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->
⟨Δv1㎧⟩ Hello, I'm new to Principia. I'm sorry if this is a bad question, but I did my best to look through the concepts page and FAQ and couldn't find anything on this. In the non-purple-noodle land it's possible to create maneuvers with MechJeb at specific orbital elements (e.g. creating a maneuver precisely at apoapsis to circularize, creating a maneuver precisely at the AN/DN to change inclination, and so on). Is there a similar kind...
<queqiao->
... of functionality in the Principia flight planner, or is it always necessary to adjust the t initial until it matches the time to the orbital element? Thanks.
<queqiao->
⟨Al₂Me₆⟩ Unfortunately, it's full-on manual with Principia (with the exception of the recently added +/- orbit buttons).
<queqiao->
⟨Δv1㎧⟩ Alright, thanks!
<queqiao->
⟨Δv1㎧⟩ Sorry, another question. Right now, when I use the "warp to maneuver" button in the flight planner it always takes me to 1 minute before the burn is scheduled to start. Is there a way to change the time that button takes you to?
<queqiao->
⟨lpg⟩ no
<queqiao->
⟨lpg⟩ fwiw, mechjeb maneuvers do work with principia, they're just not aware of the "real" orbits. for simple things like "raise Ap to roughly X", it can be good enough and quicker than the principia way
<queqiao->
⟨Δv1㎧⟩ Is it possible to capture into a relatively (<30deg) lunar equatorial orbit directly from a higher-than-lunar-inclination launch site like the cape? I was under the impression that the Apollo missions captured into low-inclination lunar orbits, but I'm struggling to do so in Principia.
<queqiao->
⟨Δv1㎧⟩ Hmmm... it's definitely possible by combining TLI with an inclination change maneuver, but that raises TLI dv to >3200m/s and insertion dv increases too. Is there a cheaper way to do it?
<queqiao->
⟨Butcher⟩ Yes, it is.
<queqiao->
⟨Butcher⟩ The cheap way is to tweak the timing to give you the correct approach vector to the moon.
* umbralraptop
might be misreading things, but Apollo didn't do direct captures like LADEE, but had to do an orbit insertion burn
<queqiao->
⟨Butcher⟩ You should be able to arrive at any inclination for a minimal (1-2 m/s) cost in delta v by changing your exit burn timing slightly.
<queqiao->
⟨Δv1㎧⟩ ⟪umbralraptop⟫ might be misreading things, but Apollo […] ⮪ Sorry, bad terminology on my part. I meant a trajectory featuring a single TLI burn and a single insertion burn, with no midcourse correction or post-insertion maneuvers.
<queqiao->
⟨Butcher⟩ I'm just launching a lunar rover as it happens.
<queqiao->
⟨Al₂Me₆⟩ ⟪Butcher⟫ Looks a bit like this. ⮪ Request: for the two windows, indicate if the transfer is at the AN or DN?
<queqiao->
⟨Butcher⟩ Hmm, I could probably do that - I currently don;t actually calculate any properties of the transfer itself, but I think I could work it out from the shape of the orbit.
<queqiao->
⟨test_account⟩ Just tested this wonderful mod: 1. It works, 2. So far I don't understand how to use it
<queqiao->
⟨test_account⟩ This is what I got, it's fine
<queqiao->
⟨Butcher⟩ 11 hours?
<queqiao->
⟨test_account⟩ Yes, that too
<queqiao->
⟨Butcher⟩ Normally I would set my transfer delta v to around 3140 and scrub time until the trajectory is close to the moon, then tune delta v to get an intercept.
<queqiao->
⟨test_account⟩ But the main question is that during launch, it shows inclination to launch now = -28.55
<queqiao->
⟨test_account⟩ Negative, and I didn't look at it and just launched north, as usual
<queqiao->
⟨test_account⟩ Still there is a TLI window in 12h?
<queqiao->
⟨Butcher⟩ From the cape that's much the same orbit I would think.
<queqiao->
⟨lpg⟩ if your target inclination is your latitude, there's no difference between -x and +x, they're both due east
<queqiao->
⟨Butcher⟩ If you're launching at the window time it's at the point where the azimuth switches from north to south so they're approximately equal.
<queqiao->
⟨test_account⟩ Hmm indeed
<queqiao->
⟨test_account⟩ well, yes, i didn't launch north, i launched east 🤦
<queqiao->
⟨Butcher⟩ You're meant to.
<queqiao->
⟨lpg⟩ I'm guessing that 12h "window" is because you input 3150 then advanced time until you got an intercept. that's the wrong way to do it
<queqiao->
⟨Butcher⟩ North azimuth is say 89.99, south is 90.01. they give the same orbit to within a small margin.
