Safale in 1G batches of cider, how much headroom?

Well, I have a cider kit to use up and rather make another 40 pints of god awful urine, I've devised a much better plan, and a way to test some ingredients and yeasts in the process - make a couple of 1 gallon batches using more concentrate and less sugar than recommended on the pack.

Here's the plan -

1) Mix concentrate with water until OG lies at around 1.060 (OG of real apple juice.)

2) Split into 1 gallon demi-johns

3) Add other ingredients and yeasts, and ferment out.

I've bought a packet of Safale S-04 yeast to try, as I'd hopefully like to get these fermenting quicker than with montrachet/champagne yeast, although they will be ageing for a while of course. From what I gather, fermentation is considerably more vigourous (and as I've just brewed my first lager, I've seen it first hand, it looked like a damn tornado) than with wine yeasts. How much headroom should I leave for a 1 gallon batch? I've got plenty of carboys so I'm happy to split it up.

beer fermentation is more vigorous because of the proteins and such in the wort. I've used S-04 in ciders and not need more than an inch of headroom. Cider must can't really foam up the same way as wort.

Ah! that's good news. thanks for the help old bean

But how important do you think it is to have a small headroom ?...I tend to not bother too much with my ciders to top up to small headoom and you got me thinking here that this may be one of my problem.

only when they are done fermenting does large headroom become a problem. while fermenting you can have lots of space. if you have cider in an airlock-sealed primary with large headroom and leave it there once it's done done, that headroom is filled with CO2 and therefore can't oxidize your cider. if you rack it then any space above your cider is now air (ie rich in O2, bad for cider!) so it becomes very important. think a half-drunk bottle of wine resealed for a week, doesn't taste so great. after a month, tastes like white wine vinegar

I don't think people need to worry too much about headroom. I've let cider sit in a half-empty carboy for 6 weeks without any off-flavors. Even after fermentation, there is still CO2 in suspension, which will slowly off-gas and displace air. Unless you actively try to aerate, I don't see how the air can have that much of an effect.