I am making a sp map and you start out in a pelican, and I got marines in the seats but after awile they jump out? My script is:

(script startup start
(sound_looping_start levels\b30\music\b30_01 none 1.00)
(ai_place covenant1)
(ai_allegiance human player)
(object_create 1)
(object_create 2)
(object_create 3)
(object_create 4)
(object_create 5)
(object_create 6)
(object_create 7)
(object_create 8)
(object_create 9)
(ai_attach 1 human)
(ai_attach 2 human)
(ai_attach 3 human)
(ai_attach 4 human)
(ai_attach 5 human)
(ai_attach 6 human)
(ai_attach 7 human)
(ai_attach 8 human)
(ai_attach 9 human)
(unit_enter_vehicle (unit (list_get (players) 0)) gas P-riderLF)
(unit_enter_vehicle 1 gas P-riderLB02)
(unit_enter_vehicle 2 gas P-riderRB01)
(unit_enter_vehicle 3 gas P-riderRB02)
(unit_enter_vehicle 4 gas P-riderRB)
(unit_enter_vehicle 5 gas P-riderLB)
(unit_enter_vehicle 6 gas P-riderRM)
(unit_enter_vehicle 7 gas P-riderRF)
(unit_enter_vehicle 8 gas P-riderLM)
(unit_enter_vehicle 9 gas P-riderLB01)
(recording_play gas foehammer_first_in)
(sleep 1000)
(recording_play gas foehammer_first_drop)
(unit_exit_vehicle 9)
(unit_exit_vehicle 8)
(unit_exit_vehicle 7)
(unit_exit_vehicle 6)
(unit_exit_vehicle 5)
(unit_exit_vehicle 4)
(unit_exit_vehicle 3)
(unit_exit_vehicle 2)
(unit_exit_vehicle 1)
(sleep 100)
(recording_play gas foehammer_first_exit)
)

Are they doing the unit_exit_vehicle before they are supposed to? And why aren't the (recording_play gas foehammer_first_drop) and (recording_play gas foehammer_first_exit)
playing ingame, their just shutting down after the first animation. Did I miss a command? And how do I make it so you CAN'T exit a vehicle untill a certain script?