When to circularize burn around Duna?

[chunglii]

Hello all! I'm new to the forums but have lurked for a small time and have about 90 hours on KSP. I've built a spacecraft capable of getting me to Duna but I'm having a few problems circularizing an orbit around the planet:

1. I know its SOI is about 47 million meters, but do I start my circularization burn immediately once I'm in the SOI? I've tried waiting until I'm closer to the planet but I time warp too fast and fly right past the planet.

2. Am I supposed to be able to see Duna once I arrive at it and I'm within its SOI? When I go to the Mun I'm able to see my spacecraft falling towards it once I'm within it's SOI but with Duna I never see it.

3. I'm using a nuclear engine for my transfer burn and it works great, but is it what I should be using for my Duna circularization burn too? Is it strong enough?

Thank you.

[EtherDragon]

Hello all! I'm new to the forums but have lurked for a small time and have about 90 hours on KSP. I've built a spacecraft capable of getting me to Duna but I'm having a few problems circularizing an orbit around the planet:

1. I know its SOI is about 47 million meters, but do I start my circularization burn immediately once I'm in the SOI? I've tried waiting until I'm closer to the planet but I time warp too fast and fly right past the planet.

Don't time-warp too fast - your circularization burn should be executed at the Periapsis - it's the most efficient time to do it.

2. Am I supposed to be able to see Duna once I arrive at it and I'm within its SOI? When I go to the Mun I'm able to see my spacecraft falling towards it once I'm within it's SOI but with Duna I never see it.

Duna's SoI is pretty large, similar to Kerbin - it may not be possible to easily spot Duna when you are near the edge. But you should be able to identify your periapsis distance in Map mode.

3. I'm using a nuclear engine for my transfer burn and it works great, but is it what I should be using for my Duna circularization burn too? Is it strong enough?

Depends on your total mass, but it should be strong enough to circularize around Duna - might be a long burn though.

[Vanamonde]

1. Burning at the closest approach, the periapsis, is most efficient. It doesn't make a big difference if the periapsis is high, but you can lower it by burning 90 degrees from your flight path in the direction of the planet. And yeah, don't warp so fast.

2. Many of the planets have SOIs so large that the planet is just a distant speck. You can find it by rotating the ship until you are pointed straight down (the middle of the brown side of the navball), and then sight along the length of the ship.

3. Nuke engines work fine for just about anything but launch from a large world, though the burn will take longer.

[TheDarkStar]

1. While burning at periapsis is best, Duna has an atmosphere. This means that the most efficient way to slow down is to aerobrake - lower your periapsis into Duna's atmosphere to something like 15km-17km (but don't use parachutes) while still far away from Duna.

2. See what Vanamonde wrote.

3. You have enough time to burn that you should definitely use the LV-N; it is more than twice as efficient as the next best engines. Make sure that you don't use very many of them, though - they are heavy.

[federicoaa]

Basically, some days before arriving to Duna SOI, burn lateral (the blue vector in the node) to reduce the periapsis inside the atmosphere. The farther you are from duna, the less fuel you need to burn to change the PE, so do it a week or a month earlier to entering the SOI. Then you will use Duna's atmosphere to help you reduce the speed (it's called aerobraking). If you come in too fast it may not be enough to get trapped, so you may sill have to burn some fuel to get the AP inside Duna's SOI.

Hope it helps

[Skorpychan]

The burn to get an orbit around Duna should be done around periapsis. Yes, use the nuclear engine for it, ISP of 800 is good.

Aerobraking will rip off extended solar panels, and may slam you into the ground or hurl you back out into Kerbol orbit if you get it wrong, and engines aren't as efficient in atmosphere. Watch out.

[Trueborn]

The Kerbal Alarm Clock mod can help you never miss an SOI or maneuver node again.

http://kerbalspaceprogram.com/kerbal-alarm-clock-2/

[SRV Ron]

Quick save before setting up the aerobraking maneuver. Try for about 15k. While this maneuver will save tons of fuel, if too high, you will skip back out into deep space. Get it too low and you will aerobrake to a unscheduled landing. Be sure to retract all solar panels before hitting the atmosphere.

