For travel throughout cislunar space, I’ve long been an advocate of having depots on both ends of the journey. The LEO depot provides a refueling stop at the first practical point after leaving the ground, and also a spot for bringing vehicles back from lunar space for refueling for their next trip out. The lunar orbit depot plays a similar role for flights to/from the lunar surface, as well potentially, as being a staging location for departures into interplanetary space. By launching from a lunar facility near the top of earth’s gravity well, it’s both possible to use low-thrust trajectories in and out of cislunar space, as well as to do an earth swingby with a departure burn at apogee for high-thrust departures taking maximum advantage of the Oberth effect.
One important question however has been where to place the lunar orbital facility.
Lunar Orbital Facility Orbit Options
A recent FISO telecon presentation by Ryan Whitley and Roland Martinez of NASA JSC describes and discusses several of these staging orbit options. I’ll be reposting snapshots of a few of their slides to introduce the orbits, but here you can find their full presentation:
They discuss most of the commonly cited options including Low Lunar Orbit, Frozen Orbits, L2 Halo Orbits, Distant Retrograde Orbits, and a more recently discovered option, Near Rectilinear Orbits.
This slide shows some of the smaller lunar orbit options and descriptions:
And this slide shows all of the orbits relative to each other to give you a better idea of what they look like:
Comparison of Options
While Whitley and Martinez in their FISO telecon focus on evaluating the various staging orbits from the standpoint of NASA missions using the Orion capsule, they still provide a lot of useful information for evaluating options for the location of a lunar orbital facility/depot. To me, some of the considerations for locating a lunar orbital facility are:
- How frequently do you have opportunities to travel from a LEO facility to the lunar facility, and how frequently you can travel the other direction?1
- How much delta-V does it take to go between the facility and LEO and the facility and the lunar surface?
- How long is the transit between the location and the lunar surface?
- How useful is the orbit for supporting deep space missions?
- How hard is it to reach various lunar surface destinations from the lunar orbital facility location?
- What is the thermal environment like in the orbit?
- And how much of the lunar surface to destination delta-V can be provided by some sort of propellantless lunar launch scheme2?
Based on these considerations, I’d like to focus the rest of this post on the pros and cons of the two options I consider most interesting–L2 Halo Orbits and Near Rectilinear Orbits.
Pros and Cons of EML-2 Halo Orbits
EML-2 orbits have been my favorite option ever since learning about the low delta-V cost of reaching them via powered lunar swingbys. They have a lot going for them, including:
- One of the lowest delta-V stopping points in the lunar vicinity, requiring only ~3.43km/s of delta-V from LEO.
- Easy access to/from a LEO facility on every LEO-lunar or lunar-LEO window.
- Any-time access to/from anywhere on the lunar surface.
- Low stationkeeping delta-V3
- Benign and cold thermal environment4
- Continuous communications with Earth, and most of the farside of the Moon.
- Good staging point for both deep-space and lunar missions.
- Could become a starting location for a lunar space elevator.
But EML-2 does have a few drawbacks:
- Long LEO-EML2 and EML2-LEO transit times5 for the low delta-V powered-swingby option.
- Long EML2 to lunar surface (and vice versa) travel times6
- It wasn’t clear that a propellantless lunar launch option located at either pole could launch easily to EML2. An elliptical orbit from such a launcher would have its line of apsides pass through the launch location, which would be orthogonal to the Moon-EML2 line. You could launch into a polar LLO, and then do multiple burns from there to EML2, but the propellantless launch option would only provide the first leg of the trip (surface to LLO).
The long trip times mean that the vehicles taking people between LEO and EML2 and between EML2 and the Moon will require much more extensive life support and accommodations than would be needed if the trip were shorter. That will drive up the dry mass of those systems, and by extension the propellant and overall cost of moving people to and from EML2.
Pros, Cons, and Questions Regarding Near Rectilinear Orbits
Starting several months ago, some of my astrogator friends started telling me about NASA’s interest in Near Rectilinear Orbits for exploration missions. After all the talk about Distant Retrograde Orbits, this sounded a bit like the “flavor of the week” syndrome, but the FISO presentation helps explain some of the allure of such orbits:
- Only slightly higher delta-V to/from LEO to NROs compared to LEO to EML27.
