In line propellant tanks

[Notebook]

All of the rockets I've looked at have the Lox tank above the Kerosene tank. Any reason for this, besides the obvious one of keeping the Lox as far from the engines as possible?
Also have any rockets been made with concentric tanks, i.e the Lox tank wrapped around the kerosene, or vice-versa?

Many thanks,
N.

 

[Urwumpe]

All of the rockets I've looked at have the Lox tank above the Kerosene tank. Any reason for this, besides the obvious one of keeping the Lox as far from the engines as possible?
Also have any rockets been made with concentric tanks, i.e the Lox tank wrapped around the kerosene, or vice-versa?

Many thanks,
N.

I suspect looking at the density is one good way to explain it, having the Kerosene low puts the center of gravity closer to the engines.

 

[lennartsmit]

The kerosene has to be used first cause it needs oxygen from the lower atmosphere. LOX is oxygen and can be used in space.

 

[Notebook]

To Urwumpe above.

Yes, that makes sense.
If the centre of mass of each tank, is just the centre of mass of the liquid remaining(the tank mass is neglible w.r.t. the liquid). Then the c.o.m moves down as the rocket flies?

If its burning Lox twice as much as kerosene, and this is compensated by the fact you have twice as much Lox as kerosene, then...
I'll have to have a think about that

N.


-----Post Added-----

The kerosene has to be used first cause it needs oxygen from the lower atmosphere. LOX is oxygen and can be used in space.

I'm not sure I understand you, I'm talking about old fashioned rockets not these new-fangled air breathing things.

I just mean the physical location of the tanks, if you look at Atlas or Soyuz, Europa, the Lox tank is placed on top of the kerosene tank.

N.

 

[Urwumpe]

The kerosene has to be used first cause it needs oxygen from the lower atmosphere. LOX is oxygen and can be used in space.

Wrong. Badly wrong. The exit pressure of the rocket engine is significant higher than the ambient air pressure (otherwise, there would be no thrust produced). Both kerosene and oxygen have to be injected into the main combustion chamber over the injector, so they mix and combust inside the combustion chamber. The outside air does not even get close to the combustion process - the high pressure of the exhaust gases pushes it away.


-----Post Added-----

Then the c.o.m moves down as the rocket flies?

No, it is a bit more complex. Empty a bottle in small steps and measure it's CoG, you will sure notice that first it really moves down, as long as the liquid weights much more than the bottle, but then, the CoG moves up again quickly, approaching the CoG of the dry structure.

 

[Notebook]

No, it is a bit more complex

Now there's a surprise!

I see, as the tank empties, the tank structure isn't neglible any more.


Intriquing stuff, I'll get my spreadsheet out and have a play.

N.

 

[RocketMan_Len]

All of the rockets I've looked at have the Lox tank above the Kerosene tank. Any reason for this, besides the obvious one of keeping the Lox as far from the engines as possible?
Also have any rockets been made with concentric tanks, i.e the Lox tank wrapped around the kerosene, or vice-versa?

Concentric tanks turn out to have more empty weight than inline tanks... a 16-percent increase, when compared to inline tanks with a common bulkhead. Multiple tanks are even worse - 62% above inline.

The LOX may be put above the kerosene to shift the CG forward - the more forward the CG is from the CP (Center of Pressure), the more stable the rocket is.


-----Post Added-----

I suspect looking at the density is one good way to explain it, having the Kerosene low puts the center of gravity closer to the engines.

Actually, that's backwards. Kerosene is LESS dense than LOX, and since you're using 2kg of LOX for every 1kg of kerosene (approximately...), you're shifting the CG *forward* by putting the oxygen tank on top.

 

[cjp]

With hydrogen in the lower tank the COG is much higher.
I always thought it was for a different reason:

The fuel from the lower tank can reach the engines directly. The fuel from the upper tank somehow has to pass the lower tank through a long tube or something like that. AFAIK that tube is usually on the outside (see Shuttle ET for instance). If that tube somehow starts leaking, then it could only cause fire if it contains the hydrogen/kerosene. These can react with O2 in the air, but O2 from a leaking tube can't react with O2 in the air, so it's less likeky to cause fire.

Plausible?

 

[Andy44]

No, because the LOX feed pipes in the Saturn V all went through the middle of the fuel tanks.

http://jkbaker.com/gallery/d/921-1/SATURN-V-CONFIGURATION.gif

 

[movieman]

From what I remember, another issue is that the fuel and oxidiser are at very different temperatures, so you have to arrange the tanks so that that one doesn't freeze and the other doesn't boil off; you don't want your fuel to freeze as it passes along pipes through the LOX tank, for example.