Author Topic: Trans-Earth Rendezvous for Human Lunar Return Missions  (Read 6364 times)

Offline sdsds

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The work presented in http://www.esoc.esa.de/external/flydynmx/MA_Workshop_2007_Presentations/LP5_Trajectories_to_from_the_Earth_Moon_Lagrangian_Points_L1_and_L2_for_the_Human_Exploration_of_the_Moon.pdf shows that an ascent from the lunar surface headed in the direction of Earth-Moon L1 requires on the order of 2.5 km/s of delta-v.  But then a rendezvous at EML1 requires another burn, possibly providing as much 0.75 km/s more delta-v.

The question is, "Why stop at EML1?"  Assuming the command and service module for the Earth-return has been loitering at EML1, could it let the lunar ascender fly past and then execute its own burn to match velocities for a rendezvous that takes place during the Trans-Earth cruise phase?

A superficial analysis seems to show a primary advantage in fuel savings.  Fuel for the ascender rendezvous burn is eliminated, and this also eliminates the descent fuel required to soft-land the rendezvous fuel on the lunar surface.

Looking a bit deeper:  since the lunar ascent module is now tasked with only a single burn, could the motor for that burn be much simpler?  Could it be accomplished with, for example, a super-high-reliability solid rocket motor?
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Offline yinzer

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #1 on: 07/10/2009 09:43 pm »
The question is, "Why stop at EML1?"  Assuming the command and service module for the Earth-return has been loitering at EML1, could it let the lunar ascender fly past and then execute its own burn to match velocities for a rendezvous that takes place during the Trans-Earth cruise phase?

A superficial analysis seems to show a primary advantage in fuel savings.  Fuel for the ascender rendezvous burn is eliminated, and this also eliminates the descent fuel required to soft-land the rendezvous fuel on the lunar surface.

What happens if something goes wrong?  You can probably get to EML1 even if you don't get it exactly right the first time.  If your navigation is a bit off, or if the burn gets cut short for some reason, you can try again.  If the CSM is waiting at L1 for the LM to fly by and it misses the departure burn by an hour, or the departure burn gets cut short by 100 m/s, everyone is screwed.
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Offline sdsds

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #2 on: 07/10/2009 10:19 pm »
What happens if something goes wrong? 

Well yes, the safety of a Trans-Earth Rendezvous (TER) is a huge concern!  Fault-tolerance is certainly one path to improved safety.  But is TER really less fault tolerant?

Quote
You can probably get to EML1 even if you don't get it exactly right the first time.  If your navigation is a bit off, or if the burn gets cut short for some reason, you can try again. 

We're agreed the safety concern dictates that for a rendezvous, the spacecraft making the burn must be able to execute a correction burn as required.  In the suggested TER scenario the spacecraft with that capability is the CSM.  The lunar ascender takes the role of passive target.  Is that inherently more dangerous for the crew aboard the ascender?

Quote
If the CSM is waiting at L1 for the LM to fly by and it misses the departure burn by an hour, or the departure burn gets cut short by 100 m/s, everyone is screwed.

No matter what the mission design, if a rendezvous burn during the  Earth-return is badly botched you're in trouble.  In a TER failure scenario where the CSM irrecoverably botches the rendezvous burn, the crew in the ascender are at least headed in the direction of an Earth flyby.  Conceivably an emergency rendezvous with a LON CSM could bring them home safely.  Admittedly the crew would make the Earth transit without the radiation shielding and other amenities of a CSM.  But isn't that acceptable in a contingency scenario?
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Offline yinzer

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #3 on: 07/11/2009 12:28 am »
We're agreed the safety concern dictates that for a rendezvous, the spacecraft making the burn must be able to execute a correction burn as required.  In the suggested TER scenario the spacecraft with that capability is the CSM.  The lunar ascender takes the role of passive target.  Is that inherently more dangerous for the crew aboard the ascender?

I think corrections in trans-earth cruise are going to be more difficult than around EML1.  The wrong halo orbit will still stay around EML1, but an incorrect trans-earth injection will steadily drift away from the desired course.
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Offline mmeijeri

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #4 on: 07/24/2009 08:00 pm »
Minimising total delta-v is not the only consideration. Minimising the delta-v of individual hops to make them fit within the capabilities of existing upper stages is another one. LEO to L1 is cheaper than LEO to LLO. A Delta-IV upper stage will get an Orion from LEO to L1 with enough fuel to get back, but cannot do that for LLO.
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Offline sdsds

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #5 on: 07/24/2009 10:55 pm »
I think corrections in trans-earth cruise are going to be more difficult than around EML1.  The wrong halo orbit will still stay around EML1, but an incorrect trans-earth injection will steadily drift away from the desired course.

The math for EML1 maneuvers looks quite challenging.  Intuition based on orbits around gravitational masses supports your view, but the chaotic nature of a halo orbit maneuver (and thus a rendezvous) might be counter-intuitive, to say the least.  Couldn't even a small error in an orbital maneuver at EML1 inject a vehicle onto a path that moved away from EML1 quite quickly?

In comparison, course corrections while in trans-earth cruise seem trivially simple -- approximate calculations could even be done in a non-rotating (and thus inertial) frame of reference!
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Offline sdsds

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #6 on: 07/24/2009 10:57 pm »
Minimising total delta-v is not the only consideration.

That's a really good point!  The space enthusiast community needs to present all the diverse reasoning that supports establishment of a "Halo Station".  Capabilities of existing vehicles support it in the near term; exploration objectives outside the Earth-Moon system support it in the long term.
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Offline mmeijeri

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #7 on: 07/24/2009 11:22 pm »
I'm slowly upgrading my still very limited orbital mechanics fu and I haven't reached libration orbits yet. What are the advantages of halo orbits over Lissajous orbits?
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Offline mmeijeri

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #8 on: 07/24/2009 11:26 pm »
More arguments:

- continuous global access to/from Earth and to/from the moon
- delta-v almost independent of launch latitude (although I prefer ISS staging since it doesn't require HLVs)
- cheaper initial target than going deep inside the moon's gravity well (the Augustine Flexible Path argument)
- libration points surround the moon and Earth from all sides and can serve as staging points for rescue craft
- libration points are excellent locations for depots
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Offline sdsds

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #9 on: 07/26/2009 12:37 am »
I'm slowly upgrading my still very limited orbital mechanics fu and I haven't reached libration orbits yet. What are the advantages of halo orbits over Lissajous orbits?

Judging advantages looks difficult.  For a good intro to the subject take a look at the 2006 book Dynamical Systems, the Three-Body Problem, and Space Mission Design by Koon, Lo, Marsden, and Ross. It's available for download at http://www.cds.caltech.edu/~marsden/books/Mission_Design.html

Chapter 2 ("Motion near the Collinear Equilibrium Points") is obviously good reading.  Also, Ch. 6 section 9 begins,
Quote from: Koon, Lo, Marsden, and Ross
We will start with the computation of the Lissajous trajectories (two dimensional tori) and the halo orbits (one dimensional tori) using the Lindstedt-Poincar´e method.

Maybe some hints could be found there about the advantages of each?
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Offline mmeijeri

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #10 on: 07/26/2009 12:50 am »
Great link, thanks! I didn't know they had an online textbook. It's going to take me some time to work through all that...
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Offline mmeijeri

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #11 on: 07/26/2009 12:59 am »
Anyone interested in an online study group for this?
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Offline Hop_David

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Re: Trans-Earth Rendezvous for Human Lunar Return Missions
« Reply #12 on: 07/27/2009 03:41 pm »
Anyone interested in an online study group for this?

I am.

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