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Mechjeb delta v per stage


stefwilliams
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Maybe I'm missing something obvious here, or maybe it's not possible for some reason, but can anyone tell me how to get Mechjeb to show the amount of delta v per stage independent of other stages in the VAB?

What I mean is, let's say I have my main payload stage, which has 1000 delta-v. Mechjeb shows this correctly in the VAB.

But when I add a booster stage, let's say it has delta-v of 2000, the delta v of my first stage will be lower (ie, as if it was trying to push the full weight of the craft, including the booster stage), maybe 750.

Likewise if I add another booster stage, let's say solid rockets, with a delta-v of 1500, both previous stages will show lower delta v values again.

It makes it quite difficult to calculate how much total DV I have, because rather than telling me I have the 3500 total for my booster stages, and 1000 for my main craft, it may show 1500 for stage 1, 1000 for stage 2, and 300 for my main craft...

Am I making sense? As I say, maybe I'm missing something obvious, but if anyone could clue me up, I'd appreciate it...

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Ummm, maybe I didn't explain myself properly.

From those equations, the problem I have is that Mechjeb is showing me the delta v of late stages based on the total mass of the rocket, rather than based on the mass that the rocket will be after the stages are spent...

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I generally design from top-down and yes, MechJeb (even dev build #685) has issues in the VAB with calculating dV per stage (especially if you fiddle with upper stages after lower stages are attached).  However, when out on the launchpad, it's accurate so when I roll it out with KRASH, I can quickly see if I've done something really wrong.

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  • 2 weeks later...

I have never seen MechJeb do this unless your rocket isn't stacked correctly.  MechJeb assumes stages are dropped when their engines can no longer obtain fuel--it does this by looking at your staging plan, not by examining the rocket itself.  Beware of confusions caused by crossfeed--1.2 allowed crossfeed on many parts that didn't support it before and it can really mess up MechJeb's idea of how your bird will fly!  (And it will behave badly in flight, also--if you allow the couplers to crossfeed it will fail to jettison asparagus or onion stages when they are emptied.)

I have managed to confuse it with a rocket where two separate engines feed from the same fuel supply at different times but there's no way it could possibly know when I plan to stage the rocket so I don't mind it being wrong there.  (Although I would like it to try to give the TWR of the second stage.)  (The rocket isn't as insane as it sounds--the core of the rocket is a big fuel tank destined to be part of a mining base and it's in a skycrane arrangement.  By the time I built the crane I had enough fuel so I simply slapped my ascent engines underneath the space engines, jettison after the ascent burn but before the circularization burn.)

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Meant to reply to this ages ago... 

Yeah, it's a problem with my rocket... Tried it on a new rocket and all is well, so yeah it's either the staging or something else wrong with the rocket, or a minor bug in MJ. Either way, the delta v reading does work as it should under normal circumstances...

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