Visiting Dres, stop at Duna?

[KerikBalm]

I was playing around with the transfer window calculator, and I had assumed that by stopping at Duna and refueling there, I could get to Dres with significantly less dV than leaving from Kerbin... but that the total dV would be higher, as would the trip time.

Kerbin to Dres dV (80 km orbit start)

Departure

Year 1, day 344 at 3:50:24

Arrival

Year 3, day 80 at 0:21:36

Total dV: 4048 m/s

(Note: for 4212 m/s you can depart at Y1 d424, and arrive at Y3 day 26)

Kerbin to Duna, aerocapture (or gravity assist capture) 80km to 60km orbit:

Departure

Year 1, day 231 at 0:14:24

Arrival

Year 2, day 56 at 4:14:24

dV: 1046

Duna to Dres:

Departure

Year 2, day 269 at 3:48:00

Arrival

Year 4, day 290 at 3:18:00

dV: 2387

Total dV: 3433 m/s -> significantly less than the most efficient direct hohmann from Kerbin (4048).

You arrive much later though... but wait... there is a 2,621 m/s burn that gets you there on Y3 d190.... not so far behind the direct departure from Kerbin (granted, you must leave earlier for the Duna transfer)

Total dV in that case: 3667 m/s... still much less... and you get to spend time at Duna.

You could save even more using Ike, and not capturing into a circular orbit, but staying quite eliptical

How many people stop at Duna on the way to Dres?

Maybe I should ask how many people even go to Dres?

[Haruspex]

I have an ion probe with science stuff on it in polar orbit around Dres. Once in a while a contract for science from Dres comes along, so it's there to stay. But more than that? Meh.

[Birdco_Space]

In general, there is a lot of dV expended in achieving a circular orbit from a Transfer, as there is a lot of hyperbolic excess dV that needs to be shed to achieve the Orbit. Thus, any mission to Dres (or another world) with an actual stop in between will cost more dV than a straight transfer to the final destination.

Even more in general, Gravity assist flybys only really make a big difference in 2 cases. Moho Via Eve ( This can knock total dV to a stable Moho Orbit down to about 4.5km/s) and Eeloo via Jool (Eeloo stable orbit for just about 600-800m/s more than the holmann to Jool)

[KerikBalm]

Well, I had done these calculations before for Eeloo and Jool, before ISRU, and it didn't save any dV.

I never even bothered to look at the numbers for Dres until now... and I was surprised that it seems to be the one case where its relatively simple to "make a stop" at another planet, and get to the destination cheaper.

(sure there are ways to get to Moho cheaper by using Eve gravity assists, but you wouldn't actually want to capture at Eve... although... maybe... with a veyr highly inclined capture orbit + waiting a long time for a window.... it might work)

It could make a Dres mission more appealing, if the first stop is Duna.

I wonder if a capture at Laythe could work to save dV going to Eeloo.

If I use the transfer calculator, it comes out almost the same if I assume I start in circular Jool orbit at 27184km (the SMA of Laythe), but I'll get more Oberth effect from low laythe orbit... but also its more of a gravity well to escape... unless I capture into highly eccentric orbit that is mostly aligned with the ejection burn needed for the next transfer window (I don't want to wait centuries for the right window)...

Anyway, I'm just noticing that Kerbin->Duna->Dres seems to be the simplest case where you can capture at a planet that is not your destination, and still use less dV than a hohman from Kerbin.

Therefore, a Dres visit should include a Duna visit... that plus the asteroids makes a mission to Dres sound much more interesting than it did before.

[mikegarrison]

Normally the best strategy for dV is not to establish an intermediate orbit. And especially it is bad to drop into a gravity well that you then have to climb back out of.

I think what is happening here, though, is a special case. First of all, Duna is basically in plane with Kerbin, so there are no losses there. Then, with the aerocapture, what you are actually doing is letting the drag from Duna's atmosphere accelerate you into Duna's higher-energy orbit. In essence you are getting a free circularization burn into Duna's solar orbit. And finally, Duna has such a weak gravity well that climbing back out of it does not eat up all your gains.

This strategy would not work with a high gravity planet, so Eve and Jool are out, and it would not work with a vacuum planet, so Dres, Eeloo, and Moho are out. That means Duna is the only planet that could be used in this way. (It might work to help you get to Jool and Eeloo, too.)

Edited September 29, 2015 by mikegarrison

[KerikBalm]

Duna Doesn't help for Eeloo (I guess due to reduced Oberth), any help it is foor jool is nullified by the greater boost from aerobraking at Jool/Laythe (you get there with more relative velocity, but then you get more aerobraking).

Stopping at Jool (high orbit.. Laythe-ish) seems to be break even for reaching Eeloo

[cantab]

A refuel at Ike should be viable. Anything rated for Dres will handle Ike.

You can alternatively use Duna gravity assists, but that takes some figuring out. I was able to use two Duna assists to change my plane to match Dres's orbit which would have helped cut the capture burn at Dres (but I only did flybys on that mission anyway). Time consuming though.

[KerikBalm]

Yea, well, a refuel at Duna is pretty easy too, the gravity isn't so strong, and a poodle works just fine in that atmosphere.

But I've already got a lot of assets at Duna/on the way to Duna.

I've got a LDO fuel depot, a LIO fuel depot, a duna miner+ tanker to maintain the fuel depot, an ike miner+ tanker to top up the ike fuel depot, some LV-N tugs, a claw tug, surface habs for both Duna and Ike, a rover/small fuel truck (with a claw to attach to anything for fuel transfers), a solar powered plane (modded), and a modded Xenon ISRU* craft

I'm sort of turning Duna into the main hub of my space program... I think I'll just have a mission depart from there to Jool rather than returning to Kerbin and sending a new mission from Kerbin.

