Delta V map issue

[dawfedora]

When I use a spreadsheet to calculate the dV needed to go from an 80km orbit to a kerbosynchronous orbit using a Hohmann Transfer, I get 1098.9 m/s.

The Delta V maps I've found say 1115.

I make my best 80km circular orbit, then do my best to get a nice clean circular kerbosynch orbit at 2863.33 km without overshooting too much or flipping to do a retrograde adjustment.

I start with 1536 dV, and end up with 437 dV for a  change of 1099 dV.  As close as nevermind to the spreadsheet calculation.  And the period is 5:59:09, which checks out with the sidereal day.

So what gives?

[HebaruSan]

Which maps are you looking at?

[Empiro]

The delta-V maps are just estimates and often are on the conservative side, just in case your TWR is very low or if you are slightly off and need to make corrections. 

Edited March 15, 2021 by Empiro

[dawfedora]

[1.7.3] CommunityDelta -V Map 2.7 , for example

 

[jimmymcgoochie]

Delta-V maps are average values. If you had a really low TWR then you’d have higher cosine losses on your burns by burning in a direction other than purely prograde, meaning you actually get less delta-V than expected and so have to use more fuel to reach the target orbit. You’re doing better than the delta-V map expects, I’d be more annoyed if it was the other way around and I ran out of fuel despite having enough according to the delta-V map.

[king of nowhere]

18 hours ago, dawfedora said:

When I use a spreadsheet to calculate the dV needed to go from an 80km orbit to a kerbosynchronous orbit using a Hohmann Transfer, I get 1098.9 m/s.

The Delta V maps I've found say 1115.

 

so there is a 1.5% difference. what's the issue?

[paul_c]

There's some manoeuvres where  you can fairly precisely "fly" or "control" things - for example a Hoffman transfer from one orbit to another, where you have a big TWR and can do a short burn pretty much at the optimal time . These could include intercepts of Mun/Minmus/other planets (if you calculate the optimal time and ejection angle etc to do them at). Or a take-off from a body with no atmosphere, big engine and flat terrain.

And there's others where its much more variable, for example take-offs from something with an atmosphere (the drag losses vs gravity losses vs orbital speed requirement energy would be a complex equation!); or a non-ideal interplanetary transfer.

Of course the correct thing would be to put a variance band on the figures - but then it would be needlessly complicated! So, the delta V maps are typically shown with a single figure for each transfer. But you'd be foolish not to appreciate there is some variance and to take excess fuel/performance. I don't think anyone looks up the figures, arrives at "oh its 6056m/s to land at xxxxx on the Mun" then builds a rocket with 6056m/s dV.

[Spricigo]

 

The original deltaV map is quite old and apparently no one bothered to adjust it for each small change in the game if the difference is small enough.

 

 

Quote

Kerbin Changes

...

1.0.5

• Kerbin actually spun up to have a 6 h synodic day.

...

 

 

 

[Guest]

The delta-V maps are more of a rough guide; there's no way you can create a generic figure that fits perfectly for everyone as there's too many variables.

However, as you're achieving 99.99% accuracy with your spreadsheet, why do such things as maps designed for mere mortals trouble you my lord?

[ajburges]

Just wait till you have a lander with TWR low enough the gravity losses demand more dV than the map suggests! 

Edit:

Why are you doing a Hohmann Transfer to synchronous orbit? Just launch to there and you use even less dV!

Edited March 20, 2021 by ajburges