I don't recall exactly where the CP is, but I can tell you that my recollection of my RockSim model was that the rocket was "over" stable with either an Estes D or even and Estes E engine in it. This model was built exactly like the plan, just replacing the BT20 stuffer/engine mount with a similar length of BT50.
From personal experience, I know this works without any nose weight. Great, straight-as-an-arrow, nice high flights. The Citation Patriot is a fabulous D engine rocket for demos. Great looking, lots of noise and smoke, nice high (but not too high) flights. Es make the thing *really* cook, but there are better birds for cloudbusting than to waste E's on the Patriot, IMO.
Sorry I can't answer the question about a 2 or 3 x 18mm (C) cluster arrangement; I never did that sim.
I would just re-open that file, but you can't save them with my demo version.
Am I off target in assuming that the basis of your question was to see if you could increase the power without changing much else?
You're right one with your assumptions. Looking to build an interchangeable motor mount in the "as designed" 18mm, a 24mm, and a cluster (probably 3) configuration. I guess I'll sit down with VCP and try to get an approximation.....Unless someone comes up with something in the meantime.
Now that I'm home I slapped together the Citation Patriot on RSim again (it's a rather easy rocket to model.)
Anyway, the CP is located at 22.128" from the tip of the nose cone via Barrowman, 22.398" via the RSim method. Don't bother breaking out your ruler - this is almost exactly at the point where the leading, or "front" edge of the fins meet the body tube. Or, to say it another way, it's the leading point of the root edge of the fin.
Loaded with 3 x C6-7's it's still very stable (margin of about 2.5 calibers, or the CG being over 4" in front of the CP.)
-PNC 60AH (from the database)
-Estes BT60, 18" long (from database)
-Fin set, free from layout, 0.125" thick balsa, three fins. The (x ,y) coordinates, starting at the leading point of the root edge, then going counter clock-wise: (0,0); (2.75,0); (3.5, 0.75); (3.875, 3.375); (2.25, 3.5)
-Inside tube of 8.65" in length, even with the base of the owning airframe tube. This is the motor mount tube. Use a BT50 for 24mm power, BT20 for 18mm power. Put an appropriate thrust ring 2.5" from the base of the owning motor tube. I always use a centering ring from the database for this (Apogee CR 13-18 for an 18mm motor block, Apogee CR 18-24 for a 24mm motor block). For an "E" mount, move the thrust ring forward 1" (to be 3.5" from the base of the owning motor tube.)
-Use the cluster wizard to cluster 3 motor tubes for the 18mm motor mount; otherwise leave it alone. Don't use centering rings between the motor tubes and airframe on the cluster mount. Rather, add 15 grams of mass object centered between the motor block and the base of the rocket airframe to account for epoxy covered wadding to fill gaps.
-For the 24mm mounts, add 0.05" thick cardboard centering rings between the motor tube and the airframe tube wherever you like. I put them at the forward end of the engine tube, and about .5" from the base of the owning airframe.
-Don't forget the mass object engine hook, centered between the thrust ring and the base of the airframe. I omitted the hook on the cluster mount, assuming a friction fit.
-I used an 18" plastic parachute from the database.
-I used a mass object of a 48" long 1/4" wide flat elastic shock cord, placed an inch or two in front of the engine tube.
-Two 1/8" launch lugs from the database, each being .5" long. Make one about 3.5" from the front of the body tube, the other flush with the base of the body tube.
Did I forget anything?
Just balancing mine on the side of my index finger, comparing the CG on the actual bird to the calculated CG on my 24mm RSim model (no engine in place), it's pretty darn close.