Help me understand midflight correction burns

[thiosk]

I now have a great working understanding of how to get transfers between SOI. I can correct inclinations (although my recent Jool visit had me coming in RIGHT over the pole and flung down south around the planet). (off topic: Should have gone all the way around for a second aerobrake but decided to correct and snag the inclined moon for some more science points, couldn't get a stable orbit and had to ditch the mission after the final transmission)

What I'm trying to do is get better at it so I can carry less fuel and make manned return missions feasible. In my recent ike visit (haven't landed on Duna yet) I got on the ground, did science, then back to Kerbin. I thought I had set up a midflight burn to get ~40,000 m periapsis to Kerbin. However, I had to do a HUUUUUUGE burn (~1500 m/s) to pull the orbit off, because something happened between my spot-on preflight burn and getting into the SOI. I was 1.5 million out+ and almost ran out of gas to get in atmo. That would have been rather disasterous.

As I alluded at the top, I had the Jool really good coming in at 128,000 meters, but right over the pole (showed 0 inclination difference on instruments)

There's something I'm not getting, and it might be maneuver node based. When I set up the nodes, I do tons of zooming in, zooming out, to the point my wife is annoyed at the continuous mousewheel sound, my finger hurts, and I have difficulty telling what the actual encounter periapsis is supposed to be, and the fine corrections are just tough.

As such, i hope to obtain tips that will make aerobreaking a feasible option for interplanetary return missions. I'll need it.

~thiosk

Edited November 7, 2013 by thiosk

[Geschosskopf]

Midcourse corrections are very rough, approximate things because what the game shows on the map isn't very accurate at the distances between planets, and what you see when you do the burn will change fairly significantly by the time you get close to the target. So tweaks in the 1st half of the trip are best used just to get an encounter at all, like to change your inclination to meet the target if you left Kerbin in its plane. But don't expect more of them than that.

To get fine control over what you'll have when you change SOI, you need to wait until you're at least 3/4 of the way there, maybe more. It depends a lot on the total length of the trip. Like for Jool, I usually wait until I'm close enough that the remaining length of blue line before the SOI change is about as long as the orange line through Jool's SOI, but with Duna, I can do such things earlier.

But even this is still just close enough for government work. You really can't set your Pe in stone until you're in the SOI and fairly close to the planet (which varies by the size of the planet). But tweaks this close in cost more, so you don't want to have to do but very minor final adjustments. This means you need to have already done most of the work outside the SOI, usually with several small burns spaced out along the trip as you feel necessary.

[csanders]

There's something I'm not getting, and it might be maneuver node based. When I set up the nodes, I do tons of zooming in, zooming out, to the point my wife is annoyed at the continuous mousewheel sound, my finger hurts, and I have difficulty telling what the actual encounter periapsis is supposed to be, and the fine corrections are just tough.

~thiosk

I get that. I hope they fix it. They've done a wonderful job with the maneuver node interface, but it's hard to see the result without fine mouse pointing.

Often, I set up a maneuver node in the right direction I know it should be. Then I point my craft in that direction, stop rotation and kill the maneuver node. Then I burn with my mouse pointer over the periapsis (it's almost always the periapsis I'm interested in. It could be something else, in which case I hover over that). Then I kill thrust when it hits the value I want. UI wise, I find that to be easier.

If you don't know where you should be pointing, and you have RCS, you can try this:

You know the -v- pointer that shows where your craft is pointing? I typed that out with two minus signs, and a 'v'. Have the direction the minus signs are pointing perpendicular to the horizon on the nave ball (if that's possible). That way you know forward/back thrust is prograde antigrade. Left and right thrust are radial-in, radial-out, and up is normal (if the ground in your navball is to the left) and down is anti-normal.

That was a bit technical, here's picture:

Ok, orient your craft that way. Fire up the RCS. Put your mouse pointer on the node you want to monitor.

Fire ikjlhn, see which one gets you closer the easiest. Take note. Setup a maneuver in that direction (not to large, say 10m/s), then point craft at maneuver node, remove maneuver node, burn and watch with your mouse hovering over the node of interest.

