A Question on explosive decompression (In space)

[Leader Bee]

Right, well...

 

Were all familiar with most films including this scenario involve some guy in a spaceship or whatever colliding into an asteroid or acidic blood from zenomorphs burning a hole in his ship. Suddenly WHOOOOSH! he's sucked out of a hole and thrown into the vacuum of space; and this is where my problem begins.

 

I'd been of the understanding that because space is empty that theres nothing there to "suck" with, it's really just the pressure within the spaceship "pushing" out and escaping to fill a lower pressure volume, right? If my line of thinking is correct it'd kinda support my second hypothesis of why a spaceship doesn't just get torn apart by the huge vacuum of space trying to get hold of all that atmosphere; Is it just that spaceships don't necessarily need to be that strong once in space (ignoring space debris and the chance of colliding with anything that is!) because the amount of atmosphere within the ship would never be enough to rupture it?

 

I think i know the answer but TL;DR atmosphere in the spaceship pushes outwards and is NOT sucked out by space?

[mooeypoo]

I'd been of the understanding that because space is empty that theres nothing there to "suck" with, it's really just the pressure within the spaceship "pushing" out and escaping to fill a lower pressure volume, right?

 

It doesn't quite have to do with "empty" but rather with pressure differences. For that matter, the same thing (only less "extreme") would happen (and did) to a plane at full-altitude. The pressure INSIDE is higher than the pressure OUTSIDE, and when there's a hole, the gas inside "wants" to go to equilibrium with the outside, which means it is sucked out.

 

Since this often happens (in movies, and again, in a plane just a few weeks ago) through relatively small holes, we see it as a wind-sucking thing. The pressure makes the air expand rapidly, and when it's through a small hole, it's "sucking" out things.

 

This may help: http://en.wikipedia.org/wiki/Uncontrolled_decompression

 

It's actually not quite right to call it an "explosion", but rather a rapid (uncontrolled) decompression. Sometimes it's called "Explosive decompression", specially when referring to planes.

 

So it seems the way you think of it is generally correct, but reading about uncontrolled decompression can make it more accurate.

 

If my line of thinking is correct it'd kinda support my second hypothesis of why a spaceship doesn't just get torn apart by the huge vacuum of space trying to get hold of all that atmosphere; Is it just that spaceships don't necessarily need to be that strong once in space (ignoring space debris and the chance of colliding with anything that is!) because the amount of atmosphere within the ship would never be enough to rupture it?

I am not sure I understand this question?

 

Spaceship hulls (and astronaut suits, too) must be well protected. Small holes can lead to rapid decompression that's very VERY bad. In movies they might not emphasize this, but there are quite a number of real life companies working on making the hull of space-instruments (and the ISS, and astronaut suits) protected and safe. In hulls there's usually a mesh of support "veins" that keep it together even if there's a hole somewhere. Same goes with a spacesuit, though I believe this would likely be much worse for the person in it (since the volume is smaller, and air escaping would affect pressure much faster and with more disastrous effects).

 

Think of a balloon for instance. It holds up just fine when it's blown up (pressure inside is higher than pressure outside). But when we rupture the balloon, it doesn't just shrink, it pops -- the rubber literally rips apart quite violently. On the other hand, if you tape a balloon with packing tape and rupture THAT part, the balloon would lose air but will not pop. The packing tape strengthens the structure and prevents it from ripping uncontrollably.

 

That's the same principle in space ships. At least, it's supposed to be. Some sci fi shows make a point of saying that, specially when a decompression occurs, and some don't. If the ship is not made of something very strong that can prevent rips from spreading, it would likely pop like a balloon.

 

I think i know the answer but TL;DR atmosphere in the spaceship pushes outwards and is NOT sucked out by space?

 

Sorry, I am not sure I got this part... TL;DR? and.. what do you mean the atmosphere pushes outwards and not sucked out? if there's a hole in the ship, it will be sucked out.... I'm not sure I understand the question or confusion?

 

~mooey

[Leader Bee]

TL;DR - too long, didn't read as I felt I was babbling by this point and thought best to quickly sum up my query. When I mention the atmosphere "pushing out" rather than being sucked out it's because of the higher pressure wanting to escape into the lower pressue volume and the confusion arises by "sucking" inferring that the area outside of the high pressure is acting upon the high pressure rather than the high pressure simply wanting to move into equilibrium.

