# A variation on the helium balloon problem.

sonhouse
Posers and Puzzles 17 Jan '06 20:43
1. sonhouse
Fast and Curious
17 Jan '06 20:43
On my first post on this subject, I had a spacecraft like the
shuttle fuel tank, a large cylinder filled with STP air, shirtsleeve
environment and a dude or dudette places a balloon in the center
of that space and then a rocket is ignited at one end, the question
being what happens to the balloon?
Variation: The same situation, this time the volume is filled with
STP of helium not air. Rocket ignites as before. NOW what happens
to the balloon?
2. 17 Jan '06 21:38
It burns
3. 18 Jan '06 04:41
Originally posted by sonhouse
On my first post on this subject, I had a spacecraft like the
shuttle fuel tank, a large cylinder filled with STP air, shirtsleeve
environment and a dude or dudette places a balloon in the center
of that space and then a rocket is ignited at one end, the question
being what happens to the balloon?
Variation: The same situation, this time the volume is filled with
STP of helium not air. Rocket ignites as before. NOW what happens
to the balloon?
not enough info..... does the rocket initiate movement sieze movement ...decelerate and then accelerate ..... is there a gravitational force acting on the bodies .... is the ballon filled up with air, fluid , solid ... if air what type ... hydrogen which is less dense than helium ,nitrogen ,... if hydrogen are there any sparks , is the ballon tied at the end????????????????????????? ???????????? ???????????? ???????????
4. 18 Jan '06 04:49
Originally posted by acubed123
not enough info..... does the rocket initiate movement sieze movement ...decelerate and then accelerate ..... is there a gravitational force acting on the bodies .... is the ballon filled up with air, fluid , solid ... if air what type ... hydrogen which is less dense than helium ,nitrogen ,... if hydrogen are there any sparks , is the ballon tied at the end????????????????????????? ???????????? ???????????? ???????????
.........hmmm ...oh yeah ... and what color of blouse was the dudette wearing
5. sonhouse
Fast and Curious
18 Jan '06 21:11
Originally posted by acubed123
not enough info..... does the rocket initiate movement sieze movement ...decelerate and then accelerate ..... is there a gravitational force acting on the bodies .... is the ballon filled up with air, fluid , solid ... if air what type ... hydrogen which is less dense than helium ,nitrogen ,... if hydrogen are there any sparks , is the ballon tied at the end????????????????????????? ???????????? ???????????? ???????????
Er, the title should suggest what is in the balloon....
Its in space so there is almost no gravity, the rocket ignites
and gives an acceleration of one g of force. Before the rocket fires,
the balloon has been placed carefully in the center of the structure
which has a helium atmosphere. The only new info I gave here is
to specify a g force from the rocket, if the structure weighs 10 tons
on earth, then to get one g of accel, the rocket needs to thrust
the same, 10 tons. So what happens to the balloon?
6. AThousandYoung
All My Soldiers...
21 Jan '06 20:09
Originally posted by sonhouse
On my first post on this subject, I had a spacecraft like the
shuttle fuel tank, a large cylinder filled with STP air, shirtsleeve
environment and a dude or dudette places a balloon in the center
of that space and then a rocket is ignited at one end, the question
being what happens to the balloon?
Variation: The same situation, this time the volume is filled with
STP of helium not air. Rocket ignites as before. NOW what happens
to the balloon?
It drifts slowly toward the tail end of the rocket, as the rubber in the balloon is more dense than helium.
7. 21 Jan '06 22:47
i think it does the same as before, drifts in the direction the rocket is moving because of increased pressure at the back of the rocket
8. sonhouse
Fast and Curious
22 Jan '06 04:00
Originally posted by AThousandYoung
It drifts slowly toward the tail end of the rocket, as the rubber in the balloon is more dense than helium.
You mean it moves towards the rocket motor?
Just trying to make sure we know what we are saying.
9. sonhouse
Fast and Curious
22 Jan '06 04:01
Originally posted by ketch90
i think it does the same as before, drifts in the direction the rocket is moving because of increased pressure at the back of the rocket
You mean you think it moves away from the rocket motor?
A thousand young I think says the opposite.
10. AThousandYoung
All My Soldiers...
22 Jan '06 05:151 edit
Originally posted by sonhouse
You mean it moves towards the rocket motor?
Just trying to make sure we know what we are saying.
The helium balloon will move opposite of the direction of the spaceship's acceleration. This means that it will move towards the rocket motor if it's like the rockets on the space shuttle.

Well technically this answer isn't correct. I'll be perfectly clear:

The space ship you describe looks like this:

~~~~~~~~~~)=========>

The ~'s represent a trail of exhaust gases. The ) represents the tail end of the space ship. The ='s represent the body of the space ship, and the > is the tip. The spaceship's center of gravity begins with velocity = 0 and position = 0 (x=0, v=0). The ship is oriented on the x axis with increasing values of x to the right.

The helium balloon is also at x=0 at time (t)=0. As the rocket motor fires up, exhaust gases accelerate into a negative velocity relative to the space ship. The space ship accelerates into a positive velocity relative to it's initial velocity.

The helium balloon will also accelerate into a positive velocity relative to the space ship's initial velocity, but one with a lesser magnitude than the space ship's. Relative to the spaceship's current velocity, the helium balloon will have a negative velocity, which will mean the tail end of the space ship - and the rocket motor - will move forward relative to the helium balloon. Or, in other words, the balloon will move in the negative direction relative to the space ship, towards the tail of the space ship and towards the rocket motor.

Clear enough?
11. sonhouse
Fast and Curious
23 Jan '06 00:561 edit
Originally posted by AThousandYoung
The helium balloon will move opposite of the direction of the spaceship's acceleration. This means that it will move towards the rocket motor if it's like the rockets on the space shuttle.

Well technically this answer isn't correct. I'll be perfectly clear:

The space ship you describe looks like this:

~~~~~~~~~~)=========>

The ~'s repres ip, towards the tail of the space ship and towards the rocket motor.

Clear enough?
A bit more than we required, just going back towards the rocket would
have made it clear what you thought! Before I chime in, lets see
what Ketch90 has to say.
BTW, nice graphic, using symbols that way to represent the situation
without drawings.
12. sonhouse
Fast and Curious
25 Jan '06 19:31
Originally posted by sonhouse
A bit more than we required, just going back towards the rocket would
have made it clear what you thought! Before I chime in, lets see
what Ketch90 has to say.
BTW, nice graphic, using symbols that way to represent the situation
without drawings.
This problem is a variation of the original one where the cylinder
was filled with air and the balloon with helium, so when the rocket
fires, the balloon goes away from the rocket motor to the opposite
end of the craft.
But replacing the atmoshere in the main tank with helium means
the density of the gas is the same inside the balloon as outside,
the balloon would be a bit denser because it is forcing the rubber
to expand to a sphere. So between the slightly denser helium and
the weight of the rubber, there is no lifting force so it drives back
towards the rocket motor end, same as falling from a height on
earth.
13. AThousandYoung
All My Soldiers...
25 Jan '06 22:121 edit
Originally posted by sonhouse
This problem is a variation of the original one where the cylinder
was filled with air and the balloon with helium, so when the rocket
fires, the balloon goes away from the rocket motor to the opposite
end of the craft.
But replacing the atmoshere in the main tank with helium means
the density of the gas is the same inside the balloon as outside,
the ...[text shortened]... orce so it drives back
towards the rocket motor end, same as falling from a height on
earth.
I believe that the pressure inside the balloon is exactly the same as that outside, no? If it wasn't, the balloon would experience a net pressure from either the inside or the outside, which would force an expansion or a contraction.