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MPDerksen

Mid-course corrections on Interplanetary transfers

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Here we go.  Fairly done with Mun and Minmus.  KAC is installed and we're off to Duna, Eve and beyond.  Watching various YouTube videos on Interplanetary transfers.  But THIS is my favorite place to learn, since there is so much experience and, frankly, decency, compared to other forums.

  • The videos I watch indicate getting about halfway to a planet and THEN adjusting the orbit to get an encounter.  Why is that more efficient than near Kerbin?  I would think changes get MORE expensive as you get closer to the target.  
  • This will become as easy as EVA and docking, at some point (both were nearly impossible at first, and now are quite simple), but actually getting an encounter is currently difficult.  I only just installed the KAC, and hope that helps a great deal.  Once I get the arrows close, I just stare at the map view and have NO idea what to change to get it to an actual encounter
  • Nuclear engines?  Best option for transfer burns?  Should I be using normal fuel tanks and just remove the Oxidizer?

That's enough for now.  I need to put a Sentinel around Eve and do a fly-by of Duna.  Both seem simple if I can actually get the encounter.  I know how to burn prograde to Duna and Retrograde to Eve.  Beyond that, I could use some advice.

Michael

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25 minutes ago, MPDerksen said:
  • The videos I watch indicate getting about halfway to a planet and THEN adjusting the orbit to get an encounter.  Why is that more efficient than near Kerbin?  I would think changes get MORE expensive as you get closer to the target.  

For the same reason you put the fulcrum of a teeter-totter in the middle of the board. It's cheaper to change the location of your encounter at the midpoint.

Quote
  • This will become as easy as EVA and docking, at some point (both were nearly impossible at first, and now are quite simple), but actually getting an encounter is currently difficult.  I only just installed the KAC, and hope that helps a great deal.  Once I get the arrows close, I just stare at the map view and have NO idea what to change to get it to an actual encounter

This is what Maneuver nodes are for. I've played for who-knows-how-many-thousands-of-hours and I don't actually know what to do. I just move the tuggies on the nodes and alternate between them, only going the direction that gets me closer. When that stops getting me closer, I move to another node.

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  • Nuclear engines?  Best option for transfer burns?  Should I be using normal fuel tanks and just remove the Oxidizer?

Nuclear engines are not necessary at all. I frequently don't even use them for Moho. You'll have more accuracy with your ejection burn if you use chemical rockets because the burn will be over faster, so will be closer to the projected "0-second" maneuver node.

Nailing a transfer window even remotely well is hard without mods. I suggest Transfer Window Planner to both get the correct times and also get information about WHY they're the correct times. KAC gives you dates but doesn't explain why, as does Astrogator (my personal choice but I already know about when to burn, I'm just lazy). You CAN figure it out in stock but there are no real tools, you just have to work it out and I personally don't like to do math when video gaming.

Edited by 5thHorseman

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1 hour ago, MPDerksen said:
  • The videos I watch indicate getting about halfway to a planet and THEN adjusting the orbit to get an encounter.  Why is that more efficient than near Kerbin?  I would think changes get MORE expensive as you get closer to the target.

It's not that it's more efficient; it's necessary due to how orbits work. Let's review what a simple Hohmann transfer looks like (please excuse the 5-minutes-of-mspaint sloppiness):

8vNwYrt.png

You start at a planet on the blue orbit (assume everything is counter-clockwise), you burn when it's at the "DEPART" X, and you follow the purple orbit to encounter your destination at the "ARRIVE" X.

Now suppose the destination orbit is inclined such that it is to the north, "above" the transfer orbit at the arrival point (towards us in the third dimension in the above illustration). If you add a normal component to your initial burn, this has the effect of tilting your transfer orbit, turning your departure point into your solar AN relative to your starting body's orbit, the point where your transfer orbit intersects Kerbin's orbit in a northward direction (by definition, since you're starting at Kerbin's orbit, and you're going northwards from it). That means the point opposite becomes the DN, which is the point where your transfer orbit again intersects your starting orbit, but in a southward direction. But the point opposite your departure point is the arrival point! So we've established that if you only burn in Kerbin's SOI, you're doomed to return to Kerbin's solar orbital plane at the point when you're hoping to reach the target orbit. If the target body's orbit goes to the north of that point, you need a correction burn from some other point in your transfer orbit to tilt your orbit up in that direction.