 

Thanks for the detailed explanation, however, Mooey.

[swansont]

"TL;DR" = "too long, didn't read" Or, as the older generations would say, "summary"

 

Vacuums don't suck. Pressure pushes.

[Leader Bee]

Vacuums don't suck. Pressure pushes.

 

Thank you Tom, thats exactly what I thought and just needed clarification.

[mooeypoo]

Okay, I'm allowing myself to be a pedantic pain in the rear because it seems the main question was answered. If this develops into anything more than my own misunderstanding, I'll move it to its own thread.

 

I was under the impression that calling space a "vacuum" at all is wrong, since it's NOT a vacuum. It does, of course, have much (MUCH) lower pressure than the 1 atmosphere pressure our bodies requires (and hence, what would likely exist in a spaceship) but calling it "a vacuum" is not quite right.

 

Also, another pedantic question, I get that pressure pushes, but isn't the case of "sucking" something almost ALWAYS the case of changing pressures? When we "suck air" we create negative pressure in our lungs. Positive pressure "pushes" the air into our lungs. But.. we're "sucking" in air.

 

I can't think of an example where "sucking" is not, in practice, a change of pressure.. and hence while in terms of pure forces higher pressure pushes, shouldn't it also be true that lower pressure sucks?

 

~mooey

[swansont]

Okay, I'm allowing myself to be a pedantic pain in the rear because it seems the main question was answered. If this develops into anything more than my own misunderstanding, I'll move it to its own thread.

 

I was under the impression that calling space a "vacuum" at all is wrong, since it's NOT a vacuum. It does, of course, have much (MUCH) lower pressure than the 1 atmosphere pressure our bodies requires (and hence, what would likely exist in a spaceship) but calling it "a vacuum" is not quite right.

 

Also, another pedantic question, I get that pressure pushes, but isn't the case of "sucking" something almost ALWAYS the case of changing pressures? When we "suck air" we create negative pressure in our lungs. Positive pressure "pushes" the air into our lungs. But.. we're "sucking" in air.

 

I can't think of an example where "sucking" is not, in practice, a change of pressure.. and hence while in terms of pure forces higher pressure pushes, shouldn't it also be true that lower pressure sucks?

 

The pedantry here is really boils down to the presence or absence of a minus sign. But sign conventions can be important.

 

~mooey

 

It is pedantry, but sometimes physics is about pedantry because we define things in a certain way.

 

There are multiple definitions of vacuum. One is the complete absence of everything, and in that sense there is no such thing as a vacuum. But we also mean it to be a region where the pressure is significantly lower than the surroundings, and from the point of view of an experimentalist, that's the definition I tend to use. You have a vacuum system, and it has a pressure. That defines how good your vacuum is.

 

As far as sucking goes, the physics question is: what is exerting the force? It's not the vacuum. Colloquially you can talk about a vacuum sucking the air out of a chamber, but in much the same way as we, as physicists, don't talk about cold transfer (though one might say "you're letting the cold in") or dark as an entity ("it's dark in there"), the proper physics approach is that it's the pressure exerting the force, not the absence of pressure.

 

The pedantry here is basically the presence or absence of a minus sign, but sign conventions can be important. You need to keep track of the sign.

[mooeypoo]

The negative sign is a good point, I didn't think about it like that.

 

I get the idea of where the forces are going (and in that aspect, that positive pressure always pushes), but isn't the word "suck", in general, a misnomer completely, then?

 

Even when we literally "suck" things -- like with a syringe -- what we are REALLY doing is creating negative pressure and "allowing" the positive pressure to push outwards. I don't see any example of anything that can actually be sucked without having the higher-pressure being the thing that exerts the force.

 

In that aspect, then, if *all* instances of anything "sucked" are, really, instances where the matter/liquid/gas in higher pressure is, in fact, pushed, won't that just make the term "suck" a proper term to use (with the knowledge that common day-to-day language is a misinterpretation of it)... ?

 

 

We misinterprate many terms, for that matter..

