800 m/s of dV left for Duna insertion, but maneuvers always use over 1300 m/s of dV

[dbubbis13]

I've gotten to Duna orbit before, but my maneuvers always use over 1000 m/s of delta-V for circularization/orbital insertion. When I mean insertion, I mean slowing down while in Duna's SOI to get an orbit. I am putting a station into orbit and have no plans of getting back to Kerbin, so there is no need for reaching escape velocity if I get into orbit. Here's my situation right now:

https://imgur.com/a/EPUiV9M

Keep in mind, I've tried to aerobrake in the past (working out horribly by either crashing or not shaving off enough velocity), and my station is very prone to heating.

 

Edit: If there isn't any way that I can fix this (it uses too much fuel or just isn't feasible), could you give me some tips on how to get better transfers? I keep hearing about things like delta v maps and transfer window planners, and they just make no sense too me and using complicated terms that I don't understand.

Edited May 6, 2023 by dbubbis13

[18Watt]

Your screenshot omits a lot of useful information, like numbers from the top and bottom of screen.  It's hard to make suggestions when there is information omitted from the screenshot.

If you are using the orbital planner, your entry at Duna is probably not ideal, you may be entering Duna's SOI with more velocity than you could be with a better transfer.

Your screenshot does show your Duna PE at over 500 km.  Your entry burn will be more efficient if your PE is as low as possible.  Duna's atmosphere only extends up to 50 km, and even below that the atmosphere is pretty thin.  Perhaps try shooting for an arrival PE of 60-90 km.  Do that burn to adjust your PE about 90 degrees prior to reaching Duna's SOI- doing that correction far away from Duna will make it more efficient.  Use the Maneuver Planner for that so you aren't just guessing and burning in random directions until you find what works.

[king of nowhere]

Sorry to say, but your transfer is terrible. Judging from how little the trajectory is deviated by the gravity, it looks like 1-2 km/s of intercept speed.

and of course you can't aerobrake; if you go too fast, you melt.

so, learning to make better transfers will both reduce your insertion deltaV, and make it more likely that you could aerobrake.

 

And the key to a low intercept is that your trajectory must be as similar as possible to that of the target body. the planet is moving very fast. your ship is moving very fast. when you intercept, you have to pay the difference in speed. but think if it was two cars instead. two cars moving at 100 km/h. if they are going in opposite direction, the speed of a frontal crash would be 200 km/h. that would also be the amount of deltaV needed for them to equalize their speed. if they are hitting at 90 degree instead, it means one car has to slow down by 100 km/h in the Y direction, and gain 100 km/h in the X direction; by pitagora's theorem, it's an intercept speed of 142 km/h. but if they are moving in the same direction, they are standing still compared to each other.

the best way to transfer is called hohmann transfer. here you get an example

you can see, the transfer orbit is an ellipse having earth(kerbin) on the closer point to the sun, and mars(duna) in the highest point. this way, when you reach duna, both the planet and your ship are moving in the exact same direction. duna is moving faster than you, and you have to compensate for that, but it doesn't take much. instead, if your solar apoapsis would extend way beyond, you're moving across duna's orbit, and you're going to have a horribly high intercept speed. which is your case.

a transfer window is the time when the planets are aligned so that you can leave kerbin on a hohmann transfer orbit, and reach duna at apoapsis.

orbital inclination makes things more difficult; for duna you can just make a plane change at a node, but if the difference in inclination is high, a plane change is hellishly expensive (can be as high as 2 km/s for moho) and there can be better strategies. but that's not a concern right now. at the moment, your objective is to get a hohmann transfer right. try one.

 

another thing to consider is the Oberth effect; it basically means that a prograde/retrograde burn is more efficient if you make it closer to a source of gravity. so going from kerbin to duna is cheaper if you launch from low orbit than if you launch from a higher orbit. And a capture burn is cheaper if you perform it closer to the planet. try to plan the maneuver, you should see that your capture deltaV should lower considerably if you lower your apoapsis from 500 to 50 km. though not enough to salvage the mission, probably, in your case.

if you have low thrust, and your capture burn lasts several minutes, you're going to get less benefits from oberth effect - because you're performing part of your burn far from the planet. oberth is bigger on bigger planets, so it's important to get close for a capture at duna, and even more at eve or kerbin, while on a minmus intercept it makes very little difference.

[Vanamonde]

Aerobraking at Duna can be helpful but also nerve-wracking because the air is so thin you may have to get down as low as 13kms to have much effect. The good news is that there's not much heating because of that. So it may be useful, and you might also play around with passes by Ike.