Terminal Velocity

A thread of 2 messages, 20 July 1992 to 20 July 1992, in the order they were sent.

#4477 Re: Terminal Velocity

Mark F. Cook · 20 July 1992 · reply to #4475

Scott sez:

> Mark sez:
> }The accepted terminal velocity for a falling human body is
> }around 120 MPH (200 kph).
>
> While high drag jump suits allow lower speeds. (90Mph)
>
> Hmm... those figures still seem fairly close.  What do you mean
> by 'accelted terminal velocity'?  Is that a person wearing a
> streamlined suit in a tucked down position?

Yup.  That's essentially the case.  When a skydiver goes for max. rate
of descent, they fold their hands flat against their sides (hips), spread
their legs at approximately 20-30 degrees (for stabilization), and arch
their backs and necks (basically facing into the wind).  This will offer
a head-downward angle of attack while presenting the smallest cross-section
of resistance.  I'm sure everybody has seen it before in the movies.

>                                              If it is, then I
> can see the 200kph speed being relevant for grav belt travel.
> After all, a person in a grav belt is trying to travel as fast
> as possible, and a parachutist is trying to travel as slowly as
> possible.  (To allow for better aim, and safety)

I can think of a number of arbitrary (that is, not required by physical
limits) reasons for the 300 khp limit.  Unaided respiration becomes very
difficult at those speeds.  Windburn (or actual heating) might start to
be a factor.  And then there's the "better aim and safety" that you mention.
Granted, none of these reasons present insurmountable obstacles.  A vacc-
suit with a really hot sensor suite would eliminate all those excuses. :^)

Regarding Capt. Kittinger and his high-altitude jump:

> However, Mark does say that at his highest velocity, he did have
> a 6 foot drag 'shute out which may have kept him from actually
> hitting Mach 1.  Thus, it seems to me that it could indeed be
> *theoretically* possible for a person in a grav belt (and a thin
> atmosphere) to actually go supersonic.

Maybe so.  In fact, maybe he *did* go supersonic at a somewhat lower
altitude.  If you check the Speed-of-Sound vs. Altitude graph in the
CRC Handbook of Chemistry and Physics (pg. F-169 in the 51st edition),
you'll find that the speed of sound is all over the spectrum depending
on your altitude.  Kittinger jumped at 102,800 ft., where the speed of
sound is about 307 m/sec. (or about 690 mph).  At around 82,500 ft.,
the speed of sound drops down to about 295 m/sec. (or approximately
664 mph.

Hmmm.  Well, according to those numbers, and my earlier research on
Kittinger's jump, he still didn't break the sound barrier, since his
documented top speed was 614 mph, which translates to about 273 m/sec.
According to the CRC, the speed of sound bottoms out at around 282
m/sec., at altitudes above 75 kilometers.

Later,
	- Mark C.

#4475 Terminal Velocity

[unnamed sender] · 20 July 1992

Mark sez:
}The accepted terminal velocity for a falling human body is
}around 120 MPH (200 kph).

While high drag jump suits allow lower speeds. (90Mph)

Hmm... those figures still seem fairly close.  What do you mean
by 'accelted terminal velocity'?  Is that a person wearing a
streamlined suit in a tucked down position?  If it is, then I
can see the 200kph speed being relevant for grav belt travel.
After all, a person in a grav belt is trying to travel as fast
as possible, and a parachutist is trying to travel as slowly as
possible.  (To allow for better aim, and safety)

The documentary I was referring to was an interview with the
guy Mark spoke of however (Capt. Kittinger)  In the interview he
claimed that he had indeed gone supersonic.  Now, Mark's source
of data sounds much more precise than the rather sensationalistic
documentary I saw, but I find the discrepancy rather disturbing.
After all, there was the guy telling how he went supersonic,
without an aircraft.  Now, Mark sez he only hit Mach 0.9.  :-(

However, Mark does say that at his highest velocity, he did have
a 6 foot drag 'shute out which may have kept him from actually
hitting Mach 1.  Thus, it seems to me that it could indeed be
*theoretically* possible for a person in a grav belt (and a thin
atmosphere) to actually go supersonic.

Catie:
Drag is a non linear function as the velocity of travel
approaches the speed of sound.  I'm no aerodynamics jock, but if
I recall, the non-linear terms build up around the speed of sound
and then drop off.  The function is discontinuous at the speed of
sound.

Thus, you need a certain amount of thrust to get to the speed of
sound and then once beyond it, less thrust is needed to maintain
speed.

Halliday and Resnick are dealing with nice sophomore physics
students who don't have to worry about supersonic stuff.  :-)

Scott "2G" Kellogg
(Then again, *MY* fave freefall bit was Col. Coon in
_Dr. Strangelove_!  Talk about *TERMINAL* Velocity...
WAAAHOOOOO!!! :-)