A JS32 FLT CREW FORGETS ABOUT ASSIGNED HEADING AND INSTEAD FLIES RWY HEADING.

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ATIS WE RECEIVED INDICATED RWY 15 IN USE FOR DEP. WE RECEIVED CLRNC TO IAD INCLUDING L TURN TO 220 DEGS AFTER TKOF (THIS IS NOT THE PROC USED FOR A RWY 15 DEP). WE DISCUSSED PROCS FOR 1) ENG FAILURE DEP PROCS; THEN 2) OBSTACLE DEP PROC; THEN 3) THE STANDARD DEP; ALL 3 USING DIFFERENT NAVAIDS; HEADINGS AND ALTS. WHEN WE CALLED FOR TAXI; WE WERE GIVEN RWY 24 FOR DEP; WHICH REQUIRED WE BRIEF THE DIFFERENT SINGLE-ENG AND OBSTACLE CLRNC DEPS RELEVANT TO RWY 24. UPON DEP ON RWY 24; WE FLEW RWY HEADING (240 DEGS) VICE 220 DEG HDG ASSIGNED IN OUR ORIGINAL CLRNC. THE CONFUSION IN HEADING FLOWN WAS PRECIPITATED BY SEVERAL EVENTS. 1) CHANGE OF RWYS AFTER BRIEFING INITIAL DEP PROCS; 2) THE COMPLEXITY OF AN OBSTACLE DEP PROC DIFFERENT FROM THE ENG FAILURE PROC; WHICH INCLUDED DME'S OFF OF 2 DIFFERENT VOR'S AND 1 LDA FREQ; 3) NO MENTION OF HEADING TO FLY CONTAINED IN THE ACTUAL TKOF CLRNC. THE FLT CREW MISSED/FORGOT THE ORIGINAL HEADING OF 220 DEGS; BUT THE ERROR WOULD HAVE BEEN AVOIDED IF EITHER THE TWR CTLR REPEATED THE HEADING IN OUR TKOF CLRNC (BEST SOLUTION?) OR WE HAD REVIEWED AND CONCURRED ON THE HEADING TO FLY AS WE CROSSED THE HOLD SHORT LINE. WE WERE AT FAULT; BUT THE COMPLEXITY OF VARIOUS DEP PROCS; IN ADDITION TO A RWY CHANGE DURING TAXI; CAUSED US TO LOSE TRACK OF THE MOST IMPORTANT POINT -- HEADING ON TKOF.