Thrusting engines shouldn't be creating so much drag

[innociv]

An engine that's turned on or off seems to make the same amount of drag according to the aero overlay.
That shouldn't happen.  And it's why people put nose cones on the backs of RAPIERs.

Couldn't thrusting parts be made to have lower drag the more thrust there is?

I mean, in fact, basically how they work is by creating negative drag pushing them forward.

[juanml82]

But... the engines (well, the stuff in front of them) create drag. That's fine. The engines, when firing, also create thrust.

[innociv]

I didn't say anything to the contrary to that.

[The_Rocketeer]

First, why would a thrusting engine generate less drag than a non-thrusting engine? Can you justify this with science or is this just something you heard somewhere?

Secondly, since drag and thrust are basically opposed forces (assuming stable level flight), there's mathematically no difference between negative drag and positive thrust. I think what you're asking for is just more thrust...

[icedown]

15 minutes ago, The_Rocketeer said:

First, why would a thrusting engine generate less drag than a non-thrusting engine? Can you justify this with science or is this just something you heard somewhere?

Secondly, since drag and thrust are basically opposed forces (assuming stable level flight), there's mathematically no difference between negative drag and positive thrust. I think what you're asking for is just more thrust...

It actually would since there is now a low pressure area directly behind the engine where the propellant normally comes out.  This would cause an increase in drag while the engine is not in operation. Drag and thrust are not necessarily opposed forces thrust direction can be offset from the vector that drag is acting at.

Edited June 6, 2016 by icedown

[The_Rocketeer]

@icedown, let's assume I accept all that stuff about low pressure (since I do, but I was hoping the OP would back up his own point).

Your argument:

• Engine is moving forward.
• Motion causes low pressure behind engine.
• Low pressure causes drag.

(This part ^^ is fine. But then you go on:)

• Propellant increases pressure behind engine.
• Increased pressure reduces drag.

First, surely you realise that this is only true if the thrust and drag vectors are aligned. If they're not, the argument makes no sense - vertical engine thrust doesn't affect horizontal drag (at least to any significant degree).

Second, increased pressure behind the engine is the mechanical principle of thrust. In other words, if thrust and drag are not opposed, whether the engine is on or off makes no difference to the drag, and if they are then the difference is attributable entirely to the effect of thrust. So there's no reason to model a decrease in drag, because it's mathematically intrinsic to the thrust output.

[icedown]

16 minutes ago, The_Rocketeer said:

@icedown
Second, increased pressure behind the engine is the mechanical principle of thrust. In other words, if thrust and drag are not opposed, whether the engine is on or off makes no difference to the drag, and if they are then the difference is attributable entirely to the effect of thrust. So there's no reason to model a decrease in drag, because it's mathematically intrinsic to the thrust output.

Thrust is not generated by anything behind the engine, it is generated almost entirely in the combustion chamber.  This is where the propellant is accelerated producing thrust.  This does extend down into the nozzle a little but not much.  When the engine is shut down becomes a low pressure area that adds to the total drag of the component.  The engine can be in operation, produce no thrust but reduce drag also. If the exit velocity of the gases equals the surrounding airflow, this would reduce the drag but produce very little thrust. The high  pressure area behind the engine is due to the expansion of gases, while this is what creates thrust in the combustion chamber, throat, and part of the nozzle, once it exits it no longer produces any form of thrust, but it does still contribute to the reduction in drag due to the air flow does not have to fill the void behind the engine as it does when it is off.

Edited June 6, 2016 by icedown

[The_Rocketeer]

1 minute ago, icedown said:

Thrust is not generated by anything behind the engine, it is generated almost entirely in the combustion chamber.

FWIW I was mostly picturing aeroplane jet engines rather than rocket engines. For one thing, why on earth would you want less drag at the bottom of your rocket? That's just asking for instability problems...

Thrust and drag are related in the sense in which you're describing. The extent to which the drag is reduced is directly proportional to the amount of thrust generated, because the amount of thrust generated directly affects the level of pressure behind the engine. So, mathematically, any nuance of the effect of drag reduction can be presumed to be bundled into the overall thrust force.

 

 

[icedown]

Just now, The_Rocketeer said:

Thrust and drag are related in the sense in which you're describing. The extent to which the drag is reduced is directly proportional to the amount of thrust generated, because the amount of thrust generated directly affects the level of pressure behind the engine. So, mathematically, any nuance of the effect of drag reduction can be presumed to be bundled into the overall thrust force.

