Few questions about large coils

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jason Johnson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <hvjjohnson13-at-hotmail-dot-com>

Hello,
 
I'm in the process of designing/acquiring parts for a large coil ~17KVA right
now, and since I've never done any work on a coil this big I thought I'd throw
out a few questions to those more experienced than I.
 
First I'll give some of the planned specs
Transformer- basically I have up to 15 KVAC at most any current I need
Capacitor- 110nf 30kvdc MMC (5x15 2000vdc 330nf snubbers, made by GE, similar
to geek group caps)
Gap- can be sync (120-480bps) async (0-700bps). triggered or airblast static
Secondary coil- either 8.75" x 60" wound with 21 guage, or 12.5" x 54" with
same 21 guage
Primary coil- depends on secondary
Topload- large tractor (or truck) tire inner tube covered in AL tape, not sure
how big yet
 
So far I think that I will go with 480 bps synchronous AC -at- 15kv with the 12.5"
secondary (mostly because the 8.75" is cardboard .5" thick). This should put my
firing voltage at about 25000 volts with 110 nf and pushing most or all of the
16.8 KVA (240volts 70amps) I have available through the coil. This is mostly
because this seems pretty safe to me I don't want my cap to die because I'm
going to be broke after I buy it. My main questions are:
 
1. Does anyone see anything majorly wrong in these specs?
2. Would DC be better to run my coil? I'd need almost 700 BPS without the
resonant kick I could get with AC to raise my firing voltage that extra 4kv,
for the same throughput. I don't really like this because MMC calc seems to
tell me that high BPS is hell on caps and I want this thing to last for quite a
while. Comments?
3. I'm leaning towards a flat primary, because I have had great success with
flat primarys on smaller coils. But the primary on this coil wouldn't have much
affect on the top turns 4.5' away, unlike most of the smaller coils I've
constructed. Would a sloped primary be better? 
4. If I get everything right on this coil (which probably won't happen at first
light) John Freau's famous formula tells me I could get upwards of 18 feet of
spark off this thing (YEAH!!!). Does this NST type formula hold well for big
coils?
 
Jason Johnson