Velocity Reviews > Physics question

# Physics question

->RINGO-
Guest
Posts: n/a

 12-04-2003
I understand (I think) how rockets engines propel the rocket when there is
air, by 'pushing' against it. How do the engines work in space, where there
are no air molecules to push against? Isn't space 'empty'?.

Just looking for a very basic answer, nothing too technical.

Thanks

°Mike°
Guest
Posts: n/a

 12-04-2003
It's got nothing to do with pushing against air. It's to
do with Newton's third law of physics - "Every Action has
an Equal and Opposite Reaction?" - which means that
something pushing matter behind itself will move in a
forward motion.

On Wed, 3 Dec 2003 22:33:28 -0500, in
<(E-Mail Removed)>
->RINGO-< scrawled:

>I understand (I think) how rockets engines propel the rocket when there is
>air, by 'pushing' against it. How do the engines work in space, where there
>are no air molecules to push against? Isn't space 'empty'?.
>
>Just looking for a very basic answer, nothing too technical.
>
>Thanks
>

--
Basic computer maintenance
http://uk.geocities.com/personel44/maintenance.html

Liz
Guest
Posts: n/a

 12-04-2003
"->RINGO-<" wrote:

> I understand (I think) how rockets engines propel the rocket when there is
> air, by 'pushing' against it. How do the engines work in space, where there
> are no air molecules to push against? Isn't space 'empty'?
>
> Just looking for a very basic answer, nothing too technical.

How jet propulsion engines work
http://tinyurl.com/xne4

Ed Morgan
Guest
Posts: n/a

 12-04-2003
The rocket thrusters they use to maneuver in space do expel mass. In a
vacuum (no wind resistance) it doesn't take much force (change of momentum)
to change the attitude of the vehicle.

I worked on a project 30 years ago where a rapid fire cannon was mounted
below the center of gravity line of an aircraft. The rate of mass leaving
the plane (heavy depleted uranium bullets) caused such a force on the plane
to dip the nose severely. We had to increase the size of the control
surfaces for the fire control system to keep the guns pointed at the target.

Ed

"->RINGO-<" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I understand (I think) how rockets engines propel the rocket when there

is
> air, by 'pushing' against it. How do the engines work in space, where

there
> are no air molecules to push against? Isn't space 'empty'?.
>
> Just looking for a very basic answer, nothing too technical.
>
> Thanks
>
>

gangle
Guest
Posts: n/a

 12-04-2003
http://quest.arc.nasa.gov/space/teac...rinciples.html

"->RINGO-<" wrote
> I understand (I think) how rockets engines propel the rocket when there is
> air, by 'pushing' against it. How do the engines work in space, where there
> are no air molecules to push against? Isn't space 'empty'?.
>
> Just looking for a very basic answer, nothing too technical.
>
> Thanks
>
>

BuffNET Tech Support - MichaelJ
Guest
Posts: n/a

 12-04-2003
Ed Morgan wrote:

> The rocket thrusters they use to maneuver in space do expel mass. In a
> vacuum (no wind resistance) it doesn't take much force (change of momentum)
> to change the attitude of the vehicle.
>
> I worked on a project 30 years ago where a rapid fire cannon was mounted
> below the center of gravity line of an aircraft. The rate of mass leaving
> the plane (heavy depleted uranium bullets) caused such a force on the plane
> to dip the nose severely. We had to increase the size of the control
> surfaces for the fire control system to keep the guns pointed at the target.
>
> Ed

Something tells me I know which project you're talking about. If I'm
right, Thank You for making one of the Butt-Ugliest planes known to
man... My brother flies one, and has been safe through some hostile
airspace.

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(BEHOLD! The power of the BOFH!)

Jimmy Dean
Guest
Posts: n/a

 12-05-2003
On Thu, 04 Dec 2003 04:04:17 GMT, "Ed Morgan"
<(E-Mail Removed)> wrote:

>The rocket thrusters they use to maneuver in space do expel mass. In a
>vacuum (no wind resistance) it doesn't take much force (change of momentum)
>to change the attitude of the vehicle.
>
>I worked on a project 30 years ago where a rapid fire cannon was mounted
>below the center of gravity line of an aircraft. The rate of mass leaving
>the plane (heavy depleted uranium bullets) caused such a force on the plane
>to dip the nose severely. We had to increase the size of the control
>surfaces for the fire control system to keep the guns pointed at the target.
>
>Ed

I believe the A-10 WartHog tankbuster does something like this. The
charge propelling the "bullet" (40 mm calibre?) resembles the old
style milk bottle. A few such projectiles can demolish most tanks.

jd

Guest
Posts: n/a

 12-05-2003
->RINGO-< wrote:

> I understand (I think) how rockets engines propel the rocket when there is

No, you don't.

> air, by 'pushing' against it. How do the engines work in space, where there

No, that's not how...

> are no air molecules to push against? Isn't space 'empty'?.

....or they *wouldn't* work in a vacuum.

