Most efficiënt way to get to duna.

[badjass]

So I am trying to get to duna with as little delta v usage as possible. Is there an efficiënt and doable way to use gravity assists to get to duna?
I know of the oberth effect (sorry if I misspelled it). I just need to know how i can do an efficiënt gravity assist to duna. thx

Edit: thanks for the comments so far. But to clarify, I do not want to refuel.
An idea: would it in theory be possible to get myself in a 10% slower orbit than kerbin and then use a kerbin gravity assist. And would this even be feasable .

Edited January 22, 2018 by badjass
clarify my goals

[Dman979]

Hi, @badjass, and welcome aboard!

I've moved your question about gameplay to the Gameplay Questions subforum. You'll get more relevant answers there.

 

[sevenperforce]

Mun swingby, but that's about it.

[Rocket In My Pocket]

These .gifs always helped me to understand gravity assists.

However, the only thing between you and Duna is the Mun and Minmus...you aren't going to get much of an assist from them. (It may even hurt you in the big picture as you'll be losing Kerbin's oberth effect.)

Really the most important thing is when you go. You should check out Transfer Window Planner if you haven't already.

Lastly, welcome to the forums!

Edited January 21, 2018 by Rocket In My Pocket

[foamyesque]

@Rocket In My Pocket: There's also Ike, at the other end, for deceleration.

[Cpt Kerbalkrunch]

@badjass, for a literal answer to your question, a transfer to Duna with the lowest amount of delta v possible (for the transfer itself) is to head to Minmus, refuel in orbit, then burn to drop your Kerbin Pe to 70-75km, and burn for Duna at Pe. You can transfer for just a few hundred delta v (I forget the number exactly, but this includes the 150m/s you need to drop back to Kerbin). However, this isn't actually saving delta v, since you had to transfer to Minmus in the first place. But it will save fuel, and allow you to stretch your ship further than it would have ordinarily. Not sure if that really helps or not.

[GoSlash27]

badjass,

 The super efficient way to do it is by refueling in low Munar orbit and then proceeding directly to Duna from there. Don't dive down to Kerbin, just go straight to Duna. I've done the math on it, and Duna is actually "closer" (in DV terms) to low Munar orbit than the Munar surface is.

*edit* Just reran the numbers. 467 m/sec DV from low Munar orbit to Duna.

Best,
-Slashy

Edited January 22, 2018 by GoSlash27

[foamyesque]

13 minutes ago, GoSlash27 said:

badjass,

 The super efficient way to do it is by refueling in low Munar orbit and then proceeding directly to Duna from there. Don't dive down to Kerbin, just go straight to Duna. I've done the math on it, and Duna is actually "closer" (in DV terms) to low Munar orbit than the Munar surface is.

Best,
-Slashy

 

You gotta get your fuel resupply to Mun orbit if you do that, which saves you nothing if you've gotta haul it from Kerbin, and if you're ISRUing, you lose a lot less going from Minmus-surface->orbit than you do trying to loft it from the Mun's surface.

Edited January 22, 2018 by foamyesque

[GoSlash27]

foamyesque,
 1) You don't necessarily have to bring the fuel from Kerbin, since you can ISRU, but 2) even if you *do* bring the fuel from Kerbin it's still worth doing because it allows you to use a smaller and cheaper ship for the Duna trip.

Best,
-Slashy

[adsii1970]

HyperEdit.

[foamyesque]

Just now, GoSlash27 said:

foamyesque,
 1) You don't necessarily have to bring the fuel from Kerbin, since you can ISRU, but 2) even if you *do* bring the fuel from Kerbin it's still worth doing because it allows you to use a smaller and cheaper ship for the Duna trip.

Best,
-Slashy

 

I am skeptical of that, since putting something in orbit around the Mun requires more dV than a Duna transfer burn. A Mun-orbit refuel only saves you on your craft's mass if you're ISRUing and even then it's marginal at best because if you put your craft in orbit around the Mun, you could've got it almost into a Duna capture orbit (possibly all the way there with aerocapture), at which point you aerobrake the rest of the way. Minmus, on the other hand, requires less hardware in every respect, and breaking orbit requires far less than breaking orbit from the Mun. It's by far the better location for refuelling depots.