 

 

Actually drag reduction only progresses to the point that pressures in the propellant flow equal those of the outside atmosphere, after that there is no longer a reduction in drag. This is true of aircraft jet engines as well, though they never actually (Hopefully) shutdown in flight, they are always idling and therefore producing thrust.

[The_Rocketeer]

8 minutes ago, icedown said:

Actually drag reduction only progresses to the point that pressures in the propellant flow equal those of the outside atmosphere, after that there is no longer a reduction in drag. This is true of aircraft jet engines as well, though they never actually (Hopefully) shutdown in flight, they are always idling and therefore producing thrust.

Disagree. If the air pressure behind a craft is higher than the pressure in front of/beside it, the craft is effectively experiencing what I'll call anti-drag, AKA thrust.

Also, what I said about the mechanical principle of thrust is correct. 'Accelerating the propellant' is the same as 'high pressure behind the craft', just that 'behind' in the instance also happens to be 'inside' the combustion chamber. Since the combustion chamber isn't closed, this is the same thing as behind. The 'jet effect' of the nozzle just increases the pressure. See what I mean?

Edited June 6, 2016 by The_Rocketeer

[icedown]

Here is the best way to show what I mean.  The first picture is flow when the engine is operational. A solid cylinder is used to represent the exhaust plume that is separated from the rest of the airflow because of it's energy.  The second shows the airflow at the same speed with the engine not in operation

Spoiler

Spoiler

 

[innociv]

Actually it's that there is a high pressure area behind it.
Rocketeer said what I said earlier. "Thrust" is that high pressure behind the engine pushing it forward. This is the opposite of drag, yet the game models drag purely based on the shape and doesn't account for that high pressure.

It's a bug in the game, really.
One that I would think would be pretty easily fixable without having to put backwards nosecones on your RAPIERs to min/max.

[icedown]

That's not how a thrust producing engine works, it's the acceleration of the propellant, not the high pressure behind the engine that produces the thrust. These may seem like the same thing but they are not.

[The_Rocketeer]

@icedown yes it is -exactly- how a thrust producing engine works: The acceleration of the propellant is caused by the propellant moving down a pressure gradient. The number of molecules exerting force on the craft is highest where pressure is highest, be that inside a combustion chamber or around a nose-cone. If the particular effect of this pressure is a reduction in velocity we term it drag; if its an increase in velocity, we term it thrust, but the physical process behind it is exactly the same.

Your images are illustrative of the point about drag induction on a shut-down engine, but the thing that reduces the drag (even in your own images) is the high-pressure propellant, which can be simply expressed as thrust. For the formula for calculating the net effect of forces in KSP, there's no difference between increasing thrust on one side of the equation or reducing drag on the other side - in this instance they are completely interchangeable.

[Van Disaster]

For clarity, I believe he's referring to the reasons behind this technique: https://en.wikipedia.org/wiki/Base_bleed . It's also a large thing in car aerodynamics, attempting to fill the area of low pressure behind the car ( and the reason slipstreaming is a thing ). Note in either case there's no actual thrust involved. For KSP rocket engines, there being more drag with no mass flow out of the engine sounds sensible... for jets it's a bit more complicated ( compressor stalled? intake still open? is it a ramjet? ) but if it really bothers you that much, use FAR

[innociv]

The low pressure area behind the blunt rear of a shell is representative of why the blunt rear of engines in KSP create a lot of drag compared to a backwards nose cone.

But when the engine is turned on and producing thrust that affect doesn't exist, except in KSP it continues to exist, and that's exactly why I made my thread as it's essentially a bug in the aero modeling that I'd very much like to see fixed.

It would seem pretty straight forward to fix, no...?
Just make them more aerodynamic like nose cones when producing thrust.  Or actually model thrust to be negative drag on that localized area which would mean the aero overlay wouldn't be showing drag there, no?

Edited June 6, 2016 by innociv

[The_Rocketeer]

To clarify, I'm saying that the effect can be considered to already exist in KSP if you assume that rather than literally diminishing drag it's simply abstracted into the effect of thrust. Increasing throttle causes a forward-force we call thrust, but the first little fraction of that force might as well be considered as a reduction in drag. The desired outcome is exactly the same as what happens as you throttle up anyway!