****In terms too simple to really be completely accurate*****, but which might
give you a basic grasp:

Imagine a blown up balloon. The pressure inside is pushing equally in
all directions. So it sits there, because the forces in all directions
are self cancelling -- any direction in which the air is pushing on a
side of the baloon, it's pushing equally on the opposite side. It's a
a tug of war with no winner -- no motion. Now, open the neck of the
balloon. The pressure there, is no longer being held back. But there's
pressure in *all* directions, remember? This means that the pressure on the
"front end" of the balloon - opposite from the neck - is no longer being
balanced by the pressure on the neck, which has simply become a stream
of escaping air. So......with no offsetting pressure at the rear, the
pressure on the "front end" pushes the ballon "forward". The air stream
at the neck is not "pushing on the outside air" -- it's simply failing
to offset the "forward" pressure, by leaving the balloon, instead.

For your rocket, just replace the balloon with a hard casing instead
of the rubber, and replace the pressurized air, inside, with a burning
fuel mixture that's trying to expand in all directions. The hole is now
in the rear of the *rocket* (remember the open neck of the flying balloon),
and the "forward" forces of the expansion - not offset by the rearward
pressure - push the rocket forward.

Yes, fellow scientists, I *said* that wasn't the whole story. The
guy thought the rocket was "pushing on air", and he's not going to get
the whole story in a minute's read, so let's trade Scientific American
for *relative enlightenment*, this time.

--
Blinky - - - - - - - - - - - - - - - - - - - - - - - Linux RU 297263
[still no new sig]

Dick M.
Guest
Posts: n/a

 12-05-2003
Nobody has explained where the rocket gets the
air to throw out its nozzle. Of course, in outer
space it can't use air, so it must use some mass
from inside the rocket instead.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Imagine a blown up balloon. The pressure inside is pushing equally in
> all directions. So it sits there, because the forces in all directions
> are self canceling -- any direction in which the air is pushing on a
> side of the balloon, it's pushing equally on the opposite side. It's a
> tug of war with no winner -- no motion. Now, open the neck of the
> balloon. The pressure there, is no longer being held back. But there's
> pressure in *all* directions, remember? This means that the pressure on

the
> "front end" of the balloon - opposite from the neck - is no longer being
> balanced by the pressure on the neck, which has simply become a stream
> of escaping air. So......with no offsetting pressure at the rear, the
> pressure on the "front end" pushes the balloon "forward". The air stream
> at the neck is not "pushing on the outside air" -- it's simply failing
> to offset the "forward" pressure, by leaving the balloon, instead.
>
> For your rocket, just replace the balloon with a hard casing instead
> of the rubber, and replace the pressurized air, inside, with a burning
> fuel mixture that's trying to expand in all directions. The hole is now
> in the rear of the *rocket* (remember the open neck of the flying

balloon),
> and the "forward" forces of the expansion - not offset by the rearward
> pressure - push the rocket forward.
>
> Yes, fellow scientists, I *said* that wasn't the whole story. The
> guy thought the rocket was "pushing on air", and he's not going to get
> the whole story in a minute's read, so let's trade Scientific American
> for *relative enlightenment*, this time.
>
> --
> Blinky - - - - - - - - - - - - - - - - - - - - - - - Linux RU 297263
> [still no new sig]

gangle
Guest
Posts: n/a

 12-05-2003
In a rocket it's not about air. It's about the ACTION of the
burning fuel hitting the rear of the rocket outake, which creates the
REACTION of the rocket being propelled in the opposite
direction with an equal force.

"Dick M." wrote
> Nobody has explained where the rocket gets the
> air to throw out its nozzle. Of course, in outer
> space it can't use air, so it must use some mass
> from inside the rocket instead.
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Imagine a blown up balloon. The pressure inside is pushing equally in
> > all directions. So it sits there, because the forces in all directions
> > are self canceling -- any direction in which the air is pushing on a
> > side of the balloon, it's pushing equally on the opposite side. It's a
> > tug of war with no winner -- no motion. Now, open the neck of the
> > balloon. The pressure there, is no longer being held back. But there's
> > pressure in *all* directions, remember? This means that the pressure on

> the
> > "front end" of the balloon - opposite from the neck - is no longer being
> > balanced by the pressure on the neck, which has simply become a stream
> > of escaping air. So......with no offsetting pressure at the rear, the
> > pressure on the "front end" pushes the balloon "forward". The air stream
> > at the neck is not "pushing on the outside air" -- it's simply failing
> > to offset the "forward" pressure, by leaving the balloon, instead.
> >
> > For your rocket, just replace the balloon with a hard casing instead
> > of the rubber, and replace the pressurized air, inside, with a burning
> > fuel mixture that's trying to expand in all directions. The hole is now
> > in the rear of the *rocket* (remember the open neck of the flying

> balloon),
> > and the "forward" forces of the expansion - not offset by the rearward
> > pressure - push the rocket forward.
> >
> > Yes, fellow scientists, I *said* that wasn't the whole story. The
> > guy thought the rocket was "pushing on air", and he's not going to get
> > the whole story in a minute's read, so let's trade Scientific American
> > for *relative enlightenment*, this time.