egg changed the topic of #principia to: READ THE FAQ: http://goo.gl/gMZF9H; The current version is Cesàro. We currently target both 1.2.2 and 1.3. <scott_manley> anyone that doubts the wisdom of retrograde bop needs to get the hell out | https://xkcd.com/323/
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<awang>
About how much dv should I expect to save using a low-energy transfer to the moon?
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<awang>
Uh
<awang>
So Principia is showing a maneuver taking ~4000 m/s of dv
<awang>
And the stage I'm using has about 5000 m/s
<awang>
But Principia is asking for a burn of about 3 minutes, when the stage only has ~2.5 minutes in it
<awang>
Any idea why?
<awang>
Also, I get that the delta-v counter isn't supposed to change
<awang>
Is the maneuver node marker supposed to not move, too?
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<egg|afk|egg>
awang: depends whether you check inertially fixed or not
<egg|afk|egg>
so if it's inertially fixed, it points in a fixed direction (e.g. towards a distant star)
<egg|afk|egg>
if it's not, it tracks the Frenet trihedron (e.g. it keeps being tangent)
<egg|afk|egg>
(tangent being prograde)
<awang>
So burns in a tangent direction will only be tangent at burn start if inertially fixed is true?
<awang>
Wait
<egg|afk|egg>
yup
<awang>
I see
<awang>
Makes sense
<awang>
Although if inertially fixed is off
<awang>
The navball indicator doesn't move then, right?
<awang>
So how do you know where to point?
<egg|afk|egg>
it does, it tracks the frenet trihedron too
<awang>
Oh
<awang>
So it's just the countdown that doesn't change
<awang>
?
<egg|afk|egg>
(in particular if your burn is tangent the navball marker will stay tangent)
<awang>
Guess I wasn't paying enough attention to the maneuver node marker
<awang>
Ah
<egg|afk|egg>
awang: well it's more complicated than that
<awang>
Well, I guess that makes sense
<egg|afk|egg>
the marker in stock tries to provide guidance (in the closed-loop sense)
<egg|afk|egg>
giving you a residual
<egg|afk|egg>
we don't try to do that, because since we don't make the instant-burn assumption guidance is hard (more complicated than a residual, depends on what you're guiding for, etc.)
<awang>
So if inertially fixed is off, the maneuver node marker doesn't move relative to the tangent marker on the navball?
<egg|afk|egg>
so we just provide you with open-loop guidance
<egg|afk|egg>
i.e. "where to point" "how long to burn"
<egg|afk|egg>
with no feedback from what you actually did
<awang>
I see
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<egg|afk|egg>
the idea being that you do your burn that way, then perform a correction (looking at map view)
<egg|afk|egg>
you need TCMs in real life too anyway
<egg|afk|egg>
(but in real life you'd have closed loop guidance on the actual burn, but that's a lot of work)
<awang>
Just making sure, residual means how far off you are?