I don't think multiple copies of an afterburner have any effect.
And the problem with making it a secondary weap, IIRC, is that a weapon impact can't make you move any faster than your top speed.
There still might be a way to pull this off, however.
You'd have to have an abortable misn, that when you open the active mission dialog box, you can abort it. It could be called something like "Disable engine safties" or "Activate emergency speed" or something. This misn is started when the player buys the special afterbunrner (which isn't really an afterburner at all.)
In the OnAbort field of that misn, you have it grant you a speed/accel mod, and start a new misn with a self-destructing dude.
From here, you could then have it also either:
A. A mod that drains your armor. An unfourtinate side affect here is that your armor will drain a predetermined amount regardles if you actually use your thrusters or not.
B. Alternately, you can now use the weapon impact trick mumbling psyco just mentioned. If you use a forward firing negative impact weapon, instead of a rear firing positive impact, it might make the escorts safer from the blast. At any rate, this will require lots of tweaking to get right. I also believe that the weapon has to physicaly exit your ship sprite before its impact will affect you.
You could also try a negative speed, high recoil, (negative recoil doesn't work) but I seem to remember that not working right. I think your ship always recoils backwards for normal weapons.
As you can see, my memory is a little fuzzy about the nuances of all this, so you'll have to do a lot of testing.
Ok, so however you get your ship to go faster, you still have that self destructing dude misn running now.
So you make this dudes death delay however long you want the "emergency speed" to last.
In the OnShipDone field you then remove the speed mod, and special items that were granted earlier.
Then you start the first misn again, so the player can activete it again.
When the player sells the special afterburner, you abort the first and second misns. This will clear them both out.
This post has been edited by Desprez : 22 April 2006 - 12:27 PM