Inaccurate burn times in orbit with NERV engines

[eminerti]

Hi all,

I've just started making spaceplanes with a combination of RAPIERS and NERV engines for long distance travel.  I've noticed that once I get into orbit with the RAPIERS and then start planning manoeuvre nodes, the burn times and my remaining delta-v are completely wrong because it assumes the burn is with all my engines.  However, I shut down the RAPIERS once I get to orbit as they're less efficient.  The only way that I have been able to get the correct figures is to set the thrust of the RAPIERS to zero, or toggle them to airbreathing mode.  Doing this will change the burn time for a 560m/s burn from 24 seconds to 140 seconds for example, so it's a pretty big difference.

Is this a common problem?  Is there something I'm missing?

Cheers!

[AHHans]

1 hour ago, eminerti said:

Is this a common problem?  Is there something I'm missing?

Maybe. It's a common enough issue, but I don't really consider this a problem. I also think that you got the gist of it right.

KSP cannot read your mind. So if you have RAPIERs and NERVs in the same stage, then KSP assumes that you'll want to use both of them at the same time and calculates dV and burn time accordingly. If you don't want to use the RAPIERs in orbit (which is fairly common) and still want the calculations to be correct, then you need to help KSP understand that you are not going to use them. I myself do that by having the RAPIERs and the NERVs in different stages. Except for activating the RAPIERs the for first time I don't use the staging system to activate engines on my spaceplanes, I use action groups instead. I also move the stage with the currently active engines to the bottom of the staging list, to help KSP get the calculations right.

And welcome to the forums.

[Zhetaan]

11 hours ago, eminerti said:

The only way that I have been able to get the correct figures is to set the thrust of the RAPIERS to zero, or toggle them to airbreathing mode.  Doing this will change the burn time for a 560m/s burn from 24 seconds to 140 seconds for example, so it's a pretty big difference.

Is this a common problem?  Is there something I'm missing?

@eminerti:

Welcome to the forum!

@AHHans covered the important points.  I'll add that you can use action groups to toggle your engines on and off.  Also, the reason that there is such a big difference in burn time is because the Nerv has such low thrust compared to the Rapiers.  If the choice were between a Nerv and a Terrier (which has the same thrust), then the burn time would be nearly the same.

[eminerti]

@AHHans @Zhetaan thanks for the responses!

Everything you say makes sense, however I did actually have my RAPIER engines turned off (I used an action group to do so).  Because they're turned off, I would expect the game wouldn't factor them into the burn calculation.  However it's only when I switch them to air breathing mode that the calculation is updated just for the NERV engines.  That's why I thought it may be a bug (or that maybe I was doing something dumb).

[Zhetaan]

On 1/29/2020 at 8:33 AM, eminerti said:

Because they're turned off, I would expect the game wouldn't factor them into the burn calculation.  However it's only when I switch them to air breathing mode that the calculation is updated just for the NERV engines.  That's why I thought it may be a bug (or that maybe I was doing something dumb).

You are not; I don't know whether they've fixed this in the most recent version of KSP, but it used to be that the calculator could not give you a time until you burned the engines at least once each time you took control of the vessel.  This wasn't an issue for simple missions that you flew in one sitting, but long missions that play out over several sessions suffered from this (this was one of the great draws of Kerbal Engineer when it originally came out).

I'm far from being the person to ask about how KSP recognises which engines are running in terms of the underlying code, but it is possible that the game is not respecting staging or anything else because you have had them both active.

Are you on console?  The only universal fix that I know is not to trust the readout and use Kerbal Engineer.  That is part of why I don't know whether the issue has been fixed in the most recent version:  I have never looked back after making the switch.

[Guest]

Switch the RAPIERs and NERVs to different stages. You can even do this in flight. Then it'll compute the dV right.

Edited February 6, 2020 by Guest

[Cavscout74]

This is why I still use Better Burn Time mod even after fixing the worst of the issues with the stock burn time indicator - it is able to recognize engines are shut off and just calculate burn time for the active engines.   (And it has a great suicide burn countdown as well)