[GoSlash27]

foamyesque,

 I found an old chart where somebody had calculated the DVs for these scenarios, and you're correct in the assertion that transferring Minmus -> Kerbin -> Duna is actually cheaper than Mun->Duna. This chart has it 306 m/sec your way and 414 m/sec my way.

As for the rest of it, I prefer using the Mun for close planets because the transfer windows are easier to plan and I don't have a moon in my way to potentially wreck things, but YMMV. The fact that it takes more DV to get to LMO than Duna isn't a concern so long as you plan on a return trip. A ship that can get from LKO to LMO can also get from LMO to Ike and back or LMO to Gilly and back.

But you're right about Minmus being cheaper.

Best,
-Slashy

 

[Cpt Kerbalkrunch]

6 minutes ago, GoSlash27 said:

foamyesque,

 I found an old chart where somebody had calculated the DVs for these scenarios, and you're correct in the assertion that transferring Minmus -> Kerbin -> Duna is actually cheaper than Mun->Duna. This chart has it 306 m/sec your way and 414 m/sec my way.

As for the rest of it, I prefer using the Mun for close planets because the transfer windows are easier to plan and I don't have a moon in my way to potentially wreck things, but YMMV. The fact that it takes more DV to get to LMO than Duna isn't a concern so long as you plan on a return trip. A ship that can get from LKO to LMO can also get from LMO to Ike and back or LMO to Gilly and back.

But you're right about Minmus being cheaper.

Best,
-Slashy

 

I think it was my suggestion, but glad to see there's a chart to back it up. Before I learned to make more efficient launches and transfers, I refueled everything at Minmus. Got pretty good at timing the drop-down technique. It works really well, if you don't mind spending all the time and effort. I've since abandoned it for expediency.

[foamyesque]

I like using Minmus as a stop for my Eve missions. Because an Eve return ship is necessarily huge, if it's carrying its own ISRU (for refuelling at Eve), you can use it in the launch to LKO, and then transfer up to Minmus, refuel there, then proceed to Eve. Landing on Minmus using the RCS thrusters installed for control during the Eve entry never stops being funny

On a Duna run though, even a Minmus refuel is sketchy. I think the most efficient (while staying relatively simple) profile would be an unpowered gravity assist at the Mun to put you into your Duna insertion orbit, then another off Ike on your way in, then aerocapture and apsis lowering. It'd be a real pain to plan out, though; easier to just load up a little extra dV.

Edited January 22, 2018 by foamyesque

[Spricigo]

6 hours ago, Rocket In My Pocket said:

 

Really the most important thing is when you go. You should check out Transfer Window Planner if you haven't already.

Lastly, welcome to the forums!

This ^

You can reach Duna's surface with 1,1km/s from low Kerbin orbit. That is actually less than to Mun surface (mind you, returning is a different story). You can save a bit with a gravity assist or top the tanks along the way, but, outside of a challenge, not really worth the trouble.

 

In any case, a gravity assist is just a colision with a celestial body. You will bounce in a given direction depending on how you "hit" the CB. You can experiment with any vessel in low orbit place a maneuver node (just plan it, don't execute) make your trajectory cross Mun SoI and see how it is deflected, change your aproaching angle a bit and see the effect. 

(People smarter than me, or at least no so lazy, may describe it with math. But just messing with maneuver nodes can give an idea of the workings)

Edited January 22, 2018 by Spricigo

[Blaarkies]

2 hours ago, Spricigo said:

In any case, a gravity assist is just a colision with a celestial body. You will bounce in a given direction depending on how you "hit" the CB.

I think bounce/collision might be confusing for readers here, since a bounce in an orbital mechanics scenario would not be able to slow down the craft. 

[Spricigo]

6 hours ago, Blaarkies said:

I think bounce/collision might be confusing for readers here, since a bounce in an orbital mechanics scenario would not be able to slow down the craft. 

Well,  Collision is the physical term for what is hapenning: two bodies meet and momentum is transfered between them. I guess "bounce" is less technical but still a reasonable analogy.

The craft gain/lose momentum taken from/given to the celestial body.* Notice that is in the parent frame of reference, in the CB's frame of reference the craft just change direction. 

*Except that KSP don't simulate the effect on the celestial body. And the effect  is meaningless for the celestial body to begin with.