Or at least, this is how I think it works, and so far no one contradicted me the last time I said it. Maybe I'll get lucky and learn something new this time. :science:

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38 minutes ago, 5thHorseman said:

You CAN figure it out in stock but there are no real tools, you just have to work it out and I personally don't like to do math when video gaming.

I"m 100% with you on this point.  I prefer Simulations to Shooters, but adding up dV from the map is enough already!

Is there a better way to move my maneuver node around the orbit?  I have to zoom in to grab it, but then need to zoom out to see it's effect.  Bad enough for Minmus, but for Duna it's extra tough.  I try and grab it and can't make small changes, or end up pulling on a handle by mistake, and have to start all over.  That part of the interface has me scratching my head, when I know WHAT I want to do, but can't get the interface to comply.

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51 minutes ago, 5thHorseman said:

For the same reason you put the fulcrum of a teeter-totter in the middle of the board. It's cheaper to change the location of your encounter at the midpoint

Assuming this is also true for Xfer burns to Mun/Minmus, it could explain why my dV requirements for off-angle captures (like a rescue at the Mun Pole) cost me so much more than I expected.

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1 hour ago, MPDerksen said:

Is there a better way to move my maneuver node around the orbit?  I have to zoom in to grab it, but then need to zoom out to see it's effect.  Bad enough for Minmus, but for Duna it's extra tough.  I try and grab it and can't make small changes, or end up pulling on a handle by mistake, and have to start all over.  That part of the interface has me scratching my head, when I know WHAT I want to do, but can't get the interface to comply.

I use the new Maneuver Mode editor in the lower left. Here's the Squad blog post on it:

https://kerbaldevteam.tumblr.com/post/184089345124/kerbal-space-program-17-room-to-maneuver-is

Before that I used PreciseNode.

Both allow you to edit the maneuver node without it being on the screen, so you can pay attention to your target instead of the node itself.

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2 hours ago, MPDerksen said:

Why is that more efficient than near Kerbin?  I would think changes get MORE expensive as you get closer to the target.  

Midcourse corrections are extremely small. Almost always less than 10 m/s. Usually less than 1 m/s. Efficiency and expensiveness are irrelevant. You don't need a maneuver node at all. Just adjusting the orbits by eye is perfectly effective.

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3 hours ago, HebaruSan said:

It's not that it's more efficient; it's necessary due to how orbits work. Let's review what a simple Hohmann transfer looks like (please excuse the 5-minutes-of-mspaint sloppiness):

8vNwYrt.png

You start at a planet on the blue orbit (assume everything is counter-clockwise), you burn when it's at the "DEPART" X, and you follow the purple orbit to encounter your destination at the "ARRIVE" X.

Now suppose the destination orbit is inclined such that it is to the north, "above" the transfer orbit at the arrival point (towards us in the third dimension in the above illustration). If you add a normal component to your initial burn, this has the effect of tilting your transfer orbit, turning your departure point into your solar AN relative to your starting body's orbit, the point where your transfer orbit intersects Kerbin's orbit in a northward direction (by definition, since you're starting at Kerbin's orbit, and you're going northwards from it). That means the point opposite becomes the DN, which is the point where your transfer orbit again intersects your starting orbit, but in a southward direction. But the point opposite your departure point is the arrival point! So we've established that if you only burn in Kerbin's SOI, you're doomed to return to Kerbin's solar orbital plane at the point when you're hoping to reach the target orbit. If the target body's orbit goes to the north of that point, you need a correction burn from some other point in your transfer orbit to tilt your orbit up in that direction.

