Code:
function up() {
	var i = 0;
	while(height <= 1531 && !key_x) {
		i += 0.1*time_step;
		i = clamp(i,0,2);
		height += i*time_step;
		wait(1);
	}
}


dont know what type fo aceleration you are seaching for but his is the most easly thing to use

"empty"