[time-nuts] Project GREAT - Galloping Galileo version

[Bob kb8tq]

Hi

On a normal OCXO or TCXO design you would put the product in a
package with a hermetic outer wall to keep out crud and moisture. 
Per the papers cited you would put a small vent hole in a “space grade” 
product. Every time I see an eBay listing with a nutty price on a “space
OCXO” I wonder if it’s been sitting “vented” on a nice humid / dusty shelf for 
decades.

Indeed the only way to test / use  product vented like that is to put it in a 
thermal / vacuum chamber (or fly it to outer space). While my basement 
has a reasonable selection of this and that, I don’t seem to have missed 
out on picking up a thermal vac chamber …. 

How different is the performance with air in the package? It turns out to
be very much a “that depends” sort of thing. Usually not as big a deal
on a TCXO as on an OCXO. 

Bob

> On Dec 8, 2018, at 9:29 PM, jimlux <jimlux at earthlink.net> wrote:
> 
> On 12/8/18 4:52 PM, Bob kb8tq wrote:
>> Hi
>> What, no 0.046 +/- -.002” vent hole?
> 
> Not on the physics package of the CSAC.  As I understand it, the vacuum around the physics package is more for thermal isolation than anything else.  You can tell that the vacuum is fading because the heater current starts to rise
> 
> 
> Long digression, near rant, on venting requirements follows
> 
> 
> We use a Volume/Area ratio <2000 inches  (oddly, in US customary units, not metric, unless you want to spend time analyzing the launch pressure profile and your orifice flow rates).  I have no idea where the specific value came from, other than it's the lowest tick mark on figure 1 in this paper.
> 
> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980236692.pdf
> 
> In particular that paper cites a reference from 1970.
> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710018690.pdf
> which alludes to failures, and also talks a lot about how you can do venting wrong (put your vent hole where there's a shock wave, let hot gases in, etc.), but doesn't really address the venting of a box within a box scenario.
> 
> 
> 
> 
> I find that there is often little actual detailed rationale for such requirements, other than "it worked before, it's easy to meet, so why bother arguing".
> I'll bet that paper (from 1998?) is basically an attempt to provide an analytical rationale for the "rule of thumb" that probably dates back to the turn of the 19th century in some way.  Maybe Lord Rayleigh wrote about it?
> 
> 
> 
> .
> 
> 
> 
> 
>> That used to be a requirement on this sort of thing.
>> Bob
>>> On Dec 8, 2018, at 6:35 PM, jimlux <jimlux at earthlink.net> wrote:
>>> 
>>> On 12/8/18 11:30 AM, Bob kb8tq wrote:
>>>> Hi
>>>> You might be surprised by how well the CSAC does in orbit. There have been a lot of cases
>>>> over the years where a device has done much better once it is away from “poking fingers”
>>>> like pressure and other semi-random stuff ….
>>>> Bob
>>> 
>>> Oh, I'm pretty sure it will do well in a very benign environment - I joked with the reps that if we could figure out how to vent the enclosure after on orbit, the whole "getter filling up" issue would go away.
>>> 
>>> I don't know that I'll be able to measure the performance.  We sort of added it at the last minute, to be able to demonstrate the ability measure & calibrate an OCXO without a GPS 1pps, and didn't give a huge amount of thought to how to do real performance measurement.
>>> 
>>> 
>>> 
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