[AR] Re: Project update - propellant tank hydro tests
- From: Michael Moser <micha_moser@xxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sat, 23 Nov 2013 16:35:23 +1100
Hey Robert,
Date: Thu, 21 Nov 2013 21:29:19 -0600
From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
Subject: [AR] Re: Project update - propellant tank hydro tests
I was going to use a stainless steel snap ring inside the LOX tank. But
I hadn't thought about the screws on the outside of the tank becoming
brittle. I assumed they would just get stronger as they're the normal
alloy cap screws (180 ksi typ). Anybody have data on or practical use
of alloy steel fasteners at cryo temps? I could certainly use stainless
since the load on the washer that holds in the seal isn't very much.
The main tensile force imparted to those screws isn't due to the tank
pressure but instead due to the tank deformation. I selected the
thickness of the seal retaining washer specifically to be thin enough to
allow it to deform and not rip out the #4 screws and yet be thick enough
to hold in the seal under pressure.
I've done some reading up on the steel-at-low-temperature-problem
recently (Canadian/Russian winter, not cryo).
TLDR: I'd only trust austenitic stainless (304 or 316) and possibly
Aluminium fasteners at Lox temperatures.
Roughly speaking, Carbon steel will have a brittle-transition between 0
and -20 deg C. Alloy steels can do better, with brittle transitions
below -80 deg C.
The main factors seem to be:
- Alloyed elements: Nickel good, Carbon bad.
- Phase: Austenitic good, Martensitic ok, Pearlite bad.
- Grainsize: Small is good, big is bad.
A good resource for your case would be "Low temperature metals" by A.
Hurlich, a copy can be found here:
<http://www.bnl.gov/magnets/staff/gupta/Summer1968/0311.pdf>
Cheers,
Michael
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