It's time to split the hull! The basic idea is to cut on the horizontal centerline until about 6" from the stern. I wanted to leave the last 6" as a single structure so it could support the control surfaces, etc.
It was relatively easy to mark the cut lines, but the cut. ohh what a pain that was. Lost foam is not ike a moulded hull where you can leave yourself 1/4" or more extra hull in the mould to trim off. In lost foam anything you cut is gone. If you take a saw that has a 1/16" blade width then when you mate the two halves after the cut they will be 1/16" short (unless you build it up again with a very careful resin pour). Ouch.
But I did not want to do that. So I looked for my finest saw. You know that balsa saw that cuts through balsa wood like it is a ultra low density soft wood. But you would not really think of using it on any material with a strength greater than butter.
That balsa saw, with it's 0.013mm blade, does cut through fiberglass very slowly...
I have tried to block this cut from my memory. When I say it was slow I mean it. I would saw and saw and saw and see a little progress. That was on the thin test hull. On the real hull with 3/32" hull thickness I kid you not it took me over 4 hours to complete the cut. I stretched it out into multiple sessions on multiple days. After the cut I had a numb spot on my index finger for over a month! Brrr.
But it was worth it. You had to be really carefull because you did not want the saw to jump out of the cut and scar the hull. The blade would heat up and then start binding more and cutting less.
But because it was such a slow process you had fabulous control over the cut. Over the full 60" length of cut I am sure that I was always within 1/2 a pencil mark width of perfect. If I had a finer pencil the cut would have been even more accurate. You did have to be careful because even the slightest course correction could create a little force that would tilt the blade off vertical. But you get used to that.
But in the end I got a cut like this:
I know it's out of focus, but even if it was in focus you could barely see it and it was true.
Of course the hulls do not fall appart because there is still 7" of foam holding them together. No problem... Just run the hull passed my hot wire cutter and ta-da. Wrong. First the wire was too thick for the 0.013mm cut! I had to go back to the local music store and get the finest wire they had. Then I had to adjust the voltage on my cutter because it would burn out the thinner wire. But eventually I was ready.
Notice how the test hull goes first... I am glad it did. As I pushed the hull onto the wire it went smoothly for the first couple of inches, then the full length of wire was busy cutting 7" of foam and it started to slow down... Then it stopped. The wire had bumped into a peice of glue (used to join the hull sections). That did not just melt away. And as I looked and thougtht about it I noticed the wire start to cut into the fiberglass of the hull!
There was now way the real hull was going through that! So in the end I took one of my trusty balsa saws and cut the back edge support off it. This way I could actually insert the full depth of the saw into the hull and cut the foam.
Besides cutting all the foam I could I also used this tool to ensure that the corner at the back end of the hull where the longtitudinal cut hits the vertical cut was truly complete. I would have hated to have the hull at that corner break or fray as I pulled the hull sections appart. It was difficult to do this part of the cut with the un-modified saw because the back edge of the blade could not go into the cut the blase was always cutting at an angle, and I did not want to overshoot the intersection point, so that left a little corner un-cut. With the modified saw I could get a nice square and complete cut from both the horizontal and vertical.
After doing the full length of the hull this way there was still an inch or more of foam in the center that was un-cut. So I put on heavy gloves and grabbed that fine piano wire and pulled with a gentle sawing motion....
There you have it. The hull parted into halves.
You can see the outline where the copper alignment tube was.
You can see the tail of the hull as a single unit so that all the control surfaces can be mounted to it.
You can see the bottlecap in the end of the hull. I used this when I was marking the hull. It gave me a nice flat surface to mark the center point of the hull.
Well then it was "lost foam" time
That is a lot of foam to remove. I started off using an xacto knife with a circular carving blade on it. The dinosours would have been re-born before I was done. So I switched to a large utility knife and I would make cuts down the full length of the hull. The first cut would be vertical, the second on an angle so that it intersected the first. Then I could remove a long triangular peice of foam. Much faster.
I would work to get a thin layer of foam next to the hull, then I could just grab the packing tape and pull the rest out. For the end peices (where it is still a cone shape) I took a large drill bit and twisted it into the foam (just using my hands, no power tools). When that removed enough I could grab the packing tape and pull out the rest.
And that left...
I almost forgot to mention the best bit. When you take the two halves of the real hull and put them together. It's perfect. You cannot see or sense the missing 0.013mm cut from the balsa saw.
It was worth having a numb finger for that month.