Use separate random sequences for geometry versus materials,

and keep seed values on redo (letting user edit them if desired)
so spaceship design only has to change on each new invocation.

__init__.py

@@ -2,7 +2,7 @@
     {
         "name" : "Spaceship Generator",
         "author" : "Michael Davies, Lawrence D'Oliveiro",
-        "version" : (1, 2, 2),
+        "version" : (1, 3, 0),
         "blender" : (2, 82, 0),
         "location" : "View3D > Add > Mesh",
         "description" : "Procedurally generate 3D spaceships from a random seed.",
@@ -19,6 +19,7 @@
     from . import spaceship_generator
 #end if
 
+import random
 import bpy
 from bpy.props import \
     BoolProperty, \
@@ -32,10 +33,15 @@ class GenerateSpaceship(bpy.types.Operator) :
     bl_options = {"REGISTER", "UNDO"}
 
     df = spaceship_generator.parms_defaults # temp short name
-    random_seed : StringProperty \
+    geom_ranseed : StringProperty \
       (
-        default = df.random_seed,
-        name = "Seed"
+        default = df.geom_ranseed,
+        name = "Geometry Seed"
+      )
+    mat_ranseed : StringProperty \
+      (
+        default = df.mat_ranseed,
+        name = "Material Seed"
       )
     num_hull_segments_min : IntProperty \
       (
@@ -97,6 +103,17 @@ class GenerateSpaceship(bpy.types.Operator) :
       )
     del df
 
+    def invoke(self, context, event) :
+        maxseed = 1e6
+          # [0 .. 999999] is enough to be interesting by
+          # default. Users can always replace seeds with
+          # anything they like.
+        self.geom_ranseed = str(random.randrange(maxseed))
+        self.mat_ranseed = str(random.randrange(maxseed))
+        spaceship_generator.generate_spaceship(self)
+        return {"FINISHED"}
+    #end invoke
+
     def execute(self, context) :
         spaceship_generator.generate_spaceship(self)
         return {"FINISHED"}