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zapman987

Closing in on a planet

Question

Making my first trip to Duna. Want to make sure I actually get to orbit/land (sending several ships), and therefore save some fuel.

Whats the best way and when to close distance on the planet, before setting up an orbit? Eg how can you lower your periapsis before acting on it?

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Two days away from intercept.

I just made that up; but it is a bit arbitrary. If you are too far away, it is difficult to get "fine" control. But the closer you are the more dV it will take. As long as you get close to where you want to be before you enter the duna SOI; then you can do the fine adjustment after you enter the SOI.

Often your transition between the SOI will cause changes that require correction anyways, so I usually do a rough adjustment a couple days out and another shortly after I enter Duna SOI.

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Edit your game's settings file and change the line

CONIC_PATCH_DRAW_MODE = 3

to read

CONIC_PATCH_DRAW_MODE = 0

You will then see the line of your projected path displayed close to the planet itself. At the halfway point of your transit (the most efficient time), place a maneuver node and tug it around until you have fine-tuned your arrival at the planet to be the way that you want it. But be sure to reduce your warp setting to x10 or less during the entry to the target's SOI, so that nasty glitch doesn't fling your periapsis someplace screwy.

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The bottom line is that you want your Duna Pe somewhere around 50km while you're still a few days out from the SOI. That's about as close as you need to be at that point. Note that it's pretty common to get a fly-by encounter with Ike so you really can't fine-tune your Duna Pe until you're inside Ike's orbit.

Aerocapture is the way to go at Duna. Once inside Ike, quicksave, then drop your Pe into Duna's atmosphere (-radial burns work best for this) and see what happens. Somewhere between 11-12km is usually good, but it all depends on your speed and the Ap you want afterwards. Anyway, if you don't get the Ap you want, reload the quicksave, tweak the Pe up or down a little, and try again.

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At the halfway point of your transit (the most efficient time), place a maneuver node and tug it around until you have fine-tuned your arrival at the planet to be the way that you want it. But be sure to reduce your warp setting to x10 or less during the entry to the target's SOI, so that nasty glitch doesn't fling your periapsis someplace screwy.

Is it better to do maneuvers at a node (decension in this case) or at halfway? Ive a node before halfway.

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It is easier and cheaper (dV wise) to do maneuvers at a node, but it is not required. Depending on where your node is a puff or two of RCS can get you on course where as it might take a couple hundred dV closer to your target. As with anything patience is key, and knowing how to slow down is good. (If you are playing with deadly reentry pack a heat shield.)

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Is it better to do maneuvers at a node (decension in this case) or at halfway? Ive a node before halfway.

You mean at ascending or descending nodes? If you want to match inclinations, you pretty much have to do it at either of those.

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What Vanamonde said. Changing your conics mode makes midcourse correction burns vastly easier. With that said, doing everything with a single course correction can be fiddly unless you have a small enough engine on your probe (trying to change velocity by tenths of a metre per second with an LV909 gets old very fast!). I tend to do a 'good enough' mid course correction and then fine tune about 2 days out from the SOI, as per Alistone's advice.

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I tend to do a 'good enough' mid course correction and then fine tune about 2 days out from the SOI
Since it's just about impossible to the mid-course burn with exactitude, yes, some fine-tuning is usually needed.

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Is it better to do maneuvers at a node (decension in this case) or at halfway? Ive a node before halfway.

If you want to dip into atmo to aerobrake, the earlier the better. And don't do do it with prograde and retrograde burns, as you aren't trying to establish a permanent orbit yet. Burn tangentially from your path.

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Duna requires little dV to achieve orbit at 50km but be aware that although parachutes and drogues help slightly, you WILL need to retro-thrust your landing. It will be easier than a mun landing though as your chutes should keep you perfectly vertical so you need only worry about speed control. Don't forget to use Ike for the gravity-assist both into and out of Duna's SOI - I typically do my best to use Ike to scrub off as much velocity as possible and tweak it to give me an aerobrake periapsis onto duna - just make sure you do a good plane correction during the interplanetary transit and you should arrive on a low relative inclination approach with Ike (and it costs relatively little dV at the edge of Duna's SOI to get into a good path).

Of course if you are running Mechjeb all of this is exceedingly simple to pull off, and if you use kethane you can just use Ike as a service station on your way to/from Duna if fuel is a concern.

Edited by hammyhamm

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Aiming at Ike gravity slingshot requires exactly the same level of precision as aiming for aerobraking so I don't see much point in using that on arrival except if you use Deadly Reentry mod. On departure, Ike might help a bit but I'm not sure it's really worth the trouble. Gravity slingshot matters the more the more your trajectory gets deflected and with Ike's low gravity and usual speeds needed to transfer to Kerbin this deflection is minimal. Notice that to make use of Oberth effect you need to perform as much of the transfer burn as possible in low Duna orbit and you'll save more dv by that than by Ike slingshot.

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Sorry for the late bump, took a break from the game. back now!

Ok so Im hearing conflicting things. Going off the logic that earlier equals less fuel: Ive just set an intercept. my PE is 28million. I need to get that down. Thats my objective.

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The bottom line is that you want your Duna Pe somewhere around 50km while you're still a few days out from the SOI. That's about as close as you need to be at that point. Note that it's pretty common to get a fly-by encounter with Ike so you really can't fine-tune your Duna Pe until you're inside Ike's orbit.

Aerocapture is the way to go at Duna. Once inside Ike, quicksave, then drop your Pe into Duna's atmosphere (-radial burns work best for this) and see what happens. Somewhere between 11-12km is usually good, but it all depends on your speed and the Ap you want afterwards. Anyway, if you don't get the Ap you want, reload the quicksave, tweak the Pe up or down a little, and try again.

Mechjeb's landing guidance can predict your orbit after aerocapture, so you can change your pe to achieve your desired Duna orbit.

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If your onlygoal is to lower your PE it's easier (But not necessarily cheaper dV wise) to wait until you are just inside the SOI of the body you are going to orbit. If your body has an atmosphere you can aerobrake to save some dV getting into orbit.

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Myself, I just do it the easy way and burn inwards to my orbital path. Don't waste all the fuel trying to decelerate, just burn up/down/inward/outward for low fuel requirement adjustment. Let atmo do the braking work. Duna is a bit more difficult cause you have to actually get pretty low to aerobrake from orbital speeds.

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