Let's not forget that the turbulence/drag behind a rocket is a teeny tiny force compared with the leading-edge drag at the other end of the rocket!

As for sticking nosecones on Rapiers, personally I view that as an exploit. It doesn't make RL sense, so to me it shatters my suspension of belief.

Edited June 6, 2016 by The_Rocketeer

[innociv]

Ehh... either way the aero overlay is bugged, even if you consider the engines to have extra thrust to overcome their improper aerodynamics modeling.

If it worked correctly, people wouldn't stick nosecones and stuff on the back of RAPIERS. They also wouldn't need to overly crowd engines in a small space, and other things.

Edited June 7, 2016 by innociv

[Rath]

The reason we stick nosecones on rapiers is because the free node is draggy.  Free nodes are always draggy.

[innociv]

That's... not contradicting the issue I'm pointing out here at all.  It's just backing up what I said.

[Van Disaster]

20 hours ago, The_Rocketeer said:

Let's not forget that the turbulence/drag behind a rocket is a teeny tiny force compared with the leading-edge drag at the other end of the rocket!

I wonder about that, I guess there's numbers somewhere. On the other hand a rocket with an area of low pressure behind it isn't going to space... either way rockets are very low drag devices. I totally agree with building engine-off drag into the engine thrust also, the issue is basically a thrust tax.

 

20 hours ago, The_Rocketeer said:

As for sticking nosecones on Rapiers, personally I view that as an exploit. It doesn't make RL sense, so to me it shatters my suspension of belief.

Makes no sense with the engine on, definitely. Might make some vague sense with it off, though.

[Sereneti]

On 6.6.2016 at 7:46 AM, The_Rocketeer said:

First, why would a thrusting engine generate less drag than a non-thrusting engine? Can you justify this with science or is this just something you heard somewhere?

Secondly, since drag and thrust are basically opposed forces (assuming stable level flight), there's mathematically no difference between negative drag and positive thrust. I think what you're asking for is just more thrust...

because you have a low-pressure area behind your vessel
https://en.wikipedia.org/wiki/Base_bleed

[The_Rocketeer]

@Sereneti this point has been made and answered. Better to read the whole thread before posting.

[EpicSpaceTroll139]

10 hours ago, innociv said:

Ehh... either way the aero overlay is bugged, even if you consider the engines to have extra thrust to overcome their improper aerodynamics modeling.

If it worked correctly, people wouldn't stick nosecones and stuff on the back of RAPIERS. They also wouldn't need to overly crowd engines in a small space, and other things.

I don't think this can really qualify as a bug, so much as a feature that just plain was never implemented. The KSP stock aero model wasn't made to simulate every aspect of real life aerodynamics. Just really enough to allow your average Joe to tell it's there when launching a spacecraft and launch a plane with. Of course it was pretty bad for a while, so it got upgraded in 1.0.0. It is of course still nowhere like real life (you still need FAR for that). KSP still doesn't model stalls reasonably, so why would we expect it to model the drag reduction of running engines? (Still would be a nice feature ofc for those of us that know about this, but I'm guessing lots of people wouldn't notice or understand it, and would think it was a bug)

As for the putting nosecone on the back of engines, even if the engines got reduced drag when running, people would still put nosecones there because there's a node. KSP physics mean that nodes will produce drag, regardless of the engine thrusting or not, so the argument about adding nosecones is a bit arbitrary. This is an entirely separate bug.

As for "overcrowding engines" ... I don't see what you're getting at. Presumably clustering engines close together would reduce cross-sectional area when looking down the length of the rocket, and thus drag. This sounds perfectly logical. Could you explain what you're talking about exactly?

[Deddly]

Although I understand where you're coming from, @The_Rocketeer, I don't think I agree with the idea that the decrease in drag can be assumed to be bundles into the thrust. You say OP is effectively asking for an increase in thrust, but an increase in thrust would also affect the engine in vacuum, would it not? Drag changes depending on the makeup of the atmosphere and how thin it is at altitude, whilst thrust is a completely different animal. The image by Icedown illustrates it perfectly: the "solid cylinder" in that picture will make a difference to drag inside an atmosphere but certainly not increase the thrust inside or outside an atmosphere.