Delta V readout Wrong

[Racescort666]

 

I don't know if this is a problem with MechJeb or KER or maybe something else but my dV readouts are different in the VAB than in orbit. Mass and fuel load are the same between VAB and orbit.

Pertinent info:

KSP: 1.0.5 (1028)

Windows 32bit

Mods:

• KER 1.0.19.4
• MechJeb2 2.5.6.0
• Asteroid day (no AD parts on this craft) 1.0.5
• Contract Configurator 1.9.6
• Distant Object Enhancement 1.6.4
• Docking Port Alignment Indicator 6.2
• KAC 3.5.0.0
• Module Manager 2.6.20
• Planet Shine 0.2.4.2
• Precise Node 1.2.1
• RCS build aid 0.7.7
• SCANsat 14.8 (no scan parts on this craft)
• Stock Bug Fix 1.0.5a.4
• Toolbar 1.7.10
• Transfer Window Planner 1.4.0.0

Not sure how much of that is pertinent but CKAN has a nice convenient list. This is an entirely fresh install as of last Saturday with all mods downloaded then so as far as I know everything is up to date.

I haven't tried to fly it yet but I'm wondering what's going on here.

[FancyMouse]

Picture 1 shows S2 (stage 2) while picture 2 shows S1 - most likely the 2k dV is calculated by this: they assumed that you stage engines, then immediately stage decoupler to get rid of the top stuff, and SRBs burn their fuel against themselves (when payload is thrown away) and that's 2k dV.

Most likely it happens because you're using the SRB as root part, not something from your payload - all those mods assume root part stays with payload and calculates delta V from there.

[Racescort666]

I didn't think about the root part being an issue. Thanks.

[Streetwind]

8 minutes ago, Racescort666 said:

I didn't think about the root part being an issue. Thanks.

I've generally found that in order to get accurate dV calculations, I'll need to build top down. Any propulsion I try to fit into a stage that is higher in the order than my root part will almost certainly break the math.

And since I don't know what other game logic might get confused by this, I consider building exclusively top-down a best practice that I recommend to everyone.

[Racescort666]

1 hour ago, Streetwind said:

I've generally found that in order to get accurate dV calculations, I'll need to build top down. Any propulsion I try to fit into a stage that is higher in the order than my root part will almost certainly break the math.

And since I don't know what other game logic might get confused by this, I consider building exclusively top-down a best practice that I recommend to everyone.

The problem is that I built it and saved it as a subassembly so I could launch it by Spaceplane.