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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<_whitenotifier> [Principia] EduRenesto starred Principia - https://git.io/JlkZO
<queqiao-> ⟨r​sparkyc⟩ Ok, getting tired, so I need to go to bed, but I'm currently explaining how to plot out and execute the initial TEI maneuver. Going to need to add some more animated gifs showing adjusting it for an Eve intercept, then we'll do the fun part of adjusting it for a free return to kerbin.
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<queqiao-> ⟨s​ichelgaita⟩ This looks really cool!👍
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<queqiao-> ⟨E​lias⟩ Quick question. How much delta V do I need to get a lunar orbit?
<queqiao-> ⟨S​tonesmile⟩ That is very dependant on how you get to the moon in the first place, for a normal hohmann transfer you need about 800 m/s for a low circular orbit
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<queqiao-> ⟨E​lias⟩ So if i sent circa 1km/s I should be fine?
<queqiao-> ⟨E​lias⟩ I usually do the polar orbit transfer where it takes 2-3 encounters
<queqiao-> ⟨S​tonesmile⟩ 1 km for insertion should be fine, just make sure to actually capture on the first pass if you plan on lowering the aposelene over multiple burns
<queqiao-> ⟨S​tonesmile⟩ 1 km/s for insertion should be fine, just make sure to actually capture on the first pass if you plan on lowering the aposelene over multiple burns (edited)
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<queqiao-> ⟨E​lias⟩ I'll try my best. Gotta figure it out somehow ^^
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<queqiao-> ⟨E​lias⟩ I'm checking out the various rcs fuels (still don't have hydrazine), as your comment made me question "why is that weird?" "it's got 201.2N vs htp's 193.6N" but now I see that somehow htp is a lot better DeltaV wise. How come it's so much better?
<queqiao-> ⟨B​utcher⟩ HTP has much higher Isp.
<queqiao-> ⟨B​utcher⟩ It's also denser so requires less tankage.
<queqiao-> ⟨E​lias⟩ I don't properly understand Isp yet. could you explain perhaps?
<queqiao-> ⟨B​utcher⟩ Isp is a measure of rocket engine efficiency.
<queqiao-> ⟨E​lias⟩ and the less tankage I can understand. just more juice and therefore more deltaV
<queqiao-> ⟨e​gg⟩ I_sp = exhaust velocity
<queqiao-> ⟨B​utcher⟩ Higher Isp means you get more delta V for the same mass.
<queqiao-> ⟨B​utcher⟩ Higher Isp means you get more delta V for the same propellant mass. (edited)
<queqiao-> ⟨B​utcher⟩ Though density plays a part as if you have a really low density fuel your dry mass from the huge tanks can offset the gains.
<queqiao-> ⟨E​lias⟩ yeah
<queqiao-> ⟨B​utcher⟩ Hmm, N2O is better than I though.
<queqiao-> ⟨E​lias⟩ it seems good but
<queqiao-> ⟨E​lias⟩ n2o
<queqiao-> ⟨B​utcher⟩ I normally use HTP then Hydrazine, then Az50.
<queqiao-> ⟨E​lias⟩ htp
<queqiao-> ⟨E​lias⟩ it really is due to the amount it can carry
<queqiao-> ⟨B​utcher⟩ Yes, but the latter will have higher mass.
<queqiao-> ⟨E​lias⟩ true.
<queqiao-> ⟨E​lias⟩ but it's better to use the htp to get the lunar orbit instead of carrying a big engine and fuel tho.. right?
<queqiao-> ⟨B​utcher⟩ Mass is generally a harder limit than volume.
<queqiao-> ⟨B​utcher⟩ For non-RCS uses I would tend to favour the denser fuel. For RCS it's not as much difference.
<queqiao-> ⟨E​lias⟩ the htp is 39 more kilos in this case
<queqiao-> ⟨E​lias⟩ and yeah. it's for the lunar orbit
<queqiao-> ⟨B​utcher⟩ You really need to compare equal mass craft with the fuels.
<queqiao-> ⟨E​lias⟩ hmm
<queqiao-> ⟨E​lias⟩ I'll try
<queqiao-> ⟨B​utcher⟩ Unless N2O is unfeasibly large.
<queqiao-> ⟨e​gg⟩ but besides being the exhaust velocity, as the name indicates, I_sp, « specific impulse » is « impulse [gained by the rocket] by mass [of fuel expanded] »
<queqiao-> ⟨E​lias⟩ I'll take a look. I'll empty the avionic and just use a tank to measure
<queqiao-> ⟨e​gg⟩ i.e. how fuel-mass-efficient your engine is
<queqiao-> ⟨E​lias⟩ baloon tank for good measure
<queqiao-> ⟨B​utcher⟩ You should be using a tank anyway, avionics have poor utilisation.
