Easy workaround for Delta-IW heavy problem?

Suppose the Delta-IV heavy problem is indeed caused by a fuel sensor
being uncovered early due to slosh/vortex/etc. .

On the other hand, the CBC seems to work OK in the Delta-IV single.
The differences in operating conditions a The Heavy tries to
throttle closer to depletion, there is less acceleration at this point
(for the center CBC, anyway) and the trajectory is different (may be a
slightly different local "down") in the two cases.

Any sensor uncovering problem will be helped dramatically if there is
more fuel in the tank, and we know the CBC works OK when the thrust
reduction occurs when 24 sec of fuel is left.

So here's the workaround - throttle all CBC engines back when 24 sec of
full thrust remain (like the Delta IV regular) instead of when 6 sec
remain, as the D-IV heavy tried. Assuming a reduced thrust of 60%,
this will add 12 sec to the burn time, both of the outer CBCs, and the
center one.

Of course, the reason they don't normally do this is because it
increases gavity losses, but by how much? Assuming 5G at full thrust,
and 3G at reduced thrust, and a very crude geometrical gravity loss
approximation, we get

18 sec at 3G rather than 5G loses 12.6 m/s or 40 fps
12 extra sec at 3G loses 20.4 m/s or 65 fps

Both of these losses occur on both the outer CBC end of thrust, and the
core CBC end of thrust, for a total of 66 m/s or 210 fps. The actual
loss will be somewhat less, I think, since the velocity is a
substantial fraction of orbital already at this point, which is not
considered in the above arguments.

Now, how much must the payload be reduced so the second stage can
supply an extra 66 m/sec? Assuming demosat = 6000 kg, empty stage 2 =
3500 kg, full stage 2 = 31000 kg, exhaust vel = 4500 m/s, then the
second stage adds 4500*ln(37/9.5) or about 6100 m/s. To get 70 m/sec
more, we need to reduce the final (and initial) mass by about 200 kg.
So reducing the payload by 200 kg should do the trick.

This should be OK for all current practical purposes, since there are
only two payloads, and these were originally designed for the Titan,
which is rated at 5760 kg to GSO.

Lou Scheffer

They know how long the transient low-level indication lasted --
according to AW, the low-level indication had already gone back to
normal as the engines began their power-down.

Why not say "Our low-level sensitivity is too great" and add a
requirement that the indication must last for T+delta milliseconds
before the engines are shut down (where T is the transient duration, and
delta is some reasonable fudge factor that takes into account the amount
of fluid remaining in the system when a valid low-level indication is
detected).

--
Reed Snellenberger
GPG KeyID: 5A978843
rsnellenberger-at-houston.rr.com

"Reed Snellenberger" wrote in message
...
They know how long the transient low-level indication lasted --
according to AW, the low-level indication had already gone back to normal
as the engines began their power-down.

Why not say "Our low-level sensitivity is too great" and add a requirement
that the indication must last for T+delta milliseconds before the engines
are shut down (where T is the transient duration, and delta is some
reasonable fudge factor that takes into account the amount of fluid
remaining in the system when a valid low-level indication is detected).

There's a bigger problem here than timing (and we're talking seconds, not
milliseconds). The LOx line cavitated. That is not a condition under which
you want to run your engine. Boing is going to have to find out why the LOx
behaved the way it did. Patching the problem with time delays is not the
answer - that's a band-aid. Everyone knows band-aids don't stay on forever,
and sooner or later that particular band-aid will find some way to come back
and bite you.

-Kim-

Kim Keller wrote:

There's a bigger problem here than timing (and we're talking seconds,
not
milliseconds). The LOx line cavitated. That is not a condition under
which
you want to run your engine. Boing is going to have to find out why
the LOx
behaved the way it did.

Throttling back earlier should also help this problem. A lower flow
rate will lead to lower pressure drop along the line, which should help
the cavitation problem. This might explain why the single CBC had no
problem.

Patching the problem with time delays is not the
answer - that's a band-aid. Everyone knows band-aids don't stay on
forever,
and sooner or later that particular band-aid will find some way to
come back
and bite you.

Ignoring short term sensor data because it was bad last time would
indeed be a band-aid. But if you understand the conditions under which
the line cavitates, and stay away from those conditions, that's
engineering. It all depends on how well they understand the problem.
Lou Scheffer

wrote in message
ups.com...
Throttling back earlier should also help this problem. A lower flow
rate will lead to lower pressure drop along the line, which should help
the cavitation problem. This might explain why the single CBC had no
problem.

The CBC core saw the same condition and also shut down early.

Ignoring short term sensor data because it was bad last time would
indeed be a band-aid. But if you understand the conditions under which
the line cavitates, and stay away from those conditions, that's
engineering. It all depends on how well they understand the problem.

The sensor data was not bad - it was reporting exactly what conditions were.
*All* the flow sensors reported identically.

-Kim-

Kim Keller wrote:
wrote in message
ups.com...
Throttling back earlier should also help this problem. A lower
flow
rate will lead to lower pressure drop along the line, which should
help
the cavitation problem. This might explain why the single CBC had
no
problem.

The CBC core saw the same condition and also shut down early.

But the central CBC on the heavy throttles back much later than than
the CBC on the single. So you get into the regime where you have both
a high flow rate and not much liquid in the tank. All the heavy CBCs
run this way, but the single does not (when the tank is low, it's
running at low flow rates).

The sensor data was not bad - it was reporting exactly what
conditions were.
*All* the flow sensors reported identically.

That's good to know.

After troubles like this, I'm even more amazed they launched the
shuttle manned on the first flight.

Lou Scheffer