.

However, CR3BP is only an approximation of the real world. We live in an n-body system were n > 3 . The Trojan asteroids seem to indicate the Sun-Jupiter L4 and L5 are Sargassos. How did that come to pass? My guess is the influence of Saturn and other bodies conspired to thwart  the barriers to the Forbidden Realm.

With the sun's help, could we toss dumb loads to EML4 and L5? I don't know.

Playing with 4 body sims (Sun, Earth, Moon, plus satellite of neglible mass), I haven't found a route.

My 4-body sims do seem to indicate the Earth-Moon L4 and L5 regions aren't stable. The sun seems to wreck EML4-5 over time. In the past I was attracted to L4 and L5 because I thought stuff would stay there with no station keeping expense. Now I'm not so sure.

Even though O'Neill daydreams don't fire me up like they used to, it'd still be neat to find ways to toss loads to EML4 or 5.

Offline alexterrell

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Re: Throwing dumb loads to EML2
« Reply #7 on: 09/20/2009 05:23 pm »
I'm a bit sceptical of the O'Neil concept because any pellet that misses the catcher leaves the lunar orbit and goes into Earth orbit, and creates a space debris problem.

My thinking would be to lower a catcher on a tether from L1, perhaps 1,000km to the moon, and make this point the apocynthion of the pellets orbit. Pellets would be fired from near the point under L1, which becomes the pericynthion. Pellets that miss the catcher would return to the launcher, so you'd elevate the launcher by a fraction so they'd actually impact some 10s of km behind.

Once the catcher is full, the cargo is reeled up to L2.

Offline sdsds

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Re: Throwing dumb loads to EML2
« Reply #8 on: 09/20/2009 09:57 pm »
I was wondering if lunar material could be thrown from the moon's surface in such a way that it would be retrievable with a tug sent from L2.

Without need for extreme precision the launcher might be able to send payloads on trajectories that loiter in the vicinity of L5 for quite some time, awaiting the arrival of a tug.

Given an ideal CR3BP (Circular Restricted 3 Body Problem), I don't think so. The L3, L4 and L5 are in a region known as the Forbidden Realm. Orbits are time reversible.

This notion of considering orbits in the CR3BP as time-reversible deserves consideration.  It's obviously true, if you could run time backwards.  But reversing the spacecraft's velocity is not equivalent to running time backwards.  Instead, wouldn't one need to reverse the velocity of all three objects to get the equivalent reverse orbit?
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Offline Hop_David

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Re: Throwing dumb loads to EML2
« Reply #9 on: 09/21/2009 03:16 pm »
This notion of considering orbits in the CR3BP as time-reversible deserves consideration.  It's obviously true, if you could run time backwards.  But reversing the spacecraft's velocity is not equivalent to running time backwards.  Instead, wouldn't one need to reverse the velocity of all three objects to get the equivalent reverse orbit?

Yes. To get the film to run backwards, you would need to flip the velocity vector of every object 180 degrees.

If you reversed just one object's velocity vector but not the others, you'd get something different than a rewind.

Common CR3BP scenario is two bodies with mass moving in circular orbits about the barycenter with the third massless body parked in L4 or L5.

Revserse the velocity vectors of three such bodies and you get a movie running backwards. In this case the movies would look about the same except in one the bodies would be moving in clockwise circles, the other counterclockwise.

If L4 or L5 are stable locations, a body parked there doesn't drift off.

Now imagine a body coming in from the outside and settling into the L4 region for a good long stay. Reverse the velocity vectors of every body in the system and you will get a movie running backwards: a body parked in L4 for a time and then drifting off.

In a 3 body system it's hard for me to imagine an outside body coming to dwell in the L4 region (unless burns are applied).

An n-body system where n > 3 is a different story.

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