As previously mentioned, the payload bay and aft section airframes of my New Glenn scale model are not of a standard size, so I will have to make those tube sections myself. In order to make the airframe tubes, I will also have to make the mandrels, which are to be the subject of this post. I previously printed a few mandrel negatives so I could cast the mandrels with polyester resin. I think I made an error printing the aft mandrel negatives, so I will have to re-evaluate and try again. Each negative is 3 inches tall (to save on printer material), so mandrels will be cast over multiple sessions. I bought a couple of 2 foot segments of 4-inch PVC piping to reduce the amount of resin I would need to cast. I did a sample pour with the forward mandrel to prove my concept and noticed a few things. First, it was difficult to maintain a good seal at the bottom. This could probably be solved by planting the PVC and mandrel negatives in a base of clay or something similar. I also imagine this would only be an issue in the first pour. Second, the fumes from the polyester resin were overwhelming. I think the only properly-ventilated place to do this task is outside. The fumes also stick around a bit after the resin has cured, so I’m not a big fan of this method. The results of my first pour were pretty encouraging, though. I only used a few ounces of resin (so as to avoid waste from rookie mistakes), which is why the segment in the photos is so short.
As you can see, the alignment between the mandrel and the nosecone (which will fit into the tube made by this mandrel) is pretty good, so I am encouraged by the results. Following the casting of this short segment of mandrel, I purchased some air-dry modeling clay, which I think will actually be much faster to cast and have no fumes. To increase the hardness of this clay mandrel, I will glaze the clay mandrel with a thin layer of polyester resin (outside), which should also give a a few thou of tolerance between the manufactured airframe tube and the parts that mate into it.
Ramblings of a Rocketeer 