Most efficiënt way to get to duna.

[Foxster]

Just now, sevenperforce said:

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.

Really? Never tried it but it seems that it would be easier to get an Ike encounter when within Duna's SOI than trying to get a Duna encounter when in Kerbin's. Guess it depends on your ninja maneuver node skillz. 

[sevenperforce]

2 minutes ago, Foxster said:

Really? Never tried it but it seems that it would be easier to get an Ike encounter when within Duna's SOI than trying to get a Duna encounter when in Kerbin's. Guess it depends on your ninja maneuver node skillz. 

Two points here. First, you don't really need the Ike encounter, not if you have heat shields. Aerocapture at Duna from a nice clean Hohmann transfer is pretty forgiving. And if you want to make the most of your dV, then you'd want to plan the Ike encounter during your Kerbin escape burn, which is easier said than done.

Second, it's easier to get a gravity assist from the Mun than you think. The optimal transfer window from Kerbin to Duna has a Kerbol periapse at Kerbin and a Kerbol apoapse at Duna. So your ejection needs to be roughly in the same direction as Kerbin's direction of motion around Kerbol. Thus, you'll want to get close to your optimal transfer window (which is when Duna is 36.5 degrees ahead of Kerbin; use this calculator for precise help), then let the Mun's gravity fling you out along the correct vector.

You can expect to get roughly 90 degrees of turn (with a nice solid gravity-assist kick during) out of the Mun. It would be less if you were burning to, say, Jool or Eeloo, but for Duna that's a good approximation. So simply wait until Kerbol, Kerbin, and the Mun are all lined up, do a standard Hohmann to the Mun, and simply keep burning, with your Munar periapse as low as you dare. Your initial trajectory is directly away from Kerbol, but the Mun will bend it into a nice prograde ejection vector along a Hohmann to Duna.

LMK if you need a diagram.

[badjass]

27 minutes ago, Foxster said:

Really? Never tried it but it seems that it would be easier to get an Ike encounter when within Duna's SOI than trying to get a Duna encounter when in Kerbin's. Guess it depends on your ninja maneuver node skillz. 

I did this before and had no problem aerobraking in duna's orbit without overheating problems. I did not even need a second pass.
I did not use heat shields but do note that my craft was very light.

Edited January 22, 2018 by badjass

[Cpt Kerbalkrunch]

1 hour ago, Laie said:

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

You're probably thinking of this one. It's been linked and referenced many times, and I've seen quite a few arguments over the meaning and conclusions reached. Best explained by the man himself, but anyone who wants can see the link below.

 

[OhioBob]

2 hours ago, 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?

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.

It does depend on how much hyperbolic excess velocity you need.  For every transfer there is something called a gate orbit, which is a function of the required hyperbolic excess velocity.  The gate orbit is also the dividing line between whether it is most efficient to lower the periapsis to perform the burn or not.  If your initial orbit is below the gate orbit, then just burn straight away from the current orbit.  Lowering the periapsis in that case is actually counterproductive.  However, if your initial orbit is higher than the gate orbit, then you can save delta-v by dropping the periapsis and performing the burn closer to the planet.  The farther you lower the periapsis, to more savings there is.

Although you can potentially save delta-v by lowering the periapsis, execution can be difficult.  You can't just lower the periapsis at any old place.  You have to plan it so the periapsis is at the correct location for the placement of the ejection maneuver node.  And you have to arrive at periapsis at the correct time.  Therefore you have to weigh whether or not the savings in delta-v is worth the execution nightmares.

[GoSlash27]

3 hours ago, 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?

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.

Actually, your example is backwards. It is worth it to dive back to LKO from Minmus for a Duna transfer, but it is not worth it to dive back from the Mun.

Contrary to popular belief, the Oberth effect does not always trump the gravity well. There is an optimal altitude where the transfer costs the least DV. Lower than this, and the penalty of the gravity well is more than the Oberth savings, and vice versa when you go higher. This optimal orbital radius is known as the "gate orbit".

 For a trip from Kerbin to Duna, the gate orbit is 7.8Mm. For a trip to Eve, it's 11Mm. The farther the transfer, the lower the gate orbit.

Best,
-Slashy

*edit* D'OH! Beaten by @OhioBob

Edited January 22, 2018 by GoSlash27

[Blaarkies]

15 hours ago, Spricigo said:

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.

Thats a very loose analogy you got there buddy 

Can't we explain it in a more correct way? What about a rubber band? (except that elastic rubber has a force profile that increases with distance)

[foamyesque]

Unpowered gravity assists are basically throwing a tennis ball at a semi-truck moving at highway speeds

[Spricigo]

3 hours ago, Blaarkies said:

Thats a very loose analogy you got there buddy 

Can't we explain it in a more correct way? What about a rubber band? (except that elastic rubber has a force profile that increases with distance)

 

The explanation is correct. 

 

...or someone will point my mistake. Until then I have nothing to retify.