Conference 7.286::space

  The TDRS and it's booster are very large and take up most of the space in the
cargo bay. It's possible that they were short on room.

  Also when you load an aircraft or space craft there are things to consider
other than gross weight. For example you must calculate the center of gravity
of the payload and it has to be within certain limits.

  Keep in mind that these have to be calculated not only for accent but also
for an aborted landing.

  George

>  Keep in mind that these have to be calculated not only for accent but also
>for an aborted landing.

and for descent having deployed the primary payload

Having said this, is there any method for "adjusting" the CG dynamically? Is 
there some portion of the water/waste/fuel that is pumped end to end as needed?

>Having said this, is there any method for "adjusting" the CG dynamically? Is 
>there some portion of the water/waste/fuel that is pumped end to end as needed?

  Not that I've ever heard of. That would involved a lot of extra weight
to support the pumps and extra tanks.

  George

They can't move the fuel around, but OMS propellent can be dumped or
burned to adjust the CG.  This is often done prior to entry and propellant
dumps are part of the various abort sequences.

There are over 2 tons of propellants which can adjust the center of gravity
up to 7 inches.


- dave

    Note also that the capacity used to be advertised as 65,000 pounds
    (32.5 Tons), since Challenger, it has been much lower.  40K or so?
    
    Burns
    

    
    The reason shuttles go up under-loaded can't be because of Center
    of Gravity problems.   1. Pot-luck payloads can be balanced and 
    2. the shuttle CAN adjust to a changing center of gravity, just 
    look at what happens during launch, the shuttle starts with a fully 
    loaded ET and 2 SRBs, that are gradually emptied and then dropped 
    off. If that doesn't change the center of gravity nothing will.
    
    It must be because they want to keep  the extra fuel margin as a 
    safety margin as reply .5 (Burns) sugests. Or that they consider 
    pot-luck payloads not to be worth it  (for some strange reason) 
    or a mixture of both these reasons.
    
    As a guess-timate the 
    TDRS (2.5 tons) + IUS (7 tons) + cradle (.5 tons) = 10 tons
    Leaving lots and LOTS of safety margin (20 tons worth)  !!! 
    
    Still at shuttle prices the cost of NOT putting things in LEO
    is expensive, can we afford it.
    
    			 1 ton	= $ 15 Million
    			 5 tons = $ 75 Million 
    			10 tons = $150 Million
    Gil
    

    Re: reduced Shuttle payload

    As I understand it (and I welcome corrections here), the reduced
    payload figure corresponds to the maximum weight at which the orbiter
    can land.  This is critical in any launch abort situation involving
    RTLS or TAL, since there's obviously no opportunity to dump the payload
    prior to "touchdown".  

Re: .6

I'm not sure how you are so sure of that, but I will point out that
there is a difference is compensating for CG during ascent by modifying
the gimbaling of the SSME's through the (changing) center of gravity and
modifying the CG during an unpowered descent.   [Any glider pilots out there
who would care to comment on how critical CG is to the descent profile of
an aircraft -- anyone care to speculate how critical it might be to a flying
brick? :-) ]

The facts are that in addition to carefully laying out the payload bay, lead
weights are placed under the nose wheel and in the aft compartment to properly
balance the shuttle.   OMS propellant dumps are also MANDITORY for both
descent and RTLS/abort scenarios -- and there is a finite limit (7 inches)
to what the propellant dumps can achieve.


There are a variety of safety and flight dynamic factors which determine
payload weight and distribution.  I fail to see how anyone can dismiss
CG out-of-hand -- though I have no knowledge that this is the critical
or limiting factor.


-dave

  I agree with .7. When landing from an abort the Shuttle must be able to glide
in for a landing and the weight distribution becomes very important. 

  Also as I mentioned earlier there is the space problem. The TDRS and it's
booster are very large and they don't leave much room for anything else in the
cargo bay.

  There are other considerations as well. Most payloads require some sort of
power which is in limited supply. Also the TDRS must be kept super clean. It
would be a real waste if the "floating spider" experiment from Blaston Jr High
fell apart and a spider drifted into the wrong place damaging a $100 million
communication satellite. 

  Then there is also the problem of limited crew time which would be taxed by
any experiment requiring human intervention. 

  In general NASA seems to be very efficient in planning missions and the
aero-space industry in general is very much aware of weight restrictions and
use of payload space. It is the combination of factors including weight, center
of gravity, space, crew safety, shuttle safety, payload safety, power, heat,
oxygen use, water use, crew time, and other factors that limits what can fly in
any one mission. 

  George 

    CG is very important during ascent for a vehicle with asymmetric thrust
    (i.e. the shuttle). It plays a major part in determining the gimbal
    profile for the SSMEs and SRBs as the 'center of thrust' has to be
    aligned with the CG. As you know, there isn't a lot of margin in the
    shuttle's lift capabilty so every factor counts.
    
    For glide, the relationship between the CG and center of pressure (or
    lift) becomes important, but for any given design there is usually a
    range of 'safe' CG locations. Keep the CG within that range and the
    bird will behave predictably for reasonable angles of attack. For low
    lift vehicles (aka flying bricks), there is usually a fairly wide range
    of acceptable CGs.
    
    I don't know enough about reentry to comment on the importance of CG.
    None of my rocket gliders have ever had to deal with that :-)
    
    I suspect that the lack of small payloads (GAS cans etc) may be more
    related to more stringent safety qualification requirements in the
    post-Challenger era. Unmanned recoverable capsules are probably
    cheaper. Maybe the Russians could make some money by flying modified
    Vostok or Soyuz spacecraft as GAScan carriers?
    
    gary
    
    p.s. It used to be said that to qualify a vehicle for spaceflight, NASA
    required a stack of paper at least as tall as the vehicle. To qualify
    something for a manned spaceflight required a stack of paper as tall as
    the altitude you intended to reach.