Lowest cost for orbiting Duna?

[Rocksummit]

Is there a way to achieve orbit around Duna without spending 2400 dV?

I'm launching from Kerbin and hit Duna with an escape trajectory and to achieve orbit it takes 2400 to get a stable orbit. Is there anyway I can aim to reduce this cost?

Edited June 16, 2015 by Rocksummit

[Empiro]

Yes, you should use a launch window planner like this one: http://alexmoon.github.io/ksp/ and launch at the correct time. It should take no more than a total of 1100 m/s to get to Duna, and you can aerobrake around Duna for very low cost

[cybersol]

Another launch window planner with porkchop plots is here: http://alexmoon.github.io/ksp/

You are coming in too hot, which is typical for your first interplanetary transfers. The move you are doing is referred to as the insertion burn in that planner. If you leave at the first window of the game on day 231, and perform an optimal Hohmann transfer where your apoapsis just barely arrives at the planets orbit at same time the planet arrives there, then the planner says the insertion burn should only cost you 640-650 m/s dv. However as Empiro points out, if you can take the heat then you can reduce the dV require down to <100 m/s by aerobraking in Duna's mid-to-upper atmosphere.

[Rocksummit]

I used kerbal alarm clock to tell me when to leave for Duna so I had a pretty efficient burn to get there. But I wasnt expecting such a costly insertion burn. If I want to aerobreak I need to add another 100 dV which isnt bad but by that time I've used up too much already.

Does my insertion burn depend on where I hit Duna's influence?

[Empiro]

How much total Delta-V did you spend from LKO to get to Duna? As I stated above, if you're using much more than 1100 then your trajectory is sub-optimal, meaning that your capture will be more expensive as well.

Aerocapture should save delta-V if you put your PE inside Duna's atmosphere from far away.

[Rocksummit]

I think I spent around 2200 including leaving Kerbin orbit which would be 950 so that would put me pretty close to 1100.

But for me to put my PE inside Duna's atmosphere wouldnt that require me to first make the insertion burn?

[Red Iron Crown]

There are two big burns in a traditional transfer: the ejection burn and the insertion burn.

The ejection burn is the one when leaving Kerbin, it should be done entirely in low Kerbin orbit as it is much costlier to escape Kerbin first and then burn to get your intercept with Duna. This burn should be almost all prograde and cost a bit less than 1000m/s during a good time for a transfer (a transfer "window").

The insertion burn (sometimes called the capture burn) is done at Duna periapsis to match velocity with the planet. This burn should be retrograde and cost about 600m/s for a low orbit (again assuming a good window). This is the one that can be significantly reduced by aerobraking in Duna's atmosphere.

Almost always it is useful to make correction burns between the ejection and insertion to fine tune the way the vessel will approach Duna and set the desired Duna periapsis. These burns should be all radial, antiradial, normal, or antinormal (depending on how the approach needs to be changed) and should only be a few dozen m/s at most.

[KarateF22]

If all you want is Duna orbit the easiest way to get there is with an ion Microprobe, their tiny mass and sky high delta-v essentially lets them do laps around the solar system, let alone Duna.

[Empiro]

I think I spent around 2200 including leaving Kerbin orbit which would be 950 so that would put me pretty close to 1100.

But for me to put my PE inside Duna's atmosphere wouldnt that require me to first make the insertion burn?

It sounds like your transfer is less than efficient then -- the 1100 m/s I cited is for everything, including the escape burn (getting to Duna is less than 200 m/s on top of escape). Because of the Oberth Effect and because Kerbin is much larger than Duna, spending an extra 1000 on the escape will probably translate into a 2000 m/s capture burn.

As for aerocaptures, no you don't need to perform an insertion burn first (which is why it's called an aerocapture). Assuming you did your transfer properly, your craft should enter Duna's SOI and then zip through. If you perform a correction (usually half way there), you should be able to tweak your path through Duna's SOI until it pass through the planet's atmosphere.

[FancyMouse]

In fact, the most efficient place to do correction is as early as possible (Oberth effect FTW). Mid-course correction is just for precision and inclination. Say for Duna, you probably first want to shoot for 50~200km Pe for ejection burn (engine throttle limit helps a lot), and then correct to ~17km for aerocapture. If you just aim for an encounter with a high Pe, you'll spend somewhat more fuel to do correction mid-course, and you'll spend really a lot more fuel if you don't correct and wait until entering SoI to do capture yourself.

[Red Iron Crown]

In fact, the most efficient place to do correction is as early as possible (Oberth effect FTW). Mid-course correction is just for precision and inclination. Say for Duna, you probably first want to shoot for 50~200km Pe for ejection burn (engine throttle limit helps a lot), and then correct to ~17km for aerocapture. If you just aim for an encounter with a high Pe, you'll spend somewhat more fuel to do correction mid-course, and you'll spend really a lot more fuel if you don't correct and wait until entering SoI to do capture yourself.

Earlier corrections are cheaper, but not because of the Oberth Effect. Corrections burns should almost always be in the plane of radial/antiradial/normal/antinormal, those sorts of burns are actually less efficient the faster you are going.

The reason early corrections are cheaper is simple geometry: An early correction to adjust periapsis might be a trajectory direction change of a fraction of a degree; the same adjustment to periapsis when much closer might be a direction change of several degrees and thus cost more delta-V.

[Rocksummit]

Ok. I haven't been doing any correction burns. I figured it was all the same once I make Duna's SOI and the insertion burn would be dependant on my timing.

