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I could not think of any use for the word "ullage" until I found this on the Internet"
Basically, the ullage is the gaseous part of a container when partially
filled with a liquid. A sealed soda bottle has a small ullage of C02 at
the top. Rocket tanks often contain a pressurized ullage of helium to help
deliver the propellant to the engine under pressure. And helium is often
chosen because it's light and non-reactive.
In zero-G, there is no "top" for the ullage to rise to, so prior to an
engine restart, small jets give the rocket a small boost forward which
allows the liquid propellant to settle back to the bottom of the tank.
John Broman
Beginner satellite observer
Professional rocket builder
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From transcript of a space mission:
[Cernan - "At ignition, the tanks were about three quarters full of fuel and one quarter pressurized helium. The trouble was that, in zero gravity, you didn't know where the fuel was in the tank. Therefore, you had to apply a little acceleration and drive the heavy (that is, denser) fuel downward towards the outlet, towards the engine, so that, when you started the main engine you don't get a fistful of helium. Because we were flying with the landing gear and the main engine forward, to get proper ullage we had to fire the RCS jets briefly against that forward motion. We had the computer programmed to fire the thrusters for a few seconds prior to ignition; but, if Jack hadn't confirmed ullage, I would have used the handcontroller to give us manual ullage."
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Anonymous
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so does the vessel have to be a closed system in order for it to have ullage? ullage seems from these excerpts (and BTW, thanks Dr. Bill!) to have a great deal more to do with quantifiable pressure and such than simple volume of space unoccupied by liquid, as i had imagined it to be this morning when i first heard the word.
would it, then, be incorrect usage to say that the quality of a restaurant is generally directly proportional to the ullage allowed by the waiter who is pouring your cabernet?
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The Apollo 13 mission went to hell in a handcart when the oxy tanks were stirred for ullage. FWIW.
The idiot also known as Capfka ...
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Any reason we can't discuss the WAD in the underused Weekly Themes forum, where it belongs?
[deceased-equine e]
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Uh-oh, 'scuse me for harrumphing at Rod--didn't realize you held the trademark, Anna! 
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bill, we really don't have to resort to rocket science to use this word (and bridget* has the right spirit). it's a very old word whose original sense is:
1. The amount of wine or other liquor by which a cask or bottle falls short of being quite full (originally the quantity required to make good the loss by leakage or absorption).
1706 Phillips (ed. Kersey), Ullage of a Cask, is what such a Vessel wants of being full. 1833 Loudon Encycl. Archit. §1324 It is usually tunned into hogsheads of a hundred gallons each, leaving a few gallons ullage. 1835 Marryat Olla Podr. III. 297 (Moonshine), I held the bottle up to the candle to ascertain the ullage. 1885 W. Ecockes in Civilian 3 Jan. 141/2 A work+comprising tables of ullages of casks, whose bung diameters range from 15 to 40 inches.
and, at the risk of angryfying the anglo-saxon police, here's some *really early usage:
1297 Chanc. Misc. (P.R.O.) Bd. 2 No. 15 (5), Tradidi etiam eidem vnam pipam pro oliagio predictorum doleorum.
when I used this as the daily wwftd, a subscriber wrote to say that this is actually used in the restaurant where he works to refer to the dregs left over in glasses and bottles!
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I should have said that its use in space technology made it much more interesting to me.
The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
cold duck (Kalte Ente)
SYLLABICATION: cold duck
NOUN : A beverage made of sparkling Burgundy and champagne.
" when I used this as the daily wwftd, a subscriber wrote to say that this is actually used in the restaurant where he works to refer to the dregs left over in glasses and bottles!"
Dear tsuwm: I think your correspondent who called beverage left overs "ullage" was simply ignorant. "Ullage" is the gas above the liquid. "Cold duck" is a fairly old name for a mixture made from leftovers of several kinds of wine, said to have orginated with European waiters and kitchen help cleaning up after banquets.
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The space usage is probably the most common - or at least the best known - usage of ullage these days. Because I have friends who run wineries or are mad-keen home wine makers, I have heard the term from time to time in its "original" usage. Actually(r) I thought that the usage by the rocket scientists was one of the better ones from the perspective of keeping the original meaning! 
The idiot also known as Capfka ...
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>: I think your correspondent who called beverage left overs "ullage" was simply ignorant.
I don't think we need to be that harsh. the OED goes on:
2. a. (See quots.)
1832 S. Roose Ullaging 5 By knowing the vacuity, and subtracting it from the whole content, leaves the Ullage or the quantity of liquor then in the cask. 1867 Smyth Sailor's Word-bk. 705 Ullage, the remainder in a cask or package which has leaked or been partially used. 1883 Encycl. Brit. XVI. 28/2 The quantity of liquor contained in a cask partially filled and the capacity of the portion which is empty are termed respectively the wet and dry ullage.
b. slang. (See quots.)
1874 Slang Dict. 332 Ullages, the wine of all sorts left in the bottoms of glasses at a public dinner. 1889 Pall Mall G. 21 Aug. 2/1 ‘Pray what is “ullage”?’ ‘The washings out of casks, sir,’ replied my friend.
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