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Scratchbuilding the “Princess Cecile” – the perils of resin casting

July 3, 2012

If you read the previous post on my scratchbuilding project, the Princess Cecile from David Drake’s Lt. Leary/RCN novels, you know that I have been working with resin casting a bit, trying to duplicate my master for the “thruster clusters.”  I have a couple of issues with the design of the thruster clusters yet, including the fact that they are big – not big on the model, but in the 1:200 scale of the model they would be about 15-20 feet across! Each rocket nozzle would be about 5 feet tall. However, the ship is supposed to be 230 feet long and it should displace about 1300 tons, so moving such a massy object would take some pretty big thrusters. In particular, the thrusters on a warship should be oversized compared to a commercial vessel so that they can maneuver quickly in a battle situation. The Sissy must be nimble!

So, I’ve been trying to get better results making resin castings of the master. The first mold I made yielded only far results, with some bubbles in the surface because I was overzealous in using mold release. I also used the wrong mold release agent – stupid! I used the aerosol kind intended for making resin castings, not the liquid stuff used to keep the master from adhering to the master.

I decided I needed to make new masters. I tried making them in the other orientation – with one nozzle pointing down – and making it in two halves that would be poured without putting them together. The problem there was that two thrusters would be made in halves, and the downward-facing one would be hard to pull from the mold. The technical term is undercut, meaning that the bottom of the nozzle has a larger diameter than the throat above it. That would require me to pull the resin copy out by forcing it through the rubber.

I actually tried a couple of these and they, frankly, sucked. I decided that I would just make a better version of the first two-part mold.

The master, sitting in clay, for the mold that didn’t work.

The master that didn’t work, both parts.

Both halves of the new mold, which looks a lot like the first one. I used big toothpicks to create the air bubble release holes instead of wire.

And I was so excited that I forgot to spray the inside of the mold before I poured the resin, and got this:

Ugh! And the nozzle broke off trying to pull it out of the rubber.

I sprayed the mold and tried again. This time I got a better result:

It was easier to pull out of the rubber this time, but it was still dicey.

The problems pulling the copy out of the mold were reduced, but not eliminated. Those long slabs on the image above are the air bubble holes, filled up with resin. The resin flowed through the mold well, however, and there were no large bubbles. The other problem is that the very bottom of the mold is made of clay, not rubber. It helped hold the master upright when making the mold. A little comes loose with each pull, even after spraying it with mold release. This one looks a lot cleaner, though, and except for removing the plugs on the ends of the nozzles, this one will take much less time to clean up.

By the way, while this one was curing, I cleaned up the other one to see if it was salvageable.

I still have to find a way to glue that nozzle on.

It would need more precision sanding before I would be ready to prime it.

I’ve considered having a more complex shape made in CAD and then having it grown in a 3D printer. I just don’t have the chops with a 3D program to make that happen. I tried a couple of programs, but the free ones don’t seem to work very well and the cost of the higher-end versions – whew!

There is an advantage to using these big thrusters on the Sissy, though. In the books, the riggers sometimes had to remain out on the hull while the ship moves in real space, not just in the Matrix. In the Matrix the ship moves by adjusting the direction of the sails, but once the ship was out, it often jumped right into the thick of battle. One of Daniel Leary’s skills is being the best astrogator this side of his uncle Stacy Bergen, who plotted many of the routes from star to star still used in Leary’s time. Leary could drop out of the Matrix closer to a planet than almost any other captain. This advantage meant that often the ship dropped out of the Matrix and right into the fighting, without having the time for the riggers to come back inside. Often they stayed outside anyway to cast off broken rigging before it fouled the guns or the missile tubes.

So anyway, if the thrusters were smaller and closer to the hull a rigger could be in the way of the exhaust of the rocket engines. Bigger clusters put the nozzles higher, farther away from the hull, where they would be less danger to the riggers. At least, that’s my rationalization!

One other thing. I’m playing with this possible High Drive motor:

I don’t think I like it. And i’s too big.

This thing is about an inch and three-quarters tall. Since an average person is about 3/8″ in this scale, this motor would be almost 30 feet tall in scale! In one of the books – I think it’s “The Far Side of the Stars” – the crew has to install some High Drive motors on another ship while both are on land, not in the water. Adele had hacked into the other ship’s computer and re-routed the plasma thruster controls to the High Drive motors. Using High Drive motors anywhere other than in vacuum is very dangerous, because they operate by matter-antimatter annihilation. The result was that the ship was badly damaged, but the was repaired by the crew of the Sissy and sent back as a sort of Trojan Horse. Since the High Drive motors were destroyed when they were used on the ground, the crew pulled some motors off of a “country craft” that was abandoned in the jungle nearby.

The description doesn’t make it sound as if moving the motors was an insurmountable problem, just with the resources carried on the two ships. It is mentioned that each motor weighs about half a ton. This is about the same weight as a SpaceX Merlin first-stage rocket engine probably weighs. I could see the High Drive motors as about the same size as a Merlin as well. This puts them about the same size as one of the rocket nozzles on the thrusters.

Part of the problem of the prototype above, besides size, is that the wire winding (which is to represent that used to create the magnetic field required for an motor using antimatter) is too big and probably beyond my skills to make prettier. A High Drive motor should be a device that generates and manipulates magnetic fields to control the mix of matter and antimatter.

In the second book the High Drive motors are on the bottom of the main hull, all together. Later they are described as being on the outriggers, in case something bad happens. However, how do you send antimatter to the engines if they are on the outriggers? That’s almost a bigger problem than building the engines!

The motor may not even need a physical nozzle, since the antimatter can’t touch it. I think I have to look into antimatter engines more. Robert Forward described how to make ships operate using “mirror matter,” but not in detail for small motors like this. And most of his required just a little antimatter mixed with more regular matter, usually hydrogen, and superheats the un-annihilated hydrogen to use as reaction mass, as a sort of super-rocket engine.

I know, you can’t study this so close or the physics breaks down! How do they make antimatter, anyway? They even have missiles with small High Drive engines on them, so controlling the magnetic fields must be old hat for these crews. I know – just ignore some of this and read the damned stories!

But to build something that looks believable – well, I have to get my head into the RCN universe and extrapolate. A lot.

My next project after this is the Alliance Space Ship Vorpal Blade, from the John Ringo/Travis Taylor book. It was built from the former USS Nebraska, a ballistic missile submarine! And it has a gigantic – well, sword – on the front! Doc Travis gets the science as right as possible, within the limits of storytelling. Giant intelligent chinchillas on antigravity golden surfboards?

(I have to do this one because I was a “red shirt” in the book. Long story, but John Ringo put a bunch of folks in as red shirts and then of course got them killed off. My character, an intel geek who completely misses the evidence pointing to an incoming invasion of “demon” dog-like creatures, gets torn apart by one of said creatures. And hey, John, I wear glasses and I suppose I’m kinda geeky, but my ears do not stick out! :))

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One comment

  1. […] of you who have followed my ramblings as I slowly work on a (mostly) scratchbuilt model of the starship Princess Cecile from David […]



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