If it's possible to hit Duna with 600dV I've been doing it all wrong. I'll try to do some correction burns to fine-tune my approach.

Thanks for the help everyone

[Laie]

In fact, the most efficient place to do correction is as early as possible (Oberth effect FTW). Mid-course correction is just for precision and inclination.

When angling for the most fuel-efficient Jool transfer, I came across situations where doing a sizable mid-course correction was cheaper than doing it all in LKO. The magnitude depends on the launch window (mine was in year 7, iirc), the difference isn't all that huge, and it's a hassle to set up. So I do by no means recommend it. I just want to point out that it can be a valid fuel-saving technique.

[Dr.LoveJoy]

It all depends on what you are taking there, methinks. And specifying this is terms of Delta V doesn't tell us any of that.. but I am going to assume you want to be uber conservative on fuel.

Everyone who says "use a launch planner" is 300% right.. first, plan the most efficient time to launch from planet A to Duna.

Secondly, the type of craft you use depends on your payload.

I've had some success in ultra conservative fuel payload transfer systems using Hooligan Labs "Death Star" torpedo shaped airship envelope. You can do ultra heavy loads like this.

So for this idea, what you do is make a payload + orbital transfer + interplanetary stage (with a primary pusher like nuclear rockets, so you can use 100% liquid fuel) that will fit symmetrically between an even number of envelopes. Then you size the envelope number onto your superstructure so that the payload is essentially weightless up to at least 25k. Easy to test.. just launch this by itself and wait. If it reaches above 25k, you're all set. You attach the payload stage with docking clamps.

Secondly, you attach a segmented, single winged body on the outside of each airship envelope and attach Jet Engines, fuel, control, electrics, etc. I like putting Kerbals into everything, personally, so I use Mk1 cockpits on each side, but you could use probe bodies for everything. I use a probe body for the orbital transfer stage. You want to keep the tanks symmetrically distributed along the center of of the airship envelope, and THEN attach wings to determine lift characteristics.

To fly it, launch it from the runway at low thrust.. it should lift very easily, or it may even liftoff without thrust..if it does that, lower the buoyancy or set the altitude control to 0.00 meters per second.... you want to bite into the air a bit to get forward momentum.. and just be gentle with the throttle and controls. Don't get to high speeds until you are above 10k.

It takes some learning to get used to.. and I lost a lot of them.. but I have gotten the profile down pretty well. With this idea, you can lift huge payloads to orbit with minimal fuel expended. You may have to add nuclear rockets to the Airship rig to maintain thrust at very high altitudes, but here the trick is to get higher altitudes with longer dwell time because this craft will have a lot of drag.

To recover is very very easy. Don't try to use the altitude control while plummeting... it ends up making you like a cork bobbing up and down.. unless you want to just leave it running for 3 hours while it figures itself out :-) Anyhoo, I would set the altitude control to -20 meters per second or so and have a few chutes for safety, but the wing body should bring it all down pretty easily on a glide. If you are gentle, you won't even see reheating.

My next step is to try to figure out how to take the whole thing into orbit, thus removing the need for an additional stage :-)

Anyway, FWIW. There you go! This probably doesn't matter for smaller payloads and you'll want to go the spaceplane route for those, but I think for heavy and bigger loads, this design wins.

Maybe at some point I'll perfect it so its more aesthetically nice and better performing and post! :-)

--DLJ

PS: I have had ideas of using structural wing parts tom completely enclose the envelopes and give the whole thing good aerodynamics and some extra lift.. like some of the lifters people make from girders and wing parts.

[ajburges]

I find inclination changes (normal burns) are best done midway. Any earlier, and you have trigometric loses. Later and you have a shorter distance traveled with the changed momentum. I strive to place Pe on the orbit I want so I can do a elliptic capture followed by a cheaper plane finalization.

Radial changes are only to adjust arrival time, earlier is better. Best is to avoid them.

Prograde changes are to correct trajectory. Earlier is better.

[nekogod]

Presumably Ike can help with a slingshot into Duna orbit

[Sharpy]

Does my insertion burn depend on where I hit Duna's influence?

Yes, the more similar the orbit the smaller the burn. An encounter on the opposite side from Kerbin, on a tangent trajectory will be less dV-hungry than just crossing the path at any random point.

You can "simulate" the encounter dV by placing a node at (slightly before) the encounter point and tuning it in such a way that the resulting orbit will 100% overlay the target orbit. You'll know both the dV and where your "retrograde relative to target" direction lies.

But for me to put my PE inside Duna's atmosphere wouldnt that require me to first make the insertion burn?

Yes. If you make it 100 days before the encounter, the burn will be of order of 3m/s of delta-V.

Just don't delay your insertion burn until you enter Duna's SOI. Make the first corrective maneuver good 100 days or more from the target ("Focus View" on Duna, turn in such a way that the node (somewhere far out there) is visible, then adjust the node so that the trajectory passes through the upper atmosphere. While performing the burn stay in the same view and instead of trying to get 0 on the maneuver dV clock, finish the burn at lowest thrust possible (if needed, set thrust limiter of your engine to 5%, this increases throttle accuracy 20x) and watch as your actual trajectory approaches the surface. Hover over the periapsis and when it enters the atmosphere, cut the thrust (and reset the thrust limiter back...)

Seriously, if done early enough, the burn changing a "distant encounter" into "aerocapture" can be of order of 1m/s.