egg changed the topic of #principia to: Logs: https://esper.irclog.whitequark.org/principia | <scott_manley> anyone that doubts the wisdom of retrograde bop needs to get the hell out | https://xkcd.com/323/ | <egg> calculating the influence of lamont on Pluto is a bit silly…
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<queqiao-> ⟨itsRyan⟩ You won't lose that much efficiency if you don't modify the script, but a free return that gets close to the moon is going to be faster/higher energy than a following transfer. To lower your Earth Pe, use more TLI dv or raise your lunar Pe.
<queqiao-> ⟨TheSquirrelPatrol⟩ So that's easy enough, but it doesn't really help with my key questions. I plot a burn that takes me out past the moon and back, and "near the moon" isn't really that close. I adjust that closer to the moon, and now I'm not coming back anywhere near Earth on the return. Basically I don't know what combination of tangent, normal, binormal, or timing I need to be adjusting to refine one end without blowing out the other
<queqiao-> ⟨itsRyan⟩ Moar speed. Start with something like 3165 m/s
<queqiao-> ⟨TheSquirrelPatrol⟩ So yeah, more tangent dV was what I added to bring my earth return down into the atmo, but that pushed my lunar Pe out past 41 megameters and 5 days, at 3174m/sec
<queqiao-> ⟨itsRyan⟩ You need to adjust your TLI time as well.
<queqiao-> ⟨Quadrupole⟩ Hence adjust the script if you are very off-plane
<queqiao-> ⟨itsRyan⟩ I mean the burn start, just talking seconds here. Using the 4 day plane for a 3.2 day transfer isn't optimal, but it's not much off.
<queqiao-> ⟨TheSquirrelPatrol⟩ I'm just getting a vicous feedback cycle. Adjust burn time a few seconds, I get closer to the moon, but farther from the earth. Add dV to get back to Earth, and now I'm far from the moon again...
<queqiao-> ⟨NathanKell (he/him)⟩ I mean, yes? It's just you should be gradually closing in on a solution rather than getting worse?
<queqiao-> ⟨itsRyan⟩ A lunar Pe at 5days with 3174 m/s also sound odd. Make sure you're encountering the moon on the way out and not the way back
<queqiao-> ⟨TheSquirrelPatrol⟩ So I flew past the DN while tweaking my plan, and now the same maneuver at the next DN is a 7 day transfer...
<queqiao-> ⟨itsRyan⟩ 7 days is not a Hohmann transfer, something is very off. I'd start over with 3165, pure tangential, and adjust time to get your lunar Pe at 3ish days, then go from there.
<queqiao-> ⟨TheSquirrelPatrol⟩ I did that, and now I have what looks like a good start. But as I swap back between more dV/earlier burn to pull either the lunar Pe or earth return Pe closer, I don't see any progress, they seem to adjust at basically the same rate, so it's all going faster, but not getting me any closer to a solution
<queqiao-> ⟨itsRyan⟩ If you're going in front of the moon I have no idea why it wouldn't converge. 🤷
<queqiao-> ⟨itsRyan⟩ It might be easier to see the earth Pe from ECI
<queqiao-> ⟨TheSquirrelPatrol⟩ When it's close yeah, but watching the DN hover way far away was a good proxy
<queqiao-> ⟨itsRyan⟩ I think 100k lunar Pe will be very fast. Do you need to be that low? It's doable, but 3174 probably isn't enough.
<queqiao-> ⟨TheSquirrelPatrol⟩ So right now I have a 64km perigee, but a 3.3Mm periselene at 3d11h, 3179m/sec So that seems like I'm in the ballpark for the maneuver you want. But I simply cannot converge both
<queqiao-> ⟨TheSquirrelPatrol⟩ I mae a 2 second change to my burn point, periselene is 700km, perigee is 42Mm
<queqiao-> ⟨itsRyan⟩ Getting there? Increase speed and readjust your timing, rinse, repeat.
<queqiao-> ⟨TheSquirrelPatrol⟩ So I did that adjustment. 2 seconds earlier, moon is where I want it, earth is now way off. Add about 17m/sec, earth is back where I want it, and Moon is... back at 3.1Mm
<queqiao-> ⟨TheSquirrelPatrol⟩ I mean, I did get 200km closer, so I guess that's progress
<queqiao-> ⟨TheSquirrelPatrol⟩ But then I repeat the process, and I'm at 3.5Mm, so not progress at all
<queqiao-> ⟨TheSquirrelPatrol⟩ Should I expect my orbit to remain flat, or deflect polar as I do my lunar flyby? Should I be adding some binormal to keep it flat?
