[PSUBS-MAILIST] O-rings and cold temperature

[Alec Smyth via Personal_Submersibles]

Thanks Sean! That's a bit different reasoning than the one given in the
article (that is wasn't the cold on launch day that did it, so much as a
permanent set taken on days prior when it was colder.)

I do think that, fortunately for us, there's something fundamental that
makes O-ring seals problems more forgiving on subs than on spacecraft. Its
the fact that if an O-ring is going to seal poorly it tends to happen at
low pressure, which for subs means on the surface. But spacecraft start
with "high" pressure that tapers off as they climb. I'm guessing that's why
Challenger blew up a little over a minute into her flight.

On Wed, Jan 28, 2026 at 4:20 PM Sean T. Stevenson via Personal_Submersibles
<personal_submersibles at psubs.org> wrote:

> The problem with the shuttle O-rings was their dynamic performance
> charactetistics, as opposed to the quasi-static ones.
>
> Interestingly, there was no actual functional reason for the SRBs to have
> the O-ring sealed joints at all. The contract for manufacture of the SRBs
> was awarded to Morton Thiokol out of state for somewhat suspect reasons,
> but regardless, this necessitated sectioning the booster in order to
> accommodate inter-state transport by rail, instead of manufacuring them as
> a single part in a facility adjacent to the launch complex.
>
> So, the SRB sections were designed with those joints, comprising a double
> radial O-ring seal protected from the propellant exhaust gases by a
> non-incendiary putty and insulation on the inside. The cold temperature on
> launch day, in addition to shrinking the O-rings and reducing the static
> sealing contact pressure as installed, also reduced their resiliency by
> increasing their stiffness and how readily they could both deform under
> applied pressure and return to their original shape when that pressure was
> relaxed.
>
> The SRB is not simply a tube filled with solid propellant that burns away
> from the bottom end. In order to maximize specific impulse, you want to
> burn as much propellant as possible in as little time as possible, so the
> SRB actually has a hollow core which is ignited from the top down to set
> the entire core alight, and furthermore, instead of being a simple
> cylinder, the shape of that core void is convoluted in order to
> maximize its surface area for the initial burn. The result is very high
> exhaust gas pressures achieved very rapidly. Ordinarily, the O-rings would
> rapidly expand and deform into their service (sealing) shape in the
> presence of the applied pressure, but in the case of Challenger,
> this pressure was applied too quickly for the cold and stiffened O-rings to
> keep up with the deformation rate required to maintain the design sealing
> force through their transition from initial installation shape to their
> maximum service pressure sealing shape , resulting in blow-by of the
> hot exhaust gases which ultimately failed the joint structurally.
>
> Sean
>
> <https://proton.me/mail/home>
>
>
>
> -------- Original Message --------
> On Wednesday, 01/28/26 at 12:45 Alec Smyth via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
> Here's one for the PSUBS brain trust.
>
> I was reading today about the Challenger accident 40 years ago. The O-ring
> that failed was Viton, and the failure was caused by cold weather in the
> days prior to the mission. The low was 18F. Per the engineers interviewed
> for the article, when an O-ring contracts due to extreme cold it sets at
> its newly reduced size, without regaining its prior shape and elasticity as
> temperatures go back up.
>
> Here's my question. The variant of Viton used on Challenger has a service
> temperature range with a lower end of -20F, way colder than +18F. Why would
> 18F have damaged it?
>
> The O-rings on Shackleton are Buna-N, which is rated to -40F, but I'd
> still like to understand the failure mechanism, since the weather forecast
> calls for 3F (-16C) in a few days' time, and I'll be diving Shackleton in
> Florida two weeks from now.
>
> Best,
> Alec
>
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