What Happened to the MMU?

What ever happened to the MMU? Why are they no longer used? On the two
flights I remember they worked out pretty well. I'm sure this is an
asked/answered topic, but I thought I would post on topic for a change.

--
Jim in Houston
oSpAm
Nurse's creed: Fill what's empty, empty what's full, and scratch where it
itches!!

Jim wrote:
What ever happened to the MMU? Why are they no longer used? On the two
flights I remember they worked out pretty well. I'm sure this is an
asked/answered topic, but I thought I would post on topic for a change.


It had something to do with the post-Challenger accident investigation,
but I forget what exactly what it was.

Pat

"Jim" wrote in message
...
What ever happened to the MMU? Why are they no longer used? On the two
flights I remember they worked out pretty well. I'm sure this is an
asked/answered topic, but I thought I would post on topic for a change.

IIRC it doesn't really work that much better (if at all) than maneuvering
the orbiter and using the RMS arm. Plus, it introduces a whole bunch of
extra failure modes, has hard limitations (i.e. fuel) and makes the crew
activity plan much more complex. So overall it's much simpler to just nudge
the orbiter in close to the target and snare it with the arm.

One of those situations where the bleeding-edge of technology is actually
worse than lower-tech solutions.

--
Terrell Miller


"One machine can do the work of fifty ordinary men. No machine can do the
work of one extraordinary man."
- Elbert Hubbard

"Terrell Miller" wrote in message
.. .
"Jim" wrote in message
...
What ever happened to the MMU? Why are they no longer used? On the two
flights I remember they worked out pretty well. I'm sure this is an
asked/answered topic, but I thought I would post on topic for a change.

IIRC it doesn't really work that much better (if at all) than maneuvering
the orbiter and using the RMS arm. Plus, it introduces a whole bunch of
extra failure modes, has hard limitations (i.e. fuel) and makes the crew
activity plan much more complex. So overall it's much simpler to just
nudge the orbiter in close to the target and snare it with the arm.

One of those situations where the bleeding-edge of technology is actually
worse than lower-tech solutions.

In fact, maneuvering the orbiter turned out to be a bit easier than
expected. The SSF program seemed to flip/flop between berthing and docking
the shuttle to the station. Berthing sort of assumed that the shuttle
wouldn't be able to maneuver all that well close to the station, so the plan
was to grab the shuttle with a short robotic arm (or two) and then use the
robotic arm(s) to berth the shuttle to the station. Docking sort of assumes
that you can control the shuttle's velocity and attitude very tightly to
meet the velocity and attitude requirements for docking.

Eventually the US settled on docking when the Shuttle/Mir program got going
and the US agreed to use the Russian developed docking system. I'm sure
part of this decision was due to the fact that maneuvering the shuttle
precisely turned out to be easier in practice than the engineers were
expecting.

Jeff
--
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor
safety"
- B. Franklin, Bartlett's Familiar Quotations (1919)

In article ,
Jim wrote:
What ever happened to the MMU? Why are they no longer used?

They're in storage, I believe. Basically, because there hasn't been a
good use for them lately.

The MMU was built on the assumption that the orbiter wasn't capable of the
delicate maneuvering needed to dock with a satellite, e.g. for repairs,
especially if the satellite was spinning. But this turned out to be
untrue. In particular, it turned out that the best way to take the spin
off a spinning satellite is to just have spacewalkers in the cargo bay
reach up and grab it. (Moreover, the more complicated plans involving
the MMUs did not actually work terribly well on the Solar Max repair
and the Palapa/Westar salvage.)

That pretty much removed the MMU's reason to exist. For more mundane
tasks like working around ISS, it's easier and safer to crawl along the
structure, or have the arm hold you in position, than to fly free.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

In article ,
Jeff Findley wrote:
...The SSF program seemed to flip/flop between berthing and docking
the shuttle to the station. Berthing sort of assumed that the shuttle
wouldn't be able to maneuver all that well close to the station, so the plan
was to grab the shuttle with a short robotic arm (or two) and then use the
robotic arm(s) to berth the shuttle to the station. Docking sort of assumes
that you can control the shuttle's velocity and attitude very tightly to
meet the velocity and attitude requirements for docking.

