Author Topic: An Alternative Lunar Architecture  (Read 325084 times)

Offline kfsorensen

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An Alternative Lunar Architecture
« on: 01/24/2006 01:37 AM »
In next month’s issue of “Aerospace America”, there is a remarkable article by the renowned American astrodynamicist Robert Farquhar on the subject of an alternative architecture for lunar exploration.  In this article, he contrasts the benefits of using the Earth-Moon L2 point as a staging location for the CEV and LSAM vs. the current plan to use a low lunar orbit.  Here are some excerpts from the article:
________________________________________________________________

A Halo-Orbit Lunar Station
By ROBERT W. FARQUHAR

In the summer of 2005 the President's Vision for Space Exploration led to the development of a comprehensive "Exploration Systems Architecture Study" for lunar exploration in the next decade and beyond. A key item in this plan foresees an orbital rendezvous between a lunar lander and command module (CEV) in a low lunar polar orbit.  However, for reasons that will be discussed here, it may very well be better to locate the staging point in a "halo orbit" around the translunar libration point, L2.

The halo orbit has previously been considered as a possible location for a lunar far-side data-relay satellite. A relay satellite following a halo trajectory will always maintain line-of-sight contact with the Earth and the Moon's far side. Moreover, the entire halo orbit, when viewed from the Moon's surface, would subtend an angle of only 6.2 deg. Of course, station-keeping will be required to keep a satellite in halo orbit, but the control techniques are extremely simple and the annual fuel expenditure is quite reasonable (DV ~400 ft/s per yr). Detailed control analyses for halo satellites exist.

The Exploration Systems Architecture Study calls for two spacecraft, the Crew Exploration Vehicle (CEV) and the Lunar Surface Access Module (LSAM).  In a typical mission sequence the CEV will be used to transport personnel and cargo from an Earth-orbit to lunar orbit.  Staging in low lunar orbit would employ a conventional lunar transfer. Staging at the L2 position, on the other hand, would use a powered lunar-swingby trajectory of the type shown in the next illustration. This maneuver substantially reduces the DV requirements for braking to the vicinity of L2. After arriving at L2, the LSAM disengages from the CEV and descends to the lunar surface.

How do low-lunar-orbit and L2 compare in this strategy?
Operational Considerations: The most important reason for requiring a lunar orbiting vehicle in the future lunar program could be its function as a communications and control center for all lunar surface and orbital activities. The communications and control tasks would include
1. Control of rendezvous and docking operations for the LSAMs.
2. Monitoring and control of the ascent and descent trajectories of an unmanned LSAM.
3. Navigation and control of unmanned lunar surface vehicles.
4. Communications and navigational support for manned surface expeditions.
5. Control of unmanned remote-manipulator vehicles in the lunar vicinity. These vehicles require continuous communications and minimal transmission delay times for efficient operation. They would be used mainly for satellite maintenance and repair.
6. Command, control, and monitoring of all elements of the lunar program.

These tasks can be conducted very efficiently from a CEV at L2. It will give continuous communications coverage for all far-side lunar operations directly-without dependence on relay satellites. It likewise permits uninterrupted direct contact between the CEV and Earth. Moreover, by placing a single relay satellite at the cislunar libration point, L1, the CEV will always be able to communicate with almost every point on the Moon or in orbit about it. This type of communications and control network offers the additional advantage of being quasi-stationary with respect to the lunar surface. Finally, it should be noted that Earth stations already cover near-side lunar operations.

A CEV in a typical 100-km low lunar polar orbit, would be particularly ill-suited for the communications and control functions, for the following reasons:

1. A lunar lander on the surface would not have any direct contact with the CEV for periods as long as 11 days. Furthermore, the line-of-sight contact time would only be about 10 min per orbit even when the CEV passes over the base site.

2. Continuous direct contact between the CEV and the Earth would only be available for two 3-day periods each month. At other times, line-of-sight contact would be interrupted during every orbit.

3. The CEV would be almost completely dependent on satellite and/or Earth relay links for control of certain critical lunar operations (e.g., a surface rescue mission). Furthermore, two simultaneous relay links would usually be required and switchovers would occur every hour.

The halo orbit offers inherent operational advantages for logistics staging. For instance, the DV requirements for transfers between the halo orbit and the lunar surface are almost identical for any landing site, since plane changes can be made quite cheaply at the halo orbit. (The difference in DV cost is usually less than 200 ft/s.) It is also worth noting that, because of the quasi-stationary characteristic of the halo orbit with respect to the lunar surface, the launch window for transfers between L2 and the lunar surface is infinite.