<queqiao->
⟨test_account⟩ Yes, I can also vary the duration of the flight by changing dV
<queqiao->
⟨lpg⟩ you enter 3150 (or whatever), find the point within one orbit that gets closest, then play with dv to get closer, then keep finetuning
<queqiao->
⟨Sunscreen⟩ test_account After our conversation yesterday about giving principia a go and my bumper having axis issues, I went off and did some reading up in intermediate axis theory. I found a great explanation video and a great visual explanation of the forces at work. I just wanted to feed that back to you and thank you for pointing me in the right direction.
<queqiao->
⟨Sunscreen⟩ * issues while spin stabilised,
<queqiao->
⟨Sunscreen⟩ * on
<queqiao->
⟨Sunscreen⟩ Here it is if you're interested. Not sure if I can delete the embed as I'm on my phone. https://youtu.be/1VPfZ_XzisU
<queqiao->
⟨Δv1㎧⟩ ⟪Butcher⟫ About 7 seconds difference and just […] ⮪ I've tried a couple of times to get this to work and I'm having no success. Changing either the tangent component of the burn or the burn timing does reduce my inclination, but it simultaneously increases my periselene, and so far any attempt to decrease my periselene increases the inclination again.
<queqiao->
⟨Δv1㎧⟩ Is there something else I might be doing wrong here?
<queqiao->
⟨Δv1㎧⟩ I'm happy to provide any more information needed.
<queqiao->
⟨test_account⟩ Maybe try playing with dv and burn timing too
<queqiao->
⟨test_account⟩ 3117 is near the minimum possible, maybe there is just not enough flexibility with such transfer
<queqiao->
⟨test_account⟩ * Sorry,misread, its 3137, anyway try changing thungs
<queqiao->
⟨test_account⟩ * things
<queqiao->
⟨Δv1㎧⟩ I'm asking if there's something else I might be doing wrong because I've been continually trying different combinations of t initial and tangent delta-v and none achieve both a low inclination and a low periselene like the kind Butcher was able to get.
<queqiao->
⟨lpg⟩ I think day-of-the-month may play a role in what's achievable
<queqiao->
⟨Δv1㎧⟩ Is there a way to work out ahead of time when certain orbits might be achievable?
<queqiao->
⟨lamont⟩ you more or less need to write an n-body optimizer to get that
<queqiao->
⟨test_account⟩ So... Midcourse correction i guess
<queqiao->
⟨Butcher⟩ I normally tweak both. Generally though, timing will have a larger effect on inclination. Though I think coming from high inclination increases the effect.
<queqiao->
You could always launch into a higher inclination parking orbit.
<queqiao->
⟨Butcher⟩ I would normally use timing to tune the inc and delta v to tune periselene.
<queqiao->
⟨Butcher⟩ Also which side of the moon you fly past will have an effect of course.
<queqiao->
⟨Δv1㎧⟩ That was the issue for me. Changing either timing or delta-v changed both inclination and periselene.
<queqiao->
⟨Butcher⟩ If you're really struggling a little binormal may help but I've not generally needed that.
<queqiao->
⟨Butcher⟩ Yes, they both change but you should find that one has a greater change.
<queqiao->
⟨Butcher⟩ So timing changes inclination a lot a periselene a little and delta v vice versa.
<queqiao->
⟨Δv1㎧⟩ Unfortunately, that didn't seem to be the case in my transfer.
<queqiao->
⟨Butcher⟩ Also for a very accurate encounter you'll need to rcs tune anyway so a rough approach with engines would be adequate.
<queqiao->
⟨Butcher⟩ Might need a mid course then. About 10-12 hours in is a good place.
<queqiao->
⟨Δv1㎧⟩ Yeah, I ended up going with one of those. I'll try a high-inclination parking orbit next and see if that improves things.
<queqiao->
⟨Butcher⟩ I've not played from Cape in a while.
<queqiao->
⟨lpg⟩ the two LANs may also give different results
<queqiao->
⟨Butcher⟩ Fwiw the launch timing addon only drifts about 15 seconds off true with principia.
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<queqiao->
⟨Δv1㎧⟩ ⟪Butcher⟫ Looks a bit like this. ⮪ What exactly does the launch now incl bar indicate?
<queqiao->
⟨Al₂Me₆⟩ If you wanted to launch at this current moment instead of waiting for the next opportunity to launch due east. Basically an off-plane transfer.
<queqiao->
⟨Al₂Me₆⟩ * a polar
<queqiao->
⟨Al₂Me₆⟩ (The inclination changes such that the LAN ends up at the right location relative to the Moon.)
<queqiao->
⟨Δv1㎧⟩ Hmm... I can never seem to actually reach the first window. As I approach it (and am about 4-6 hours out) it increases again.
<queqiao->
⟨Δv1㎧⟩ Launching when the "launch now" inclination is my launch site inclination seems to work, though.
<queqiao->
⟨Butcher⟩ That's what the window should be showing.
<queqiao->
⟨Butcher⟩ I have not tested cape with principia so it might just be buggy.