[time-nuts] Project GREAT - Galloping Galileo version

[jimlux]

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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