I don't think it's all that often that you hear about either the Rotring Core or the Rotring Primus anymore. I think the Rotring Core has an odd sort of notoriety since not that many people were able to appreciate its odd looks. The Primus is very rarely mentioned on FPN at all. It wasn't available in the US. Anyway, when I first started using fountain pens, the Rotring Core was very easy to find in the states and quite readily available. I have always had a soft spot for Rotring, but despite this, I didn't really appreciate the distinct design of the pen. My sister really loves the Rotring Core though, and she collects them. I never appreciated the Rotring Core until someone sent me a Rotring Core Eternium which is the light apricot, light grey, and silver pen with dark steel blue grey letters on the barrel. It's quite a large pen, but due to the way the grip is designed, it's not actually unwieldy or huge in the hand. It's certainly an eccentric pen in a number of ways, but that's not to say that it isn't well-designed because it is.
The basic design of the pen is very ergonomic. The pen is well balanced, and it is designed to have minimal weight. The grip of the pen is quite special and is a grip that is really only possible with a fountain pen if one wants to use standard refills that are readily available. Ballpoint and rollerball refills are designed for pens that need more cylindrical grips. The design is done so that the fingers lie below the axis of the nib. This allows the hand and fingers to be relaxed in a lower position than the top of the nib. Basically, you can just put it in your hand, and it will sit in a proper position so to speak. I think it's a good design for beginners since it forces a beginner to hold the pen in a specific way, and allows the beginner to hold the fountain pen properly. It limits the ways one can hold the pen, and forces one to adapt a more relaxed grip. In my experience, it seems to get rid of the habit of holding the pen in a death grip since one can just rest the pointer and the thumb on top of the pen with very minimal pressure, and the pen will write. This grip design is unique in the fountain pen world. If one has never tried writing with a grip like this, I have to say that it's quite nice. Oftentimes, if I see a grip like this, I try it out the way I think the designer intended to maybe try to see what was going through their mind when they designed it. I think though that the really odd grip shape really scares off a lot of people who are used to more traditional fountain pens with cylindrical grips. I still think though that it's one of the best designs I've seen for an ergonomic fountain pen grip. And, no, I don't think I would have had the guts to admit that before. Once I wrote with this pen, I was able to adapt a more relaxed grip with my other pens.
In this picture, you can see how the grip is below the top of the nib. You can't do this with a rollerball or ballpoint that uses regular cartridges. The Rotring Core and Primus still take international cartridges which you can find in a lot of places.
Moving to the filling system, these pens use international cartridges or converters. When using a converter it's important to use a new converter and dedicate it to the specific pen you are using. The connector has a more narrow diameter than a lot of other pens, so if you use a pre-used converter, it might fall off. The same is true for the Pelikan Pelikano. Also, the regular Schmidt converter doesn't fit very well. It might get stuck in the barrel. One should use Rotring converters or older international converters. The Schmidt converter has the ring on the body high enough that it doesn't fit into a lot of pens with tighter tolerances in the barrel. I have a bunch of other pens that fit other international converters but can't use the Schmidt one because of this. In any case, one of the most important things to note about the filling system is that the pen has a reserve function. This reserve function isn't seen in pens that use cartridges very often. It's mostly seen in pens with self-filling mechanisms. Some German school pens that are piston-fill have a tab under the nib for the reserve function. So, how does it work? The pen has a cartridge connector that is several mm longer than normal as well as a small piece with tabs that click into a groove in the barrel. When the barrel is unscrewed until it stops, this piece pulls the cartridge out of the grip section slightly. This allows the ink that was trapped around the cartridge connector to flow into the pen. When using the filling system, when the pen stops writing, one should unscrew the barrel until the first stop. The pen will then be able to write some more. When the pen has stopped writing, one can then slowly unscrew the barrel all the way. Often one can hear the tabs disengage from the ring in the barrel once the barrel is turned past the first stop. If one does not stop at the first stop and keeps turning the barrel, the tabs will not be able to disengage. In other words it's important to stop at the first stop. Once the barrel is off, the cartridge or converter should remain attached to the grip section. One can then reset the ink reserve function when using a converter by pushing the converter all the way in then fill the pen as normal. In the case of the cartridge, the ink reserve function is automatically reset when pushing in a new cartridge. In the second generation Core, which is a very rare pen. My sister has one, but I haven't seen another one. There is no first stop, and the grip unscrews all the way. There aren't tabs in this version, and the cartridge connector and the tail of the feed are shortened. This is a very rare variant, and I don't think any of them ever made it to the US officially. I still wonder how long it took for Rotring to figure out how to design this system. It's hard to make a system that works as reliably as this one does. When used properly, the tabs engage and disengage very predictably.
Here is a diagram to show how it works a little better
Here is the barrel unscrewed all the way
This is what the tabs are supposed to look like when the barrel is unscrewed all the way
Here is a side view. They are nearly flush against the sides of the barrel
This is how it looks when the pen is freshly refilled and the reserve function is reset
Here is a side view. You can see how the tabs stick out
Here is the Core and Primus side by side. The Primus is in the reset position. The Core isn't
This is the Primus packing. You get a cartridge and stickers in a triangular box.
This all said, this still makes me wonder what would have happened if Rotring made a more traditional looking pen with the ink reserve feature where one would unscrew a red ring to a stop to use the feature, then unscrew the barrel behind the red ring to remove the barrel. It might have been an interesting idea. Or maybe a more traditional-looking pen with a version of the odd grip. One must admit that the cryptic instructions included with the Core didn't do much for the ink reserve feature or any other feature in the pen. The instructions included on the box of the Primus (in full color!) explain the ink-reserve feature very well. The instructions on the Primus also describe the other features of the pen in simpler, more understandable language than the instruction leaflet included with the Core. The pen may have been more of a success if they made a bunch of companion models and took the time to highlight the unique features of the pen more carefully and perhaps made a more traditional version to appeal to regular, somewhat older fountain pen users. I also still wonder why they allowed the Core to be posted. Maybe it is to teach people not to post pens? It's the same length as the Primus, which should be decent enough to use without posting. Maybe the pen would look off if it didn't post since there isn't an excuse to put a larger piece on the end to prevent posting. I also still wonder why the Core wasn't exactly idiot-proofed since there are features that don't work as well unless one knows how to use them properly.
I must admit that these pens are not necessarily my normal realm in a sense. Most of the pens I have and prefer are smaller, prettier, and come with more graceful, smoother shapes, but I suppose I can still appreciate something like this. If anything, I suppose the Core does look sophisticated next to the Primus, although that's not really a word one uses with the Core. I guess Rotring had its own world with its own rules and design language. The Core stands out if you put it next to any pen, but I guess it does look more at home next to other modern German school pens.