On the other hand, with the staging point in a 100-km polar lunar orbit, the nominal staytime for lunar surface sorties would probably be constrained to 14-day intervals. Otherwise, due to precession of the polar orbit, a plane change would be necessary when the LSAM returns to the CEV. A graph on page 61 shows the DV penalty for this plane change as a function of surface staytime.

The differences in launch-window flexibility for transfers between the lunar staging point and an Earth parking orbit are not as clearcut as in the case of LSAM operations. Launch opportunities for economical TLS transfers are limited by certain varying geometrical factors. With a CEV based in low lunar orbit, these factors include Moon's position, nodal regression of the Earth parking orbit, and orientation of the CEV orbit with respect to the Earth-Moon line. Transfers to L2 would not be subjected to the third constraint, but the transfer times would be somewhat longer than those required for the low lunar orbit.

Another oft-stated argument in favor of a CEV based in low lunar orbit has it an ideal base for a rescue LSAM. As can be seen from the graph F-4 on page 61, however, the plane change DV penalty can become rather high when a surface rescue mission is needed at an inopportune time. Notice that the DV cost is not very sensitive to the maximum allowable transfer time. For a rescue tug stationed at L2, the tradeoffs are quite different, as the graph (F-7) just at left shows. From a DV standpoint, neither concept has a clear advantage for all rescue situations.

Finally, the station-keeping requirements of the two staging concepts should be considered. Although the normal DV costs for the two concepts are almost equal (~400 ft/s per yr), the CEV could remain in the vicinity of the L2 point (with some occultation) at a cost of only 100 ft/s per yr. Without orbit control, a CEV based in low lunar orbit would impact with the lunar surface in about four months.

Briefly, it has been contended that a CEV based in low lunar orbit would "provide a highly stable, safe, and flexible operations base." The factors just reviewed cast doubt on this claim.

The performance for a particular mission mode can be evaluated by calculating the normalized propellant weight. Normalized propellant weights for the assumed mission modes are given in the graphs above.  Notice that the CEV in low lunar orbit is rather sensitive to plane changes at the lunar polar orbit. These results show significant performance gains with L2 staging.

Conclusions and Recommendations: A L2-based CEV could offer important operational and performance advantages compared to a CEV based in low lunar orbit in a lunar program. Therefore, it is recommended that the present strategy for the lunar-program portion of the ESAS be reexamined. Comprehensive tradeoff should be initiated of several mission modes for lunar systems using low-lunar-orbit and L2-based rendezvous. These studies would provide the information needed to select unequivocally the most effective strategy.

________________________________________________________________

Now most of you readers who have been members of the AIAA since 1972 will of course by this point realized I have been pulling your leg.  This article is NOT coming out next month in Aerospace America, rather it CAME out in June 1972 in “Astronautics and Aeronautics”.

Here is the real article for your enjoyment.

But hopefully you will see that the implications of Farquhar’s results are just as pertinent today as they were in 1972, perhaps even more so.

Offline Flightstar

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RE: An Alternative Lunar Architecture
« Reply #1 on: 01/24/2006 01:38 AM »
VERY nice find. Will need to read this some more, but on skipping over this is an excellent find.

Offline Jamie Young

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RE: An Alternative Lunar Architecture
« Reply #2 on: 01/24/2006 02:06 AM »
I can't find one image showing a translunar libration point, L2 orbit. Can anyone help explain what this is for those of us that don't know what this is all about?

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #3 on: 01/24/2006 02:32 AM »
Quote
Jamie Young - 23/1/2006  9:06 PM

I can't find one image showing a translunar libration point, L2 orbit. Can anyone help explain what this is for those of us that don't know what this is all about?
The libration points are locations of gravitational balance in a three-body (major body, minor body, spacecraft) system.  The word "libration" comes from the Latin "libra", or balance.  Here is a schematic of the libration points formed by the Earth-Moon and Earth-Sun system.  The location of interest in this article is labeled "LL2" in this figure.

Offline spacester

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RE: An Alternative Lunar Architecture
« Reply #4 on: 01/24/2006 02:38 AM »

Offline Jamie Young

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RE: An Alternative Lunar Architecture
« Reply #5 on: 01/24/2006 03:07 AM »
Thanks. That helps :)

Offline CuddlyRocket

RE: An Alternative Lunar Architecture
« Reply #6 on: 01/24/2006 12:16 PM »
Alternatives are always worth considering. However, the advantage of the lunar orbit approach is that we know we can do it because we've already done it. Once we have the infrastructure in place and demonstrated the capability, then thoughts can turn to possible alternatives.