 

 

[RizzoTheRat]

14 hours ago, GoSlash27 said:

The super efficient way to do it is by refueling in low Munar orbit and then proceeding directly to Duna from there. Don't dive down to Kerbin, just go straight to Duna. I've done the math on it, and Duna is actually "closer" (in DV terms) to low Munar orbit than the Munar surface is.

*edit* Just reran the numbers. 467 m/sec DV from low Munar orbit to Duna.

Surely it would be even more efficient from a high Munar orbit?  If you're refining fuel on the Mun then that fuel is effectively free, so may as well lift it to a high munar orbit with a local tanker rather than make the Duna ship do it, plus you need less dV to get the Duna ship in to the higher orbit in the first place. 

If that is the case, then would it be more efficient to burn to Duna from a high Munar orbit, or drop the Mun PE first and then use the Oberth effect as you pass the PE to accelerate towards Duna?

I appreciate this isn't what the OP asked but I'm just in the process of installing a Mun mining base and refuelling tanker to try this as a way of reducing the dV requirements of my ships.

 

Edited January 22, 2018 by RizzoTheRat
spieling mistaks

[GoSlash27]

Rizzo,
 I'd have to rerun the numbers to be sure, but I believe low munar orbit is preferable in all cases because it's a relatively weak gravity well and Oberth is where the savings are coming from. The more V_xs , The lower your gate orbit will be.

Best,
-Slashy

[Laie]

19 hours ago, badjass said:

An idea: would it in theory be possible to get myself in a 10% slower orbit than kerbin and then use a kerbin gravity assist. And would this even be feasable .

In principle yes, It should be possible to leave Kerbin at the price of a Munar encounter, approx. 850m/s, and steal the remaining dV on a later Kerbin flyby. I don't think it's physically possible to get any cheaper than that (unless you use Principia). I also do not know how to plan such a maneuver.

I can at least get you partway there, though: Consider this simple Munar gravity assist to Duna (ordinary transfer shown for comparison):

I've written a short tutorial on how to plan such a maneuver without getting insane. Aiming for a follow-up encounter will compound the maneuver-plotting difficulties by a lot (and I for one wouldn't even know what to aim for).

 

[sevenperforce]

Just wait until the Mun is going to be tangential to your desired Kerbin SOI ejection angle and do a low-altitude swingby to take you into the Duna SOI. Adjust at Munar periapse to get the Duna periapse you want.

[Spricigo]

57 minutes ago, RizzoTheRat said:

  If you're refining fuel on the Mun then that fuel is effectively free, ..

Well, there is no free lunch. A refining base 'costs' the time* you expend with the management ( transfering fuel, launching it to orbit, delivering to the refuelling place,...), time that you would otherwise use for something else. If doing something else can pay for the fuel you can get from the refining base, that is actually cheaper.

Also consider the initial cost you incur to put the infrastructure in place, that need to be recovered somehow.

 

 

 

*while in game time may be considered, we usually are concerned with real life time.

[Foxster]

I think that practically your best bet is a good transfer window, a gravity brake in front of Ike's orbit and then aerobrake at Duna. 

[sevenperforce]

Question (although I think I might be able to work it out for myself): is it actually more efficient to spend the dV to dive to a low periapsis to get more of an Oberth effect? Or does it depend on how much excess hyperbolic velocity you need?

My gut is the latter. For example, if you're at Minmus and you only need to leave Kerbin SOI to go rendezvous with a nearby asteroid, it's trivial to show that you'd be better off simply burning straight from Minmus's orbit to escape. However, I'm suspecting that there's a point where a very large burn (like, down to Moho or out to Eeloo) would benefit from the Oberth effect significantly, and so the dv you "waste" dropping down to Kerbin is more than compensated for by the Oberth boost to your escape burn at periapse.

No idea where that cutoff is, though.

6 minutes ago, Foxster said:

I think that practically your best bet is a good transfer window, a gravity brake in front of Ike's orbit and then aerobrake at Duna. 

Duna is not too terribly hard to aerobrake at. You'd be better off planning a gravity assist at the Mun than worrying about a gravity brake at Ike.

[Laie]

Just now, sevenperforce said:

Question (although I think I might be able to work it out for myself): is it actually more efficient to spend the dV to dive to a low periapsis to get more of an Oberth effect? Or does it depend on how much excess hyperbolic velocity you need?

@OhioBob has explained that in some thread a long time ago. Sadly I can't recall any search terms, hence paging.