<queqiao-> ⟨TheSquirrelPatrol⟩ Ok yes, that did it, now I've got a 814km periselene and a 74km perigee, but I had to throw in a bunch of binormal
<queqiao-> ⟨itsRyan⟩ It won't be flat, you're not in the lunar plane from the Cape. But it should be enough "in-front" for a free return.
<queqiao-> ⟨TheSquirrelPatrol⟩ Any idea what I should be aiming for as my lunar velocity perigree in a Gemini?
<queqiao-> ⟨itsRyan⟩ That is one option. Mid course would be a lot cheaper if you have the burn. I've never had that much trouble with pure tangential from the cape, and I don't modify from the 4 day plane.
<queqiao-> ⟨itsRyan⟩ Not off the top of my head, but it should be something like a 3 to 3.5 day transfer.
<queqiao-> ⟨itsRyan⟩ I'm still thinking more speed. My 3165 number was my memory from a flyby, not a LLO injection. I believe Apollo was closer to 3200 m/s, and with an undoubtedly better parking orbit. Using Quadrapole's suggestion to change the script's assumed travel time would help there.
<queqiao-> ⟨TheSquirrelPatrol⟩ I did that, changed the mintraveltime value to 2 from 3
<queqiao-> ⟨TheSquirrelPatrol⟩ and I'm burning 2187 tangent, 260 binormal, for a 3198.548m/sec burn
<queqiao-> ⟨TheSquirrelPatrol⟩ going to be about 600km lunar, and 84km earth return
<queqiao-> ⟨TheSquirrelPatrol⟩ thats more than good enough for a sim to verify everything is working
<queqiao-> ⟨TheSquirrelPatrol⟩ and I'm burning 3187 tangent, 260 binormal, for a 3198.548m/sec burn
<queqiao-> ⟨itsRyan⟩ Cool. Close enough. I'm surprised at the binormal, but the energy of the transfer sounds close.
<queqiao-> ⟨TheSquirrelPatrol⟩ And of course after all that, overburn and I have a lunar impact, not a free return.
<queqiao-> ⟨itsRyan⟩ We need a symbol for residuals.
<queqiao-> ⟨itsRyan⟩ IRL nobody did it without a correction burn though.
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<queqiao-> ⟨DRVeyl⟩ Anyone know how close the free-return trajectories aimed for in terms of Pe?
<queqiao-> ⟨DRVeyl⟩ "If we have a failure between the main burn and the correction burnm, we hit the moon" does not seem to be "free return" 😛
<queqiao-> ⟨TheSquirrelPatrol⟩ My rcs was off enough with the centaur still in use that just reorenting the capsule for thermals is the difference between hitting the moon or not...
<queqiao-> ⟨lamont⟩ Battin gives an example of a free return in his book with a perilune altitude of 1,180 miles
<queqiao-> ⟨TheSquirrelPatrol⟩ Apollo 8 was 110km lunar orbit, but I don't know if they performed multiple burns to get that.
<queqiao-> ⟨TheSquirrelPatrol⟩ Ok, if I burn a bit before closest approach, I can get a 120km circular orbit of the moon for less than half my dV.
<queqiao-> ⟨TheSquirrelPatrol⟩ AW, the Gemini Portholes don't appear in the IVA
<queqiao-> ⟨itsRyan⟩ I take back my comment about binormal. Since this is off plane, it's very likely that going that close to the moon with a pure tangential TLI would get you into a very high inclination lunar encounter, which won't work since you're no longer "in front" of the moon. So the binormal on TLI is probably needed for a low lunar Pe free return. Good job finding it!
<queqiao-> ⟨itsRyan⟩ +or mid-course correction
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<queqiao-> ⟨egg⟩ ⟪NathanKell (he/him)⟫ I mean, yes? It's just you should be […] ⮪ yeah, a lot of the slider fiddling is gradient descent by hand
<queqiao-> ⟨egg⟩ ⟪DRVeyl⟫ "If we have a failure between the main […] ⮪ Yeah; I suspect chasing a low perilune from the main burn is at odds with the « failsafe » idea of free return…
<queqiao-> ⟨egg⟩ (I would also advise against using the word pseudorandom which generally means something other than what it seems to mean to some people here)
<queqiao-> ⟨Butcher⟩ I've never managed a low perilune that also has a low earth encounter.
<queqiao-> ⟨Kirk (He/Him)⟩ You can't hit moon space low and get a safe free return
<queqiao-> ⟨Kirk (He/Him)⟩ At least not without a good window
<queqiao-> ⟨Kirk (He/Him)⟩ The Apollo missions would do a course correction to leave the free return trajectory about a day in
<queqiao-> ⟨egg⟩ independently of free return, if your main burn gives you a low perilune, it is a small error away from slamming you into the ground, so your plan does not fail safe
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