Mk3 Engine Mount aerodynamics

[HebaruSan]

What are the aerodynamic properties of this part?

Does the big flat part cause drag if you put engines on it? Or does it count as a smooth mk3-to-3x1.25m adapter?

Edited February 8, 2016 by HebaruSan
tag spelling

[RA3236]

It actually has similar drag properties to most other MK3 parts because KSP doesn't know the difference between a sphere and a brick.

 

[PrimeDirective96]

It and other M3 parts like the Cargo bays have may more drag than they ought to, even for KSP, just press f12 and take a look at the in-flight aerodynamics overlay, the drag lines are insanely long for those parts, longer than the drag lines from the Cockpit where you would think most of the drag is occurring. Probably just a glitch with the node attachments for those parts not reading them as occluded, hopefully it get's fixed for version 1.1.

Edit: Since apparently the drag lines aren't an accurate measure, I decided to take a look at the drag values using the aero-data in the action menu's for each of the parts in a very simple Mk3 plane with a fuselage consisting of a Nose Cone, Mk3 Cockpit, Mk3 Cargo Bay (Long), Liquid Fuel Fuselage (Short) and a Mk3 Engine Mount with 3x J-X4 Turbo Ramjet's on the engine nodes.

I put the plane into a cruise at approximately Mach 1 (According to Kerbal Engineer) at an altitude of almost exactly 500 m and took the drag values of the parts consisting the fuselage and engines and rounded them to the nearest half.

Recorded drag values (kN):

Aerodynamic Nose Cone: 23

Mk3 Cockpit: 82

Mk3 Cargo Bay CRG-100: 103.5

Mk3 Liquid Fuel Fuselage Short: 27

Mk3 Engine Mount: 154

J-X4 ''Whiplash" Turbo Ramjet: 0.17

One can see from the values that the Engine Mount has a more drag than any other part on the plane.

I recorded the values for the Engine Mount a second time but with two engines, under the same conditions as before and recorded a drag value of 184 kN.

I then looked at a single engine and no engine by decoupling it at high speed and noticed a fairly substantial drag increase there too.

This makes designing Shuttle's that fly like the real shuttle near impossible.

 

Edited February 11, 2016 by PrimeDirective96
Correcting and additional comment based on new information

[sal_vager]

The drag lines are not an accurate measure of anything, they only show more or less and direction, if you want the actual magnitude you will need to enable the aero data in the action menu via the debug screen.

Being flat, the part is as draggy as other flat parts of the same size such as fuel tanks, but filling those nodes does help to reduce the drag.

You can see the values for the drag cube for this and all other parts in the partDatabase.cfg, KSP has not treated parts as spheres for drag since 1.0.0 unless of course you enable that option in the debug screen.

[Araym]

On 9/2/2016 at 1:43 PM, PrimeDirective96 said:

<snip: various drag consideration about the part, compared to other Mk.3 parts>

This makes designing Shuttle's that fly like the real shuttle near impossible.

 

Basically my conclusion, done by more "emphirical" gameplay:

my mk.3 shuttles (standard configuration: orbiter + external tank + 2x boosters mounted sideway to E.T.) have rather the orbiter ending with any of the mk.3 tanks than the mk.3 shuttle mount, because

• any design using it had a deadly drag in the glinding/reentry phase, and magically using an empty tank in the same layout of (space)plane resolved its flight properties
• it have some useful role of sort (a tank... fuel, fuel+oxi, monoprop), in a comparable lenght fuselage
• empty, as a simple "structural part", with a tank there is no any particulary mass penalty
• the "Vernor" SSME engine is surface-attackable, so it's not so a problem a x3 engine layout shuttle-alike
• in surface attack, i can set symmetry for the "Vernor" engines as I like: the engine mount does not accept ANY symmetry, radial or mirrored, in their nodes
• KSP rule "open node creates drag": a tank has only ONE node... a mount has FOUR ones... BETTER "a tank" to manage/cover it
• ... and for my ease of design, mods like "editor extension" (to set more free assembly capability) and "no offset limits" (to be capable, freely, to offset parts with the editor tools even out of the parent collider properties... a GOD SENT MOD tha correct some annoying mk.3 parts behaviour of surfaced attacked parts) saved my life

 

... so, basically, DO NOT USE it...

... unless you have some creative use of it, that forgive the bad properties of it overall, like Cupcake:

(see his creations here: http://forum.kerbalspaceprogram.com/index.php?/topic/25343-cupcakes-dropship-dealership/)

[AeroGav]

Well now, it turns out I've been using, (or rather, not using ) this part wrong all this time.

The three outer attach nodes are for 1.25m parts.  Leave them empty, or attach something with a diameter that is not 1.25m,  and you get a big open node drag penalty x 3.

The centre attach node is for 2.5m parts, and likewise must be occupied or a gargantuan penalty is applied.

The problem is its appearance gives rise to the commonsense notion that it's designed for 3 x 1.25m parts or a single 2.5m engine, NOT both at the same time, since there isn't possibly room for all those items to fit without overlapping.    So, a node gets left naked and terrible drag ensues.

I did a really simple test with this part.   Stuck it to the back of a mk3 cockpit, with a pair of Thuds mounted radially to the cockpit.  Enable Hack Gravity and Infinite Propellant, accelerate to 250m/s just above the runway with prograde assist set on the SAS.

With a 2.5m rocket nose cone mounted to the central node, and the three 1.25m left uncovered, we get 56kn drag.

When I add 1.25m nose cones to each of the small attach nodes as well, the mk3 engine mount has just 9kn drag, though of course the cones are visually clipping into each other.

I am now rethinking my attitude to mk3 spaceplanes.   Is there a good combination of 1.25m and 2.5m powerplants you'd want to use this way?  Wouldn't any cargo design still suffer from CG problems if you put all your engines right at the back?

[fourfa]

Wow, great discovery and should have been obvious now that you say it. Going to try the usual designs clipped around a 2.5m nose cone. I always scratched my head at how hard it was to get a STS-looking ship to SSTO