How to Read a Delta-V Map

[Little Kerbonaut!]

Hello, currently I am having trouble reading a Delta-V map. Mostly I can’t understand what the numbers mean like do they mean for all stages or first stage or just the second stage and so on.

[bewing]

You can think of the total deltaV in a rocket as a budget. Each phase of your journey is going to require a piece of that budget. And that's generally how deltaV maps are set up. Each number on a deltaV map usually tells you how much of your total budget you are going to need to use to get to your next destination. KSC to LKO, LKO to Duna orbit, Duna orbit to Ike orbit, Ike orbit to Ike surface, and like that. The numbers tend to be close to optimal, so you also need to include a small safety margin.

[Zhetaan]

9 hours ago, Little Kerbonaut! said:

Mostly I can’t understand what the numbers mean like do they mean for all stages or first stage or just the second stage and so on.

I suppose the correct answer is 'any stage'.  As @bewing mentioned, it's similar to a budget, and in the same vein that, for example, your grocery budget doesn't care whether the money you put into it is what comes out of your wallet first or last, the delta-V budget doesn't care whether the stage that provides the thrust is first or last.  You may care, but that is a different factor.

The numbers on the map tell the total delta-V needed for that leg of the journey.  Your role is to ensure that your rocket can supply that amount over however many stages.  You can do it in one stage, or you can combine several.  If you do not supply enough to account for the entire mission, then the mission becomes one-way unless you change your flight plan.

[Little Kerbonaut!]

Oh ok, but am I able to split it up into different stages like if I needed 500 Delta-V can I try and put 250 of it in the first stage and the other 250 in the second stage?

[bewing]

5 hours ago, Little Kerbonaut! said:

Oh ok, but am I able to split it up into different stages like if I needed 500 Delta-V can I try and put 250 of it in the first stage and the other 250 in the second stage?

Yes.

[Spricigo]

6 hours ago, Little Kerbonaut! said:

Oh ok, but am I able to split it up into different stages like if I needed 500 Delta-V can I try and put 250 of it in the first stage and the other 250 in the second stage?

Yes. But keep in mind you may also have "twr requirements". You may eventually reach the Mun with a horrendously low TWR (if you make several small burns each time you pass your kerbin periapsis) but there is no way you can land and return from it in that conditions. 

 

 

[Little Kerbonaut!]

Oh ok, but if I don’t have enough TWR how  can I increase it?

[Superfluous J]

1 hour ago, Little Kerbonaut! said:

Oh ok, but if I don’t have enough TWR how  can I increase it?

The correct nomenclature is - ahem - MOAR BOOSTERS.

More technically speaking, add more engines or swap those you have for larger ones.

Note this will also probably require more fuel, which may lower your TWR. It's just how it works. You don't need 15 TWR to get anywhere. 1 is fine. More than, really. I frequently go interplanetary with less.

Edited June 10, 2020 by Superfluous J

[Little Kerbonaut!]

Oh ok, but some of my rockets that I want to go to Duna  have the largest engines I have but I still can’t reach the places I want to go.

Edited June 10, 2020 by Little Kerbonaut!

[Spricigo]

9 hours ago, Little Kerbonaut! said:

Oh ok, but some of my rockets that I want to go to Duna  have the largest engines I have but I still can’t reach the places I want to go.

One way to effectively have MOAR BOOSTERS™ is to remove NOT BOOSTERS™. Same fir MOAR FUEL™/NOT FUEL™

Give us a picture of your craft so we can see what are you carrying around. 

 

[Little Kerbonaut!]

For some reason pictures on Imgur aren’t working for me. But my first stage is made up of the Mammoth Engine and Large boosters I forgot the name. The second stage is made up of a single Rhino Engine. And that’s pretty much all.

[king of nowhere]

how much fuel?

a mammoth engine can go single stage to orbit with the largest fuel tank, but not much useful mass left. and then once you are in orbit fuel is the only thing that really matters

[Cavscout74]

Also, despite the KSP lore of MOAR BOOSTERS, bigger is often not better - you can often go further with smaller craft, since they are less massive and therefor require less fuel to move.  It all depends on what you are trying to get to Duna.  But for a smallish manned mission (like 3 kerbals in a Mk1-3 pod), a Rhino should be overkill.  Most of my orbital/interplanetary craft have acceleration around 0.3 - 0.6g at full fuel.   And the main reason I try for at least 0.3g is that I'm impatient.

But for you original question, the dV map.  Here is a snip of the path to Duna:

You need roughly 3400 m/s to reach orbit, then another ~930 m/s to reach the edge of Kerbin's sphere of influence (SoI), and another ~130 m/s to get to Duna by the map.  Typically, the 930 & 130 will be done in a single maneuver (so roughly 1060 m/s to go from Kerbin to Duna) but it doesn't matter if that is all done by a single stage or two or ten.  It will be easier to figure out how long the maneuver will take if its all done on a single stage.   That other 10 m/s above the orange line is to adjust your inclination since Duna isn't orbiting at 0 degrees like Kerbin is.

Once you get into Duna's SoI, you will need to do another maneuver to capture your craft into Duna orbit

That is the next step on the map.  It will need about 250 m/s to capture with an Ap near the edge of Duna's SoI, and 360 more to get a circular orbit in low Duna orbit.  Again, this is typically done as a single maneuver (~510 m/s) at your periapsis.  The final 1450 is technically to land - but since Duna has an atmosphere, drag and parachutes will make most of this "free" as far as fuel goes, and you can usually land with just a short use of your engines during the last few seconds.

Edited June 11, 2020 by Cavscout74