 

 

I hope I'm being clear on that, my "problem" isn't really the words, it's more the concept. I always told my friends that they don't know what "sucking" actually is, rather than say "there's no such thing"... see my point?

 

 

 

EDIT/ADD:

 

The term "Weight" is similar. It's not wrong to use it when refering to, say, how many tomatos I bought in the store today (because tomatos DO have mass, and hence weigh...) but it's usually misunderstood by the general public. We often don't push that point because (at least one earth) it's close enough to the physical meaning, and because it serves its purpose -- everyone understands what you want to say.

 

In that aspect, isn't the term "Suck" the same? It's not what people THINK it is, but it's consistently used in the proper context, and physicists are attempting to not be as pedantic as to correct people. You should be able to say "it was sucked out" though, if you understand that this doesn't mean the force was the other way around.

[DrRocket]

It is pedantry, but sometimes physics is about pedantry because we define things in a certain way.

 

There are multiple definitions of vacuum. One is the complete absence of everything, and in that sense there is no such thing as a vacuum. But we also mean it to be a region where the pressure is significantly lower than the surroundings, and from the point of view of an experimentalist, that's the definition I tend to use. You have a vacuum system, and it has a pressure. That defines how good your vacuum is.

 

As far as sucking goes, the physics question is: what is exerting the force? It's not the vacuum. Colloquially you can talk about a vacuum sucking the air out of a chamber, but in much the same way as we, as physicists, don't talk about cold transfer (though one might say "you're letting the cold in") or dark as an entity ("it's dark in there"), the proper physics approach is that it's the pressure exerting the force, not the absence of pressure.

 

The pedantry here is basically the presence or absence of a minus sign, but sign conventions can be important. You need to keep track of the sign.

 

Right, and if anything deserves to be called a vacuum as a practical matter, it is "outer space", which is much nearer a perfect vacuum than anything in a laboratory.

 

For purposes of macroscopic stress analysis or gas dynamics, space is a vacuum.

 

 

 

 

Right, well...

 

Were all familiar with most films including this scenario involve some guy in a spaceship or whatever colliding into an asteroid or acidic blood from zenomorphs burning a hole in his ship. Suddenly WHOOOOSH! he's sucked out of a hole and thrown into the vacuum of space; and this is where my problem begins.

 

I'd been of the understanding that because space is empty that theres nothing there to "suck" with, it's really just the pressure within the spaceship "pushing" out and escaping to fill a lower pressure volume, right? If my line of thinking is correct it'd kinda support my second hypothesis of why a spaceship doesn't just get torn apart by the huge vacuum of space trying to get hold of all that atmosphere; Is it just that spaceships don't necessarily need to be that strong once in space (ignoring space debris and the chance of colliding with anything that is!) because the amount of atmosphere within the ship would never be enough to rupture it?

 

I think i know the answer but TL;DR atmosphere in the spaceship pushes outwards and is NOT sucked out by space?

 

Spacecraft hulls, like rocket motors are designed as pressure vessels. That puts the skin in a generally tensile stress state, which allows a relatively lightweight structure, when compared to, say, a submarine hull which is in compression and subject to compressive buckling. A hole in a thin-skinned pressure vessel can result in a marked change in the local stress state, resulting in tearing and enlargement of the hole -- a problem if you are in space. The only diference between the stress in space and at sea level on Earth is the external pressure, 0 in space and about 15 psia on earth. That is not a huge difference. Other structural concerns may dominate, and mitigate against tearing -- not the least such concern would be adequate margins to specifically preclude tearing following micrometeorite impact.

 

When gas flows through an orifice there are two notable flow regimes, subsonic flow and supersonic or choked flow. In choked flow the gas speed is the local speed of sound at the "choke point" and supersonic as the gas expands downstream. The precise choke point varies somewhat with the geometry of the orifice and subtleties of the gas dynamics. Choke flow occurs when the ratio between the internal pressure and the external pressure exceeds a critical value that is determined by the thermodynamic properties of the gas, the ratio of specific heats being a major factor. As I recall, the critical pressure ratio for air is somewhat less than but very roughly 2. Since the pressure outside a spacecraft is essentially 0, the precise critical ratio is unimportant. The flow will be choked. Flow velocity, "wind speed" very near the hole will be nearly mach 1, and one would not want to be caught in this very high speed flow regime. But that regime can be very localized. At points farther removed from the hole the velocity will be much lower, negligible if the hole is small and the volume of the spacecraft is large.