Berthing does have some technical advantages -- much lower impact loads,
much less machinery cluttering up the mating areas (and thus easier
accommodation for, among other things, bigger hatches), and generally
better control of the process -- and so SSF had pretty much settled on it.

And then along came Shuttle-Mir, which required using the Russian docking
system (developed for Buran) that was already in place. After that, it
seemed easier to just keep using that for ISS, and not develop a new
berthing system. Arguably penny wise and pound foolish, but that's the
story of ISS...
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

On Feb 2, 8:40 am, (Henry Spencer) wrote:
In article ,

Jim wrote:
What ever happened to the MMU? Why are they no longer used?

They're in storage, I believe. Basically, because there hasn't been a
good use for them lately.

The MMU was built on the assumption that the orbiter wasn't capable of the
delicate maneuvering needed to dock with a satellite, e.g. for repairs,
especially if the satellite was spinning. But this turned out to be
untrue. In particular, it turned out that the best way to take the spin
off a spinning satellite is to just have spacewalkers in the cargo bay
reach up and grab it. (Moreover, the more complicated plans involving
the MMUs did not actually work terribly well on the Solar Max repair
and the Palapa/Westar salvage.)

That pretty much removed the MMU's reason to exist. For more mundane
tasks like working around ISS, it's easier and safer to crawl along the
structure, or have the arm hold you in position, than to fly free.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

The plans for Solar Max were in fact overly complicated, but I don't
think a failure of the MMU per-se contributed to mission problems.
The MMU docking pin on Solar Max had a screw placed in the a wrong
place (at build) that prohibited the docking, with the MMU interface.
(if I recall correctly the screw had no mission functionality at all).
The astronaut piloting the MMU (Pinky?) showed he could approach and
hit the target despite the spin speed. Now limitations as to how many
approaches he could make is entirely another story. With Palapa I'm
not so sure it was the MMU again, The docking mechanism was large and
ungainly (although built for the MMU) and repeated attempts failed,
and I'm not sure if a determination was made as to why. the docking
apparatus (I beleive it was made to fit inside the engine bell). I'm
not sure at higher spin speeds the arm would have done much
better ...................Doc

"Henry Spencer" wrote in message
...
Berthing does have some technical advantages -- much lower impact loads,
much less machinery cluttering up the mating areas (and thus easier
accommodation for, among other things, bigger hatches), and generally
better control of the process -- and so SSF had pretty much settled on it.

And then along came Shuttle-Mir, which required using the Russian docking
system (developed for Buran) that was already in place. After that, it
seemed easier to just keep using that for ISS, and not develop a new
berthing system. Arguably penny wise and pound foolish, but that's the
story of ISS...

Berthing the shuttle to ISS would certainly simplify operations on a
resupply mission since the MPLM could stay in the shuttle's cargo bay for
the duration of the mission. This would eliminate a lot of time spent
simply moving the MPLM around, connecting/disconnecting umbilicals in that
small pressurized space between the MPLM's hatch and the node's hatch, and
etc.

Jeff
--
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor
safety"
- B. Franklin, Bartlett's Familiar Quotations (1919)

On Fri, 2 Feb 2007 14:54:49 -0600, Jeff Findley wrote
(in article ):

connecting/disconnecting umbilicals in that
small pressurized space between the MPLM's hatch and the node's hatch

.. . . it's called the Vestibule and I spent way too much of my brief SSF
career working on stuff for it. :-p

--
You can run on for a long time,
Sooner or later, God'll cut you down.
~Johnny Cash

"Herb Schaltegger" wrote in message
.com...
On Fri, 2 Feb 2007 14:54:49 -0600, Jeff Findley wrote
(in article ):

connecting/disconnecting umbilicals in that
small pressurized space between the MPLM's hatch and the node's hatch

. . . it's called the Vestibule and I spent way too much of my brief SSF
career working on stuff for it. :-p

Would berthing of the shuttle, instead of docking, helped make your job
easier, or do you think you'd have done much the same work due to the fact
that many of the connections between more permanent modules (than the MPLM)
are still done in the Vestibules?

Jeff
--
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor
safety"
- B. Franklin, Bartlett's Familiar Quotations (1919)