Does drag reduce as fuel is used?

[whistlehead]

As most of you probably know, KSP makes the unrealistic assumption that cross-section is proportional to mass in its drag model.

I'm just wondering if it bases this entirely on the full mass of parts, or if it updates as the ship's mass reduces (i.e. as you use fuel), thereby reducing drag as fuel is used.

Thanks

Chris

[capi3101]

http://wiki.kerbalspaceprogram.com/wiki/Drag#Drag

The game makes the assumption that cross-section is based on mass, so drag should be decreasing at a rate proportional to the decrease in mass.

[KvickFlygarn87]

This could easily be tested with MJ, build a rocket, note drag on the pad, burn away fuel, check if drag is less.

Also, if you're on FAR, nope.

[wasmic]

Effective drag doesn't decrease, though. As "drag" is actually a drag coefficiency, draining fuel won't change your terminal velocity, as terminal velocity varies with average drag coefficient. Thus, the only way to slow down a ship is to make parts with a drag coefficiency of more than 0.2, but a parachute with a drag coeff of 200 that weighs 1 ton will slow your ship down more than one weighing in at 0.2 tons, as it will weigh more in the average drag coeff calculation.

[numerobis]

Drag (in kN) definitely falls with fuel usage, which is pretty silly. You can test by flying steady at a given altitude with infinite fuel, set throttle so that you're exactly in a circular orbit (effective gravity is zero, so drag = -thrust), then turn off infinite fuel. As fuel drains, you speed up, but thrust doesn't change -- this proves that drag is falling. Since you're at steady altitude, drag is falling because your mass is falling (of course, because you sped up, soon you will be climbing, and drag will be falling because of pressure).

[rosenkranz]

Ummmmm, maybe I'm wrong here but the aircraft is going faster because your thrust to weight ratio is increasing as your fuel is used. That has nothing to do with your drag.

IIRC, your Cd is calculated in real time so your drag never changes as fuel drains. What does change is your center of mass though which will change your flight characteristics.

Most all parts have a 'drag' of .2 per ton. If all your parts have the same drag then your drag will be .2 per ton. Nosecones now have a a drag of .1 which will help that end point into the 'air stream' such as it is. However since the nosecones are very light, they won't effect the over all drag of the vehicle much.

[Supernovy]

I think wasmic has it about right here, as fuel drains mass decreases, so drag force decreases. However, since mass decreases, drag acceleration stays the same.

Stock drag is a little bit like gravity in that respect; acceleration is independent of mass.

[numerobis]

Ummmmm, maybe I'm wrong here but the aircraft is going faster because your thrust to weight ratio is increasing as your fuel is used. That has nothing to do with your drag.

TWR matters when you're fighting gravity, but I specified that you're in a circular orbit, so you aren't fighting gravity. The thrust to drag ratio matters when you're fighting drag, and in the situation I gave, that's all you're fighting.

wasmic is only mostly right; if you are burning fuel in parts with higher than average drag (e.g. an SRB), drag falls faster than mass -- another reason it's nice to burn SRBs first in a launch.

[Supernovy]

I think he means that as mass decreases, acceleration increases for a given thrust, which could mask the change in drag acceleration.

[Jason Patterson]

I think he means that as mass decreases, acceleration increases for a given thrust, which could mask the change in drag acceleration.

But that's not the case in numerobis's situation. The way it ought to work is that as mass decreases, nothing happens, since the net force on the craft should remain at 0, with thrust and drag equal in magnitude and opposite in direction.. F = ma only really makes sense when F is the sum of all of the forces on the object (or if you wanted to sum up all the accelerations, I suppose.)

That only applies if drag and thrust remain constant (or at least equal in magnitude and opposite in direction.) We know that the engine's thrust doesn't change, and since the plane accelerates, that means that the drag must.