how fast were drum memories?

[Paul Koning]

> On May 10, 2018, at 11:33 AM, Peter Corlett via cctalk <cctalk at classiccmp.org> wrote:
> 
> On Thu, May 10, 2018 at 10:29:06AM -0400, Paul Koning via cctalk wrote:
>> [...] So far so good. He goes on to suggest that such a drum might spin at
>> 1000 revolutions per second, i.e., 60,000 rpm. That seems amazingly high. I
>> could see it being physically possible for a drum of only 40 mm radius, but
>> it sure doesn't sound easy.
> 
> Looking at modern hard disks, I'm unconvinced we could even mass-produce
> something like that today.
> 
> A 40mm radius is comparable to a 3.5" disk, which are generally 5,400-7,200
> RPM. 15,000 RPM is the fastest available, but those tend to be low-capacity and
> expensive, and are often 2.5" drives with a huge heatsink. We could perhaps
> rotate a very narrow smaller cylinder faster still but then the capacity
> suffers further, and the seek time would start to dominate.

Drums are head per track devices, so there is no seek.  Yes, modern drives do 10k rpm max on 3.5 inch disks, while 15k rpm disks uses 2.5 inch platters.  As I understand it, the reason is air resistance and the desire to limit drive motor power.

Chuck mentioned a CDC effort to have a drum spin in vacuo.  That obviously avoids the air friction issue, but at the cost of losing the ability to have flying heads.  It probably makes sense to use much reduced pressure, maybe 1% of standard, which still gives you some lift on the heads.

Note that the document I quoted wasn't talking high density.  I'm guessing 3 mm between tracks, which is easy enough (the RF11 is similar, perhaps somewhat denser if I remember right).  Van Wijngaarden mentions 1000 bits per track (and 100 tracks on a "few decimeter" length, so about 3 mm per track which is similar to the track spacing of the DEC RF11).  80 mm diameter means about 250 mm circumference, so we have 4 bits per mm, which is clearly easy enough and doesn't seem to require flying heads.

	paul