How To Read the dV Map

[Poppa Wheelie]

This tutorial is in 3 parts:

1. How to Read the dV Map
2. Mission Planning with the dV Map
3. Does the KSP2 Mission Planning tool work?  (short answer, "no")

Part 1:  How to Read the dV Map

Spoiler

What dV Map?

There are a few dV maps out there, made for various versions of KSP1, some of which include dV information for planetary mods, or additional information on transfer windows.  I use the dV Map below, which I find “accurate enough” for both KSP1 and KSP2.
 

Spoiler

As far as I know, this is the latest version of this map.  Credits are the last paragraph on the upper left, and the license information is on the lower left.

Segments

You can see that this dV Map is made up of a bunch of colored line segments.  Each segment has the following:

• A number written inside it, representing the dV required to change your trajectory from one state to another
• An optional number written outside of the colored line, representing the dV required to adjust for inclination
• An optional triangle inside the colored line, indicating that the specified dV of this segment could be achieved by aerobraking, instead of by firing a rocket engine, if the mission planner user (that’s you) desires (and builds accordingly)
• An icon on each end of the segment.  These indicate the state of the trajectory before and after the dV is applied.

Icons

On each end of each segment is a black circle or ellipse with white interior.  Inside each of these is one of the following icons:

Description: No icon, just white interior

• Meaning:  A celestial body (CB):  planet, moon, or sun

Description: No icon, white interior, rays extending from the outer circle

• Meaning:  Kerbol

Description: A second black circle directly inside the outer circle

• dV Map Key for this Icon:  Jet Engine Operation Possible
• Meaning:  Some of the dV required by the attached segment can be provided by jet engines. Notice that this icon is only used for Kerbin and Laythe.

Description: A second black circle directly inside the outer circle, a black dot in the middle, and a small line connecting the inner circle to the black dot

• Meaning:  Keostationary Orbit

Description:  A black dot inside of a circle

• dV Map Key for this Icon:  Low Orbit
• Meaning:  Low Orbit around the CB 

Description:  A black dot inside an ellipse

• dV Map Key for this Icon:  Elliptical Orbit to SOI Edge
• Meaning:  Sphere of Influence (SOI) escape or capture (depending on direction)

Description:  A black dot beside a curved line

• dV Map Key for this Icon:  Intercept
• Meaning:  Flyby encounter through the SOI of the CB

Other Information On the dV Map

There is plenty of additional information on the dV Map, such as time (from Kerbin) for each flyby, orbit altitudes, total dV to each CB, communications antenna information, etc.  We won’t be using any of that additional information for this tutorial.

Direction Is Important

Each segment is showing us the dV required to change your trajectory from one state to another.  The two icons, one on each end of each segment, tell us which state the trajectory is coming from and which state it is moving to.  Therefore, when reading the dV Map, the direction in which we read each segment will determine the trajectory change we will see.

Let's Do An Example

I think all of the above will become much more clear if we run through an example.  Let’s “read” the dV Map for a Kerbin to Ike and Return mission.

I’ve zoomed in a little on the dV Map, highlighted the Kerbin, Duna, and Ike portions of the map, and marked 14 specific “segments” of the round-trip journey.
 

Spoiler

Here’s how to “read” the dV Map for this mission.  For each segment, “read” this text:

“At the appropriate location, apply [insert dV] m/s of dV to change the trajectory from [insert From] to [insert To].”

For example, read segment 3 like this:  “At the appropriate location, apply 140m/s dV to change the trajectory from a Kerbin SOI escape to a Duna encounter flyby.”

Segment	dV (m/s)	From	To
1	3400	Kerbin Surface	Kerbin Low Orbit
2	930	Kerbin Low Orbit	Kerbin SOI Escape
3	130+10	Kerbin SOI Escape	Duna Encounter Flyby
4	250	Duna Encounter Flyby	Duna SOI Capture
5	30	Duna SOI Capture	Ike Encounter Flyby
6	150	Ike Encounter Flyby	Ike Low Orbit
7	390	Ike Low Orbit	Ike Surface
8	390	Ike Surface	Ike Low Orbit
9	150	Ike Low Orbit	Ike SOI Escape
10	30	Ike SOI Escape	Duna SOI Escape
11	250	Duna SOI Escape	Kerbin Encounter Flyby
12	130+10	Kerbin Encounter Flyby	Kerbin SOI Capture
13	930	Kerbin SOI Capture	Kerbin Low Orbit
14	3400	Kerbin Low Orbit	Kerbin Surface

