Delta v maps are crazy!

[StarBRA]

Ok thet are acurate about everything, but on how much I need to get to other planets. For example..... I get out o kerbin´s sprere of influence with 950 delta v. No problem. But it says that I require like 80 delta v to go to Eve, or, 350 to dres, or, 110 to duna. How can I possibly make it to these planets using only that or a little more delta V???? Please help explaining me how.

[Dedjal]

80 Dv to Eve?

I think your map is wrong?

[EdFred]

80 additional dV. And no, that's about right. 950+80 = 1030

[Tortoise]

According to this map, It'll take 1390 Dvto get there, since Ever isn't that far.

Plus the Dv to slow down your orbital velocity in order to get into a lower orbit and eventually land if needed.

EDIT: And add 950 to that. You get 2340. (And again, add pack additional Dv for landing/getting in low orbit)

[Pecan]

You have to add-up ALL the steps along the path from where you start to where you want to finish. Figures can be reduced by aero-braking in the atmosphere of your target but otherwise for Eve, for instance, you'll need 2,880m/s according to the deltaV map I use (which I can't find the link for *sigh*). Have a look at chapter 4 of the tutorial in my signature.

Oh - and remember that it all depends on your first burn - it's no good if you "get out of Kerbin's sphere of influence" in completely the wrong direction when you start.

ETA: 90 from Kerbin escape on my map PLUS 430 for plane-matching, even if you aerobrake for all the rest.

Edited June 28, 2014 by Pecan

[Claw]

Without knowing which you are looking at, dV maps are usually cumulative.

So the dV to get to the planet is in addition to the dV required to exit Kerbin's SOI. Most maps also assume an optimal Homann transfer to the target planet. So there will be a specific time that you would have to leave Kerbin in order to use those numbers. You can use AlexMoon's transfer window calculator.

http://alexmoon.github.io/ksp/

(haha, I knew I'd get ninja'd, but I didn't think it would be that much.)

[xcorps]

Delta V is a measure of change in velocity. If your velocity is 1000 ms and you want an orbit that will give you a capture for planet x at a velocity of 1050 DV then you need 50 DV to make the transfer.

9284.5 m/s Kerbins velocity

10 811 to 11 029 m/s velocity of Eve

[FenrirWolf]

Which map are you trying to follow? Could you link it here?

Also, you might be confused by the fact that every segment in a delta-v map is additive. By that I mean that you need to add up the costs at every point along from your starting point to your ending point. For example, taking off from Kerbin with the intent to land on Duna looks something like this on a delta-v map:

delta-V to low Kerbin orbit + delta-v to Kerbin escape velocity + delta-v to Duna encounter + delta-V to Duna capture + delta-v to low Duna orbit + delta-v to Duna landing = total delta-v required

Some of the individual segments might have a small number, but on a whole your costs are anything but small when you account for everything from start to finish. You generally want a little more fuel than advertised as well to give yourself a margin of error as well.

[Superfluous J]

Do NOT EVER just burn to get out of Kerbin's SOI and then get out there, and then burn to another planet.

EVER.

If you burn to Kerbin's SOI and then keep burning right then to get to Eve, it will cost that 950+80.

If you burn to Kerbin's SOI, fast forward until you are out of Kerbin's SOI and the burn to Eve, that second burn will cost about 950+80 (and probably a little more!). So you're burning 950 to leave Kerbin's SOI and then another 1030 to get to Eve.

Hohmann is KING. Learn his transfer methods and you will save SO much fuel.

[LethalDose]

I kinda want to agree with the fact that 80 m/s to transfer to Eve sounds ludicrous, but then I have to acknowledge my Eve transfer burn last night was less than 100 m/s. That was after I dropped down from a Minmus parking orbit and dropped my Pe down to ~ 85km above Kerbin, but that was still a "closed" orbit (no escape).

The thing about the maps is that there are other big assumptions that they usually fail to mention, like parking orbits. For example, since your orbital speed is lower at 250 km above Kerbin than it is 75 km above Kerbin, it will take more dV to reach escape velocity from the 250 km than from 75 km.

Also the ejection angle from the parent body is pretty critical to get those really low transfer values. Also, information not included on the map.

From these first two points, you can see where person A can do some x'fer with 200 m/s dV while person B needs 2km/s dv. This begs the question "what did person A do differently than person B". The maps rarely give information about the transfer details, and you need to rely on transfer calculators like the ones here or here. And those are obviously not maps.

