Why do engines have drag (while thrusting)?

[g00bd0g]

So I was watching @Luch video about using structure fuselage as drag reduction on engines. It got me thinking.

Any propulsive engine (rocket or jet) should have ZERO rear drag as it is filling that space with reaction mass. If it had any drag you would not accelerate. Thrust is the opposite of drag. There should be no drag while thrusting greater than the unthrusting drag. Squad should fix this.

Thoughts?

 

 

Edited March 26, 2017 by g00bd0g

[bewing]

The sides of the engine are exposed to the air and have some drag -- that's where it comes from. It's not about the drag from the back end.

[g00bd0g]

Did you watch the video? Adding a structural fuse to the REAR of the engine reduces drag. If the drag was coming from the sides, this would not work.

[bewing]

That only works on stackable engines -- rockets and rapiers. An engine with an open node on the back end has extra drag because of the open node, of course. That's why the reversed nosecone trick works (which is a better answer than stuctural fuselages, because those still end in another open node.)

So if you don't like the fact that open nodes cause drag, then I'd definitely suggest using the reversed nosecone trick on your engines.

 

Edited March 26, 2017 by bewing

[g00bd0g]

2 hours ago, bewing said:

That only works on stackable engines -- rockets and rapiers. An engine with an open node on the back end has extra drag because of the open node, of course. That's why the reversed nosecone trick works (which is a better answer than stuctural fuselages, because those still end in another open node.)

So if you don't like the fact that open nodes cause drag, then I'd definitely suggest using the reversed nosecone trick on your engines.

 

A node is a logical construct, not a physical one. Logical constructs should not have physics applied to them. i.e. something should have drag because of its physical shape and placement, not because it's an "attachment point". It doesn't seem like it should be that hard to reduce the drag by the thrust, until the thrust is greater than the drag, at which point there is no rear drag. If they want to leave some "side drag" in there for engine parts, fine. But the effect as depicted above should not happen. It doesn't work like that in real life, it's just an artifact of the simulation, and one that can/should be corrected.

Edited March 26, 2017 by g00bd0g

[diomedea]

4 hours ago, g00bd0g said:

 It doesn't seem like it should be that hard to reduce the drag by the thrust, until the thrust is greater than the drag, at which point there is no rear drag.

Congratulations. You have just negated not only physical evidence, but even the mathematical notion behind continuous functions. Drag is a continuous function (including rear drag), it doesn't magically cease to exist "just because" there is some mass exiting the rear. The mass exiting the engine provides thrust, there is one specific throttle setting when exhaust mass (at the exhaust speed) is exactly equal to the mass required to make continuity with the external airflow, which is also the condition to avoid flow turbulence (a result of the rear drag): that specific setting is where thrust = rear drag. Any further, we have thrust > rear drag and the engine provides net acceleration. Rear drag keeps existing, you may also conceive that as the amount of dynamic pressure the airflow exerts on the exhaust mass (as opposed to static atmospheric pressure, that we know reduces effective exhaust velocity and Isp), so this dynamic pressure also reduces the effective exhaust velocity (please note, this isn't the amount of dynamic pressure registered on the nose of a vessel, which is function of air density and mach speed; this is the pressure caused by convergence of the flow at the rear of the vessel, which is yet another way to see continuity, indeed included in the general continuum Navier-Stokes equation).

As for the video, I have no wonder different parts in KSP have different dragcubes. The rear drag is a function from the equivalent rear area from the drag cube of the rearward part in a stack. One could still say e.g. the flea has such a bad rear drag (certainly has a larger rear drag compared to side drag, than other SRBs) that even a structural fuselage makes it better. Or (given how KSP simulates things), one could say e.g. the flea has a short inefficient nozzle, so even adding a fuselage section makes it work better (which could be true in reality, but isn't how KSP works).

[DrLicor]

drag is the force 'backwards' , thrust is de force 'forward'. 

If you do thrust - drag get the resulting force a engine gives at a specific moment.

The drag a engine produces can be different on different heights and positions in de atmosphere. 

[Luch]

The engine shape is not perfect. Turbulence arises from a collision with the uneven surface of the engine. But if we have any parts after the engine, its drag just not calculated.

You can use such constructions to lower the engine drag:

[sal_vager]

The real reason is that engines never had dynamic drag cubes set up for them, ideally they would become more pointed as their thrust increased, reducing drag, though the drag does aid in rocket stability.

[Snark]

16 hours ago, g00bd0g said:

Any propulsive engine (rocket or jet) should have ZERO rear drag as it is filling that space with reaction mass.

On what are you basing the above assertion?

In particular:  You appear to be assuming that because the engine is "filling that space with reaction mass", the reaction mass should somehow be exerting a forward pressure on the ship.

Why are you assuming that?  What numbers are you using?

Bear in mind that this isn't just some high-pressure gas that's just sitting there behind the rocket.  It's gas that's moving away from the rocket, and doing so at very high speeds.  To have an effect on the rocket after the point where it leaves the nozzle, the high-pressure exhaust gas behind the rocket would have to have an expansion velocity that's higher than its net backwards velocity coming out of the nozzle.  Which seems unlikely to me.  So... why are you assuming that?

[fourfa]

Maybe we can simplify a lot of this discussion to "it's a game, not a simulator."  There are routes available to make KSP more like a simulator.  In my observations, some people who try the simulator route find the addictive gameplay of stock KSP becomes more like work than fun.  Imagine what fun these forums would be if KSP became RSS/RO overnight - I don't think anyone wants that.

Also be aware this topic might get moved to the graveyard of "Suggestions and Development Discussion" as this subforum is for discussing how to play the existing game, rather than picking fights with mods and devs about how the game should be.  ¯\_(ツ)_/¯

Edited March 26, 2017 by fourfa

[g00bd0g]

Thanks for all your input. @diomedea stated it much better than myself. I still don't believe the effect as depicted in video matches reality, but if you want to defend it as a desireable in-game feature, that's your prerogative.

If this post should be moved, that's fine too

Edited March 27, 2017 by g00bd0g

[Kryxal]

10 hours ago, Snark said:

On what are you basing the above assertion?

In particular:  You appear to be assuming that because the engine is "filling that space with reaction mass", the reaction mass should somehow be exerting a forward pressure on the ship.

Why are you assuming that?  What numbers are you using?

Bear in mind that this isn't just some high-pressure gas that's just sitting there behind the rocket.  It's gas that's moving away from the rocket, and doing so at very high speeds.  To have an effect on the rocket after the point where it leaves the nozzle, the high-pressure exhaust gas behind the rocket would have to have an expansion velocity that's higher than its net backwards velocity coming out of the nozzle.  Which seems unlikely to me.  So... why are you assuming that?

It's clearly not as simple as "engine is generating thrust, so something's there, so no drag", but the rear-node drag is turbulent drag, so it would make some sense that having something exit the engine would have some effect on it.  Just what effect it would have is beyond me, though.