Author Topic: Episode #597  (Read 5675 times)

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Online gmalivuk

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Re: Episode #597
« Reply #60 on: January 02, 2017, 03:27:04 PM »
I haven't seen the movie, but what's wrong with fire extinguisher rockets?
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Offline daniel1948

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Re: Episode #597
« Reply #61 on: January 02, 2017, 03:39:19 PM »
I haven't seen the movie, but what's wrong with fire extinguisher rockets?

Unrealistic, and they seem to last longer than they really would, IIRC. I liked the movie, because Sandra Bullock, but it was pretty unrealistic.
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Re: Episode #597
« Reply #62 on: January 02, 2017, 03:49:35 PM »
I haven't seen the movie, but what's wrong with fire extinguisher rockets?

I haven't seen it either, but here's a scene with a fire extinguisher, starting at about 40 seconds.



How much pressure would there have to be in that fire extinguisher for that to be possible? I think if they wanted to do that they should've had the astronaut traveling with barely any speed at all. Relative to the station.

Offline daniel1948

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Re: Episode #597
« Reply #63 on: January 02, 2017, 07:35:21 PM »
Even worse than the extinguisher having enough pressure and duration, is the idea that an untrained person, tumbling head over heels, would be able to shoot it in a way that would actually get her where she wanted to go. And then, when it's empty, she manages to throw the extinguisher itself in exactly the right direction to complete her trip and grab onto the station.

But I still liked the movie.
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Offline lucek

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Re: Episode #597
« Reply #64 on: February 04, 2018, 07:16:03 AM »
Quick google. Fire extinguishers have a velocity of 25M/S. The largest on the market are like 20lbs of CO2. If we go with an optimistic model space suit and average adult female human would be around 300lbs (yes i know i should convert to unites of mass) making it aprox 15:1 mass to propellant ratio. 25 M/S x 1/15 gives a delta V of 1.6 M/S repeating. In the scene she uses several orders of magnitude more delta V.
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Offline RMoore

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Re: Episode #597
« Reply #65 on: February 14, 2018, 12:42:57 PM »
I wonder if you could get more velocity just by throwing the fire extinguisher? It depends on the person's strength, of course, but here is one data point (though it should be considered an outlier): https://www.quora.com/What-is-the-fastest-object-thrown-by-a-human

Offline Friendly Angel

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Re: Episode #597
« Reply #66 on: February 14, 2018, 12:57:03 PM »
A 20-lb fire extinguisher will last 15-25 seconds.

I think the relevant principle though is conservation of momentum - and an astronaut would probably get more change in direction and/or velocity by throwing the whole tank and the spray would be for minor directional changes only.
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Offline RMoore

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Re: Episode #597
« Reply #67 on: February 14, 2018, 04:15:18 PM »
A 20-lb fire extinguisher will last 15-25 seconds.

I think the relevant principle though is conservation of momentum - and an astronaut would probably get more change in direction and/or velocity by throwing the whole tank and the spray would be for minor directional changes only.

The fact that the discharge occurs over an extended time even reduces the effect more. Part of the thrust exerted at the beginning is used to accelerate the remaining mass of the extinguisher's contents. It's the same problem with lifting a rocket into space -- you need fuel to lift the rocket, and you need more fuel to lift the fuel.

Online gmalivuk

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Re: Episode #597
« Reply #68 on: February 14, 2018, 11:20:49 PM »
I doubt anyone can throw a 25lb tank-plus-gas at 55mph.
The world is so exquisite with so much love and moral depth, that there is no reason to deceive ourselves with pretty stories for which there's little good evidence. Far better...is to look death in the eye and to be grateful every day for the brief but magnificent opportunity that life provides.

Offline lucek

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Re: Episode #597
« Reply #69 on: February 15, 2018, 01:05:19 PM »
A 20-lb fire extinguisher will last 15-25 seconds.

I think the relevant principle though is conservation of momentum - and an astronaut would probably get more change in direction and/or velocity by throwing the whole tank and the spray would be for minor directional changes only.

The fact that the discharge occurs over an extended time even reduces the effect more. Part of the thrust exerted at the beginning is used to accelerate the remaining mass of the extinguisher's contents. It's the same problem with lifting a rocket into space -- you need fuel to lift the rocket, and you need more fuel to lift the fuel.
Technically true but insignificant. The mass of the extinguisher is an order of magnitude less than the person in a spacesuit holding it. Yes you accelerate the mass of the extinguisher but 95%+ is still accelerating the payload i.e. you.
You have the power, but. . .
Power is just energy over time and. . .
Energy is just the ability to do work.

Online gmalivuk

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Re: Episode #597
« Reply #70 on: February 15, 2018, 04:40:55 PM »
Yeah it's a difference of 54mm/s in delta-V (1.667 if it's all at once, 1.613 if it's over time).
The world is so exquisite with so much love and moral depth, that there is no reason to deceive ourselves with pretty stories for which there's little good evidence. Far better...is to look death in the eye and to be grateful every day for the brief but magnificent opportunity that life provides.

Offline RMoore

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Re: Episode #597
« Reply #71 on: March 02, 2018, 05:47:03 PM »
Speaking of rocket science calculations, I give you the dumbest thing I've read all week:

http://nov79.com/en/ener.html

Online gmalivuk

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Re: Episode #597
« Reply #72 on: March 03, 2018, 05:04:26 PM »
I didn't read the whole thing (because I'd still like to use my brain today and can't afford to melt it with bullshit), but does that person know that *both* energy and momentum are conserved?

Also that there are other forms of energy than kinetic, such as the chemical energy in things like rocket fuel?
The world is so exquisite with so much love and moral depth, that there is no reason to deceive ourselves with pretty stories for which there's little good evidence. Far better...is to look death in the eye and to be grateful every day for the brief but magnificent opportunity that life provides.

 

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