Unity has a trig limit on parts.

[PB666]

Having been modding silently in the background here for several months. My latest creation is a RL10-b1 engine based on 2 photographs I have found online. Its made to scale and has the same performance characteristics.

RL10-b1 is a big engine for a deep space thruster. its 2.24 meter nozzle and 4.11 meters long and produces 110 kn of thrust at 4600 m/s exhaust velocity an amazing weight of 277 kilograms.  The concept of this engine is that if you are using INRU to resource your engines on things like dirty ice balls, comets or asteroid belt objects the end products are going to be hydrogen and oxygen. This is the engine to have.

http://www.aerospaceguide.net/rocketengines/RL10B-2.html

This version has the extended nozzle

I have alot of detail in the nozzle, composed of conic sections with 96 sides per section, 192 trigs per section and interior and exterior. Plus the aluminum rings that hold the radiators on the outside. so each engine has about 4 conic section profiles so this is around 3000 trigs, plus the trigs for connection, piping going in and out of the engine.

The photo shows irregularity in the lip of the bell, I used this to my advantage so that 24 of the pieces form the exterior limit. This lowered the collidor trig count considerable.

All and all the RL10-b engine is more like anything in the stock game, and it works. 

But I need a base engine for a big off-world base lander. Blender has a Edit tool called duplicate that allows on to make exact copy of center engine to six equilateral hexagon position engines. Kept my collder mesh the count low be enshrouding the array so that only a few point on the engine stick out.

So I figured I would make a three engine combo and seven engine combo. The three engine combo worked.

but the seven engine combo gave me this when I imported it into Unity.

Meshes may not have more than 65534 vertices or triangles at the moment. Mesh 'Circle' will be split into 2 parts: 'Circle_MeshPart0', 'Circle_MeshPart1'.
UnityEditor.DockArea:OnGUI()

Uh-oh, I broke Unity.

No meshes were made.  Apparently unity could not calculate the collider mesh, too many trigs.

[SpannerMonkey(smce)]

Hi thats a known unity gotcha with high poly (lots of faces) imports .  though I'm surprised it did not create the mesh  divided into large chunks.   Another way to do this would be to duplicate 1 engine nozzle in unity and save it as a prefab, (simply drag the mesh into the assets folder to create a prefab)  then using a little bit of easy part manipulation you can reuse the prefab nozzle  as many times as you want without needing to import larger and larger models or lower poly counts .   It worth noting that your 1 part has more geometry than many, completed stock. ready for launch rocket ships