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Titan : Mach 1 you said ?



 
 
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  #1  
Old January 18th 05, 06:00 PM
Thierry
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Default Titan : Mach 1 you said ?

Hi,

Have you recently checked all the litterature speaking about the speed at
which Huygens plunged into Titan atmosphere.
Don't have you noted what I call another "mistake of style"... ?
In fact we all know that on earth the sound speed is about 331 m/s at 0C
(or in imperial units 742 mi/h at 32F)

On Titan, mostly filled with nitrogen ( 80%) at -200C, the sound speed is
only 174 m/s. Mach 1 is thus reached at 627.6 km/h instead of 1194 km/h like
on earth.
But it is paradoxal that all pre-flight sheets dealing with this matter
written by Cassini teams kept some kind of terrestrial value (not accurate
at all).
I can understand that it is maybe to make comparisons easier, but one more
time speaking of Mach 1.5 and saying that it is 400 m/s as I read in some
PDF is a mistake because it is only 261 m/s (and even not 400 but 497 m/s on
earth or only 257 m/s at -200C).

To help you here is the converter :
in US/UK : http://www.cactus2000.de/uk/unit/massmac.shtml
in french : http://www.cactus2000.de/fr/unit/massmac.shtml

Thierry
http://www.astrosurf.com/lombry/titan-brumes.htm


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  #2  
Old January 18th 05, 06:14 PM
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I suspect that it really doesn't matter. Most of the (re)entry is so
far beyond Mach 1.0 that the energy equations still denote how well the
heat shield must (have) work(ed)

  #3  
Old January 18th 05, 09:41 PM
md
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"Thierry" . wrote in message ...
Hi,

Have you recently checked all the litterature speaking about the speed at
which Huygens plunged into Titan atmosphere.
Don't have you noted what I call another "mistake of style"... ?
In fact we all know that on earth the sound speed is about 331 m/s at 0C
(or in imperial units 742 mi/h at 32F)

On Titan, mostly filled with nitrogen ( 80%) at -200C, the sound speed is
only 174 m/s. Mach 1 is thus reached at 627.6 km/h instead of 1194 km/h like
on earth.


I think Mach 1 is defined as the sound of speed at sea level at 0 deg C. Implicitly we mean "on
earth".
Mach 1 is therefore an absolute number, and the same everywhere in the universe.
--
md
10" LX200GPS-SMT
ETX105
www.xs4all.nl/~martlian


  #4  
Old January 18th 05, 10:14 PM
Chris L Peterson
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On Tue, 18 Jan 2005 21:41:16 +0100, "md" not given to avoid spam wrote:

I think Mach 1 is defined as the sound of speed at sea level at 0 deg C. Implicitly we mean "on
earth".
Mach 1 is therefore an absolute number, and the same everywhere in the universe.


That would not be a useful definition. The Mach number is defined as the ratio
of actual speed to the speed of sound in a given medium. In the case of aircraft
on Earth, the actual speed of Mach 1 is a function primarily of altitude. Any
article on Titan that uses Mach 1 as a synonym for the speed of sound at sea
level on the Earth is clearly using the term incorrectly.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #5  
Old January 18th 05, 10:50 PM
Brian Tung
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md wrote:
I think Mach 1 is defined as the sound of speed at sea level at 0 deg C.
Implicitly we mean "on earth". Mach 1 is therefore an absolute number,
and the same everywhere in the universe.


I wondered about that, so I looked it up. As far as I can tell, Mach 1
is when an object travels as fast as sound *in that medium*. Presumably,
this is useful because interesting turbulence phenomena show up at that
speed, whatever it might be in that medium under those conditions. If
Mach 1 were defined absolutely, as you suggest, Mach 1 would no longer
be a convenient way of speaking of the speed at which those phenomena
appear.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt
  #6  
Old January 18th 05, 11:39 PM
md
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"Brian Tung" wrote in message ...
md wrote:
I think Mach 1 is defined as the sound of speed at sea level at 0 deg C.
Implicitly we mean "on earth". Mach 1 is therefore an absolute number,
and the same everywhere in the universe.


