In article <1993Apr30.101054.1@stsci.edu> hathaway@stsci.edu writes: >In article <1rq3os$64i@access.digex.net>, prb@access.digex.net (Pat) writes: >> In article <3t75nhg@rpi.edu> strider@clotho.acm.rpi.edu (Greg Moore) writes: |> | |> |	As Herny pointed out, you have to develop the thruster. |> |Also, while much lighter, you still have to lift the mass of |> |the thruster to orbit, and then the thruster lifts its own  |> |weight into a higher orbit.  And you take up room in the payload |>>bay. |>> |>  |> a yes,  but the improvement in  boost orbit to the HST is Significant, | |I do not understand what you are saying here.  What is improved, what  |is Significant, and what does this have to do with carrying more  |equipment on a servicing mission?  Also, as implied by other posters, why  |do you need to boost the orbit on this mission anyway?  Maybe you have  |something here, but could you please clarify it for us on the net?  |  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.  |> and  that means you can then carry EDO packs  and enough consumables |> so the SHuttle mission can go on long enough to also fix the |> array tilt motors,  and god knows what else  is going to wear out | |From what I've heard, the motors are fine - it is one of the two  |sets of electronics that control the motors that needs a fix.  The  |motors and electronics are separate pieces of hardware.  I expect  |to be corrected if I'm wrong on this.  |  a Whatever it is,  the problem in the tilt array is a big constraint on HST ops.