One weakness in the article, at least for near-side missions, is that the LSAM on the surgafe can communicate directly with Earth. And if communication with the CEV whilst it is on the far-side is felt desirable, a simple communications relay satellite would solve the problem.

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #7 on: 01/24/2006 12:23 PM »
Quote
CuddlyRocket - 24/1/2006  7:16 AM

Alternatives are always worth considering. However, the advantage of the lunar orbit approach is that we know we can do it because we've already done it. Once we have the infrastructure in place and demonstrated the capability, then thoughts can turn to possible alternatives.
We've done it for near-side, near-equatorial landing sites.  One of the key points made by Farquhar, and reinforced in the ESAS report, and understood by Apollo planners, is when you go to a "global-access" strategy on the Moon, lunar-orbit rendezvous loses a great deal of its advantages, because of the requirements for plane change for rendezvous and/or trans-Earth injection.  You'll note in the ESAS report that they have to carry ~1400 m/s of DV for plane change.  That's a lot more than lunar-orbit insertion (~900 m/s) or trans-Earth injection (~900 m/s).

That's a heck of a propulsive penalty to levy on an architecture.  It makes a much bigger difference than the choice between methane or hypergols.

Offline CuddlyRocket

RE: An Alternative Lunar Architecture
« Reply #8 on: 01/24/2006 12:30 PM »
Quote
vanilla - 24/1/2006  1:23 PM

We've done it for near-side, near-equatorial landing sites.
Which is what we'll be doing initially, and probably for a number of years. Once we develop from that to "global-access", alternatives like these may well be worth considering, whether for the reasons you give or otherwise. Whereas, we have to decide on LOX/Methane or not (hypergols aren't the only alternative) now.

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #9 on: 01/24/2006 01:04 PM »
Quote
CuddlyRocket - 24/1/2006  7:30 AM

Quote
vanilla - 24/1/2006  1:23 PM

We've done it for near-side, near-equatorial landing sites.
Which is what we'll be doing initially, and probably for a number of years. Once we develop from that to "global-access", alternatives like these may well be worth considering, whether for the reasons you give or otherwise. Whereas, we have to decide on LOX/Methane or not (hypergols aren't the only alternative) now.
No, I'm pretty sure we're going to the poles, right away, and that "global access" was a basic requirement of the ESAS architecture, as was "anytime abort".  Both can be fulfilled, probably better, through L2-based rendezvous.

A 1400 m/s DV penalty makes a BIG difference on an architecture.  For methane, it makes the total system mass (including the launch vehicle) 45% larger, for hypergols, it's 55% larger.  That's almost half a launch.

Additionally, in L2 the discarded LSAM upper stages that accumulate could be aggregated a form a little space station.  In lunar orbit they'll decay and crash after a while.

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #10 on: 01/24/2006 01:59 PM »
I'm sorry, I overestimated the performance penalty of the plane change by erroneously assuming it would apply to both the CEV and the LSAM...it would apply to one or the other, but not both.  That will reduce the penalty to probably 25%.

Offline dmc6960

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RE: An Alternative Lunar Architecture
« Reply #11 on: 01/24/2006 05:16 PM »
Curious, why does an object in low orbit around the moon seem to decay so quickly?  It seems that since there is no faint atmosphere to contend with that LEO objects are slowed down by, a lunar orbit would last for much longer. My only guess is the orbit becoming warped by Earth's gravitational forces enough to eventually cause impact on the lunar surface. Am I correct or completely off?
-Jim

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #12 on: 01/24/2006 05:49 PM »
Quote
dmc6960 - 24/1/2006  12:16 PM

Curious, why does an object in low orbit around the moon seem to decay so quickly?  It seems that since there is no faint atmosphere to contend with that LEO objects are slowed down by, a lunar orbit would last for much longer. My only guess is the orbit becoming warped by Earth's gravitational forces enough to eventually cause impact on the lunar surface. Am I correct or completely off?
You're correct about the drag--there isn't any.  And although the Earth does perturb the orbit, it's not the real culprit either.  The Moon's gravity field is very "lumpy" -- for reasons I don't really know -- and it leads to an evolution in the orbital elements of the spacecraft.  Basically, an initially circular orbit grows more and more eccentric until it impacts the lunar surface.  Depending on the initial conditions, it can happen very quickly or very slowly, but the orbital parameters of a CEV in a low lunar polar orbit will probably not be chosen for orbital stability, rather for operational concerns.  The way to fight it is regular propulsive maneuvers to reduce eccentricity, but the Moon is not a place to leave a passive satellite for very long in a low orbit.  The Apollo subsatellites had varying levels of orbital lifetime in their low lunar orbits.

