Jeff Findley wrote on Tue, 16 Oct 2018
07:00:33 -0400:

In article ,
says...
You mean besides it being a 1960's design?

Come on. I this a fair accusation of Soyuz? The capsule has modern
electronics, glass cockpit, automated ejection system etc. So I have to
wonder what else was upgraded over time and what is left of the originla
design.

And if the engines work well and have proven themselves over many years
and perform close to what modern engines can do, why re-inent the wheel?

Spacex use Kerosene, so it can't be all that bad.

Yes Soyuz the launch vehicle (we're not talking about the crewed Soyuz
capsule) has had some minor tweaks since the 1960s, but it's still
essentially the same. It's boosters and core stage are arranged and
function the same as they have since the R-7. The stage separation
sequence is pretty much the same as it's always been. It's a very old
design.


[The following is actually intended for you, Mayfly, since Jeff
probably already knows all this stuff.]

And how that stage separation works is a particularly elegant design.
Soyuz doesn't 'sit' on the pad like other launchers. Instead it is
suspended by the upper attachment points of the strap-ons. This means
that the same forces that hold the thing together in flight hold it
together on the pad. Once you're under thrust the force holds the
ball joint at the upper attachment point in place and the rocket is
still 'suspended by them'. When the strap-ons shut down, they start
to 'fall' backward since the rest of the rocket is still under thrust.
This motion triggers a 'separation signal' that blows the pyros on the
cabling at the lower attachment points and commands open a valve to
vent oxygen from the tip of the strap-on. The strap-ons fall backward
and the venting gas pushes the 'nose' of the strap-on out away from
the core (second) stage of the rocket, leading to something known as
the 'Korolev Cross' as the four strap-ons separate.

In the case of the failed launch, the 'Cross' looks particularly messy
and we know at least one of the strap-ons hit the core stage during
separation. My guess (and it is a guess) is that one or more of the
valves that is supposed to vent gas didn't do so, so there was no
impetus to 'push' the strap-on away from the core and it fell straight
down instead. Of course, even that explanation isn't a complete
'why', since you'd still want to know WHY the gas venting didn't
happen; valve failure, insufficient gas available, separation signal
not sent or not received, etc.


--
"Insisting on perfect safety is for people who don't have the balls to
live in the real world."
-- Mary Shafer, NASA Dryden