[AR] Re: ESAS Safety Concerns about Small SRMs
- From: "John Stoffel" <john@xxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 17 Apr 2018 13:32:26 -0400
Would it be better to use some other areostructure to help counter
that lifting (bending really) moment, instead of going full bore into
a shroud? And you only really need it for what, the first three
minutes or so? And they less you throw away each time... the less
there is to replace and build anew.
William> Another issue—which seems to me far more problematic—is that
William> lifting payloads impart a bending load on the Centaur stage.
William> For another design—not Dream chaser—that would have required
William> a stiffening cylinder around the whole Centaur stage....
William> This is the reason X-37 fits inside a payload shroud.
William> Bill
William> On Tue, Apr 17, 2018 at 10:47 AM Henry Spencer
<hspencer@xxxxxxxxxxxxx> wrote:
William> On Mon, 16 Apr 2018, Lars Osborne wrote:
"Both Atlas and Delta single-engine upper stages fly highly lofted
trajectories, which can produce high deceleration loads on the crew
during an abort..."
Is this why starliner will fly on a DEC?
William> I think you mean Dream Chaser...
William> Sierra Nevada probably isn't too worried about abort G-loads, so
long as
William> the thing isn't flying manned. The more important reason is that
the SEC
William> (Single Engine Centaur) simply isn't a good choice for launching
heavy
William> payloads to LEO.
William> Those "highly lofted" trajectories are inefficient: too much of
the
William> first-stage burn has to go to imparting vertical velocity, to
ensure that
William> the underpowered upper stage doesn't fall out of the sky before it
William> finishes its burn. SEC is meant for delivering light payloads to
William> high-energy orbits -- that being Atlas's main market -- where the
lower
William> dry mass is more important. The whole reason for retaining the
option of
William> a two-engine Centaur (DEC) was better performance to LEO;
GD/LM/ULA has
William> been explicit about this from the start.
William> (So yes, ESAS citing abort problems with SEC trajectories as a
reason to
William> reject Atlas manned launches to LEO -- DEC not being an "existing"
upper
William> stage -- was a pretty blatant case of slanting the analysis to
achieve the
William> pre-chosen conclusion. For that matter, so was making the
notional CEV so
William> bloody heavy.)
William> So far, there haven't been many LEO customers for Atlas III and V,
their
William> payloads haven't been pushing the limits, and the delays and costs
of
William> dusting off the DEC plans and getting it flying haven't been
justified.
William> And those delays and costs have probably been rising, as people
who knew
William> about the DEC design retired, agreements with equipment suppliers
expired,
William> and little hardware changes were made to SEC and its support
equipment
William> without considering impact on DEC.
William> Apparently Dream Chaser is enough of an elephant :-) that it needs
the
William> extra lift capacity. (I wouldn't be surprised if SNC does have
some
William> concerns about abort for a manned version, but I doubt that it's
the main
William> issue right now.) One of the things the lifting-body enthusiasts
don't
William> mention much is that lifting bodies tend to have a rather lower
payload
William> fraction than capsules, so they're considerably heavier for a given
William> payload.
William> Henry
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