Try this for more information.

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #13 on: 01/28/2006 01:25 AM »
These images will help show the trajectories Farquhar describes.  They also include the delta-V and time penalties for each leg of the trip.

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #14 on: 01/28/2006 01:53 AM »
You could also use L2 as an assembly and checkout site for a Mars vehicle.  At this location it has nearly all of the energy needed for Earth escape, and by using the lunar swingby to get back into a highly-elliptical Earth orbit, a trans-Mars injection burn (which will be very astrodynamically efficient since it is deep in the Earth's gravity well) will send it on its way to Mars.

Offline braddock

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RE: An Alternative Lunar Architecture
« Reply #15 on: 01/28/2006 02:10 AM »
Is a stable L2 halo orbit wide enough to avoid any occlusion by the moon so as to keep constant line-of-sight communications directly to the Earth?  Or do Earth communications from L2 still have to go through some sort of relay satellite?  The paper seemed to indicate that L2 could be used as a communications relay from the far-side lunar surface, but since the L2 halo itself is by definition centered beyond the far side of the moon I wasn't sure if they meant direct-to-Earth comms?



Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #16 on: 01/28/2006 02:53 AM »
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braddock - 27/1/2006  9:10 PM

Is a stable L2 halo orbit wide enough to avoid any occlusion by the moon so as to keep constant line-of-sight communications directly to the Earth?
Yes, there are a large number of L2 halo orbits that will not occlude the Moon and have constant line-of-sight with the Earth.  In a large fraction of the halo itself, there is very good line-of-sight with one or the other lunar pole as well.

Offline braddock

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RE: An Alternative Lunar Architecture
« Reply #17 on: 01/29/2006 01:01 AM »
Hrm, I was curious about if the ESAS study had considered the libration points.  They had at least looked at an L1 profile:

ESAS Final Chapter 1, Exec Summary, p15.

"LPR [Libration Point Rendevous] was eliminated early from the mission mode trade space. Recent studies performed by
NASA mission designers concluded that equivalent landing site access and “anytime abort”
conditions could be met by rendezvous missions in LLO [Low Lunar Orbit] with less propulsive delta-V and
lower overall Initial Mass in Low Earth Orbit (IMLEO). If used only as a node for lunar
missions, the L1 Earth-Moon LPR is inferior to the LOR [Lunar Orbit Rendevous] mission mode."

The report ESAS seems to be citing as demonstrating LLO superiority to Libration point L1 (and L2, mentioned in passing) is cited in Chapter 4 "Decadal Planning Team/NASA Exploration Team (2000–2002)"

In Section 4.2.1.2.3, an interesting note about the ESAS architecture goals not ruling out Libration point missions:

"The performance requirements associated with missions to
the ISS, lunar orbit, Earth-Moon libration points, and various Mars mission staging points are
very similar in terms of overall mission duration, crew size requirements, and basic transportation
payload capabilities. Entry speed at Earth return is the one key discriminator between
the various near-Earth destinations, all of which can be satisfied through the choice of a midlift/
drag vehicle design. Pursuit of this common vehicle design is the key to enabling a robust
exploration capability beyond LEO."

Anyone know where this Decadal Planning Team study is?

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #18 on: 01/29/2006 01:32 AM »
I have since found out that they were only considering direct trajectories to the L1 and L2 points, rather than these lunar powered swingby trajectories.  These alter the equation and tip the balance in favor of L2 rendezvous, due to the steep plane change penalties in the LLO architectures.

Offline kfsorensen

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RE: An Alternative Lunar Architecture
« Reply #19 on: 01/29/2006 05:07 PM »
I think this idea of "gaggling" together the spent LSAM upper stages in L2 should be seriously considered as the first step in developing a fully reusable lunar transportation system.  If you could imagine just launching the CEV to the trans-lunar trajectory (rather than a CEV/LSAM every time) and then rendezvousing at the L2 station with a reusable LSAM, which would then descend to the polar base and refuel for its ascent, I think that would take you a long way toward a sustainable, less-expensive lunar exploration capability.  Which is what we must have if we want this VSE plan to be a long-term plan.

Later on, the Mars vehicle (either high-thrust or low-thrust) could be assembled and checked out at L2.  If it was a high-thrust system, it would get back on a Earth-return trajectory (but with the benefit of nearly all the Earth escape energy) for a small injection burn at perigee.  If it was a low-thrust system, it could depart directly from L2 (a point on the very edge of the Earth's gravity well) and have very little spiral-out time before escaping the Earth's gravity altogether.

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