Statistical Ars Legatus Legionis
15y
54,490
michaeltherobot said:
You clearly know what you are talking about, so could you ELI5 why polar LLO costs more than equatorial LLO? My intuition that they are the same comes from KSP, in which, soon after leaving Earth orbit, you plan a miniscule burn to adjust lunar insertion from coming around the side to coming over the top.
Of course, in both those cases I then have to decelerate hard at perilune to be captured. Perhaps the flight paths NASA is considering have some way to save dV vs my hard deceleration, which don't work for polar orbits?
The added cost comes from the plane change and plane change at high velocity (low orbit) are expensive. You CAN do something similar to what you describe it just takes longer potentially much longer. The higher the perilune the cheaper the plane change becomes but the longer it takes to reach the perilune. You drop yourself into a highly elliptical orbit around the moon at the same plane as the initial orbit. You then ride up to the perlune, raise the plane to 90 degrees and lower the perilune to circular (decelerate hard).
NASA wouldn't consider doing a plane change in Earth orbit because then you can have a free return trajectory which is a risk reduction factor.
So the tradeoff of DeltaV vs time.
Compare this map to the one in the previous post.
https://arstechnica.com/civis/attachments/1772816223709-png.129833/
1772816223709.png
Significant cheaper but it adds a 3.5 day loiter riding up to the vey high perilune to become as cheap as NRHO (including the transit). To have insertion and exit cost that are 2x this you would need the same loiter on the way back. In KSP things like mission duration are quite cheap and excessive risk doesn't matter but yeah same basic concept and math.
To be clear this is really only an issue for an occupied crew vehicle. If you add a 15 day loiter then the phase change becomes essentially free. For prestaging the lander or the tanker to refuel it after a sortie neither would be harmed by a 30 day longer mission. So if LLO was used as a staging point, which I don't think it will, then there would be mission choices by SpaceX and BO on how much LLO loiter vs round trip DeltaV for sending that tanker to meet lander with the prop it needs.
