Commercial Crew

In article ,
says...

On 2019-07-14 09:05, Jeff Findley wrote:

monitor itself to insure it's on the right trajectory. If its not, it
initiates the FTS (flight termination system) in order to make sure that
it doesn't go completely off course which might endanger people who are
outside of the exclusion zone underneath the intended flight path.

My understanding is that FTS is triggered only when the rocket strays
from a cone of acceptable trajectory. So it isn't triggered as soon as
it strays off nominal trajectory since there could still be hope it
recovers. So there is logic involved in this.

Yes there is logic involved. I don't know what the error bars are, so I
didn't mention them.

Of course it does. But that does not negate the fact that the second
stage needs to know its trajectory all the way to orbit. So it would
make sense that the second stage computers are the ones to insure
mission success.

Second stage computers only need situational awareness, aka a copy of
telemetry feed and comms with first stage computers. First stage
computers needs the logic in order to land, swo it can't be a slave to
second stage.

That's not how these things work. Any 2nd stage hardware necessary for
tracking the trajectory will be on and verified as functioning properly
even before launch. The second stage is not going to wait to turn on
things like ring laser gyros and GPS receivers at first stage
separation. To insure mission success, they'll be on and operating
before first stage ignition. You don't want to get to first stage
separation only to find out your second stage guidance system is effed.

Landing happens *after* first stage separation. It's
a secondary objective not directly tied to mission success.

But still critical one because first stage could go nuts and require
termination instead of crashing in downtown Cocoa Beach.

Yes, the first stage has a flight termination system. We covered that
at length.

Why would the first stage ever give a damn about the second stage?

It needs to know if second stage is healthy or has exploded or whatever.
It should be part of the logic to decide whether to self destruct or not.

All it really needs is that one wire from the upper stage that has
positive voltage on it that means everything is "a.o.k.", else it drops
to zero voltage to initiate an abort. The logic to monitor the health
of the second stage wouldn't be in the first stage. It would be in the
second stage, where it belongs. This is because the 2nd stage has to
monitor itself after first stage separation.

It's going to know right away because its going to lose the link to the
second stage and its engines will shutdown.

Exploding tank in stage 2 might not sever the "voltage or not" line.
Consider Apollo 13. They lost one side of the command module but much of
it remained functional.

Then the computer in the second stage would detect the tank going
"boom" and initiate an abort, which would be sent to both the Dragon 2
and the first stage. Again, neither the Dragon 2 nor the first stage
gives a damn about *why* the abort was initiated.

We've gone over this what feels like 100 times. The "abort now"
wire
going to the capsule that should have a positive voltage during launch


I really doubt "man rating" a rocket would accept a single wire as the
one commanding the catrastophic abort. If you insist on an old analogue
voltage or no voltage wire, they at the very least put 3 such wires each
120° apart around the rocket and have computers at least requite loss of
voltage on 2 wires for more than x milliseconds.

Fred covered what happens when you have multiple wires. That may very
well be the case. But if there are multiple wires, zero voltage on one
of them would trigger an immediate abort. A voting system would not be
allowed when interpreting the multiple abort signals. That's because if
something in the upper stage triggered an abort, don't want to count on
the redundant wires all going to zero voltage. You want to get the eff
out of there *right now*.

But I really doubt that Musk would have gone for 1950s analogue stuff on
a modern rocket, Especually since Falcon9 would not have had such a wire
running in cargo missions that don't have abort.

We're talking about the abort system, not the flight control system.
When something absolutely has to work to save astronauts' lives, KISS -
keep it simple, stupid.

Consider also that there must be some delay betwene initiation of
Dragon2 abort, and initiation of the self destruct charges. So it can't
be the same wire.

We're not talking about a delay in the charges. That would be the job
of the destruct system, not the abort system. The destruct system would
receive the abort signal, wait the appropriate amount of time to let the
Dragon 2 escape, then blow the tanks wide open.

When designing hardware like this, you must be careful to keep the roles
and responsibilities separate.

That is because the initiation of an abort really only needs one signal
wire (and a ground as a voltage reference). We've both been telling you
this from the beginning, but you simply won't listen.

Because the use of analogue unreliable connections is not credible in a
man rated system built in 21st century. And when the user Guide mentions
"command" which implies a data packet sent.

It's not really analog, it's digital. Every single logic chip ever made
has a voltage threshold to distinguish between a 1 and a 0. Exactly
what that voltage would be would be determined by the engineers
designing the system. At any rate, an ON/OFF signal is digital, not
analog.

Sure you could let the capsule monitor telemetry from the launch vehicle
during the flight. But that is *separate* from the abort system.

So you admit Dragon2 might get telemetry? The other guy doesn't admit to it.

No, I don't know if it does or does not. What I do know is that no sane
engineer would commingle the telemetry system with the communication of
an abort. Again, keep the hardware separate since an abort is a
critical function that must work even if the telemetry system fails to
work as designed. You have to cover the case where the telemetry system
is saying everything is o.k. due to some unforeseen problem, despite the
fact that something has gone horribly wrong (i.e. structural failure of
some sort which severs the abort wire(s)).

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.