> On 12-3-2015 4:12, Alain wrote:

>>> Thomas de Groot <tho### [at] degrootorg> wrote:
>>>> Aliens are reported on a daily basis, in particular on YouTube.
>>>> However,
>>>> and invariably, they are shaky, blurred, indistinct things. Only the
>>>> POV-Ray telescope is able to show alien ships in all their pristine
>>>> splendour coming to us from deep space.
>>>> --
>>>> Thomas
>>>
>>> Hi Thomas,
>>>
>>> that is a very good start indeed, but I see some options for
>>> improvements.
>>> First, the structure looks great, but too many areas are lost in the
>>> shadows.
>>> This seems to be a general problem with space scenes, since one has
>>> only one
>>> light source (star) close by. May be a binary star can solve this. The
>>> orbiter
>>> can still have a different color in this case. The second issue is the
>>> engine
>>> glow. IMO it should have more turbulence at the edges. Otherwise
>>> really great.
>>>
>>> Best regards,
>>> Michael
>>>
>>>
>>
>> I can't think of any source of turbulence in the vacuum of space.
>> Once the gas/plasma leave the engines *realy, realy* fast, each of it's
>> particles goes in a straight line, mostly indepently from the others.
>>
>
> Interesting. No interaction between the particles at all? Hmmm... I
> suppose that would be minimal due to the eject velocity indeed, like you
> say.
>
> So turbulence is another of those things - like wings - which we should
> put in the domain of imagination :-)

I did not say "no interactions at all" but mostly independently, meaning 
very little interactions.
If you looks at the jet from a rocket in the athmosphere, you'll notice 
a central, often cone like and very smooth, part. It often display some 
repeating pattern, like a string of beads or diamonds. It's surrounded 
by a broader, more diffuse jet. On the outside of that outer layer, 
where it interact with the mostly static ambient air, is where all the 
turbulence is generated. As you progress farther from the engine, those 
turbulences progress inward.
In space, you get the inner part forming a long and sharp cone, and the 
outside part is mostly V shaped, also smooth and a lot dimer, that stay 
that way as there is almost no exterior medium to interact with.

In your later image, the exaust looks like the flame of a cheap, low 
presure, blow torch. In several cases, in films and TV shows, it's 
exactly what they used for the "rocket engines"... I've seen some cases 
whith the flame curving up for a ship going in a straight line.
If a rocket engine had an exaust like that, it would shake itself loose 
and apart very shortly, or need to be so rugged that it's weight would 
be excessive.


Alain

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