Anyway, I checked the transfer windows relative to my current game time... and I think I'll just do a direct flight to Dres, no ISRU, no fuel depot, leaving a reusable lander in orbit in case I decide to go back...

I may have the crew stop at duna on the way back though.

*This modded ISRU can produce only Xenon from the atmosphere of planets: ie works on Eve, Kerbin, Duna, Laythe, and Jool - Eve is sort of dumb because you can't lift anything up cost effectively, and Jool is dumb for obvious reasons... so its only really viable for Laythe and Duna, or free Xenon at Kerbin.

[Mitchz95]

I just use an ISRU base on the asteroids orbiting Dres to refuel.

[Starhawk]

Very cool! I hadn't thought about it like this before, but essentially Duna's atmosphere saves you loads of delta-v on the trip since you end up having so much less to get rid of when you capture at Dres.

I wonder if stopping at Duna would allow aerocapture without exploding to death at Jool. Hmmm, probably not until 1.1.

Happy landings!

[CalculusWarrior]

Aerobraking at Duna actually doesn't slow you down relative to the Sun, it speeds you up! This allows to you circularize your orbit at Duna, meaning your required burn is just from Duna to Dres, less than a comparative burn from Kerbin due to Duna's smaller gravity well. It becomes much more efficient if you combine it with a fuel depot at Ike or somewhere else in the Duna system. Someone made a really great post on how this would work a while ago:

http://forum.kerbalspaceprogram.com/threads/68420-Off-world-Refueling-A-short-explanation

[KerikBalm]

Very cool! I hadn't thought about it like this before, but essentially Duna's atmosphere saves you loads of delta-v on the trip since you end up having so much less to get rid of when you capture at Dres.

I wonder if stopping at Duna would allow aerocapture without exploding to death at Jool. Hmmm, probably not until 1.1.

I think most people don't understand the biggest problem with Jool.

It doesn't matter how much you match Jool's orbit before crossing the SOI, due to Jools gravity well, you will be going a minimum of 8km/s when you enter the atmosphere.

Maybe it will be 8.5km, or only 8 depending on how you approach jool, but you won't get less than 8... and 8 is quite deadly

[magnemoe]

I think most people don't understand the biggest problem with Jool.

It doesn't matter how much you match Jool's orbit before crossing the SOI, due to Jools gravity well, you will be going a minimum of 8km/s when you enter the atmosphere.

Maybe it will be 8.5km, or only 8 depending on how you approach jool, but you won't get less than 8... and 8 is quite deadly

This, one option might be to catch up with Laythe and then aerobrake at it, since you and Laythe is moving in the same direction the initial speed is pretty low, this also have the benefit of not going so deep into Jool gravity well, your Pe would stay around Laythe orbit.

Main problem is to finding an fitting attitude for aerobraking.

[Acet]

I think most people don't understand the biggest problem with Jool.

It doesn't matter how much you match Jool's orbit before crossing the SOI, due to Jools gravity well, you will be going a minimum of 8km/s when you enter the atmosphere.

Maybe it will be 8.5km, or only 8 depending on how you approach jool, but you won't get less than 8... and 8 is quite deadly

Shallow dip and a ridiculous number of aerobrakes???

Just wondering, I haven't gone to Jool in 1.0.x yet.

[Starhawk]

I think most people don't understand the biggest problem with Jool.

It doesn't matter how much you match Jool's orbit before crossing the SOI, due to Jools gravity well, you will be going a minimum of 8km/s when you enter the atmosphere.

Maybe it will be 8.5km, or only 8 depending on how you approach jool, but you won't get less than 8... and 8 is quite deadly

Good point. I wasn't thinking about it very clearly.

Happy landings!

[Findthepin1]

I had a Dres colony of 10 (along with 2 in orbit) until I edited the save file. I ended up with a floating hitchhiker module.

[cantab]

Shallow dip and a ridiculous number of aerobrakes???

Just wondering, I haven't gone to Jool in 1.0.x yet.

I've heard that it doesn't work, that Jool's atmosphere is "cut off" so you go from no air at all to enough air to heat you to blowing up.

[Hannu]

I will go to Dres but I have made it only once from Duna station. DV requirements from Kerbin or Duna to Dres vary very significantly between launch windows because Dres have inclined orbit. Transfers are the most economic when Dres is about Kerbin's orbital plane when the spacecraft arrives or departs there. Duna is practically in Kerbin's plane so there is no significant difference between them.

First window from Kerbin to Dres have arrival date of y3 d78 (Kerbin's time). Dres is then about 2440 Mm above Kerbin's orbital plane. It takes more than 1600 m/s normal velocity to achieve that point and total dv (ballistic orbit from 100 km equatorial to 100 km) is 4094 m/s. But if you start at second window you arrive at y4 d282. Dres is then only 20 Mm above the plane and total dv is only 3070 m/s.

Your trajectory via Duna arrives at y4 d290 when Duna is also very near the Kerbin's plane (-57 Mm). You should compare dv requrements to second window from Kerbin. You may get advantage if you use Duna as gravity assistant but typically it is not economic to brake on bodies between arrival and target bodies (if you do not have to get fuel or investigate several bodies).

I have tried gravity assists couple of times because they are interesting in mathematical and physical sense but unfortunately KSP's trajectory planning does not support such complicated trajectories and they are very laborious to do and very sensitive to errors. DV is so cheap and there are no outer planets in stock KSP so that there are not any gameplay reason to use gravity assists.