<queqiao-> ⟨B​utcher⟩ You know these props don;t work in balloons right?
<queqiao-> ⟨B​utcher⟩ You need HP tankage.
<queqiao-> ⟨E​lias⟩ so n2o is better then?
<queqiao-> ⟨E​lias⟩ cave man say: High number good. yes?
<queqiao-> ⟨B​utcher⟩ Depends.
<queqiao-> ⟨E​lias⟩ ah yeah. forgot
<queqiao-> ⟨Q​eDelphyn (Xšayāršā/-ām/-āyā)⟩ not necessarily
<queqiao-> ⟨B​utcher⟩ If you need 5 times more N2O the the mass of the 5x bigger tank might be an issue.
<queqiao-> ⟨e​gg⟩ yes, high number good, if the only thing you care about is efficiency ; but sometimes you need high thrust, you need density because of the atmosphere, etc.
<queqiao-> ⟨B​utcher⟩ If you need 5 times more volume for N2O the the mass of the 5x bigger tank might be an issue. (edited)
<queqiao-> ⟨e​gg⟩ Note that while I_sp is often reported in seconds those seconds are actually seconds times standard gravity, which are more usefully interpreted as kilograms-force times second per kilograms-mass
<queqiao-> ⟨e​gg⟩ kilograms-force times second being a unit of [imparted] impulse, kilograms-mass being a unit of [consumed fuel] mass
<queqiao-> ⟨e​gg⟩ You will sometimes see I_sp given in kN s / kg
<queqiao-> ⟨E​lias⟩ mass for mass
<queqiao-> ⟨e​gg⟩ (same, but different unit of force in the unit of impulse)
<queqiao-> ⟨E​lias⟩ htp is better
<queqiao-> ⟨E​lias⟩ htp is better (due to tank size) @Butcher (edited)
<queqiao-> ⟨B​utcher⟩ I thought that might be the case.
<queqiao-> ⟨E​lias⟩ It's a bit hard to understand all the isp things.. but i kinda get it
<queqiao-> ⟨E​lias⟩ Efficiency is good.
<queqiao-> ⟨E​lias⟩
<queqiao-> ⟨E​lias⟩ But in the same way that in stock ksp you don't really use the ion thruster for much.. since it's so weak
<queqiao-> ⟨E​lias⟩ and if you decide to use thousands of those ion thrusters.. the bulkiness becomes an issue
<queqiao-> ⟨(​ᴇxᴘᴇʀɪᴍᴇɴᴛᴀʟshells)⟩ actually the stock KSP thruster has ridiculously high thrust
<queqiao-> ⟨E​lias⟩ was just trying to say that the ion thing can't really get an orbit or so. but yeah. I don't play stock ksp. Tried it like 10 years ago and didn't like it
<queqiao-> ⟨e​gg⟩ I_sp is fuel efficiency, a bit like miles per gallon, except the thing you do in space is not consume fuel to travel more miles, but consume fuel to alter your speed (to impart some impulse), and since not everything is the same fuel it is more interesting to measure fuel quantities by mass rather than volume
<queqiao-> ⟨E​lias⟩ understandable
<queqiao-> ⟨E​lias⟩ so isp is good for planes and such though
<queqiao-> ⟨e​gg⟩ that is not a meaningful statement; it is fuel efficiency; whether it is what you need or not depends on what you are doing and what the other engineering constraints are (in the extreme case of « the bigger tank would outweigh the gain » that is obvious, but it can also be « a bigger and lighter tank would incur more drag because we have to get through that pesky atmosphere », or « you nee
<queqiao-> ⟨e​gg⟩ in the same way that if your car has wonderful mileage but can’t get to motorway speeds, it may not be suited to trips that require taking the motorway (also it might be a bicycle)
<queqiao-> ⟨E​lias⟩ I understand that It's very hard to properly understand. but the car motorway analogy helps a lot
<queqiao-> ⟨e​gg⟩ (but the analogy must be taken with care, since whereas motorways are for going large distances, most of the large distances in space are at very low Δv from each other; the motorway analogues are more « getting off the ground and into orbit » or « ejecting »)
<queqiao-> ⟨E​lias⟩ I tend to value fuel efficiency due to always wanting the cheapest thing in real life. Probably why I just applied the base thought of "efficiency good" since fuel is so damn expensive and money is tight
<queqiao-> ⟨E​lias⟩ yeah i get that
<queqiao-> ⟨e​gg⟩ once you have ejected on an adequate trajectory, flying by the entire solar system is within reach of your metaphorical Vespa https://www.youtube.com/watch?v=7BDxZV7UD9I
<queqiao-> ⟨E​lias⟩ I saw that yesterday haha
<queqiao-> ⟨E​lias⟩ it really is nice
<queqiao-> ⟨E​lias⟩ Butt. For now. I'll focus on making a light little orbiter for the moon.