Notice a few things from this:

• Look at segments 3 and 11.  They both end at what the dV Map calls the “Intercept” icon.  But at the end of segment 3, this represents an encounter flyby with Duna, while at the end of segment 11 this is an encounter flyby with Kerbin
• Look at segment 4. The white triangle indicates that we could, if we designed our rocket and mission properly, get some or all of the specified 250m/s of dV by aerobraking at Duna, converting the Duna encounter flyby to a Duna SOI Capture without using any fuel
• Segments 12 and 13 offer the same possibilities with aerobraking on return to Kerbin. In fact, you may be able to use aerobraking to capture from a Kerbin flyby (segment 12), make zero or several aerobraking orbital passes resulting in lower and lower Kerbin orbits (segment 13), and then use a combination of aerobraking and parachutes to land on Kerbin (segment 14). In this scenario, your rocket does not need to provide any of the dV for these segments.
• Look at segments 7 and 8.  On the map, we only see the Ike icon, the red bar with 390 written in it, and the Low Orbit icon.  But direction matters!  For segment 7 we are using 390m/s dV to go from Ike Low Orbit to Ike surface, while segment 8 is using 390m/s dV to go from Ike surface to Ike Low Orbit.
• Look at segment 14.  You will typically get all (or mostly all) of this dV by some combination of aerobraking and parachutes

Notice also that the dV Map does not give you any information on where and how to apply the dV amounts indicated.  This map has sometimes been referred to as a “subway map”.  This is ONLY because the visual appearance of this map is similar to many big city subway maps.  This visual likeness is the ONLY similarity.  This map does NOT tell us anything about where to build maneuver nodes or where or how to apply the various dV amounts.  It is definitely NOT indicating anything like traveling from node to node or burning additional dV at each node.

Part 2: Mission Planning With the dV Map

Spoiler

Very generally, here is how to plan a mission, and how to use the dV Map in that planning.  I’ll follow this procedure below using the same Kerbin to Ike and Return mission we discussed earlier.

Follow these steps:

1. List the stages (steps), in chronological order, that you intend to use for the entire mission
2. Starting with the very last stage (step), determine how much dV you need for that stage, including whatever safety margin you think you need
3. Design that stage so that it provides the required dV
4. Working backwards through your stages (steps), repeat steps 2 and 3, treating all stages that you have already completed as “payload”, until you have completed steps 2 and 3 for the very first stage of your mission (since you worked in backwards order)

OK, OK, here’s the example, using the Kerbin to Ike and Return mission discussed earlier.

Disclaimer:  I don't know if this vehicle will actually accomplish the mission.  I haven't tested anything.  I haven't flow this thing.  My purpose here is only to show that the vehicle meets the dV requirements specified by the dV Map.

Step 1:  List all stages in chronological order

STAGES:

1. Launch from the KSC to Kerbin Low Orbit (80km)
2. Drop the last booster stage after Kerbin Low Orbit is achieved (optionally deorbit it)
3. Make one transfer burn from Kerbin Low Orbit (at the right transfer window time and the right location on my Kerbin orbit) that results in a Duna encounter flyby (and hopefully, an Ike encounter flyby)
4. After leaving the Kerbin SOI, make one or more small correction burns, including one at the AN or DN to correct inclination, to get as close as possible to my ideal PE and inclination with Ike
5. Upon entering the Duna SOI, make one more small correction burn, if needed, to finalized PE and inclination with Ike
6. At the Ike PE, make one burn to capture into Ike Low Orbit
7. Separate (and possibly deorbit) the Kerbin to Ike transfer stage (all remaining components used to perform the previous steps)
8. Send the remaining vehicle to the surface
9. Return from the surface to Ike Low Orbit
10. Make one transfer burn from Ike Low Orbit (at the right transfer window time and the right location on my Ike orbit) that results in a Kerbin encounter flyby (hopefully, with equatorial inclination and 40km PE)
11. After leaving the Duna SOI, make one or more small correction burns, including one at the AN or DN to correct inclination and to get as close to equatorial inclination and 40km PE as possible
12. Upon entering the Kerbin SOI, make one more small correction burn, if needed, to finalize PE and inclination with Kerbin
13. Separate the transfer stage from the landing stage and orient the landing stage properly
14. Aerobrake with heatshield 1 or more times
15. Land with heatshield and parachutes

Now let’s do the whole thing, in reverse order, per steps 2, 3, and 4.

