LOX tank placement

[rubicondsrv]

why do many LV's place the heavier LOX over the LH2/RP1 it would seem that this adds structural mass to the design? <br /><br />The only design that I have seen that places LOX over the fuel is Delta II/III.<br /> <div class="Discussion_UserSignature"> </div>

 

[tap_sa]

You want heavy/dense stuff in the front and light/bulky in the aft in order to move cg(center of gravity) forward and cp(center of pressure) backward. Object moving through the air prefer to keep their cg first followed by cp (=stable flight). If you'd have them reverse the rocket would constantly want to flip around.

 

[danwoodard]

The Saturn upper stages did place the LOX at the back for this reason. However both the Shuttle and the Delta IV Heavy have widely spaced engines at the base. When the Shuttle SRBs drop off, the main engines must be able to gimble far enough the keep the line of thrust through the center of gravity. <br /><br />If you imagine the Shuttle with the LOX tank (which weighs eight times as much as the larger LH2 tank) at the base of the external tank, you can see this would place the CG just ahead of the main engines but several meters away from them laterally. There is no way they could be gimbled far enough to thrust through the CG. <br /><br />In the case of the Delta IV, in the heavy version the three engines are widely spaced at the base. If an outboard engine fails, the remaining two engines are very asymetrical and must gimble far enough to keep the thrust line through the CG. If the CG is just ahead of the engines, this can't be done.

 

[rubicondsrv]

If an outboard engine on Delta IV Fails it will result in the payload not making orbit. <div class="Discussion_UserSignature"> </div>

 

[danwoodard]

Depends on how close to the planned MECO the problem occurs, and how close to the performance limit the payload is. But at least the vehicle won't depart from controlled flight. There may have been some thought to a potential man-rated version.<br />