Earth to Moon (and Vice-versa) in an Established Space Infrastructure
Posted 08-17-2009 at 10:12 PM by pattersoncr
I had intended to work on a list of add-ons I use in my incarnation of the Earth’s “future history,” but after doing a lot of tinkering with Earth to moon & moon to Earth transfers I decided to talk about that first.
A well established space infrastructure would feature significant resources in permanent orbit around both the Earth and moon. A 2001 style Aries lander would allow you to skip the lunar orbit station, but that’s not the case with the Burchismo line (which includes separate transfer vessels and landers).
Having to dock with an orbiting station at your destination rather than directly landing significantly complicates things. Two factors add to theis complexity: Aligning planes with the destination station and phasing to allow rendezvous.
During an earth to moon transfer, plane alignment with a station in an equatorial orbit is easy since you can always set up a trajectory that puts you very nearly over the moon’s equator.
Plane control for Moon to earth transfers is a nut that I haven’t really figured out how to crack yet other than noting the following points: Twice per month, the moon is at a node with the orbit of the LEO station. Transfers at these times would minimize ΔV. If the moon is not at a node, a node will occur at some point along the transfer trajectory from the moon to Earth. A plane change at this point will reduce the required ΔV but it will still be higher than if you started when the moon is at a node.
Orbit phasing adjustment via careful control of PeT
I’m convinced that the key to efficiently operating in this type of infrastructure (as a successful commercial enterprise no doubt would), is careful control of periapsis time (PeT) to allow a rendezvous with the station quickly after entering orbit. For example, yesterday, I executed a flight from the moon station to Celestium (in LEO) using LSTS201. The trajectory I set up had a flight time of about 67hrs. After arriving in LEO, I spent 16hrs (sim time) waiting for the phasing to be correct to rendezvous with Celestium. An 83hr trajectory that proceeded to rendezvous within an orbit or two would have taken the same total amount of time but would have saved several hundred m/s worth of ΔV.
Getting the phasing right is conceptually pretty simple but there is not (to my knowledge) an existing tool to help you set it up. I use careful control of PeT to set up a situation where I arrive in low orbit slightly behind my target station. If Periapsis altitude (PeA) is chosen well, I can place my self in a lower (and therefore faster) orbit a few degrees behind my target, the perfect place from which to conduct a rendezvous. Interplanetary MFD (IMFD) and a small excel spreadsheet provide the tools necessary to do this. For the rest of this discussion, I’ll discuss an Earth to moon transfer but the principles & techniques apply (with some changes) for a moon to Earth transfer as well. I set up a transfer using IMFD course, target intercept program using offset targeting. This transfer trajectory ends with a retrograde pass around the moon with a near zero equatorial inclination.
IMFD Map can very accurately display time to lunar Periapsis (PeT). It can also display the orbit and position of the target station (Ref must be set to “moon”). To adjust phasing, I set IMFD Map up to display these then wait until the target passes my lunar Periapsis. When this happens, I write down the current MJD from the upper right-hand corner of the screen (including all 4 decimal places). With this MJD and the orbital period (t) of the target station (from Orbit MFD), I use the attached exel spreadsheet to calculate when the station will cross my periapsis on subsequent orbits. It displays the PeT (in sec and MJD) required to pass periapsis just behind the target station as discussed above.
I then make small adjustments to TIn and Rad on IMFD Target Intercept in order to get the periapsis MJD and PeA as displayed on IMDF map to match the desired MDJ calculated by the spreadsheet.
At Pesiapsis, I use IMFD Delta velocity program to conduct the LOI burn -dVf provides the braking while small amounts of dVi keeps the resultant orbit circular. Following LOI, I plan and execute a transfer and rendevous with the station using Target intercept.
I hope someone finds this useful. Please comment if you do (or if you don't and I'll put my effortd in a different direction).
Total Comments 3
Posted 08-17-2009 at 11:20 PM by dougkeenan
Posted 08-17-2009 at 11:35 PM by pattersoncr
Posted 08-22-2009 at 04:17 AM by NukeET