This particle system was designed using three emitters that are briefly discussed below: Combustion emitter, Smoke emitter and Heat emitter.
Jet Exhaust Particle System: Emitter components
Combustion
Combustion is achieved by creating a GPU Sprites Emitter with 500 particles that are put through a vector field. This vector field is designed to push the particles in a jet stream like motion. Further, the vector field is set up as local vector field with a constant 'intensity' value and a 'tightness' value of zero, meaning that it only influences the particle's velocity (as opposed to completely taking over its velocity). Therefore, the particles need to have an initial velocity when spawned. The velocity cone module is used to spawn the particles with an initial velocity along a cone with a small angle aperture that is always pointing downwards. The angle of the combustion cone is fixed but the velocity distribution is set up to be a 'Distribution Float Particle Parameter' in order to be able to control this parameter from code or blueprints. Exposing this parameter to code or blueprints allows for controlling how strong the overall combustion effect looks like and, in this particular case, its hooked to the strength of the thrusters. Which is in turn controlled by pulling the gamepad left thumbstick up and down.
It is worth mention that an alternative way to controlling the overall strength of the combustion effect could have been set up by using a global vector field instead. This means that is up to the blueprint/code to define and own the vector field allowing it to modify the vector field intensity dynamically (instead of modifying the velocity cone's velocity parameter).
A color over life module is used to create the blue to red gradient effect. Also the size over life effect allows for the sprite to grow twice its size as it ages.
Lastly, another key module to achieve this combustion effect is to use a subimage index module (sub UV). First of all, this subuv module allows for a sprite to play a sprite animation. In this case the explosion sprite atlas shown below was used.
It is worth mention that an alternative way to controlling the overall strength of the combustion effect could have been set up by using a global vector field instead. This means that is up to the blueprint/code to define and own the vector field allowing it to modify the vector field intensity dynamically (instead of modifying the velocity cone's velocity parameter).
A color over life module is used to create the blue to red gradient effect. Also the size over life effect allows for the sprite to grow twice its size as it ages.
Lastly, another key module to achieve this combustion effect is to use a subimage index module (sub UV). First of all, this subuv module allows for a sprite to play a sprite animation. In this case the explosion sprite atlas shown below was used.
Explosion Sprite Atlas
It should be noted that there are two ways to achieve sprite animations via subuvs: One option is to create a custom material that uses time to programmatically sweep the uvs horizontally and vertically across the sprite atlas. Another simpler approach, which was used here, is to create a custom material and just use a particle subuv expression. This expression automatically does the uvs horizontal and vertical sweeps (cascade tells it how many sub images there are horizontally and vertically). All there is left to do when using this expression is to provide the sprite atlas texture. When using this approach the subimage index module needs to be created in order to define how many subimages are to be used. Moreover, the amount of subimages horizontally and vertically is defined through the subuv section parameters in the 'required' module.
Finally, a couple comments regarding collision. I found that GPU collision only works when the option 'Use Local Space' is disabled in the required module. According to Unreal Forums this is because "collision on GPU particles calls from Scene Depth and cannot function in local space". However, disabling this local space option causes the particle effect to have a trailing effect when flying and I didn't like that. Hence I disabled collisions on this combustion emitter (The cascade screenshot may not reflect that tho).
Finally, a couple comments regarding collision. I found that GPU collision only works when the option 'Use Local Space' is disabled in the required module. According to Unreal Forums this is because "collision on GPU particles calls from Scene Depth and cannot function in local space". However, disabling this local space option causes the particle effect to have a trailing effect when flying and I didn't like that. Hence I disabled collisions on this combustion emitter (The cascade screenshot may not reflect that tho).
Smoke
Originally the smoke emitter was simply meant to be a simple CPU sprite emitter to simulate spark collisions when the jet exhaust got too close to the ground. This was meant to overcome the limitation of the GPU sprites from above not supporting collisions when dealing with local space. However, I couldn't get the sparks to look natural enough and eventually this module evolved into smoke. The particles in this emitter are simply shot down with an initial velocity and made to collide with any static actor in the world. A maximum of two collisions per particle was chosen and a damping factor vector that makes the particles (smoke) jump in an upward fashion when colliding. To create the sprite material a smoke alpha texture was used in a translucent material configuration. As usual, the particle color modulates the material emissive color as well as the opacity. Hence, to create smooth smoke a color over life was chosen where smoke is born light gray and dies black. Also, it is born almost translucent just to become fully opaque halfway and die fully translucent again. Finally, this is alpha texture used for the smoke sprite.
Smoke Alpha Texture
Heat
The heat effect is mainly achieved by the sprite material. This material is set up as a lit translucent surface and the main thing about it is its refraction variation. The refraction variation is being driven by a lerp module that interpolates between 1.0 and 1.6 for the refraction index input parameter in the shader. Furthermore, this lerp is driven by a modulation between the particle's color and a noise texure being panned. Finally, the normal of this material is driven by two normal maps that are panned and then added to each other in order to create random motion. With the material set up, the emitter is setup to spawn particles on the surface of a sphere centered around the combustion emitter with an initial velocity pointing up (heat rises). A drag module helps slow down the particles as they mature and grow in size. Finally, the scale color module allows for the emitter to communicate to the material to increase that modulation factor driving the refraction interpolation.
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