Stages 15 and 14:  Aerobraking and landing, no dV required

Stages 13, 12, 11, 10, 9, 8:  From Ike Low Orbit to Ike Surface, return to Ike Low Orbit, then From Ike to Kerbin

dV Map calls for:

• 390    Ike Low Orbit to Ike Surface (segment 7)
• 390    Ike Surface to Ike Low Orbit (segment 8)
• 430    Ike Low Orbit to Kerbin encounter flyby (segments 9, 10, and 11; dV = 150 + 30 + 250)
• 1210    Total dV required

See that we have 1557m/s dV, with more than sufficient TWR for Ike

Stages 7, 6, 5, 4, 3:  From Kerbin to Ike

dV Map calls for

• 930    Low Kerbin Orbit to Kerbin SOI Escape (segment 2)
• 140    Kerbin SOI Escape to Duna Flyby Encounter (segment 3)
• 250    Duna Flyby Encounter to Duna SOI Capture (segment 4)
•    30    Duna SOI Capture to Ike Flyby Encounter (segment 5)
• 150    Ike Flyby Encounter to Ike Low Orbit (segment 6)
• 1500    Total dV required

See that we have 1810m/s dV

Stages 2, 1: From Kerbin surface to Kerbin Low Orbit

dV Map calls for

• 3400 Kerbin Surface to Kerbin Low Orbit (segment 1)

See that we have 3428 to 3475m/s dV (depending on whether you're looking at the MechJeb or KER screen), and 1.29 TWR with the SRBs thrust limited to 70%

Conclusion:

This example shows how to plan a mission, determine the dV requirements for that mission, and design the mission vehicle (in reverse order) so that dV requirements are met.

Part 3: Does the KSP2 Mission Planning tool work?
Short answer, "no"

Spoiler

The KSP2 Mission Planning tool was a great idea.  And having that tool available in the VAB was exactly the right call.  However, it has several shortcomings which made its results highly misleading.  Here are the big problems with the tool, as I see it:

• It doesn't understand that Direction Matters!  If you asked the KSP2 Mission Planning tool for a Kerbin to Ike round trip plan, it would actually produce a 14 segment result.  However, the text on the entire return from Ike (7 of the 14 segments) is incorrect. The tool does not recognize that the text associated with "reading" each segment must be different, depending on which direction you are reading. For example, we talked earlier about the difference between reading segments 7 and 8.  The KSP2 tool indicates that segment 7 is for "Ike Low Orbit" to "Ike Surface" (which is correct), but then indicates that segment 8 is for "Ike Surface" to "Ike Surface" (which is definitely incorrect).
• Although it indicates which segments can be handled with aerobraking, it doesn't allow the player a method of subtracting out that dV. This is especially misleading (particularly to new players) on the return to Kerbin segments. Many mission profiles will use aerobraking to capture from the flyby and lower the orbit, and all missions will use aerobraking  (and maybe parachutes) to do the final landing segment.
• It doesn't understand that grand total mission dV pretty much never matters. In the example we did in Part 2, we made a plan that would require a total of 6110m/s of dV over the life of the mission. However, there's never a time during the planning where this total was meaningful.  And, since the KSP2 tool always includes the totals required for optional aerobraking segments (especially the 3400m/s required for the last, Kerbin Landing, segment), the tool will always state that the designed vehicle does not provide the required grand total dV.
• It cannot be used to determine dV requirements for subsets of a mission, or for anything that does not begin at Kerbin and, if a round trip, end at Kerbin

If you are planning a one-way trip, from Kerbin, and will be using no aerobraking, then the tool can possibly be useful.