Lowering periapsis at destination planet?

[dryer_lint]

Does anyone have any advise for how to lower my periapsis at a destination planet (Duna, in this case) ahead of time? I've been messing around with maneuver nodes, but I just can't manage to lower it as far as I want to, or else the encounter goes away... it would really be helpful if I was able to aerobrake at a destination. At the moment, half my fuel is wasted on just slowing down.

Edited July 11, 2014 by Nikola7007

[Red Iron Crown]

Radial burns are good for adjusting your periapsis height for aerobraking. If it's too high, burn radial - to lower it. If it's too low or impacts the planet, burn radial + to raise it. This technique can also be used to turn a clockwise orbit at your destination into the more desirable counterclockwise.

The further away you do this burn, the less fuel you'll use but the less precision you'll have. Very important not to warp through the SOI change if you've got a finely tuned encounter set up.

[NecroBones]

When you're still far away in a solar orbit, it's very tricky to get it precise. I'll usually try to fine-tun it a bit at the ascending/descending node, and it may involve all 3 axes (all 6 handles) on the maneuver node to tweak it. Usually I'll try to adjust each axis until I can find the lowest value I can get from that one, then try another to tune it further.

But I've found that the most hassle-free thing to do is just to get an encounter, lower the periapsis a little, but don't sweat getting it where you want just yet. Then, right after entering the SOI (still out at the SOI's edge), make a maneuver using only the radial and normal axes (periapsis altitude and inclination, respectively) and not the prograde/retrograde handles yet, and you can adjust your approach vector very easily, and without spending a lot of fuel here. Yes, it'll cost more than "getting it right" from much further away, but we're talking a difference of maybe dozens of meters per second, not hundreds or anything like that. So as long as you've budgeted enough fuel to make correction burns, doing the correction just inside the edge of the SOI will be a good compromise between efficiency and ease.

As noted above, the further out you can do it, the better, from an efficiency standpoint.

Edited July 11, 2014 by NecroBones

[Dartguy]

I have found that when you set up a maneuver node, click on the target planet and switch view, then it becomes MUCH more precise. Of course, the SOI changes still affect the end result.

[dryer_lint]

Ohh, radial burns! I pretty much forgot that marker was there... does this technique work from anywhere on the orbit or do I have to do it while I'm still in Kerbin's SOI?

EDIT: Ok, wow, a lot of people posted while I was replying to Red Iron Crown

[NecroBones]

Yep, you can tweak the radial/normal controls at any time, just note that large burns in those directions are not "optimal". For instance, if you're doing your escape maneuver from Kerbin, and your destination is Duna, it's better to adjust your departure maneuver's position in Kerbin's orbit and use the prograde handle, than to use the radial handles for large corrections here, since you can change your outbound trajectory the same way more efficiently. But if the tweaks are small, it's easier to do it with the radial handles, and it won't effect the burn efficiency much in those cases.

[Red Iron Crown]

It's fairly inefficient to adjust your periapsis using radial burns when you're close to it, once you're a ways into its SOI it's usually better to dor pro/retrograde burns to adjust. The further away you are, the more efficient a radial burn becomes in comparison to a pro/retrograde one. I can't say I have the math to back that up, though; it's entirely empirical in my case.

[butterknife]

It's fairly inefficient to adjust your periapsis using radial burns when you're close to it, once you're a ways into its SOI it's usually better to dor pro/retrograde burns to adjust. The further away you are, the more efficient a radial burn becomes in comparison to a pro/retrograde one. I can't say I have the math to back that up, though; it's entirely empirical in my case.

It really depends on where in the orbit you're meeting the destination. If it's Duna, you're probably slowing down on the way to your apoapsis, or you're speeding up on the way from your apoapsis. If you can hit duna right on your apoapsis, you'll be going as slow as possible, so you'll burn less fuel if you have to slow down for your orbital insertion. Burning radially when you exit Kerbin will allow you finer control over where your apoapsis actually is. So your best encounters are going to be when you actually set that point as your encounter.

You'll likely be going around 5000 Km/s when you're on your way to Duna, and in my experience, getting into it's outer SOI requires I reverse throttle until I'm at around 100-500 Km/s. That's a helluva long burn. Usually in the range of 2 minutes with poodle engines. At your midway point, you can adjust your radial burn and your vertical axis burn a little easier for fine tuning your approach.

[Red Iron Crown]

It really depends on where in the orbit you're meeting the destination. If it's Duna, you're probably slowing down on the way to your apoapsis, or you're speeding up on the way from your apoapsis. If you can hit duna right on your apoapsis, you'll be going as slow as possible, so you'll burn less fuel if you have to slow down for your orbital insertion. Burning radially when you exit Kerbin will allow you finer control over where your apoapsis actually is. So your best encounters are going to be when you actually set that point as your encounter.

We're talking about refining an encounter that has already been set, i.e. the encounter with Duna already exists but we want to set a periapsis appropriate for aerobraking.

You'll likely be going around 5000 Km/s when you're on your way to Duna, and in my experience, getting into it's outer SOI requires I reverse throttle until I'm at around 100-500 Km/s. That's a helluva long burn. Usually in the range of 2 minutes with poodle engines. At your midway point, you can adjust your radial burn and your vertical axis burn a little easier for fine tuning your approach.

