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Something I was wondering about delta-V maps.

Rusty6899
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Rusty6899

Rusty6899

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I was wondering if anyone could tell me what exactly is meant by "intercept" on a delta-V map. I had always assumed it indicated the point at which you would encounter the target object.

For example, from the delta-V map I use, it takes 950 m/s to get escape Kerbin from LKO. It then takes 965 m/s to get to Jool intercept. I was wondering if this refers to a Jool encounter or to get into an elliptical orbit about Jool with a low periapsis and apoapsis close to the edge of its SOI. I was thinking that surely if it does refer to an encounter, then there is no way the map could be correct in either direction. It would have to take more delta-V to get to LKO from a transfer from Jool that it would from Duna after the encounter was made, as you would be travelling faster coming from Jool.

Edited by Rusty6899
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Captain Sierra

Captain Sierra

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'Intercept' on a delta V map, refers to the amount of delta V needed for your ejection burn to intercept the target. It is referring to simply achieving orbital intercept, HOWEVER, it does not take into account the orbital inclination of your target, which varies depending on how the orbits line up. The necessary delta V to make a mid-course correction of inclination and possibly fine-tuning of the encounter is not taken into account.

For delta V required to establish orbit, there should be another number labeled 'Low <body> Orbit' which is a somewhat standard low orbit and the delta V displayed there is the delta V required to circularize yourself from transfer from Kerbin, to a low orbit around the target body.

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Rusty6899

Rusty6899

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That's pretty much what I assumed, but for that to be true, and for the delta-V map to be accurate for both directions, it would mean that your circularisation burn around your new destination would be the same regardless of your origin.

If I was to use the example of a return trip to Low Jool Orbit from LKO.

It takes 950 m/s to leave Kerbin's SOI (I assume that is what is meant by the figure between LKO and Kerbin Intercept.)

It takes 965 m/s to encounter Jool (I assume this refers to an encounter with a 150 km periapsis as that is the value the map gives for LJO so a 150km periapsis would be the most efficient encounter).

It takes 2630 m/s to circularise to LJO.

This is what I have always assumed the delta-V map denotes, however when considered in reverse, although I accept that the TOTAL delta-V is the same, each individual value should not be. If the map is read in the same way in reverse...

It takes 2630 m/s to leave Jool's SOI.

It takes 965 m/s to encounter Kerbin.

It takes 950 m/s to circularise to LKO.

This makes no sense though, as a LJO circularisation burn from a Jool encounter HAS to take more delta-V than a burn from LJO to leave Jool's SOI, because it is effectively the same burn in the opposite direction with the addition of the delta-V required to achieve an initial orbit from an encounter with Jool. The same has to be true of Kerbin escape/circularisation burn.

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capi3101

capi3101

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The delta-V map denotes optimal values only, and optimally, you'd use aerobraking to account for some of your delta-V.

Let's take the case of LKO to LJO and let's use the newer delta-V map since it's got inclination change values on it:

jGxK1zG.png

.

Red arrows indicate delta-V that can effectively be ignored for incoming aerobraking: so to read it, it's like this:

620 LKO to KSO

180 KSO to Mun Transfer

70 Mun to Minmus Transfer

20 Minmus Transfer to Kerbin Escape

130 Kerbin Escape to Duna Transfer

480 Duna Transfer to Dres Transfer

370 Dres Transfer to Jool Transfer

270 Inclination Change

and the rest is free:

Total LKO to LJO: 2140 optimal

For the reverse:

1880 LJO to Laythe

310 Lathe to Vall

220 Vall to Tylo

180 Tylo to Bop

60 Bop to Pol

160 Pol to Jool Escape

160 Jool Escape to Kerbin Transfer

270 Inclination Burn

and the rest is free:

Total LJO to LKO: 3,240 optimal

Thing about delta-V maps, they aren't accurate all the time. They just show the optimal case, which is fine and dandy for mission planning purposes most of the time. I'd recommend using anywhere from 120-150% of the total required delta-V indicated by a map simply because I have a tendency to screw things up on occasion and it's nice to have a little margin for error. Bigger margin for less experience.

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Supernovy

Supernovy

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I think I see what you're saying - a Jool escape has to require less Δv than a Jool escape and transkerbin orbit, due to the effort of lowering solar periapsis (and these will be the same Δv as circularising from a parabolic Jool orbit and circularising at Jool from a transjool trajectory respectively).

I believe the maps simply don't show escape and transfer as separate for non-Kerbin bodies, because the usefulness of that would be limited. Otherwise, you are correct.

FAKEEDIT: Actually I think I've got it now

It takes 2630 m/s to leave Jool's SOI.

It takes 965 m/s to encounter Kerbin.

It takes 950 m/s to circularise to LKO.

This should actually be

It takes 2630 m/s to leave Jool's SOI and intercept Kerbin.

It takes 965 m/s to go from an encounter to an escape trajectory of Kerbin.

It takes 950 m/s to circularise to LKO.

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Rusty6899

Rusty6899

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That helps a lot. I kind of figured that there was a bit of simplification in the map I was using.

Thing about delta-V maps, they aren't accurate all the time. They just show the optimal case, which is fine and dandy for mission planning purposes most of the time. I'd recommend using anywhere from 120-150% of the total required delta-V indicated by a map simply because I have a tendency to screw things up on occasion and it's nice to have a little margin for error. Bigger margin for less experience.

I generally have a design delta-V of each stage of 110% of the map value. The only time I go over that is when I am landing on Duna or exploring the Jool system, as I can't always guarantee that I will be able to rely on parachutes alone to land on Duna or get into a perfectly prograde orbit about Jool. It does help to have a little bit extra delta-V, but your total mass increases exponentially with each stage that is over the required levels so I avoid too much excess.

Cheers guys

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