 

Hollywood is more concerned with audience reaction and excitement than with the Navier-Stokes equation..

[swansont]

The negative sign is a good point, I didn't think about it like that.

 

I get the idea of where the forces are going (and in that aspect, that positive pressure always pushes), but isn't the word "suck", in general, a misnomer completely, then?

 

Even when we literally "suck" things -- like with a syringe -- what we are REALLY doing is creating negative pressure and "allowing" the positive pressure to push outwards. I don't see any example of anything that can actually be sucked without having the higher-pressure being the thing that exerts the force.

 

In that aspect, then, if *all* instances of anything "sucked" are, really, instances where the matter/liquid/gas in higher pressure is, in fact, pushed, won't that just make the term "suck" a proper term to use (with the knowledge that common day-to-day language is a misinterpretation of it)... ?

 

 

We misinterprate many terms, for that matter..

 

 

I hope I'm being clear on that, my "problem" isn't really the words, it's more the concept. I always told my friends that they don't know what "sucking" actually is, rather than say "there's no such thing"... see my point?

 

 

 

EDIT/ADD:

 

The term "Weight" is similar. It's not wrong to use it when refering to, say, how many tomatos I bought in the store today (because tomatos DO have mass, and hence weigh...) but it's usually misunderstood by the general public. We often don't push that point because (at least one earth) it's close enough to the physical meaning, and because it serves its purpose -- everyone understands what you want to say.

 

In that aspect, isn't the term "Suck" the same? It's not what people THINK it is, but it's consistently used in the proper context, and physicists are attempting to not be as pedantic as to correct people. You should be able to say "it was sucked out" though, if you understand that this doesn't mean the force was the other way around.

 

It depends on how deep into physics you are going to get. In a discussion with nonscientists, it's probably not a problem to use "suck." But similar to other terminology, there is a mental picture you construct, and if the terminology is wrong, the model breaks down somewhere. I can't think of an example offhand, but consider accelerate and decelerate. A physicist knows that decelerate is unnecessary, since it just represents a difference in direction, but breaking the concept down like that probably reinforces the misconception that acceleration is all about speeding up and slowing down, and makes the concept of direction change as acceleration more difficult. So there may be a misconception out there, based on a vacuum doing something, that can be avoided by remembering that it's the pressure doing the work, not the vacuum.

[mooeypoo]

It depends on how deep into physics you are going to get. In a discussion with nonscientists, it's probably not a problem to use "suck." But similar to other terminology, there is a mental picture you construct, and if the terminology is wrong, the model breaks down somewhere. I can't think of an example offhand, but consider accelerate and decelerate. A physicist knows that decelerate is unnecessary, since it just represents a difference in direction, but breaking the concept down like that probably reinforces the misconception that acceleration is all about speeding up and slowing down, and makes the concept of direction change as acceleration more difficult. So there may be a misconception out there, based on a vacuum doing something, that can be avoided by remembering that it's the pressure doing the work, not the vacuum.

 

Right, yeah, I think I mostly agree.

 

I'm being a bit of a pain in the rear atm, but I was just wondering about this particular terminology. I mean, "deceleration" might not be physical but it does "Get the point across", and you do see it in freshman-physics books, and, normally (at least from what I encountered) it doesn't get physicists all riled up when it's used.

 

Unlike "vacuum sucks". I am not talking about you in particular, swansont, your note above that vacuum doesn't suck but pressure pushes was on point with the thread, it just got me thinking. It's not the first time I hear this correction, and I get the feeling this *is* something that most physicists get annoyed with (I hear this one being corrected much much more often than the 'deceleration' term).

 

And yet I don't quite see a situation where "vacuum sucks" doesn't actually means pressure-pushes. So there's no potential of using it wrong, but rather misunderstanding the fundamental physics --- just like "deceleration".

 

I just wonder why this one seems to be more of a pet peeve to many physicists than other abused and misused terms.

 

~mooey