[AR] Re: ESAS Safety Concerns about Small SRMs
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 16 Apr 2018 14:46:57 -0400 (EDT)
On Mon, 16 Apr 2018, Nels.Anderson@xxxxxxx wrote:
What was the issue with small SRMs identified by the OSP report? The
second paragraph quoted makes clear that it's not just an oblique way of
referring to the infamous non-existent "black zones".
In the ESAS report's "Launch Vehicles and Earth Departure Stages" chapter:
"These small strap-on motors have entered the human spaceflight debate as
a result of the OSP-ELV Human Flight Safety Report Certification Study,
which recommended against their use for crewed flight. The basis for this
recommendation was that, although small strap-on solids are individually
reliable (estimated at 0.9987), failures of these motors serve as
undetectable initiators of liquid core explosion, which requires an
estimated 2 sec of abort warning to escape. Additionally, since small
strap-ons provide relatively little delta-V, multiple strap-ons are often
required for performance reasons, multiplying their overall risk."
"These concerns do not apply to the Shuttle-derived CLV configuration
because a single RSRB provides the entirety of the first-stage
performance; hence, the issue of cumulative risk over multiple units is
not applicable. In fact, the RSRB failure risk replaces liquid core risk,
as opposed to adding to it, as is the case for strap-ons. Moreover, as is
discussed in Section 6.8.9.5, RSRB Survivability, the RSRB does not have
the explosive potential of a liquid core stage, so the 2-sec abort warning
requirement does not apply. Finally, RSRB failures are detectable (as
demonstrated by the Challenger accident)."
"...the maximum RSRM critical distance is less than the distance between
the forward RSRM segment and the CEV, indicating that abort lead time may
not be needed for RSRM failures (assuming they do not propagate to the
upper stage)..."
In other words, let me pick the assumptions and I can prove anything.
E.g., a solid strap-on failure in an EELV derivative would inevitably make
the liquid core explode, but the far-larger SRB would never do that to its
second stage, and anyway there are things we could do to make that less
likely (even though we're not going to actually do any of those things),
and it's going to be just like the shuttle SRB (even though we're going to
end up making a whole bunch of changes), which has only killed one crew
(unlike those dangerous liquid-fuel rockets), so it's okay -- trust our
models, which must be right.
Henry
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