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# A Physics Test

Fri 23 Dec, 2022 12:11 am
There is so much nonsensical posting going on with the tag of "physics," I thought I might inject a tiny bit of real physics.

Does anyone know how a rocket works? What is the underlying functionality all rockets have in common, regardless of the details of their construction?

I promise, it's not a trick question, nor am I a stickler for being correct in every detail. I will correct wrong answers and stop when the first person gets it more or less right.

I have talked to very smart people who didn't understand this. "The New York Times" once, in the early 2oth century, famously got this completely wrong in an editorial.
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Type: Question • Score: 1 • Views: 377 • Replies: 17
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hightor

1
Fri 23 Dec, 2022 06:29 am
@Brandon9000,
Is it based on Newton's Third Law of Motion?
0 Replies

izzythepush

2
Fri 23 Dec, 2022 08:01 am
@Brandon9000,
You light the blue touch paper and it flies in the air.

Are you not allowed matches or something?
0 Replies

Walter Hinteler

1
Fri 23 Dec, 2022 08:08 am
As taught 60 years ago in school:
the fuel gases are ejected at high speed. The rocket (or more precisely the rocket motor) exerts a force on the gas particles (actio) and the gas particles in turn exert a force on the rocket (reactio).

To say it simple: The rocket pushes itself away from the ejected fuel gas.

And that's - as hightor said above - Newton's 3rd law.
Walter Hinteler

1
Fri 23 Dec, 2022 08:19 am
@Walter Hinteler,
Walter Hinteler wrote:
As taught 60 years ago in school
Correction: 58 years, since it was taught when we were 14 years old.
Brandon9000

2
Fri 23 Dec, 2022 09:44 am
Both are correct. Go to the head of the class. If asked, that this the explanation I would give.

Another way to look at it would be that the rocket gives the fuel momentum and, since a system's total momentum cannot be changed without the action of an external force, the rocket must take on the opposite momentum, so that the vector sum of the momenta of all of the parts remains the same.
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hightor

2
Fri 23 Dec, 2022 09:45 am
@Walter Hinteler,
When I was a kid there was a series of books published by Random House called the "All About Books". They kindled a love and respect for science which has remained with me my whole life. My first introduction to the theory of rocket propulsion was in this simple book by Fletcher Pratt:
Brandon9000

2
Fri 23 Dec, 2022 09:46 am
@hightor,
Thank you, Bennett Cerf, wherever you are.
hightor

1
Fri 23 Dec, 2022 09:56 am
@Brandon9000,
Bennett Cerf – last summer I found used copies of two of his books that were in the old family library, "Shake Well Before Using" and "Try and Stop Me" – full of these stories about Broadway, Hollywood, and the publishing industry. Lots of humorous anecdotes, and illustrations by Carl Rose. Re-reading them was like being immersed in the world I knew as a child.
0 Replies

Walter Hinteler

1
Fri 23 Dec, 2022 10:01 am
@hightor,
hightor wrote:
My first introduction to the theory of rocket propulsion was ...
Mine was by my late (Austrian) uncle, who was a mathematician in von Braun's team in Peenemünde (Actually I mainly remember that it was the first time to have a Coke when I was in a pub with aunt and uncle in those days - I was five or six.)
hightor

1
Fri 23 Dec, 2022 10:04 am
@Walter Hinteler,
Amazing story, Walter!
0 Replies

farmerman

1
Mon 26 Dec, 2022 03:55 pm
@Brandon9000,
and ,
, how is "lift
" calculated for any rocket engine design?

farmerman

1
Mon 26 Dec, 2022 03:58 pm
@hightor,
My firt book on rockt was by two guys fromnazi rocketry,Von Braun an Willy Ley. It as all based on Robert Goddard's work on liquid fuel rockets. Ley scaped from Germany in 1935 and came to the US where he first stablished a rocket powre mail dlivry in NY STate.
Ly was more a writer whod gotten his education In De. He authored many introductory papers and popular txts on spac exploration based on multi stage rockets
Ragman

1
Mon 26 Dec, 2022 05:27 pm
@farmerman,
Somewhat related to this topic, my dad was in college around 1928 at Clark University in Worcester Mass. Goddard was a professor at his college but I think he went on to Princeton by the time Dad went there in 1928.
0 Replies

Brandon9000

1
Mon 26 Dec, 2022 10:10 pm
@farmerman,
In my physics training, lift refers to a force perpendicular to the direction of motion. For a rocket, it would be

1/2 ρ v^2 A C

where ρ is air density, v is velocity, A is cross sectional area, and C is a lift coefficient. Is that what you had in mind?
0 Replies

hightor

1
Tue 27 Dec, 2022 06:48 am
@farmerman,
Do you remember this one?

Bonestell's illustrations were incredible!
0 Replies

hightor

1
Tue 27 Dec, 2022 06:53 am
More:
Brandon9000

1
Tue 27 Dec, 2022 09:01 am
@hightor,
Chesley Bonestell made some nice pictures.
0 Replies

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