I wondered about that, so I looked it up. As far as I can tell, Mach 1
is when an object travels as fast as sound *in that medium*. Presumably,
this is useful because interesting turbulence phenomena show up at that
speed, whatever it might be in that medium under those conditions. If
Mach 1 were defined absolutely, as you suggest, Mach 1 would no longer
be a convenient way of speaking of the speed at which those phenomena
appear.


thanks brian, I did not know that. I thought Mach1 was just an absolute number, but now I see
that's wrong.
--
md
10" LX200GPS-SMT
ETX105
www.xs4all.nl/~martlian


  #7  
Old January 19th 05, 04:46 AM
Algomeysa2
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"Thierry" . wrote in message ...
Hi,

Have you recently checked all the litterature speaking about the speed at
which Huygens plunged into Titan atmosphere.
Don't have you noted what I call another "mistake of style"... ?


If a spaceprobe makes a sonic boom in Titan's atmosphere and there's nobody
around to hear it, does it make a sound?

I'd say...you need to relax.

Mach 1 on Earth is 1000 feet/second.

When they were describing how fast this probe will be moving through Titan's
atmosphere, they were using a term so people would get the idea.

Let Titanides worry about how fast their sound is, so that when Titanide
children count the seconds from the lightning to the thunder, they know how
far away it is.....


  #8  
Old January 19th 05, 05:18 AM
Chris L Peterson
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On Wed, 19 Jan 2005 03:46:22 GMT, "Algomeysa2"
wrote:

I'd say...you need to relax.

Mach 1 on Earth is 1000 feet/second.


Or so- it varies by more than 10% over the range that aircraft fly. I'd agree
with Thierry here- the usage is just plain wrong, and it's fair to call them on
it. If they said "faster than the speed of sound on Earth" or something similar,
I'd say fine (that isn't real precise, but it's okay with a popular science
article). But "Mach" has a precise meaning, and its usage in the article makes
things more confusing, not less.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #9  
Old January 19th 05, 06:12 AM
Brian Tung
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Chris L Peterson wrote:
Or so- it varies by more than 10% over the range that aircraft fly. I'd
agree with Thierry here- the usage is just plain wrong, and it's fair
to call them on it. If they said "faster than the speed of sound on
Earth" or something similar, I'd say fine (that isn't real precise, but
it's okay with a popular science article). But "Mach" has a precise
meaning, and its usage in the article makes things more confusing, not
less.


If all they wanted was to convey the high speed, all they had to do was
use 750 miles per hour (or whatever it really was). Most people will
understand that that's fast, and if they want, they can say "about the
speed of a supersonic jet."

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt
  #10  
Old January 19th 05, 11:30 PM
Thierry
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"Brian Tung" wrote in message
...
Chris L Peterson wrote:
Or so- it varies by more than 10% over the range that aircraft fly. I'd
agree with Thierry here- the usage is just plain wrong, and it's fair
to call them on it. If they said "faster than the speed of sound on
Earth" or something similar, I'd say fine (that isn't real precise, but
it's okay with a popular science article). But "Mach" has a precise
meaning, and its usage in the article makes things more confusing, not
less.


If all they wanted was to convey the high speed, all they had to do was
use 750 miles per hour (or whatever it really was). Most people will
understand that that's fast, and if they want, they can say "about the
speed of a supersonic jet."


Hi,

Of course that everybody undertood what NASA meant, and most even don't
noticed this error.
But such sentences are published in official publications signed by two or
more investiogators of cassini team and released as PDF on various websites
dealing with the Cassini-Hygens missions. Those text are usually addressed
to advanced readers.

If they can speak technics with many details, most requesting a deep
knowledge in the field (e.g.they speak of correlator and bandwidth that need
some knowledge in radioastronomy or electronics, fusion and boiling point of
methan, that request some knowledge of chemistry, etc) they can also take
care to their vocabulary and use the right word at the right time.
Mach 1.5 on earth or on Titan are not exactly the same for a specialist.

Found such errors in documentation checked by several chief investigators at
NASA is not acceptable.

Thierry
http://www.astrosurf.com/lombry/titan-brumes.htm



Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt



 




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