Sorry for late reply
For the particle sim, basically your built-in cloud or smoke textures and built-in fire textures provided are the way to go.
You'll want to go into the thread here: http://hitfilm.com/forum/discussion/4699/learning-hitfilm-tutorials-from-basics-to-advanced/p1
Look at the anatomy of a particle sim tutorials, and look at the homemade fireworks tutorials.
Basically you'll combine multiple emitters--for your fire you'll want an emitter with orange particle (using fire textures?) in an ADD blend mode. You'll have another emitter with dark grey or brown cloud/smoke textures in NORMAL blend mode. This will help the smoke occlude the fire. You'll need a third emitter that's using spark/ember textures in an ADD mode. For this emitter some forces (turbulence) are helpful. Finally, you're want a fourth emitter kicking out mobile emitters (this is where the fireworks tutorials come in) using an ember texture for the mobile emitter itself and cloud textures for what the mobile emitter spawns. This is your chunks of flaming debris. You might look at the LIFETIME panel for each particle system so your fire starts very yellow and fades to orange and red as it ages. Make your smoke start as a lighter grey and darken later. Also the lifetime panel will let you adjust the transparency of your elements so they fade away and not pop off. All these emitters can be in the same particle system layer.
Another thing to look at is the "Creating a clone army" tutorial--this covers setting up the particle sim to use a video clip as a particle texture. You could then use a stock fire or explosion element as a particle texture to make really big explosions.
Once you get a good explosion set up, you can save it as a preset for later re-use!
Here are some example from my own work.
This is the third shot I ever did in Hitfilm--most of the explosion is particle sim (there's one stock element explosion in there as well). The Particle Sim is making most of the fire and smoke as well as the flaming bits of debris bouncing off the ground (deflectors and physics). I also added in an instance of the Fire Explosion preset and there are two instances of the Sci-Fi shockwave preset colored brown to be the billowing dust.
This second example is, again, mostly the particle sim (actually, the particle sim is making the ships, thier thrusters, the asteroids, the weapons fire AND the explosions....). In this case I am using a stock explosion element as the texture source for the main fireball. You can see other mobile emitters making flaming debris, etc. There's also an instance of the Fire Explosion preset and a Sci-Fi shockwave preset.
This final example is 100% particle sim with built in textures. As described above there are four emitters--a fire emitter, a smoke emitter, a sparks emitter and a debris emitter (kicking out mobile emitters).
So, once you delve into it, you can see the particle sim can really blow things up!
Hope this is helpful.
THANKS! Just for curiosity... how heavy were your shots when compositing and rendering?
Hmmmmm.... Let's see that shuttle crash shot rendered in about three hours, the Dalek Shot rendered in about 11 minutes and the Strike wing shot rendered in about 12 hours.
My machine is a four year old [email protected] Ghz with an NVidia 580 GTXm, and 16GB RAM. It's fairly slow by current standards.
The Strike Wing shot in particular is very complex with tens of thousands of particles, combining stills, 3D models and video as particle sources and multiple forces and deflectors.
Still it was great!
You don't need a fast PC to do this stuff!(Although it would be useful)