Henry:
Another issue—which seems to me far more problematic—is that lifting
payloads impart a bending load on the Centaur stage. For another
design—not Dream chaser—that would have required a stiffening cylinder
around the whole Centaur stage....
This is the reason X-37 fits inside a payload shroud.
Bill
On Tue, Apr 17, 2018 at 10:47 AM Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
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?
I think you mean Dream Chaser...
Sierra Nevada probably isn't too worried about abort G-loads, so long as
the thing isn't flying manned. The more important reason is that the SEC
(Single Engine Centaur) simply isn't a good choice for launching heavy
payloads to LEO.
Those "highly lofted" trajectories are inefficient: too much of the
first-stage burn has to go to imparting vertical velocity, to ensure that
the underpowered upper stage doesn't fall out of the sky before it
finishes its burn. SEC is meant for delivering light payloads to
high-energy orbits -- that being Atlas's main market -- where the lower
dry mass is more important. The whole reason for retaining the option of
a two-engine Centaur (DEC) was better performance to LEO; GD/LM/ULA has
been explicit about this from the start.
(So yes, ESAS citing abort problems with SEC trajectories as a reason to
reject Atlas manned launches to LEO -- DEC not being an "existing" upper
stage -- was a pretty blatant case of slanting the analysis to achieve the
pre-chosen conclusion. For that matter, so was making the notional CEV so
bloody heavy.)
So far, there haven't been many LEO customers for Atlas III and V, their
payloads haven't been pushing the limits, and the delays and costs of
dusting off the DEC plans and getting it flying haven't been justified.
And those delays and costs have probably been rising, as people who knew
about the DEC design retired, agreements with equipment suppliers expired,
and little hardware changes were made to SEC and its support equipment
without considering impact on DEC.
Apparently Dream Chaser is enough of an elephant :-) that it needs the
extra lift capacity. (I wouldn't be surprised if SNC does have some
concerns about abort for a manned version, but I doubt that it's the main
issue right now.) One of the things the lifting-body enthusiasts don't
mention much is that lifting bodies tend to have a rather lower payload
fraction than capsules, so they're considerably heavier for a given
payload.
Henry