a longitudinally extendible , segmented boom structure includes a plurality of hollow , substantially v - shaped segments which are pivotally connected to adjacent segments for storage . a series of load - bearing cables extend through the segments for carrying loads . the segments have uniform cross - sections and progressively increasing lengths . segments are stored in rolled - up form on a reel by pivoting the segments to form a coil of nesting segments . each segment has laterally extending side flanges through which longitudinally extend the pre - tensioned load - bearing cables . the cables pass through the flanges on opposite sides of the boom segments for supporting vertical loads and resisting side loads on the boom . a second set of auxiliary cables on the underside of the boom is automatically payed out as the boom is extended . the auxiliary cables prevent the extended segments from collapsing when inverted . the center point of the segment reel is adjusted in elevation to automatically feed boom segments at an optimum angle into a segment dispenser and boom support structure which has rollers thereupon for cooperating with corresponding lands and grooves on the segment flanges .