raptop changed the topic of #principia to: READ THE FAQ: http://goo.gl/gMZF9H; The current version is Fréchet. We currently target 1.5.1, 1.6.1, and 1.7.x. <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… | <egg> also 4e16 m * 2^-52 is uncomfortably large
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<discord->
lpg. — flight plans don't take boiloff into account, do they?
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waptdragon3. — probably not
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lpg. — (specifically, for burn duration)
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lpg. — I'm seeing a discrepancy between dv/burn time numbers in MJ and the flight plan, and the flight plan's worse numbers appear to be the ones matching reality. if it did take boiloff into account, that _might_ explain it (but probably not)
<discord->
waptdragon3. — it might continual update whereas MJ doesnt
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lamont. — MJ takes boiloff into account as well as HTP
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lamont. — and updates continually
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lamont. — (assuming we are talking stage stats)
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lpg. — we are
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lpg. — what I'm seeing: MJ thinks I have 3200m/s, and will take 5min50 to urn. A flight plan to burn 3155m/s thinks it'll take 5min54 to burn - more time for less dv. In actual practice, I run out of dv ~4s before the maneuver time elapses, and I'm indeed short of the planned trajectory. So MJ was right on burn time, but wrong on dv
<discord->
lpg. — what I'm seeing: MJ thinks I have 3200m/s, and will take 5min50 to burn it. A flight plan to burn 3155m/s thinks it'll take 5min54 to burn - more time for less dv. In actual practice, I run out of dv ~4s before the maneuver time elapses, and I'm indeed short of the planned trajectory. So MJ was right on burn time, but wrong on dv (edited)
<discord->
lpg. — boiloff seems to be minimal, but I'm not coming up with other theories
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Acer_Saccharum. — Spoolup delay?
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lpg. — not long enough to account for it (~1.5s)
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Kirk. — Off axis thrust?
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lpg. — not according to rcs build aid (or to 'dv includes cosine losses')
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lamont. — probably finite burn losses
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Acer_Saccharum. — The arc your thrust turns though might be it
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lpg. — but the flight plan does take that into account, no?
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lamont. — i haven't played with principia for awhile, does it have a "node executor" now?
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lpg. — no
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lpg. — it tells you where to point, when to start and when to stop
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lpg. — ('where to pont': puts a maneuver node on the navball, which sas can point at)
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lpg. — ('where to point': puts a maneuver node on the navball, which sas can point at) (edited)
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lamont. — dunno, i'm somewhat skeptical the MJ ∆v calc is off
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lpg. — this is a moon tli. if I was actually burning 3200m/s prograde, I'd be wayyy past the moon. instead I'm short
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lamont. — oh MJ doesn't take boiloff predictions into account because it can't be accurately predicted (depends on the sun and stuff)
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lpg. — right, I assumed as much
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lamont. — it does take it into account in terms of as the ratio changes it will reflect the loss of ∆v and correctly "drains" the tank once any fuel goes to zero
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lpg. — right
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lamont. — so the calculation MJ does is really simple
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lamont. — the burntime comes from figuring out mdot from the fuel flows, which is the most complicated part, but if you got the right burntime that went right
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lamont. — then the other variable is thrust
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lpg. — plugging MJ's dv (3200), initial mass, final mass into the rocket equation gives me 446s Isp, which is wrong. If I change the flight plan to burn for what MJ says my burn time is, I get 3109m/s; plugging _that_ into the equation gives me 433s, which is correct (matches what the engine's PAW says)
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lamont. — what does MJ's ISP calculation say?
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lpg. — where do I see that?
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lamont. — its another column in the stage stats
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lpg. — (will take a minute, currently scene changing)
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lpg. — hm... this is a crewed flight. kerbal mass shenanigans?