Also, with midcourse corrections, sometimes RCS alone can do the trick.

EDIT: Also, sometimes you can only correct things so much. For instance, if you want to align planes, and your not at an ascending (an) or descending node (dn), you can only correct it so much. However, you can try to make either the an or dn align with closest approach.

Edited November 7, 2013 by csanders

[Superfluous J]

It's all a matter of knowing when to burn for what you want to do. It takes practice and thought, and isn't always commonsense obvious.

To go faster in an orbit, slow down. To slow down, speed up.

To affect your apoapsis, burn at your periapsis, and vice versa.

To affect your ascending or descending nodes, burn at them.

To change where you hit a planet (for aerobraking or just to make sure you come in at the right angle) do it as early as possible.

Up your conic patch limit in the config to 8 or more. Also if you like mods install PreciseNode, it'll let you really fine tune.

You can click on your periapsis and apoapsis markers and their numbers will stay on the screen. This is a great way to fine tune them without having to go back and check if what you did helped or hurt. You can do this with the resultant apo/peri for maneuver nodes as well.

[allmhuran]

1) Make sure your time warp factor is low (1000x max) when an SOI transition occurs. If you leave time warp at a very high value what happens is that by the time the game has recognized your new SOI and redone its calculations you will already be well beyond the transition boundary, and so the original estimate will be waaaay off.

2) To get better visibility on exactly what is happening at your intercept target, go into your settings.cfg and change the conic draw mode to 0. Mode 0 shows the flight path through the target SOI at the target's current position. This means you can tab through the objects in map view to the target body and zoom right in on it, allowing you to see your trajectory very precisely. I talk about this in my most recent video here: http://youtu.be/MryzqX5hUjw

I also demonstrate the improved manoeuver nodes mod. This is not a mechjeb type of thing, it doesn't do any work for you. It's just a working interface in the way that the existing interface is not. It allows you to make small changes to your planned burns without having to be able to see the node itself in map view, and therefore without having to be all finicky with the camera in order to be able to click and drag the right handle, etc. It also fixes the problem with nodes closing unexpectedly (or rather, the problem becomes irrelevant because you no longer have to drag the node handles to plan your burns).

Edited November 7, 2013 by allmhuran

[thiosk]

I think the SOI transfer was a huge problem, and I now have demonstrated i can point and tune rather than rely purely on node calculations.

I nailed a dres encounter, to show off my sexy new chops.

Boom shackalacka.

To extend my previous question, I recently installed one of these new fangled dV calculators to tell me what I was really flying with. I think I took too little fuel. I've got 2277 dv left. An optimum kerbin intercept is ~1300 dV. But by the time I orbit, correct inclination, I end up in bad shape. I found an intercept... but was about 300 dv short of hitting it. I had to revert this one, so I'm sitting back on the planet wondering whether jeb and co are part of a dres colony or not (no docking ports yet).

Is it reasonably to get out maybe 800 dv and then use the inclination boost at the approaching node to give me a little more distance, then finish the burn out there?

Tips?

Or send rovers and make a lifelong dres colony? (i really wanted those points!)

[allmhuran]

Yes, Dres is a very expensive target for a return mission because it has a very high inclination difference to Kerbin, and it has no atmosphere to help with the capture. I'm not entirely sure what you mean by your question though, regarding inclination burns and distance.

As a general answer to your query, the best time to make corrections is when you are furthest away from your target, but the best time to make inclination corrections is at the ascending or descending node.

The very best time to leave, then, is when Kerbin is ascending or descending through the orbit of Dres. In fact, you don't need to make any inclination correction at all if you leave at this time, because your intercept with Dres will be at your apoapsis on the other side of the sun... again at the ascending or descending node!

But if you want to actually intercept Dres on the first pass then you also have to leave at the right phase angle for a Dres intercept. So you would have to sit around watching Kerbin and Dres orbit until you get that perfect alignment.