[TheDarkStar]

Quick save before setting up the aerobraking maneuver. Try for about 15k. While this maneuver will save tons of fuel, if too high, you will skip back out into deep space. Get it too low and you will aerobrake to a unscheduled landing. Be sure to retract all solar panels before hitting the atmosphere.

Also, feel free to use your engines to keep orbit or to slow yourself down a tiny bit more; engines are barely less efficient in atmospheres until the last 4km or so.

[Clockwork13]

Can somebody tell me what SOI means? Please PM me because I probably won't check this thread again.

[Taki117]

Can somebody tell me what SOI means? Please PM me because I probably won't check this thread again.

You may not, but others might. So here's a link to the Wiki page with some of the more common Abbreviations

http://wiki.kerbalspaceprogram.com/wiki/Acronyms_and_Initialisms

[The_Rocketeer]

The short answer is burning at the periapsis is the most efficient way to lower your apoapsis, but that will probably not give you the result you want because you are using a very low-power engine with a heavy ship travelling at a high relative velocity. Aerobraking is efficient but it's very risky and will probably take a few quicksaves to get a satisfactory result (for realists like me it would also result in unsurvivable G-forces and temperatures almost certainly causing your Kerbanauts to turn into char-grilled green smoosh on the dashboard of your Command Module). The truth is, your approach to Duna (or any planetary body) is much more complicated than just hitting the sphere of influence (SOI) and making a burn to slow down. Let me try to give you an idea what I mean.

When docking two spacecrafts in orbit of Kerbin, you probably already know that you need to create an intercept - a point where you pass close to your target. That's basically what hitting the SOI of another planet is - an intercept. Obviously there is more to docking than just passing close your target. You probably realise that when you get close to the target, matching your speeds is very important, because otherwise you'll smash right into it or drift right past and never achieve a dock. To match speeds, you thrust with your engines or RCS to slow down relative to your target (which might mean actually speeding up in your orbit). Based on your question, you've also realised all this is also true of intercepting another planet - you make an intercept, passing close to the target, and then slow down so you travel with it instead of leaving it behind.

The difference here is your orbit is almost certainly not going to be anything like Duna's. You will be climbing steeply away from Kerbol (and Kerbin's lower orbit) to extend your Apoapsis out to Duna's orbit, and timing this so that Duna will be there when you climb high enough. When you get to Duna's orbit, your orbital speed will be way, way lower than Duna's because you're coming from below Duna's orbit and are close to your apoapsis, the point of your orbit where your speed is lowest. To look at it another way, when your ship shoots through Duna's SOI, it's actually Duna that shoots past you at a much higher speed than yours.

The trick is, before you reach Duna's SOI get your speed up to as close to Duna's as possible without throwing out your intercept. That means burning prograde (green vector on navball) to speed up, and radial-in (towards the brown half of the navball) to keep your apoapsis at the same altitude above Kerbol (of course if you were heading to Eve, or coming back to Kerbin from Duna, you'd need to do the opposite to match speed with the lower orbit planet - burn radial-out and retrograde). This obviously uses fuel, but this is fuel that you would have to burn anyway if you timed your burn to your Duna periapsis.

Your speed is about right when Duna's gravity causes a 45 degree change in your trajectory (based on your planned orbit periapsis - for Duna I suggest 80,000-100,000m). Alternatively, when you get near Duna's orbital altitude compare your orbital speed with Duna's - aim to be within about 2500m/s of Duna's speed as you enter Duna's SOI (you can get Duna's speed in map-mode while still in Kerbol orbit, or on the wiki). If you can manage either of these, you should have no trouble adjusting your Duna periapsis to make a retrograde burn into a nice low orbit.

Please PM me if any of this is unclear! Best of luck

Edited January 15, 2014 by The_Rocketeer
Added details.