Pressure Variation on spacecraft

[Bradracer18]

Homework Statement

Hey guys, I'm helping my girlfriend who is in a 101 physics class...ha, I've taken enough engineering physics, and I guess I can't figure this one out. I'm thinking too much into it, and can't even understand the question(what its asking).

Is there a pressure variation(increase with depth) in a fuel tank on a spacecraft in orbit? Why or Why not?

Homework Equations

I guess I'm thinking to use P = D(density)hg

The Attempt at a Solution

If I use this, and I'm still not positive for what its asking...but if I solve for h(depth),

h = P/Dg

Decrease in gravity(like what would be in space), would cause an increase in height.

Any suggestions/help? I'm not sure what to do here.

Thanks,
Brad

 

[denverdoc]

Brad,

while there is a decrease in gravity due to the increased distance from Earth's center while in orbit, this is obviously not enough to acct for weightlessness. In other words if the rocket were stationary, apparent g would be less, but things would still have weight. yet you spill a drink in orbit, what happens?

 

[Bradracer18]

You spill the drink in orbit, it floats(I think).

So, due to the decrease in gravity while in orbit, the depth would increase, due to it "floating"...?

 

[denverdoc]

Well I would assume fluids which are subject to strong cohesional forces such as Van der Waal's forces might behave in many interesting ways in a weightless situation, I don't know that it makes any sense to even consider a column of fluid. I assume in reality that spacecraft of this sort have some kind of gas pressure arrangement. Really out of my depth here so to speak.

 

[Bradracer18]

Ok, So can anyone else help. This is just a 101 question, remember that. It can't be too hard.

 

[D H]

Liquid fuel tanks used in spacecraft come in two basic flavors: Tanks with a bladder, and those without.

Tanks with a bladder are boring. A flexible bladder splits the tank in two parts. A pressurant gas (helium or nitrogen) is on one side of the bladder and liquid fuel or oxidizer is on the other side. Gas from a separate high pressure gas tank is added to the gas side of the bladder as liquid is consumed from the liquid side. The fuel tank pressure is kept high enough to ensure the liquid behaves well. The disadvantages of bladdered fuel tanks are that they add mass (gas tank), add complexity (gas regulator), and increases the risk of failure. The advantage is that the propellant is a more-or-less well-behaved liquid (but slosh is a problem).

The behavior of the propellant in bladderless tanks is truly out-of-this-world. The propellant forms an ethereal mix of vapor and liquid. Bubbles of vapor can appear and disappear anywhere in this mix. The propellant is basically a foam. Getting liquid out of the tank is a technical challenge. Google "Propellant Management Devices" for more info.

A couple of good papers on PMDs: http://www.psi-pci.com/Technical_Paper_Library/AIAA97-2811.pdf"

 

[denverdoc]

Well tell your friend that it might become an aerosol foam and more or less filled the tank completely. I think you'd have a good argument, esp if the fuel was never subject to vacuum prior to launch. All kidding aside, you see why?

 

[Bradracer18]

No I do not see why. I still don't understand what I'm supposed to find as an answer to the question. I feel, for a 101 question, this is way more indepth than needed. This is what I was doing(to indepth), is there a more general answer here? I don't understand what its even asking, I guess.

 

[D H]

The one thing you will never won't get on orbit is an Earth-like pressure variation, $P(h) = P_{top} + \rho g h$, where $h$ is the depth. The fuel tank is in orbit: It is falling. You will get a pressure variation within the tank if the vehicle is thrusting. However, the pressure is highest in the part of the tank closest to the thrusters, not to the center of the Earth.

 

[denverdoc]

Brad, just in case you're still unsure or even mad.

Really no one is trying to mess with you. In orbit the gravitational forces are exactly balanced by "centrifugal" forces. This isn't even a "force" per se but a huge velocity that is sideways to gravity. But the net result is nothing has weight. Still has mass, and is subject to other forces, but weight is out of the eqn. So if you put fluid in a tank it will go whereever. It may atomize, form big globules, etc, so yes it surely will go up in the tank.

 

[D H]

In orbit the gravitational forces are exactly balanced by "centrifugal" forces.

There is no "centrifugal" force. The vehicle is falling, and so is everything in it. On the Earth, you feel weight because the Earth is pushing up on you, keeping you from falling. The Earth is pushing up on the bottom of the ocean, keeping it from falling. The water above "wants" to fall toward the bottom of the ocean, but it can't because water is incompressible. Pressure builds up precisely because the water is restrained from falling toward the center of the Earth.

In the fuel tank in an orbiting space vehicle, the fuel is falling toward the center of the Earth. Pressure does not build up.

But the net result is nothing has weight.

It depends on what you mean by "weight". The standard definition of weight (and the definition used by aircraft designers) is simply mass times acceleration due to gravity. With this definition, the fuel in a vehicle in low Earth orbit has about 95% of its Earth-bound weight. So why do we say that things are "weightless"? I answered that question in this post: https://www.physicsforums.com/showthread.php?p=1234372#post1234372"

So if you put fluid in a tank it will go whereever. It may atomize, form big globules, etc, so yes it surely will go up in the tank.

There is no buoyant force in a non-thrusting vehicle in orbit (or in the Vomit Comet, either). The fuel will do very weird things.

From a Physics 101 perspective, there is no pressure variation in the fuel tank because the fuel and the tank are falling with the same acceleration.

 

[denverdoc]

I think that is what i said, I put centrifigal in quotes, and reckoned there was no sense in even assuming a column of fluid. This is phys 101, i have no idea re post xxyz, cept an intuitive notion that roket fuel or spit behaves differently when its not whizzing by at 20,000 m/sec and therefore in freefall.
J

 

[denverdoc]

I'm going to add on to this. I'm still new to the game, but in this forum the notion is helping with homework. We both agree the fluid can wander, 101 is done. I never mentiond a bouyant force, nor did it cross my mind.

 

[Bradracer18]

ha...I actually understand some of what you guys are talking about. I just couldn't understand what they were asking for, or couldn't think of how to explain it to her. Anyways, thank for the help guys!

 

[denverdoc]

This was a brillant use of resources--recomenend to all your buds! But bring crib sheets and notions,