Okay, so what I am doing is using Fire to create an engine thrust flame coming out of the back of my space ship.

It looks great in still renders (plus a bulb for that nifty lens flare).

Now, when animating I want a bit of movement in the flame, but nothing drastic.  I've tried playing with the Completion setting, but even at low setting it looks like the engine is sputtering!  Too much randomness.

With no completion added the engine flame looks too static, no movement.

Any ideas?  Should I abandon using completion and maybe use another parameter?  I figure someone has to have come across this.  I can't be the only one who rushes out and does spaceship animations first thing when learning a new 3D program.  (I did the same with 3D Studio Max 12 years ago lol.)

You need to use a very low value in the Upward Speed slider, but still need to use the completion.

If you want it to move over a long period of time but not move by much, you might try setting your completion value to a small amount (say 10% to 20%) in the final frame of the animation, so that you have one keyframe at the end and one at the beginning. Then you can change the tweener between the keyframes to Oscillating, and adjust the number of oscillations until you get the look your going for.

A very simple trick is put a Spin Modifier at your flame. Rotating the Fire primitive at the right axis could create your desired effect.

I've played with a funky way of doing exhaust animation that you could try. For the duration of the animation in your timeline, set completion from 0 to 100 over the period. But instead of leaving it linear in the timeline progression, set it to random. Then adjust the settings associated with that so you get a lot of spikes. I've had a neat result from that which looks like a fast moving jet exhaust. (But perhaps this may look like the sputtering you're talking of?) Of course it doesn't hurt to play with some other stuff in the fire settings too. (They affect the solidity and flicker more than completion does. From what I can tell, completion just provides a seed value that the other variables build on, so they can progress over time.) Other than random for completion, you can also set the timeline progression to one that looks like a sawtooth pattern. (I forgot the exact one, but I suspect its a sub-setting under the oscillation mentioned already.) You can adjust the speed by changing the number of "sawteeth" during the period in the timeline.

Also it might not hurt to stack some fire on top of fire, then you can get it to look more consistent over time. Just remember to make each one a master rather than an instance so they aren't in sync. I believe the fire is additive in appearance, so it's likely you'll do a bit of other adjusting so it's not too solid looking.

Quote - I've played with a funky way of doing exhaust animation that you could try...

Sounds cool......do you have a movie we can see? 

I'll have to re-do from scratch, since I don't save every experiment. But yeah, I'll make a YouTube example if that's what you want. Might be a while though.

Attached Link: Simple Rocket Exhaust in Carrara