NavList:

   

Reply

The B-2 astro-inertial window is more clearly seen in this article.


https://theaviationist.com/2021/09/10/lets-have-another-look-at-the-b-2s-air-data-ports-and-astroinertial-navigation-system/

The outline of the removable panel that gives access to the device is
clearly visible. It can be lifted out with a small general purpose
flightline crane ("cobra crane").

Also visible are the air pressure sensing ports which the B-2 uses to
measure altitude, airspeed, angle of attack, and sideslip. (Air
temperature is measured in the engine intakes.) There are three port
locations on top of the plane and three on the bottom. At each location
are four ports because the flight control system has four identical
redundant channels. Everything is quadruplicated. A transducer is
immediately behind each port.

Since air accidents have been mentioned, I'll make special note of the
quartet in the center. They were behind the crash that destroyed a B-2
during takeoff from Guam.  Water in three of the transducers applied
extra pressure (up to 0.26 inch Hg according to the accident report).
When stationary the B-2 expects all ports to sense the same pressure
within close limits. Failure to attain that condition triggered a
message to the crew that air data calibration was required.

It's a maintenance procedure which alters the bias of any selected
transducer so its output is equal to the mean pressure sensed by all
other air data ports. A technician performed the calibration, thereby
clearing the failure indication. But it was like zeroing a scale with an
unnoticed fly resting on a balance beam. The scale becomes inaccurate
when the fly departs.

After taxiing to the runway, half a minute before advancing throttles
for takeoff, the crew turned on pitot heat. (The switch is labeled PITOT
HEAT despite the absence of any pitot tubes on the B-2.) The trapped
water began to boil out, thereby rendering the calibration invalid. By
the time the plane broke ground its flight control system sensed a false
nose down angle of attack. The resulting pitch up was uncontrollable.
The crew ejected successfully.

The investigation discovered some aircrew and maintainers knew a brief
application of pitot heat could clear the calibration message. (If you
run heat continuously on the ground the heat is excessive and can cause
damage.) But the people on scene that day didn't know the trick. The
report blamed the crash on failure to formalize this knowledge. So the
most expensive aircraft accident in history was due to a little water
and a few guys who had not heard of the hack to boil it out.

The Smithsonian Channel "Air Disasters" series devotes an hour episode
to this crash. It's fairly good. However, I don't think the part where
the maintainer performs the air data calibration is accurate. I must
admit that by the time of the crash I had been out of the picture for
years, and back in the day I never did an air data calibration. But
unless I miss my guess by a mile, you access that function via the DEP
(data entry panel — lots of buttons below a small display — also found
on airliners nowadays). In the TV show the tech has a device which
displays air pressures (inches Hg) sensed at the air data ports. In
reality, I suspect the air data cal proceeds automatically with no
feedback on the magnitude of the pressure discrepancies. Had such data
been displayed, the tech may have realized what was really wrong.


> I would have liked to watch the surveyors set the mark, but I was on
> temporary duty at another base going to school on the new system.

At the school the instructor showed us the heart of the B-52 SPN/GEANS:
an electrostatically suspended beryllium sphere spinning in a vacuum
chamber. That's the gyroscope. A pattern at the "north pole" is
optically sensed to detect any discrepancy between the spin axis and the
inertial platform. Motors adjust the position of the latter to maintain
alignment with the sphere, which is about the size of a ping-pong ball.
It's hollow, with extra mass around the "equator" to give it a natural
spin axis. The class was allowed to spin the sphere on a table to see
that for ourselves. (Nowadays I don't think that would be permitted.
Beryllium is a brittle metal and when shattered its dust is toxic.)

The vacuum chamber is within a ceramic case, split through the center.
The ball has only a few thousandths of an inch clearance. Electrodes in
the chamber wall apply the suspension voltage. I can't remember how the
ball is spun up. At rest the ball is intentionally non-spherical but
assumes the correct shape at speed, which as I recall is 650 revolutions
per second. Spin force is applied only during initialization. During
navigation the ball freewheels and can maintain speed for three years.
When the system is shut down at the end of a flight, the ball must be
stopped before suspension voltage is turned off. No power interruption
can be tolerated while the ball is spinning, else it's reduced to
powder. (Maybe exaggerated, but that's what the instructor said.)
Therefore, the SPN/GEANS includes a UPS.

According to this article, the Peacekeeper ICBM took the floating ball
one step further. The entire inertial platform was enclosed in a sphere
suspended in fluid. Allegedly it was the most accurate INS ever
constructed, with virtually zero contribution to the missile's error budget.


https://www.twz.com/30254/this-isnt-a-sci-fi-prop-its-a-doomsday-navigator-for-americas-biggest-cold-war-icbm

--
Paul Hirose
sofajpl.com

   

Reply