<queqiao-> ⟨E​lias⟩ Thanks for all the information
<queqiao-> ⟨E​lias⟩ Oh last question
<queqiao-> ⟨E​lias⟩ which side of the moon should I aim for when I want to get into the lunar orbit contract?
<queqiao-> ⟨E​lias⟩ with or against it's rotation? since one will slow me down.. but will be cheaper (I think?)
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<queqiao-> ⟨E​lias⟩ with or against it's rotation? since one will slow me down..(and probably be very unstable and result in an impact quite fast) but will be cheaper (I think?) (edited)
<queqiao-> ⟨B​utcher⟩ It makes little difference.
<queqiao-> ⟨B​utcher⟩ Even for landing the difference is trivial, the net gain from a prograde vs retrograde orbit for a lander is on the order of 7-8 m/s. For an orbiter it makes no difference.
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<queqiao-> ⟨e​gg⟩ it might make a slight difference when it comes to orbit stability but I would need to think
<queqiao-> ⟨e​gg⟩ anyway for orbit stability you need to look at graphs/tables or play lunar roulette
<queqiao-> ⟨D​amien⟩ just launch 10 redundancy satellites
<queqiao-> ⟨D​amien⟩ one of them is bound to live long enough
<queqiao-> ⟨e​gg⟩ Nevermind, retrograde doesn’t matter even for that, see Lara’s *Design of long-lifetime lunar orbits: A hybrid approach*:
<queqiao-> ⟨e​gg⟩ > Note that the figures only present the case of direct inclination orbits, because the (first order) averaging carried out introduces a symmetry between direct and retrograde inclination orbits, so that the case of retrograde inclination orbits is a mirror figure of the direct one.
<queqiao-> ⟨r​sparkyc⟩ @Elias the reason ISP is in seconds is because it is a measure of how long the propellent could offset it's own mass at 1g
<queqiao-> ⟨r​sparkyc⟩ so a fuel with an ISP of 50 could suspend it's own weight for 50 seconds (minus the weight of the engine and tankage of course - this is strictly the propellent)
<queqiao-> ⟨r​sparkyc⟩ an ISP of 100 could do it for 100 seconds
<queqiao-> ⟨r​sparkyc⟩ that's why you'll see that higher is generally better
<raptop> Apropos nothing, I think nyrath mentioned that rocket performance measures are the derivative of those for cars (Isp vs range, TWR vs top speed)
<raptop> er, Δv vs range, Isp vs MPG
<queqiao-> ⟨r​sparkyc⟩ huh - renaming the page changes the URL -
<queqiao-> ⟨e​gg⟩ That is a way of expressing it in seconds, but I would not say it is « the reason »
<queqiao-> ⟨e​gg⟩ « the reason » is that it is a polite way of saying lbf s / lbm (which is the unit used in, e.g., old Apollo documents)
<queqiao-> ⟨e​gg⟩ (those same old documents, which were dual imperial-metric, had kN s / kg for the metric column)
<queqiao-> ⟨e​gg⟩ (I would rather we called it lbf s / lbm or kgf s / kg than seconds with g₀ shoved awkwardly in formulæ where it makes no sense ; this makes it clear why it is specific impulse—and thus that it is fuel efficiency—and arcane coëfficients existing in formulæ for strange historical reasons just confuses everyone)
<queqiao-> ⟨e​gg⟩ (see the regular questions pertaining to incomprehension of I_sp in this very channel)
<queqiao-> ⟨r​sparkyc⟩ I've just gotten so used to seconds 🙂
<raptop> Ah, yes, m/s divided by 9.80665 for Reasons™
<queqiao-> ⟨e​gg⟩ meaningful hysterical raisins
<raptop> . o O (annoy everyone by listing it in dam/s and calling them metric seconds)
<queqiao-> ⟨e​gg⟩ daN s / kg if we want to see it as specific impulse rather than eggshaust velocity
<queqiao-> ⟨B​utcher⟩ Isp is microfortnights.
