Shuttle Questions

Here are some random shuttle questions I have always wondered about but
kept forgetting to ask:

1. Why do the SSMEs need to be firing when the stack lifts off?

2. Kind of related to the first question...what would happen if the
SSMEs quit right at lift off, aside from not being able to initiate the
roll program? Would the stack pitch backwards? I guess I'm wondering if
their thrust helps compensate for the orbiter's weight at liftoff, or
would the SRBs be enough to launch straight up?

3. Do the umbilical lines attacked to the sides of the orbiter under
the OMS pods help the stack stay in place on the pad, or do the SRB
bolts hold the entire stack themselves?

Thanks!

On 5 Apr 2006 08:26:18 -0700, wrote:


1. Why do the SSMEs need to be firing when the stack lifts off?

a) It allowed NASA to move a lot of the pre-ignition systems off the
Shuttle and on to the Pad, saving weight.
b) It allowed lighter structural supports between the Orbiter and the
External Tank, saving weight (the SSMEs keep the Orbiter from being
ripped off the side of the tank during Max Q.)
c) It improved mission safety, ensuring all three SSMEs are in good
working order prior to liftoff.

2. Kind of related to the first question...what would happen if the
SSMEs quit right at lift off, aside from not being able to initiate the
roll program?

Roll program is handled mostly by the SRB nozzle gimbals, although the
SSMEs help.

Would the stack pitch backwards?

Not immediately, although control would probably be lost at Max Q.

I guess I'm wondering if
their thrust helps compensate for the orbiter's weight at liftoff, or
would the SRBs be enough to launch straight up?

It would still go up, but more slowly, and control might be lost, I'm
not sure. (Shuttle weight: 4.5 million lbs., Combined SRB thrust: 5.2
million lbs.)

3. Do the umbilical lines attacked to the sides of the orbiter under
the OMS pods help the stack stay in place on the pad, or do the SRB
bolts hold the entire stack themselves?

The SRB bolts/posts hold the whole thing. the Tail Service Masts are
just that... for servicing.

Brian