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Michael Parks
09-03-2009, 09:36 AM
This one is for folks with a good grasp of physics, I think.

Given a moving object with the following attributes:

Length: about 8' long
Circumference: 1.5'
Altitude: 8,000 feet
Air speed: 3,000 mph
Surface temperature of object: 65F
Ambient air temperature: 30F
Propulsion: none (read: no heat exhaust, etc)
Notes/parameters: object is conical shaped at its front. Surface temperature of object is constant, not affected by friction of moving through air at speed (that is not to say the air being displaced wouldn't heat up by the act of being disturbed that rapidly by such a fast moving object).

My question is... would this object leave any kind of visual contrail in the sky? Why or why not?

TIA..!

waylander
09-03-2009, 01:13 PM
Con trails are trails of ice crystals left by the exhaust of engines that produce water as part of their combustion process. No propulsion = no con trail.
The only thing this object will leave is turbulence in the air which you can't see.

This object sounds pretty similar to a large artillery shell and those don't leave any visible trail (unless they're designed to)

Smiling Ted
09-03-2009, 06:02 PM
It would leave a trail if it traveled through clouds.
Since it's traveling at about Mach 4, it would also probably leave a hell of a sonic boom, and it might form a Prandtl–Glauert singularity cloud if it's traveling through sufficiently moist air.

Michael Davis
09-07-2009, 07:24 PM
At 3000 your traveling between mach 4-6 (depending on a ton of conditions). but I don't get your "65 degree" surface temp. At that speed the friction with air molecules would create very high temperatures and for most standard metals you would ablate (burn off) the outer layer of the surface. On the other hand, if it was made of high temp materials (like titanium) it might only glow at the tips. Remember the SR71 operated at high altitudes (>50000 feet) where the air is thin and even then the leading edges were extremely hot. At 8000 feet the air is still really thick. If there was a high degree of moisture in the air, its conceivable the moisture contacting the hot edges might vaporize and leave a slight trail. Depends on a ton of conditions, but it is possible at that altitude and speed.

blacbird
09-07-2009, 10:51 PM
MDavis is correct. Your assumption of "constant surface temperature" under the conditions you describe is simply not realistic physics.

caw

Michael Parks
09-08-2009, 08:28 AM
MDavis is correct. Your assumption of "constant surface temperature" under the conditions you describe is simply not realistic physics.

caw

Precisely so, which is why I put it forth. In this scifi/fantasy story, the constant surface temperature is an artificially created bubble of space (air). The laws of physics are in fact being violated to sustain this small area of "comfortable" air. The violation is immutable (meaning no external reaction/conditions can destabilize the pocket of air) which led me to question the effect of that "wall" of temperature controlled air as it burrowed through normal space. My first vision was the contrail, which fit very well with a previous, unrelated scene.

That normal space, described above, must yield to the artificial body, but with what effect? 30F air, suddenly pushed aside as it comes into contact with the the conical head of air and then its body, 65F... seems something should happen there.

I've written it so that the contrail forms as a result of the warm bubble's leakage or trails, much like the exhaust of a plane. However, since contrails usually form at 26,000 feet or above, and because of the water vapor as a byproduct of combustion. This bubble would have no such water vapor.

Combined, this scenario yields no contrail. My question then becomes, is it a detail worth worrying over?

It is addressed in this sentence:
"The warm bubble created ... leaked like exhaust into the cold air, forming the contrail.."

To me, it's a momentary visual used to tie back to a previous scene only. The sentence above could work for most readers, I'm thinking. If not, then I am considering employing clouds instead, where the passage through them creates "distrails", a reminder of the previous scene's contrail, which could serve sufficiently, I suppose.

A lot of detailed thought around a scene, yes? But the tie-back really works, as the previous scene was in a dream, and this scene is now real life.

Thanks for the review and thoughts.

blacbird
09-08-2009, 09:22 AM
Precisely so, which is why I put it forth. In this scifi/fantasy story, the constant surface temperature is an artificially created bubble of space (air). The laws of physics are in fact being violated to sustain this small area of "comfortable" air. The violation is immutable (meaning no external reaction/conditions can destabilize the pocket of air) which led me to question the effect of that "wall" of temperature controlled air as it burrowed through normal space. My first vision was the contrail, which fit very well with a previous, unrelated scene.

In other words, you're invoking "magic" essentially, which means you can do anything you want in writing fiction. Which brings up the question: Why did you pose your question in the first place?

caw

Michael Parks
09-08-2009, 09:31 AM
I posed it because the "magic" isn't dictating the laws of physics beyond the protective bubble. In trying to understand/predict the effects of the "magic", I turned to the Collective to see what would emerge.

It's not magic in the context of the story, although at that point, the reader isn't sure exactly what is powering the scenario, but are fairly certain it isn't from our world (if that helps any).