A friend recently leant me a copy of the Spacecraft Films production of "The Mighty Saturns." It contains lots of footage of Saturn rocket launchs from many angles, including inside the fuel tanks. With a camera mounted at the top of the tank, you can see the fuel drop very quickly as it is consumed by the engine(s). Once the engine shuts off, there is a sudden "whiplash effect" as the fuel is exposed to no acceleration forces. Blobs of fuel begin to float all around the inside the tank.

I know that the lower stages of the Saturn were one-time fire stages, but the third stage of the Saturn 5 had to be restarted. How was the engine restarted with the fuel flopping around inside the tank as blobs? I would imagine that unless the unused fuel was forced to the drain opening of the tank, the engine would get a mixture of liquid and gaseous fuel, which would probably be a bad thing.

I guess this question applies to any liquid-fueled engine that has to be started in "weightlessness." How do rocket engineers keep the fuel near the drain of the tank?

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"All natural, huh? So is lightning." - Mike Rowe

One solution is ullage motors (Wikipedia) to provide artificial gravity.

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Don't know about Saturn rockets, but hydrazine and nitrogen tetroxide tanks on Russian spacecraft usually contain what amounts to a nitrogen-filled balloon under pressure. As fuel gets used up, the balloon expands and keeps forcing the fuel against the intake valve; there is no free gas in the tank. I don't think this works with cryogenic fuels though -- no elastic substance retains its elasticity in contact with liquid hydrogen.

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Fiction has to be plausible. Reality is under no such constraint.

Don't know about Saturn rockets, but hydrazine and nitrogen tetroxide tanks on Russian spacecraft usually contain what amounts to a nitrogen-filled balloon under pressure. As fuel gets used up, the balloon expands and keeps forcing the fuel against the intake valve; there is no free gas in the tank.

That technique is very common for satellites that use hypergolic propellants (that's almost all of them).

One solution is ullage motors (Wikipedia) to provide artificial gravity.

A more accurate way to say that is that the ullage motors cause a small acceleration that causes the propellants to shift towards the rear of the tanks. The acceleration mimics the effects of gravity but isn't really artifical gravity.

I know that the lower stages of the Saturn were one-time fire stages, but the third stage of the Saturn 5 had to be restarted. How was the engine restarted with the fuel flopping around inside the tank as blobs? I would imagine that unless the unused fuel was forced to the drain opening of the tank, the engine would get a mixture of liquid and gaseous fuel, which would probably be a bad thing.

From this source:

One second after first stage separation, eight solid-fueled motors mounted on the first/second stage adapter ring were fired for four seconds. These provided a combined thrust of 181,000 pounds.

In addition to maintaining the positive motion of the rocket, these motors performed an ullage maneuver, forcing the second stage fuel to the bottom of its tanks in order to feed the engines. The five J-2 second stage engines were fired during this ullage burn.

...

At this point, the Saturn V had achieved a speed of 15,700 m.p.h. and an altitude of 115 miles.

Two solid-fueled ullage motors located 180 degrees apart on the third stage aft skirt were fired for four seconds to settle the liquid fuel. These motors produced 6,800 pounds of thrust.

Three seconds after second stage separation, the S-IVB third stage J-2 engine was ignited. Nine seconds later, the third stage ullage motors which fired at separation and their cases were jettisoned.

The third stage J-2 engine was fired for 142 seconds before being shut down. This initial S-IVB burn was sufficient to carry the Apollo spacecraft into a 118-mile orbit at a speed of 17,500 m.p.h.

At the end of this first S-IVB burn, two ullage motors were fired to settle the remaining fuel and provide spacecraft stabilization. These ullage motors were housed in two Auxiliary Propulsion System (APS) modules located 80 degrees apart on the third stage aft skirt.

Each APS module housed three attitude control motors and one ullage motor. The attitude control motors could each produce 150 pounds of thrust, while the ullage motors could each produce 70 pounds of thrust. All burned nitrogen tetroxide/hydrazine liquid fuel.

During two or three checkout orbits, the S-IVB attitude control motors could be fired in sequence to make any necessary on-orbit corrections. Following these checkout orbits, the ullage motors were fired for 77 seconds to settle the fuel and provide forward spacecraft momentum.

Like, what do you consider real artificial gravity?

Wikipedia: Artificial gravity:

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The effect is caused by acceleration, not gravity (artificial or otherwise).

Of course the effect is not real gravity. It's artificial, right?

If acceleration cannot produce artificial gravity, what is something that you do consider to do so. What makes the one real artificial gravity and the other -- what? -- ersatz artificial gravity?

What's wrong with the Wikipedia usage? How would you fix it?

(Next up: margarine -- real artificial butter or... what?)

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Thanks for the info guys.

BTW: I understood what was meant by "artificial gravity."

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"All natural, huh? So is lightning." - Mike Rowe

Fuel-displacing bladders keeps the fuel towards the back. The ullage motors help, but what if there's a misfire due to excessive splashing - now you're out of ullage motors.

I'd opt for both options!

I think I've also seen a design for a bladder which is anchored at both ends and the space between the bladder and the insides of the fuel tank are displaced with nitrogen. Sort of like a toothpast tube, but with less liklihood that the bladder wouldn't be injested into the fuel pump.

That would be bad. Probably fatal, even if it's at the very end of the burn.

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Mine: "Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that."

Heinlein's: "Staying young requires the unceasing cultivation of the ability to unlearn old falsehoods." "Freedom begins when you tell Ms. Grundy to go fly a kite."

Change it to "simulated gravity".

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I'm guessing that's a joke. From:

http://www.answers.com/topic/artificial

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The Leif Ericson Cruiser

With all due respect to Answers, I think the two words have somewhat different connotations. A thing can be artificial (manmade) and still identical to the "natural" version.

Now if I'd said "Processed Gravity Product", that would be joke. Not much of one, but still...

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
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Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Well, that would be consistent with the equivalence principle. But, anyway, I don't see a useful distinction in this case between "simulated" and "artificial" - which is why I suspected a joke before I googled the synonyms.

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The Leif Ericson Cruiser

Simulated means fake-- and that's all it means. Artificial just has to do with whether something's natural, not whether it's real. For example, an "artificial diamond" has the same carbon structure as a natural diamond. It really is a diamond. A "simulated diamond" isn't a diamond at all.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

The acceleration due to a rocket isn't real?

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The Leif Ericson Cruiser

I don't necessarily want to get into your battle over what qualifies as artificial or simulated, but according to the NOVA website...

[my emphasis]

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"All natural, huh? So is lightning." - Mike Rowe

Ullage motors are often solid propellant so you don't have this problem.

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Perhaps Mugaliens' point was that the ullage motors are a one-shot deal (because they are solid). If the liquid fuel hasn't settled by the time the ullage motor burns out, you don't get another chance to restart the liquid engine.

Of course, you could always include spare ullage motors, or use a restartable liquid fueled ullage motor that relies on a pressure bladder. (From Larry's link, it appears that the Apollo third stage did that, using hydrazine as the fuel.) With cryogenic fuels used for the main engines, a pressure bladder wouldn't work well because of the cold temperature and tendency for the fuel to boil into a gas.

BTW: To reduce splashing, the Saturn designers added baffles to the fuel tanks. At one point, they tried to reduce fuel sloshing by floating "beer cans" on the surface.

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"All natural, huh? So is lightning." - Mike Rowe

Yes, I think I may have misread what he was saying.

The Saturn third stage used both solid motors and liquid engines for ullage. I believe the solid motors were used prior to the first engine burn only and in combination with the APS (Auxiliary Propulsion System) engines. The APS provided ullage not only at the st