
RIght now the HST sevicing mission is listed as 11 days.  before
it was listed as 9 days.  they just kicked up the number of spacewalks
to 5,  after simulations indicated  that it was not do-able in 4.  

After all the space walking,  they are going to  re-boost the HST's
orbit.  I think right now  it's sitting  at 180 miles up,
they would like  220.  I don't know the exact orbit numbers.
I know when HST was first flown, it was placed in the Highest
possible Shuttle orbit. 

Now the shuttle can cary a thing called the EDO pallet, or extended
duration orbiter pallet.  It's mostly  LOX/LH for the fuel cells
and RCS gear,  plus more O2  and canisters for the life support
re-breathers.  maybe more nitrogen too.

THe limit on space-walking is a function of suit supplies (MASS)
and Orbiter Duration.   

In order to perform the re-boost of the HST, the OMS engines
will be fired for a long period.  Now the shuttle is a heavy
thing.  THe HST isn't light either.  THe amount of OMS fuel
needed to fly both up is substantial.   a small booster
carried up and used to boost HST on it's own will weigh significantly
less then the OMS fuel required to Boost  both HST and SHUttle,
for a given orbital change.  

From what i understand,  the mass margins on the HST missions are
tight enough they can't even carry extra Suits or MMU's.

Now if they used a small tug,  I would bet,  just a wild guess,
that the savings on amss margin  would allow carrying the
EDO pallet,  extra suits,  more consumables,  parts for the
flaky FGS sensor,  parts for the balky solar  electronics,

and still enough for a double magnum of champagne.

or the HST could even get placed into  some sort of medium orbit.
The reason they want a high orbit, is less antenna pointing,
and longer drag life.


a
Whatever it is,  the problem in the tilt array is a big constraint
on HST ops.