Trailing effect for rockets or jets?

function smokefnc(PARTICLE *p)
{
    p.alpha -= p.skill_a*time_step;
    p.size += p.skill_a*time_step;
    if (p.blue > 0) { p.blue -= p.skill_a*time_step*2; }
    if (p.red > 0) { p.red -= p.skill_a*time_step*2; }
    if (p.green > 0) { p.green -= p.skill_a*time_step*2; }
    if (p.alpha <= 0) p.lifespan = 0;
}

function smoke(PARTICLE* p)
{
	vec_set(p.blue,vector(150,150,150));
   set(p, MOVE | BRIGHT | TRANSLUCENT);
   p.alpha = 50+random(20);
   p.size = 0.1+random(1.5);
   p.gravity = -0.025;
   p.skill_a = 2; // fade factor
   p.event = smokefnc;
}


effect(smoke,1,my.x,nullvector);

effect(smoke,1,vector(my.x+random(16)-8,my.y+random(16)-8,my.z+random(16)-8),nullvector);

void p_fnc(PARTICLE* p)
{
	set(p,MOVE);
	...
	p.lifespan = 16;
	p.event = p_fnc_event or NULL;
}