Drag doesn't add up

[king of nowhere]

As my spaceplane is soaring in urlum's high atmosphere, I got curious about aerodinamic data. What I found out seems contradictory, now I'm trying to figure out how the various displays work.

I opened up the aerodinamic display, and it's telling me my total drag, at the moment, is 77 kN. I decided to see which parts were more responsible, so I also activated the display in action menu

here is the plane. 77 kN total drag from aerodinamic window, wings are 2.13 each. The inner wings are the same. Less than 9 kN accounted for, so far.

Those SOA adapters are very aerodinamic, they make less than 0.1 kN each. The nose cones make 0.02 each. Those in the front and rear have similar values. All toghether, they are maybe 1 kN. 10 accounted so far.

Those aerospikes engines are also very aerodinamic, 0.07 each. With 6 of them, we are at 10.5 kN total

the cockpit and mk1-mk2 adapters are less than 1 kN each, other parts of the fuselage vary, there are 5 pieces in the fuselage, in total it's less than 5 kN. So far, we're at 16 - even approximating up all the calculations.

I further checked the control surfaces, the other fuel tanks, none of them is dragging significantly. All together, there will be maybe 20 kN of drag accounted for. Less. I checked the parts inside the cargo bays, to make sure they are properly shrouded.

So if looking at the parts menu one by one gives me a total of less than 20 kN, and the aerodinamic window is telling me 77 kN, where the hell are the other 60 kN of drag coming from? which of the two windows is wrong? are they even telling me what I think?

[Lt_Duckweed]

Open and then close your service bays.  For whatever reason, when you close service bays, the gui glitches out and keeps using the last drag value for everything in the bay, in this case, your props.

They don't actually contribute any drag, but the GUI will still add their "drag" to the total value.  That makes up for a large portion of the drag.  The rest is lift induced drag, because the lift from wings in ksp is perpendicular to wing surface, not to velocity.

[king of nowhere]

1 hour ago, Lt_Duckweed said:

Open and then close your service bays.  For whatever reason, when you close service bays, the gui glitches out and keeps using the last drag value for everything in the bay, in this case, your props.

They don't actually contribute any drag, but the GUI will still add their "drag" to the total value.  That makes up for a large portion of the drag.  The rest is lift induced drag, because the lift from wings in ksp is perpendicular to wing surface, not to velocity.

earlier during the flight, i already opened and closed the cargo bays. Drag shown in the GUI was added and removed accordingly (circa 20 kN for the big bay, 5 for the smaller bays).

as for lift-induced drag, lift is stated as 55 kN; could the lift-induced drag be just as high?

[Lt_Duckweed]

Lift induced drag is the very bottom value in the aero gui, and for this craft looks to be pretty small.

[OHara]

For me, the sum of all the drags under 'debug' in the parts' right-click menus, 
plus 'induced drag' at the bottom of the aero-GUI window,
has always added up to the total drag that KSP applies to  the craft.

I don't see anything that you've missed above.  Maybe put the craft file on KerbalX or somewhere and one of us can check the sums in our installation of KSP ?

Edited August 11, 2021 by OHara
At high speed in upper atmosphere, the total drag will go up quite quickly if you pitch up/down, so it is tricky to note the part-drag in the same condition as total drag.