Where's my 1100 m/s going?

[Zeiss Ikon]

I'm playing 1.6.1 RSS/RO/RP-1.  I've got a Lunar flyby mission in parking orbit; it's the second attempt (first ran out of batteries in the 3 day orbit to the Moon).

The first Lunar flyby had no problem with the trans-Lunar insertion maneuver -- I set up the node, used RCS to settle the fuel, started the AJ10-37, and shut down when the remaining node velocity was close to zero, then oriented the vessel to get sun on the solar panels and warped to the SOI change -- only to find the batteries dead.

Okay, added battery, split up the avionics to get the lowest possible power draw, removed some tanks of hypergolic fuel from the final bus tank (left over from an earlier design iteration, missed in the review -- giving a net mass reduction compared to the previous iteration (spent 200+ days building, and 38 days rolling out to the pad).  Launched to parking orbit.

My maneuver for a 2000+ km Lunar periapsis shows as 3100.5 m/s, which is about what I'd expect from a 330 km circular parking orbit.  MechJeb's Delta-V display shows the stage with over 4200 m/s/  On-board avionics hold node pointing during the burn, but over the course of the burn I go from 1100 m/s to spare, smoothly down to coming up about 230 m/s short.  Somehow, I'm gradually losing 1300 m/s over the course of a routine burn that worked fine (including leaving hundreds of m/s spare dV from a 4100 m/s stage) on the previous flight.

What could do this?  Pointing isn't drifting off the node; I'm starting the 1:53 burn at -57 seconds, which ought to give an orbit I can easily correct with RCS en route (it came out perfectly on the previous flight).  There's no indication of an low-grade engine failure (I've watched the engine status through one retry).

Hmm.  I just looked at the paused game screen again, and the burn shows 1:53 for 3100.5 m/s, but MechJeb says I have 1:44 available propellant to provide 4238 m/s from the stage.  Seems like a little discrepancy there.  My burn time matches the node display -- not the MechJeb one.

I have a quick-save from about 3 minutes before the node, and I can, if it'll make a difference, cancel and recreate the node.  I'm in a stable parking orbit, though I still don't have enough solar panels to stay in orbit forever -- but another hour and a half shouldn't make much difference relative to a three day orbit to the Moon.

[AHHans]

Disclaimer: I don't know RO, RSS, RP-1, or MechJeb, I essentially only play the stock game.

8 hours ago, Zeiss Ikon said:

I just looked at the paused game screen again, and the burn shows 1:53 for 3100.5 m/s, but MechJeb says I have 1:44 available propellant to provide 4238 m/s from the stage.

That means that either the game or MechJeb got their math wrong. (Well, probably more: "have different ideas about the relevant parameters".)

What does the game show you about the start and end mass of the stage and the Isp? Are you still in atmosphere (as thin as it might be)? What is start and end mass of the stage after the burn? (I.e. did the end mass change?)

[Zeiss Ikon]

Going out the door to work, but I'll try to check those other parameters when I get home.  As a first expectation, I don't have anything leaving the stage other than propellants, there's no boil-off (UDMH and white fuming nitric acid are long-term storable), and the entire burn is at app. 330 km altitude, almost 200 km above the sensible atmosphere.

[Laie]

20 hours ago, Zeiss Ikon said:

Somehow, I'm gradually losing 1300 m/s over the course of a routine burn that worked fine (including leaving hundreds of m/s spare dV from a 4100 m/s stage) on the previous flight.

Do you have testflight? Could it be an engine failure perhaps? (MJ used to catch the resulting changes in dV, but perhaps they've thought of some new way to leak propellant)

Edited November 11, 2019 by Laie

[Zhetaan]

When you're getting different results for your burn time, that's just KSP.  When you're getting different results for your delta-V, something's wrong with your tank and fuel flow arrangement--or it's the case that MechJeb or KSP is seeing a connection that doesn't exist.  Check your tankage and the way your feed is set up.  Try detaching and reattaching the stage in the VAB.  If nothing else works, then you're going to need to rebuild the rocket and see whether you can duplicate this.

Edited November 11, 2019 by Zhetaan

[Zeiss Ikon]

There is only a single tank connected to the engine used for this burn.  The final spacecraft has a tank, but there's a decoupler between with crossfeed disabled, and I pulled the UDMH/IWFNA tanks out of it last trip to the VAB (to lighten the stage, forgot that stuff was in there from a previous design version), so that stage only ever had nitrous oxide (monopropellant for RCS).

@Laie I do have Test Flight (gives me something to cuss at, if I didn't already have enough of those), but the engine icon is staying white through the burn.  I'm just seeing the dV in the MechJeb display drop faster than the remaining dV for the node.  And I'm starting with a small red stripe at the bottom of the node dV display (1.6.1 is mostly new to me, too, I didn't catch what that was at first).  So, apparently MechJeb is practicing for his next job as a used car salesman.

[ElWanderer]

8 hours ago, Zeiss Ikon said:

there's a decoupler between with crossfeed disabled

The display is showing two different TWRs against two stages, the first of which has no delta-v. I think it is assuming you are going to decouple the extra mass before burning, giving you a TWR over 2 and 4.2km/s delta-v, but actually your TWR is more like 1.7 and your delta-v will be correspondingly lower.

It doesn't know when you intend to fire that decoupler, though you may be able to help it by moving it about in the staging menu.

[Zeiss Ikon]

Actually, I think you're mostly right, @ElWanderer.  I found I had the booster Avionics core set as root, when it should have been the Upper stage core (they sandwich that decoupler).  Stages after dropping the root part often show incorrect dV, for exactly the reason you give -- MJ makes an assumption about staging order.  The displayed dV was most likely that of the engine, tank, and avionics core, without the actual spacecraft.

I deorbited this one and fixed the root, redesigned the solar layout to actually produce enough power for the avionics, at least in optimum orientation, and added fuel.  Hopefully there's enough margin in the Achilles and AJ10 second stage to still make orbit.  Sim will tell next time I have time to play.