<queqiao-> ⟨B​utcher⟩ FFF for life.
<queqiao-> ⟨B​utcher⟩ *"In the FFF system, heat transfer coefficients are conventionally reported as BTU per foot-fathom per degree Fahrenheit per fortnight."* such beauty. 🥲
<queqiao-> ⟨e​gg⟩ no! it ought to be measured in some multiple or submultiple of the furlong per fortnight, or more explicitly, of the firkin-furlong-per-squared-fortnight fortnight per firkin
<queqiao-> ⟨B​utcher⟩ firkin-force - fortnights / firkins?
<queqiao-> ⟨e​gg⟩ yeah that works too
<queqiao-> ⟨B​utcher⟩ I kind of like exhaust velocity it has to be said. :-p
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<queqiao-> ⟨P​aculino (ŝi/ri/she/they)⟩ https://www.youtube.com/watch?v=JBzDRFJi6KQ
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<queqiao-> ⟨S​ir Mortimer⟩ Cat’s going to the doc tomorrow. Something is not good with his hips
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<queqiao-> ⟨P​aculino (ŝi/ri/she/they)⟩ Purebred issues?
<queqiao-> ⟨S​ir Mortimer⟩ Unlikely
<raptop> Dogs *are* cats, though
<queqiao-> ⟨r​sparkyc⟩ Ok, renamed back to the original URL, since I think most (if not all) of the text is done:
<queqiao-> ⟨r​sparkyc⟩ still need to add some more screenshots
<queqiao-> ⟨B​utcher⟩ I don't think my cat has quite got that idea of books.
<queqiao-> ⟨B​utcher⟩ I don't think my cat has quite got the idea of books. (edited)
<raptop> Is your cat learning from ANBOcat on how to interact w/ books?
<queqiao-> ⟨Z​eusbeer⟩ Hmm since maneuvers are integrated with principia, can you plan a suicide burn maneuver efficiently enough on a non-atmospheric planet to not explode? Or is the map view quite terrible for predicting terrain altitude?
<queqiao-> ⟨Z​eusbeer⟩ Hmm since maneuvers are integrated with principia, can you plan a suicide burn maneuver efficiently enough on a non-atmospheric body to not explode? Or is the map view quite terrible for predicting terrain altitude? (edited)
<queqiao-> ⟨B​utcher⟩ The latter
<queqiao-> ⟨B​utcher⟩ The map view is a flat sea level surface I think.
<queqiao-> ⟨Z​eusbeer⟩ Maybe we can have some kind of burn analyser that tells the terrain distance at the end of a burn :rabbit_hole:
<queqiao-> ⟨B​utcher⟩ I wrote one of those in kos... 😜
<queqiao-> ⟨Z​eusbeer⟩ With principia?
<queqiao-> ⟨s​ichelgaita⟩ Linked rsparkyc's tutorial from the main wiki page.
<queqiao-> ⟨B​utcher⟩ Yes, though I just use two body gravity since over the time scales of landing it makes little difference.
<queqiao-> ⟨Z​eusbeer⟩ Is the calculated impact point that principia provides based on terrain altitude or sea level altitude?
<queqiao-> ⟨e​gg⟩ Principia does not give you an impact point.
<queqiao-> ⟨e​gg⟩ It tells you that there will be an impact if the periapsis is underground, and it tells you that where the periapsis marker would be
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<queqiao-> ⟨B​enji⟩ The first time, some years ago, I planned a moon landing by graphically eyeballing the suicide burn in the map view.
<queqiao-> ⟨B​enji⟩ Worked really well, but probably very lucky.
<queqiao-> ⟨B​enji⟩ The first time I played with principia, some years ago, I planned a moon landing by graphically eyeballing the suicide burn in the map view.
<queqiao-> ⟨B​enji⟩ Worked really well, but probably very lucky. (edited)
<queqiao-> ⟨r​sparkyc⟩ for that, i just wait till I'm pretty close, and then use mechjeb
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<queqiao-> ⟨Z​eusbeer⟩ You could integrate by crashing, noting altitude and loading a quicksave :caveman:
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<queqiao-> ⟨Z​eusbeer⟩ You could test by crashing, noting altitude and loading a quicksave :caveman: (edited)
<queqiao-> ⟨Z​eusbeer⟩ You could test by crashing, noting altitude and trying again :caveman: (edited)
<queqiao-> ⟨Z​eusbeer⟩ You could test by crashing, noting altitude and trying again with a different burn duration :caveman: (edited)
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