5000km/s sounds absurdly fast for a Duna transfer. Getting into Duna's SOI should cost no fuel, the encounter is already set. If you mean a capture burn to stay in Duna's SoI, that can be almost eliminated by aerobraking or greatly reduced by bruning at periapsis (Oberth effect).

[LethalDose]

When I'm in a solar transfer orbit, I'll actually use the RCS translational controls to tweak my periapsis. I typically have some idea about which way to thrust to effect the changes I want, but it usually takes some trial and error, and I always quicksave ahead of time.

I place my vessel facing pro-grade with it's dorsum orientated to "solar north" (basically, its pointed forward and upright) and then thrust. If something goes to far, I just reverse the thrust to undo it.

It's fairly inefficient to adjust your periapsis using radial burns when you're close to it, once you're a ways into its SOI it's usually better to dor pro/retrograde burns to adjust. The further away you are, the more efficient a radial burn becomes in comparison to a pro/retrograde one. I can't say I have the math to back that up, though; it's entirely empirical in my case.

I usually do radial and normal/inclination burns right after I make SoI changes. When they're so far out, they're much more efficient for adjusting the Pe than are retro/prograde burns anywhere in an open orbit.

That being said, they don't necessarily help you get captured since these burns do very little to change your orbital energy, just where and when your Pe/Ap occur. I will usually combine maneuvers and burn retro & radially to get the Pe I want and start killing some energy.

Basically, burning retrograde at Pe is always the most efficient location to burn for capture, but radial/normal burns just inside the SoI (or outside of it, if you're really good) are typically the most efficient way to adjust where that Pe occurs.

Edited July 11, 2014 by LethalDose

[Pthigrivi]

When I'm in a solar transfer orbit, I'll actually use the RCS translational controls to tweak my periapsis.

Genius. Also just as a tip I usually end up aerobraking with a periapsis around 11 or 12k.

[dryer_lint]

I set the question as answered. Thanks a lot for all the help, guys!

[Laie]

[this post was nonsense that should never have been written. sorry about that.]

Edited July 12, 2014 by Laie

[LethalDose]

(nitpick)

When doing a retrograde burn, you have Oberth against you.

Nope, this is false as written.

If you're burning retrograde at periapsis, each m/s of dV that you spend changes orbital energy more than spending dV anywhere else in the orbit (in this case, you're decreasing it). It's still a case of rockets travelling at higher velocities doing more work. Oberth effect is absolutely working for you if you're burning retro at Pe (where velocity is highest and the Oberth effect is greatest).

Edited July 11, 2014 by LethalDose

[Kasuha]

(nitpick)

When doing a retrograde burn, you have Oberth against you.

That's of course not true. Powered braking at low periapsis is more efficient than at high periapsis, it takes less dv to get the apoapsis inside the SOI.

[Laie]

Powered braking at low periapsis is more efficient than at high periapsis,

I do not doubt that. But I always understood it such that Oberth was only on my side when I was accelerating further.

Hmmm. (scratches head, thinks things through). Time to edit my previous post, I guess.

[LethalDose]

I do not doubt that. But I always understood it such that Oberth was only on my side when I was accelerating further.

Hmmm. (scratches head, thinks things through). Time to edit my previous post, I guess.

Burning retrograde is acceleration, because you're changing your velocity. Oberth is a two-way street in terms of increasing or decreasing orbital energy (Why I said "changing" energy above, instead of "increasing").

Anyway, making mistakes is part of the learning process. Don't sweat it.

[Red Iron Crown]

I wrote a visualization of Oberth working in both directions in a huge Oberth effect thread a fem months ago (apologies for self-quoting):

As a thought experiment, imagine the following:

Two identical ships are docked together, with one facing prograde and the other facing retrograde. An explosive charge is placed between them and detonated at periapsis thrusting the ships apart with equal force in the prograde and retrograde directions, with each ship's kinetic energy changing by the same amount. Each ship perceives the other as fuel it has expended to change its velocity. The ship headed in the prograde direction, you will admit, gains extra kinetic energy due to the Oberth effect. Due to conservation of energy, the ship headed in the retrograde direction must have lost exactly the same amount of energy as the ship going prograde gained, so it must have experienced the Oberth effect as well.

The Oberth effect is symmetrical. The exhaust of the ship gains or loses exactly the same amount of "extra" energy as the ship itself.

Maccollo demonstrated the retrograde Oberth effect in capture burns in the same thread, in this post.

[technion]

Far and away, the solution here is the PreciseNode mod.

Make a node. Click on a planet and set your view there, then zoom in. The use the +- arrows in preciseNode to move it exactly where you want.

[KerikBalm]

Genius. Also just as a tip I usually end up aerobraking with a periapsis around 11 or 12k.

I typically use ions for this, when they buffed ions, my control became less precise

Of course, even the simplest ion system weighs more than the simplest RCS system, and can often be hard to set up as far as placement of the ion thruster. I only use them on my re-usable interplanetary tugs that have high payload fractions.

Basically, after I reach a limit with playing with the nodes, I point my ship in a direction I think I should go, light the ions, see if it gets better or worse, keep doing that until it starts to get worse/I hit my desired orbit.

Ion ISP allows for trial and error, and the low thrust is good for very precise maneuvers. My tugs also have significant dV reserves in the form of Xenon gas, for when they need to deliver a large payload, and don't have enough liquid fuel to make it all the way back to LKO (which is where the buffed ions really save time)