Or at least, this is how I think it works, and so far no one contradicted me the last time I said it. Maybe I'll get lucky and learn something new this time. :science:

I had to read it a few times to grasp it, but YES, this makes complete sense, once the lightbulb went on.  Thanks.

 

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To get your correct transfer window w/o mods you can use this:

https://alexmoon.github.io/ksp/

When moving nodes I usually "focus view" on the target and I use the mousewheel to push/pull the icons of the node.

Just for the sake of the stock game.

 

Cheers.

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Another point is that even if the arrival planet is at the exact same plane as Kerbin and you leave directly going perfectly 90 degrees, you may burn slightly too late, or early, or too radial in or out or whatever. You use less fuel burning from Kerbin but that's because there's no room for errors, it has to be precise. It's not unusual or bad to need to correct things half way (the fulcrum of the seesaw @5thHorseman mentioned) in order to delay or expedite your arrival to be at the right place at the right time.

Even if you get an encounter directly from Kerbin, halfway is the best place to fine tune your approach, to get the entry angle and PE you want, or get a gravity assist from a moon. Doing that from Kerbin would need minuscule amounts of fuel, a fine touch of the throttle, and pinpoint accuracy. Much easier to wait and do it in deep space where the margin of error is larger to accommodate for your manual and mechanical inputs.

 

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15 hours ago, MPDerksen said:

The videos I watch indicate getting about halfway to a planet and THEN adjusting the orbit to get an encounter.  Why is that more efficient than near Kerbin?  I would think changes get MORE expensive as you get closer to the target.

All other things being equal, it is more efficient to correct it right at the get-go. But if you’re far enough away, the difference between the two is pitifully small. But there is also the consideration of being precise - you need to balance being far enough away that it requires a small enough maneuver that you save fuel with being close enough that it requires a large enough maneuver you can actually control it without overshooting, requiring an additional correction that wipes out your fuel savings.

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Make note that the best time for a correction burn is not necessarily "halfway there". There is a trade-off between the velocity ratio of the maneuver and the time the changed course can propagate. Use a normal burn component and vary the time of the maneuver node to visually see what maneuver time maximizes you normal component efficiency.

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On 9/19/2019 at 8:06 PM, 5thHorseman said:

I use the new Maneuver Mode editor in the lower left. Here's the Squad blog post on it:

https://kerbaldevteam.tumblr.com/post/184089345124/kerbal-space-program-17-room-to-maneuver-is

Yes, that was an absolute game-changer in the stock game! Trying to grab/drag the maneuver node without pulling on any of the toggles (prograde, etc.) was hard enough for a simple orbital rendevous. Doing it for an interplanetary burn when you were focused on another planet was almost impossible.

 

As for the OP's question, I don't think that there is necessarily any advantage to doing a mid-course burn. Ideally, you'd nail the exact course from Kerbin orbit and the crew could sleep until the injection burn. The problem is that the transfer burn from Kerbin orbit is usually 1000+ m/s. Unless you have a very high TWR, that means you will be burning relatively far from the actual maneuver node for part of it. I might tune the maneuver node to reach Jool at exactly the periapsis I want, execute it perfectly, and still find my course is off a bit.

With really small targets it gets even harder because they have so little gravity. I just did a transfer from Eve to Gilly in a game and coming out of Eve orbit a 0.1 m/s difference meant the difference between getting an encounter with a 15-km periapsis and missing the moon altogether. Since I couldn't tune it that precisely, I launched on a 'close enough' trajectory. When I was ~ 80% of the way there, I did a correction burn to get the encounter and it was still only about 2 m/s. It was worth the extra 2 m/s because I just couldn't dial in the intercept precisely enough from the start.

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On 9/19/2019 at 4:52 PM, MPDerksen said:

The videos I watch indicate getting about halfway to a planet and THEN adjusting the orbit to get an encounter.  Why is that more efficient than near Kerbin?  I would think changes get MORE expensive as you get closer to the target.  

A couple points I haven't seen mentioned on this yet.

