Jose...

not to be too much of an anal nitpicker, but actually, props and
rockets DO work in the same manner...

The Rocket takes a small amount of matter and throws it at great speed
in X direction, getting and equal and opposite reaction in the opposite
direction Y... The prop grabs a large amount of air and throws it in X
direction, getting an equal and opposite reaction in Y... The principal
is the EXACT same.

Its the same principal as aircraft flying - while the common perception
is that aircraft fly because of pressure differential, 'really' they're
flying solely because they're imparting a downward force on the air
that is exactly equal the downward force caused by gravity- that is,
they're forcing exactly enough air downward to accelerate themselves
upward at 1 G...

The simple reality is that these are all various ways of stating the
EXACT same thing (and fundamentally, they all cancel out if you go back
down to the equasions)... Saying that an Aircraft flies via pressure
differential vs. flies via forcing air downward... or saying that a
prop or rocket flies via the same too manners (one might suggest that a
rocket flies with the same pressure differential as an aircraft, just
centered in a MUCH smaller area (inside the bell vs. outside, rather
than in front of the prop vs behind)... in the end, its the same net
result- mass gets forced backwards, I get forced forward... life is
good.


On Dec 13, 8:18 am, Jose wrote:
The thrust of the engine is not against the air. It generates
thrust as a Newtonian reaction to the prop moving air back, not
"pushing on other air." A rocket in space has nothing to push against,
yet it generates the same thrust as it did in the atmosphere.Actually, it's not that way. (but read carefully)

The thrust of a propeller engine is created when the propeller (an
airfoil) creates a high pressure area behind and a low pressure area in
front of the prop, as it pushes air back. The prop is pushing against
the air in order to do this. The air is constantly trying to get out of
the way, but it is not entirely successful, which leads to the pressure
differences. There's nothing funamentally wrong in saying that the
airplane pushes against the air to move forward. The prop (a part of
the airplane) is doing the pushing.

Rockets are different. The tail of fire coming out of the rocket does
push against the air (push the air out of the way to make room for the
fire), but it is =not= part of the rocket. If there were no air to
"push against", the rocket would work just as well, for that reason.
Where the rocket gets its thrust is the tail of fire pushing (the other
way) against the engine bell of the rocket itself. The rocket is
pushing against the fire, in essence. The fire is =not= part of the rocket.

Both cases can be viewed in the newtonian "action/reaction" paradigm,
but something has to push against something else in order to get the
thrust to happen in the first place. In a plane, the propeller pushes
against the air (to make the air go backwards fast and create thrust. In
a rocket, the engine bell pushes against the tail fire (pushing the fire
out, and the rocket forward). I suppose it may be clearer to say that
the expanding gasses of the tail fire push against the engine bell, but
the two are equivalent.

Although the expanding rocket gasses do push the air out of the way,
that doesn't help the rocket in any meaningful way.

Jose
--
"There are 3 secrets to the perfect landing. Unfortunately, nobody knows
what they are." - (mike).
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