Use only the smallest engine variants

[Foxster]

You may think that matching a rocket engine variant base size to the attached tank size will lead to the lowest drag. 

Wrong. 

There is a small drop in the drag of the tank (~2) by correctly matching the engine size to the tank but this is not the case for the engine's drag. Matching the engine base size to the tank size can lead to massively more drag for the craft.

In fact, when an engine has variants it is far and away best to use the smallest variant in all cases e.g. a Mastodon  engine can have the following drag at 5km altitude on a simple rocket:

Small variant: 9

Mid variant (matches tank size): 336

Large variant: 793

So, using anything other than the smallest engine variant can lead to huge drag losses for a craft. The exception being if you use the default engine variant size and match that to the tank it is attached to, in which case you get something like the drag of the smallest variant.  

Edited December 20, 2018 by Foxster

[ZL647]

However, due to the flight path taken by a rocket, drag quickly ceases to be an issue. 

[OHara]

I posted a bug-report last week on the (self-registration-required) bug tracker: 20683

Quote

 

Many of the new engines in Making History expansion have variants models, often with and without tank butts.

1) The PartDatabase.cfg is populated with drag-cubes that correspond to the areas and shapes of these engines with shrouds attached. This produces drag in-game that can be surprising if you chose an engine that would fit the stack without its shroud. 

2) The models with shrouds have holes passing thru so the drag areas do not fill the expected {1.25, 2.5, 3.75}-meter-diameter circles. This leaves an area mismatch in the joints of a stack, and KSP applies flat-plate drag that users would not expect, similarly to the behaviour of tubes in #19376.

The drag a player feels is very sensitive to mismatches in area where two parts join along the stack. Desired behavior is to have at least the ends with nodes have areas in the drag model matching the visible area of the model, with any holes filled in

 

 

[Foxster]

3 hours ago, ZL647 said:

However, due to the flight path taken by a rocket, drag quickly ceases to be an issue. 

It can make quite a difference, costing hundreds of dV by orbit depending on how many engines your craft has. On Eve it can easily make the difference whether you make orbit or not. 

[KerikBalm]

11 hours ago, Foxster said:

Matching the engine base size to the tank size can lead to massively more drag for the craft.

In fact, when an engine has variants it is far and away best to use the smallest variant in all cases

...

The exception being if you use the default engine variant size and match that to the tank it is attached to, in which case you get something like the drag of the smallest variant.  

I'm confused by your statements....

My best guess is that you should not care about matching engine size to tank size, and just use the smallest size *Unless* the default size matches the tank.

So use default size if it matches, or use the small size, never use a size larger than default...

Is this a correct interpretation?

[Foxster]

1 hour ago, KerikBalm said:

I'm confused by your statements....

My best guess is that you should not care about matching engine size to tank size, and just use the smallest size *Unless* the default size matches the tank.

So use default size if it matches, or use the small size, never use a size larger than default...

Is this a correct interpretation?

Pretty much. Except for "...never use a size larger than default..."  should be "...never use a size other than default (if it matches the tank) and the smallest...". This can be important where you have a large engine like the Mastodon and both the mid (which is a visible size match for 1.5 tanks) and large (the default) cause a lot of drag when attached to 1.5 tanks.  

So, use engines as you did before engine variants were introduced and match the default variant to the tank size. If you want to use another variant and drag could be an issue then only use the smallest variant and this can lead to the lowest overall drag for a craft. 

Edited December 21, 2018 by Foxster

[ZL647]

4 hours ago, Foxster said:

It can make quite a difference, costing hundreds of dV by orbit depending on how many engines your craft has. On Eve it can easily make the difference whether you make orbit or not. 

I'll agree only with Eve. On other planetary bodies their atmosphere effectively ends so low as to render the effect you are talking about to be largely irrelevant. All you have to do is throttle back some.

Edited December 21, 2018 by ZL647

[OHara]

With the new version 1.6.0, two base-game engines are affected: Terrier and Spark,
and for these engines, the largest variant works the best.

The drag is not huge, but is noticeable.  On a small early-game moon-rocket with a Terrier upper stage, the extra drag is 1/3 total of the rocket -- 1/2 the total after dumping solid-fuel boosters. The problem is that the drag is a surprise, and that it is located high in the rocket stack where it can flip a rocket as it crosses the sound barrier.

Even in version 1.6 we can still access the old models for Terrier and Spark, using the upper-left |||>> 'enable advanced mode' in the VAB and looking under 'Filter by Module' : 'Engine'

A straightforward Module-Manager patch, below, restores the former drag-configuration for Terrier and Spark.  The new engines are trickier, but I put a suggestion on the bug tracker.

Spoiler

@PART[liquidEngine3_v2]
{
    %DRAG_CUBE
    {
        cube = Fairing, 0.96,0.78,0.72, 0.96,0.78,0.72, 1.23,0.96,0.19, 1.23,0.73,0.70, 0.96,0.78,0.72, 0.96,0.78,0.72, 0,-0.4,0, 1.25,0.8,1.25
        cube = Clean,   0.64,0.70,0.72, 0.64,0.70,1.00, 1.22,0.96,0.15, 1.22,0.73,0.74, 0.64,0.70,0.72, 0.64,0.67,1.01, 0,-0.4,0, 1.25,0.8,1.25
    }
}
@PART[liquidEngineMini_v2]
{
    %DRAG_CUBE
    {
        cube = Fairing, 0.25,0.78,0.40, 0.25,0.78,0.40, 0.30,1.0,0.11, 0.30,0.75,0.11, 0.25,0.78,0.40, 0.25,0.78,0.40, 0,-0.2,0, 0.625,0.4,0.625
        cube = Clean,   0.13,0.70,0.40, 0.13,0.70,0.40, 0.30,1.0,0.11, 0.30,0.75,0.51, 0.13,0.70,0.40, 0.13,0.70,0.40, 0,-0.2,0, 0.625,0.4,0.625
    }
}

 

Edited December 25, 2018 by OHara
suggested fix