There's also some slight variation in the dV require based on whether you're hitting your target near it's Ap or Pe (although my calculations have show this is really small).

Finally, and this one I may be wrong about this one, the dV costs to travel between planets may not be reciprocal, meaning if it takes X dV to transfer from planet A to planet B, that doesn't necessarily mean that it takes X dV to transfer from planet B to planet A.

Because there's so much missing info on the dV maps I've seen, I find they're best as an "ideal" situation to I can have a bare minimum value for dV for planning a mission, but I don't rely on them at all. This isn't a criticism of the maps, mind you, they're great for displaying a lot of info quickly, but there's still lots they don't have.

Edited June 28, 2014 by LethalDose

[quantumpion]

Finally, and this one I may be wrong about this one, the dV costs to travel between planets may not be reciprocal, meaning if it takes X dV to transfer from planet A to planet B, that doesn't necessarily mean that it takes X dV to transfer from planet B to planet A.

If that were not true it would violate conservation of energy. Think about the transfer orbit between Kerbin-altitude and Eve-altitude. The velocity at Pe and Ap has to be the same regardless of which direction you are going.

[LethalDose]

If that were not true it would violate conservation of energy. Think about the transfer orbit between Kerbin-altitude and Eve-altitude. The velocity at Pe and Ap has to be the same regardless of which direction you are going.

Right, but energy to go from a Kerbin orbit to a Kerbin-Eve transfer is not neccesarily the same as going from an Eve orbit to a Eve-Kerbin transfer. The specific orbital energy or both transfers is the same, but the difference between the initial energy state and the transfer state is not necessarily the same. Again, this may or may not be true when you consider the energy it takes to capture a body into an orbit.

My perspective on this comes from Kerbin-Duna transfers and the respective returns. In my experience, the latter seems to require much less energy than the former.

When going from Kerbin to Duna, there's a large energy investment to break out of Kerbin's relatively large gravity well (relative to Duna), the transfer, and then another relatively larger investment to slow down enough for capture in Duna's relatively weak gravity. Going the other way at least seems easier because I don't have to speed up as much to break out of Duna's well to initiate the transfer, and I don't have to slow down as much to be captured in Kerbin's SoI. This may only be my perception since I can more effectively aerobreak at Kerbin than at Duna, leading to the illusion that the return takes less energy, but I'm not sure that's the case.

Again, I agree that the energy of the transfer is the same, but I think it's possible that the energy required to start and end the transfer may be different in reciprocal directions.

And even if that's not the case, it doesn't change the fact that all the information required to effectively make an efficient transfer is not always, or even frequently, presented in the dV maps. So lets not derail the thread.

EDIT: Ugh, sorry, but I just thought of a more compelling counter-point. I swear I'll let it go after this. You pointed out, and I agree, that the energy of the transfers in both directions must be the same. However, orbital energy is dependent on the velocity squared, while dV costs are simply dependent on the linear velocity term. This asymmetry is the reason that orbital states with identical energy states can be reached using maneuvers with different dV costs, and similarly, why we can 'exploit' the Oberth Effect.

So even if the energy of the reciprocal transfer is identical, the dV costs associated with the reciprocal transfers are not necessarily identical.

Okay, leaving it alone now, going to bed. Honest.

Edited June 28, 2014 by LethalDose

[mhoram]

Most Delta-V Maps assume that you burn the 950+80 m/s in Low Kerbin Orbit.

If you want to first leave Kerbin's SOI and burn afterwards to Eve, have a look at this Delta-V map that handles exactly this case.

Edited June 28, 2014 by mhoram

[rkman]

But it says that I require like 80 delta v to go to Eve

That only puts you on a flyby of Eve, and only if the phase angle is near optimal.

[GoSlash27]

The reason that map shows 80 DV to Eve is because it assumes a single burn from LKO to transfer. In that sense, it is true. It only takes 80 additional DV to get from Kerbin escape to Eve transfer, but only if you add those DV while still in LKO (due to the Oberth effect).

If you're like me and you transfer burn outside Kerbin SOI, there's no way you can set up an intercept on a measly 80 DV. If that's the case, try this map:

Best,

-Slashy

[edit] Ninja'd by mhoram![/edit]

Edited June 28, 2014 by GoSlash27