- Because the NRO orbit’s perilune is only 2000km from the Moon’s surface, once per 6-8 day orbit, the orbit lines up so that the travel time between NROs and the lunar surface drops to 0.5 days.
- Powered swingby trajectories between LEO and NROs take approximately 5 days each direction, instead of 9-11 for EML2.
- Slight lower delta-V between NROs and the lunar surface compared to EML2.
- The NRO is close enough to an elliptical polar orbit that it might be possible for a polar base to use propellantless launch techniques to fling payloads nearly into NRO, with possibly only minor adjustments and raising the perilune with a burn near apolune half an orbit later8
The benefits of shorter transit times are pretty important, but there are still a couple of relative drawbacks and open questions:
- While it’s possible to get from LEO to a given NRO orbit during every lunar injection window, the NRO facility will be at different points in its orbit during each window, which may make a first-orbit rendezvous either infeasible or it might cost additional delta-V. I’d want to get this resolved, because while this isn’t an issue for one-off, ground-launched missions like the NASA folks were thinking of, this would be a real issue for reusable spaceship flights between a LEO and NRO facility.
- Likewise, departures from the NRO may not be in the optimal part of the orbit for the Earth return maneuver when the timing is right to return to the plane of the LEO facility. This isn’t a problem if you’re doing a direct return, but once again is a big pain in the neck for reuse of space hardware. Once again this is something I’d want to analyze more before settling on an NRO orbit.
- Additionally, the NRO facility has LOS with one lunar pole about 86% of the time (while heading out and coming back from apolune), but only sees the other facility for a brief period near perilune. If you’re planning on using propellantless launch methods to send stuff from a polar lunar settlement to the NRO facility, it’s going to be in an NRO with apilune on the opposite side of the moon from your lunar settlement, meaning you’ll only be in contact briefly for maybe 1 day out of a week.
- Because the perilune is only 2000km, the heating environment is going to be warmer than EML2, with slightly higher boiloff, but this is probably only a minor difference–it should still be tons easier to keep cryo boiloff low in an NRO than in LEO.
While NRO orbits have some really interesting characteristics, I’d really want answers to those first two concerns before I’d pick it for the location of a lunar orbital facility. If you can’t get to it on a regular basis from a given LEO depot without having to do complicated trajectories, or paying big penalties in flight duration or delta-V, then that would likely outweigh the benefits. If on the other hand, it’s not a big deal to adjust the trajectory on the way to and from the NRO facility to enable rendezvous with the facility regardless of where it is within its orbit when the LEO to lunar launch window opens, then it could be a really interesting location for a lunar transportation node. I’ll have to see if I can get some of my astrogator friends to weigh in on those questions. Until then I’m probably still more of a fan of EML-2, in spite of the annoyingly long transit times.
[Update 1: After speaking with an astrogator friend who’s been looking at NROs to support lunar missions, he thinks it might be possible to put an NRO facility in an orbit whose period is synchronized with the average time between launch windows from the LEO facility. If that works, that would mean the NRO facility would be in approximately the same part of its orbit during each trip to/from the Earth. There are questions of if you can make an NRO orbit with a long enough period (~9 days) to make that work, and if the NRO facility could be made to line up both for arrivals and departures from/to Earth, but hopefully he’ll have more opportunity to dig into that further later this year.]
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- Due to the orbital motion of the Moon and nodal regression of a LEO facility, you get optimal lunar departure and/or return options about every 7-9 days IIRC. The choice of lunar orbital facility location may constrain this further.
- Lunar Slings, Mass Drivers, Launch Loops, etc. All the stuff I was supposed to write about in my “The Slings and Arrows of Outrageous Lunar Transportation Schemes” series that I still need to finish
- About 10x lower LOX/LH2 boiloff rate than LEO. In fact with passive insulation you can completely surpress LOX boiloff and even freeze oxygen at EML-2
- 9-11 days
- 3-5 days
- 3.58km/s vs 3.43km/s
- Which you’d also want to be the burn that brings you to rendezvous with the NRO facility