# Physics question

Discussion in 'Computer Support' started by ->RINGO-, Dec 4, 2003.

1. ### ->RINGO-Guest

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

->RINGO-, Dec 4, 2003

2. ### °Mike°Guest

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
<>
->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

°Mike°, Dec 4, 2003

3. ### LizGuest

"->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

Liz, Dec 4, 2003
4. ### Ed MorganGuest

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-<" <> wrote in message
news:...
> 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
>
>

Ed Morgan, Dec 4, 2003
5. ### gangleGuest

http://quest.arc.nasa.gov/space/teachers/rockets/principles.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
>
>

gangle, Dec 4, 2003
6. ### BuffNET Tech Support - MichaelJGuest

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.

--
--

BuffNET Technical Support Supervisor
(BEHOLD! The power of the BOFH!)

BuffNET Tech Support - MichaelJ, Dec 4, 2003
7. ### Jimmy DeanGuest

On Thu, 04 Dec 2003 04:04:17 GMT, "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

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

Jimmy Dean, Dec 5, 2003
8. ### Blinky the SharkGuest

->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]

Blinky the Shark, Dec 5, 2003
9. ### Dick M.Guest

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]

Dick M., Dec 5, 2003
10. ### gangleGuest

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.

gangle, Dec 5, 2003
11. ### BuffNET Tech Support - MichaelJGuest

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.
>

It's a chemical reaction - not too dissimilar to a bottle-rocket. You
bring together 2 chemicals that are, seperatly, stable... and WHAMMO!
instant exhaust gasses that push against the rocket casing as it
expands... and therefore pushes the rest of the vehicle.

SEE? I *did* learn something from Star Trek!

--
--

BuffNET Technical Support Supervisor
(BEHOLD! The power of the BOFH!)

BuffNET Tech Support - MichaelJ, Dec 6, 2003
12. ### Blinky the SharkGuest

Dick M. wrote:

> Nobody has explained where the rocket gets the
> air to throw out its nozzle. Of course, in outer

Except that I did explain that. Read what I wrote, below.
Pay attention to, "for your rocket...replace the pressurized
air, inside, with a burning fuel mixture..."

> space it can't use air, so it must use some mass
> from inside the rocket instead.

It's called burning fuel. They carry it in...follow along, here...
fuel tanks.

> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

>> 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]

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

Blinky the Shark, Dec 6, 2003
13. ### Dick M.Guest

The point being that fuel is normally used to heat air,
while in this case the fuel is used as something to
eject from the rocket and make it move.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

"Blinky the Shark" wrote
>
> It's called burning fuel. They carry it in...follow along, here...
> fuel tanks.
>

Dick M., Dec 6, 2003
14. ### docmillGuest

"Dick M." <> wrote in
news:f5fAb.23824\$ZE1.17690@fed1read04:

> The point being that fuel is normally used to heat air,
> while in this case the fuel is used as something to
> eject from the rocket and make it move.
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> "Blinky the Shark" wrote
>>
>> It's called burning fuel. They carry it in...follow along, here...
>> fuel tanks.
>>

>
>

Now you're getting into the Flaming post.

--
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No Lemmings or pseudo-plonkers were amused by this post, but I tried.

docmill, Dec 6, 2003
15. ### Blinky the SharkGuest

Dick M. wrote:

> The point being that fuel is normally used to heat air,
> while in this case the fuel is used as something to
> eject from the rocket and make it move.

Yes: fuel makes it go.

Something in your above statement makes me think you still
believe that the propellant "pushing on the outside air"
is what makes the vehicle move.

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

Blinky the Shark, Dec 6, 2003
16. ### Dick M.Guest

I would think the action of throwing the fuel out
the back end of the rocket is what causes it to
move forward. Action & reaction.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

"Blinky the Shark" wrote
>
> Something in your above statement makes me think you still
> believe that the propellant "pushing on the outside air"
> is what makes the vehicle move.
>
> --
> Blinky - - - - - - - - - - - - - - - - - - - - - - - Linux RU 297263
> [still no new sig]

Dick M., Dec 6, 2003
17. ### gangleGuest

I've already stated that the force of the burning fuel upon
the rear rocket surfaces (not the actual "throwing out" of the fuel)
is the action, thereby causing the equal force of reaction
that moves the whole rocket in the opposite direction
of that of the original force: F1 = F2
<< >>

"Dick M." wrote
> I would think the action of throwing the fuel out
> the back end of the rocket is what causes it to
> move forward. Action & reaction.
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> "Blinky the Shark" wrote
> >
> > Something in your above statement makes me think you still
> > believe that the propellant "pushing on the outside air"
> > is what makes the vehicle move.

>

gangle, Dec 6, 2003
18. ### fkasnerGuest

gangle wrote:
> I've already stated that the force of the burning fuel upon
> the rear rocket surfaces (not the actual "throwing out" of the fuel)
> is the action, thereby causing the equal force of reaction
> that moves the whole rocket in the opposite direction
> of that of the original force: F1 = F2
> << >>
>
> "Dick M." wrote
>
>>I would think the action of throwing the fuel out
>>the back end of the rocket is what causes it to
>>move forward. Action & reaction.
>>
>>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>>"Blinky the Shark" wrote
>>
>>>Something in your above statement makes me think you still
>>>believe that the propellant "pushing on the outside air"
>>>is what makes the vehicle move.

>>

>
>

False! Force is a vector quantity so F1 <> F2 but F1=-F2 .
The chemical reaction (or if no fuel is burned)the physical changes
causes the exhaust gases to exert one of the forces on the body of the
rocket. Or another waay to look at this is the momentum of the system is
zero before igniting the fuel. As the fuel expands relative to the
center of mass of the system is momentum (also a vector) increases in a
direction opposite to the desired direction of motion of the rocket. The
maintenance of constant momentum requires that the rocket acquire an
equal momentum in the opposite direction to the direction that the gases
moved away from the rocket. So the constancy of momentum remains equal
to the starting zero momentum.
FK

fkasner, Dec 6, 2003
19. ### Brian H¹©Guest

gangle said:

> I've already stated that the force of the burning fuel upon
> the rear rocket surfaces (not the actual "throwing out" of the fuel)
> is the action, thereby causing the equal force of reaction
> that moves the whole rocket in the opposite direction
> of that of the original force: F1 = F2
> << >>
>

www.absey-vine.co.uk/~temp/f1.jpg

> "Dick M." wrote
>> I would think the action of throwing the fuel out
>> the back end of the rocket is what causes it to
>> move forward. Action & reaction.
>>
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> "Blinky the Shark" wrote
>>>
>>> Something in your above statement makes me think you still
>>> believe that the propellant "pushing on the outside air"
>>> is what makes the vehicle move.

Brian H¹©, Dec 6, 2003
20. ### Mr. SmileyGuest

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
>
>
>
>
> "->RINGO-<" <> wrote in message
> news:...
>> 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

Uranium bullets? Come on!

--
the abominable toner

computer maintenance links & tips on:
http://www.hal-pc.org/~toner27/

Mr. Smiley, Dec 10, 2003

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