Alternatively you could leave Kerbin at the ascending or descending node of Dres, get your apoapsis out to the orbit of Dres, and then just stay in your elliptical orbit around the sun until you get a close approach with Dres, at which point a couple of minor corrections can close up your approach distance and give you your intercept.

If you're feeling really adventurous, you can use gravity assists to get to Dres. Head down to Eve first, use Eve to slingshot you back to a more agressive Kerbin intercept, and use Kerbin to slingshot up to Dres. This is much cheaper than burning directly for Dres, and you can use Eve and/or Kerbin to help make the inclination change to that of Dres.

Edited November 7, 2013 by allmhuran

[Superfluous J]

You don't need all that delta v to get back to kerbin. Don't bother orbiting and correcting inclinations and all that, just wait for a transfer window and (again I'll suggest PreciceNode for this) aim straight at Kerbin. When you get closer, aim to shave into the atmosphere at about 20km and you should come in for a nice (though firey) atmospheric-braked landing. According to the chart I'm looking at, that'll cost you about 1300 dV if you're orbiting dres, and maybe 1700-1800 if you're landed.

Really, you can do it with the right tools and a higher conic patch limit

[thiosk]

To clarify my question, sometimes when I try to change inclination, I get a higher orbit as a result of the speed change; one would have to add some retro to it to keep from escaping in the case of extreme burns. I was wondering if it might be viable to use some of that speed to finish the job of getting to the kerbin orbital overlap. Would probably send a rescue mission before waiting for the encounter and nodes to overlap.

edit: I will have to figure out what these conic patches are. 1300 back from dres I get. 500 to orbit makes sense. But i do not have the intuitive sense I guess I need to get the encounter.

When you mean point at kerbin, do you really mean... POINT AT KERBIN?

I've never flown anywhere by just pointing at it, preferring to use the nodes. My first attempt to point at the moon did not get me to the moon.

Edited November 7, 2013 by thiosk

[Superfluous J]

NONONONO don't point at Kerbin

Go here: http://ksp.olex.biz/?

Plug in dres and Kerbin. Use the pictures to see how Kerbin and Dres should line up when you're leaving dres. Don't worry TOO much about the numbers, you're just going for rough. Of course, F5 first

Fire roughly where the chart tells you, roughly when it tells you. That will get you hopefully a near encounter. Tweak that encounter to ideally HIT KERBN (that's what I meant by "point directly at it." not your ship, your encounter. You don't want a periapsis near Kerbin. You want it inside Kerbin.) Then ride that line all the way home.

The conic patch limit is how many SOI changed the game will draw. It's default is 3 which is probably fine for a Dres->Kerbin encounter because you'll only switch SOI twice (once to Sun, once to Kerbin) but if you encounter Mun or something you won't be able to see what your Kerbin encounter is going to be, which is why I suggest raising it. It's in a config file and PreciseNode (have I mentioned that yet? ) lets you change it on the fly.

[thiosk]

Oh right, ok yeah that makes a lot more sense. I was really perplexed by "point at kerbin"

As I said, when I gave it a shot, I came up way short. So, here goes nothin.

[Superfluous J]

F5 is your friend, abuse it like you would a... friend... hm that doesn't work. But you know what I mean.

[boomerdog2000]

It also helps if you change the

CONIC_PATCH_DRAW_MODE = 0

is the settings.cfg to 0 as shown here. This allows you to see what your predicted orbit is around the planet in question, so you can focus the camera on Kerbin and mess with your intercept from a maneuver node in deep space.

[thiosk]

Success!

My procedure was to wait for the window, launch into a nice orbit. I then burned hard 1200 of 1350 dV needed to get on the way. At the descending node, about 22 days away, I gave just enough normal and radial to get the encounter. From there, I burned an additional 30 dV pointing around trying to tune the encounter, and by an iterative trial and error process hit this:

150 dV to spare. Splashed down a bit hard, but managed to recover all the science. ~1000 points to tinker with!

Thank you all for your help, I'm now a much better flier. And, I just unlocked heavy rocketry.

Eeloo.

I'm coming for you.

[Superfluous J]

Congrats! I knew you'd be able to do it!