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About me



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  1. I got tired of doing a complex set of calculations from scratch every time I wanted to find the dV of a given Hohmann transfer, so eventually I sat down and made an excel calculator to do it for me. Which led to this: And this: These are reference tables for periapse and apoapse velocities for Hohmann transfers between numerous orbits of interest around the earth and the moon. They should be pretty self-explanatory. These won't give you dV directly; instead, you have to subtract your current velocity from your target velocity. So if you're at a low orbit and want to go up to a higher one, subtract your circular-orbit velocity (in purple) from the periapse velocity (in pink) matching your orbital altitude to the target altitude and execute that burn. Once you reach the apoapse of the Hohmann transfer at your target altitude, subtract your new velocity (in blue, matching the new altitude to your starting altitude) from your target orbit's circular velocity (in purple) and execute that burn to circularize. To drop to a lower orbit, do the same thing in reverse. The EML-1 and EML-2 points are reference for an orbit at that distance; the perigee burn is the same, but the apogee burn needs to match the lunar-circular velocity instead. Actually that's not perfectly correct (since it matches period not speed), but I'm using patched-conic anyway so it's close enough. I've made a correction to the original so that the EML-1 and EML-2 circular velocities are the period-matching velocities rather than the reference velocities for an orbit at that distance. On the lunar side those points are stationary so you don't have to match velocity at all, In the lunar reference table, the circular velocities for EML-1 and EML-2 are for orbits with that distance but at other points; if you are actually reaching one of these points, you just kill your elliptic-orbit apolune velocity. Then I decided to go ahead and create a complete dV map for all major cislunar transfers. I can't attach it here but I posted it at the following link: http://forum.nasaspaceflight.com/index.php?topic=39942.0 Here's a reduced-size version of the dV map; if you want the full-size version, you'll have to go to the link above or click here.
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