[AR] Re: Hybrid High Powered Rocket
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 14 Mar 2021 15:39:45 -0400 (EDT)
On Sun, 14 Mar 2021, Troy Prideaux wrote:
The one glaring point to laser focus on with a Chill-and-Fill hybrid
is to ensure your tank is *rated* to hold the required pressure. If
you’re making it yourself, allow at least a x3 safety factor with a
pressure relief device.
For reference, normal non-aerospace pressure vessels are required to have
a safety factor of 5x. That can be relaxed to 4x with some precautions
like X-ray inspection of welds, and sometimes to 3x in certain cases with
elaborate precautions. Safety factors are calculated on fairly-worst-case
low strength against fairly-worst-case high loads, *not* on nominal
strength against nominal loads, and all known issues are figured into the
strength or the load (that is, you shouldn't use safety factor as margin
against issues you know about -- it is solely a hedge against surprises).
Now mind you, that's for pressure vessels that are meant to have the
uninvolved public literally standing beside them. You can be a bit less
generous for things that will always be filled remotely... but only if you
make provisions to *empty* them remotely when (not "if") something goes
wrong. And properly speaking, "I'll just wait for the safety valve to
pop" is not a valid answer -- such valves are meant to be emergency
backups only, so you shouldn't be relying on them for reasonably
foreseeable situations.
Particularly if you're developing your own gear rather than flying
something off-the-shelf, situations like valve trouble or computer crashes
or long launch delays are most definitely "reasonably foreseeable".
Never mind the uninvolved public -- it's *your* head and hands that might
have to get "up close and personal" with the hardware when things go
wrong, as they will. Thinking about this beforehand is a lot better than
having to improvise ways to defuse the bomb.
Henry
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