Author Topic: EML1 lunar elevator and plane change capture?  (Read 2753 times)

Offline Asteroza

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EML1 lunar elevator and plane change capture?
« on: 09/11/2013 04:47 AM »
Is it a vaguely sane idea to have a craft move from a sun synchronous earth orbit to EML1, such that the transfer orbit is a simple apogee raising maneuver, and the remaining pergiee raising + plane change is done by grabbing an L1 center station of an EML1 lunar elevator?

The obvious problem is the shock of the grab, precisely at the middle of the tether, would probably cause the largest tension force increase at the counterweight and anchor (grabbing the lunar elevator closer to earth would make the situation even weirder). A secondary problem is, because you are coming in at such an angle, that a safe approach is hard, and would require something like a secondary momentum exchange tether rotating around the elevator center station at L1, to reduce local velocity delta during the grapple operation (a plane change tether).

Which brings up a potentially weird elevator variant. If the "upper" half of an elevator is actually made up of three of more tethers which share a common rotating joint at the center station, the multiple tethers share a common perpendicular loop tether near their ends, and these tethers swing in a circular pattern such that their path sweeps something like a cone, then if the grapple point(s) are located at the intersection of the circular tether and the upper tether(s), you would get something similar to a plane change change momentum exchange tether mounted at the center station, but perhaps better able to absorb the capture operation motions?


I suppose this would be in comparison to say, a tug using a gravity assist swing around the moon to effect the plane change to approach EML1 more gently, and an aeroassist plane change for a tug dropping off the upper end of a lunar elevator for the transfer orbit down.

Offline Robotbeat

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Re: EML1 lunar elevator and plane change capture?
« Reply #1 on: 09/11/2013 05:04 AM »
You can actually get to EML1 or EML2 with a single burn from LEO ballistically, with essentially a zero capture delta-v. So, at least in certain circumstances you could launch to something and capture it gently.
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Offline Asteroza

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Re: EML1 lunar elevator and plane change capture?
« Reply #2 on: 09/11/2013 11:39 PM »
You can actually get to EML1 or EML2 with a single burn from LEO ballistically, with essentially a zero capture delta-v. So, at least in certain circumstances you could launch to something and capture it gently.

You are probably imagining LEO inclinations not too far from from the moon's inclination (18-28 degrees) itself, which makes a transfer orbit comparatively simple because the deltaV cost for the plane change is low. SSO is usually 97-100 degrees. So, functionally, the transfer orbit from SSO is to get enough altitude to reach EML1, while the elevator does the whipsnap 80 degree-ish plane change work.

Yeah, that would be the lowest deltaV to do an immediate grab at EML1 but that puts the worst forces on the elevator. Some funky two burn and lunar gravity assist method, where the second burn is at EML2 might be more sane to get better inclination alignment for the grab at EML1. I'm still liking the EML1 lunar elevator upper tether drop plus aeroassist plane change for a returning tug to get back to SSO though.