First, when setting up transfers by hand, it's often easier to set up any correction node once you're outside Kerbin's SOI, just due to how patched conics display.  And, when you're still in Kerbin's SOI, any up/down adjustments you make are relative to Kerbin, when what you're ultimately needing is relative to the sun.  This can sometimes be confusing, so it's generally easier to wait until you're outside Kerbin's SOI.  Also, prior to the relatively recent addition of the maneuver node editor box in the lower left corner, the interface often made it very difficult to plot correction burns while you' were still close to Kerbin.  You had to focus on the target planet to see your trajectory in its vicinity but the node was all the way back at Kerbin where it was hard to manipulate.  So this is often why folks would do their corrections like 1/2way to the target.  They were outside Kerbin's SOI and close enough to the target to manipulate the node while focused on the target.  Experienced players, knowing this was how you had to work around the limitations of the interface, would bring a bit more fuel than necessary to cover any inefficiencies this method caused.  Nowadays, however, with the node editor, these concerns don't matter.

The other thing to keep in mind about waiting to the middle to do the mid-course inclination burn is that the further you get from Kerbin, the more vertical distance there is between Kerbin's orbital plane (where you are) and the plane of the target.  Thus, your ship has to go up or down at a steeper angle the farther you are from Kerbin.  Which means, when you encounter the target, you'll have more inclination relative to the target because you'll be approaching it at a steeper angle from above or below.  If you're wanting to get into an equatorial orbit at the target, then yo'u'll have a bigger inclination change to do once you get there than if you'd done the en route plane change closer to home.  If you're not concerned about this, then it doesn't matter.

 

On 9/19/2019 at 4:52 PM, MPDerksen said:

Nuclear engines?  Best option for transfer burns?  Should I be using normal fuel tanks and just remove the Oxidizer?

My rule is only use nukes if there's no other way to get the necessary dV.  The problem with nukes is that they lack thrust and if you're leaving from LKO, you need a minimum TWR of 0.75 or so.  This is to keep the transfer burn down to 5-6 minutes AT MOST, or about 1/4 of an orbit around Kerbin.  If the burn takes any longer, you lose beaucoup efficiency to cosine loss and might even clip the atmosphere.

If you've made some massive mothership, sometimes you have no choice but to accept a very low TWR and a stupidly long burn.  In such cases, you'll want to start in a higher orbit with a bigger radius, so the cosine loss is much less.  But then you lose out on the Oberth effect.  You can also sometimes split a long, low-TWR  burn over 2 orbits, but this has its limits.  The 1st orbit can only be about 1000m/s or you'll leave Kerbin's SOI instead of coming back around to finish the burn.  Also, going out to the vicinity of Minmus on the 1st part of the burn will make the 2nd half happen a week or 2 later, which throws off your carefully timed transfer window, so you'll need a bigger mid-course correction.

So all in all, keep your transfer burns 5 minutes or less.  That's plenty of time to debeer/rebeer without having a long wait staring at the screen :)  

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I use nuclears a lot and have TWRs near 0.25. You wont leave Kerbin orbit in just one burn, so after burning 450-500 dv necessary to leave orbit i delete the node and make another one in the same place, rotate once around the planet and continue burning. I lose nothing but some time (45 min to 1hr in game) which changes nothing in interplanetary transfers

 

EDIT: Press ALT and . to engage in forced warp, so you can burn AND warp at the same time. I abuse this with nuclears and ion drives.

Edited by Fierce Wolf

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I find that making a correctional burn half way to my target planet can be made more precisely than trying to do it at Kerbin.  ANd as others have dsaid, it usually isn't that costly in terms of dV.

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Technically the correction burn shouldn't be done at the midway point.  It should be done when the vessel is 90 degrees away from the arrival point.  If a plane change is needed to intercept the target, it can be done most efficiently at this point in the orbit.

wH4DwmP.png

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However, if you need to do a prograde/retrograde/radial adjustment, it is most efficient to do that at a different point in the orbit than the plane change. So in order to consolidate all those adjustments into one, then you need to compromise on "efficiency".

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