I'll contribute as my training is as a nuclear engineer and my work specialty is adjacent to nuclear fusion reactors, so I may know a thing or two that could be useful.
So currently, the TN power/engine techs below antimatter are (pulled from DB):
- Pressurised Water Reactor + Nuclear Thermal Engine
- Improved PWR + Improved NTE
- Pebble Bed Reactor + Nuclear Pulse Engine
- Improved PBR + Improved NPE
- Gas-Cooled Fast Reactor + Ion Drive
- Stellarator Fusion Reactor + Magneto-Plasma Drive
- Tokamak Fusion Reactor + Internal Confinement Fusion Drive
- Magnetic Confinement Fusion Reactor + Magnetic Confinement Fusion Drive
- Inertial Confinement Fusion Reactor + Inertial Confinement Fusion Drive
Conventional and AM+ tiers are probably fine.
The current proposed change is to remove Tokamak + IntCF, shift everything above that down one tier, and add a new high-end tech. In this case we only need 8 techs from fission through fusion instead of 9.
Having said this I would highlight a few problems with the current tech track:
- The "Improved" techs are just boring and not flavorful.
- Pebble Bed Reactors are a form of gas-cooled reactor, albeit not a fast reactor.
- There is not really a strong connection between the gas-cooled fast reactor and the ion drive, at least not compared to any other reactor type.
- Both stellarators and tokamaks are MCF devices, and having both of these as precursors to a generic MCF reactor is weird.
- "Internal confinement fusion" is too similar to "inertial confinement" and doesn't really have a clear definition.
Probably the best approach is to work out what engine techs we want. Here I think it makes sense to scrap the IntCF drive, for the reasons already given by Steve and others but also because it's frankly a bit suspect - we frankly don't have any more viable fusion methods beyond MCF and ICF (inertial) for practical purposes (you can do e.g. beam fusion but that's not going to be a viable power source). However there are some interesting MCF devices which could be used.
For the fission-era techs I would suggest including
Nuclear Radioisotope Engine (NRE) as a pre-NTE tech (moving NTE up to tier 2) as a companion to RTGs. These are such a simple kind of reactor that I think it makes sense to have the introductory TN reactors and engines be of this sort. The other tech I would suggest is a
Nuclear Gas-Core Engine (NGE) which is driven by exhaust from a gaseous fission reactor and could be an intermediate step to a plasma-driven TN ion drive.
Now we need to match this to reactor techs. Most of these are not too tricky, for example RTGs match to NREs, the existing PWR and PBR match to NTE and NPE with no problems (it may not necessarily be a close comparison in terms of the physics, but it is by now canonical so let's leave this as it is). The ICF reactor and drive match. I would connect Tokamaks to the MCF drive and stellarators to the MP drives - this is a topic of much debate, but thus far the fusion community is largely pursuing tokamaks as the MCF power reactor of choice over stellarators, however the stellarator was developed earlier and has better plasma stability (to grossly oversimplify) so it makes sense as a "pre-fusion" plasma tech step.
This leaves the two middle techs. For the ion drive, I would suggest to connect this to the
Magnetic Mirror Fusion Reactor both as another plasma/fusion type of reactor which is more specific and flavorful, and because the traditional mirror reactor design has a natural "loss cone" which could at least be readily visualized as the exit nozzle of an ion thruster (I'm not sure that's exactly how it works, but it looks cool in my imagination). For the gas-core drive, I would suggest a
Gaseous Fission Reactor in place of the gas-cooled fast reactor. On one hand this is because that is the exact reactor type to use for a gas-core rocket, on the other hand a fast reactor is I think a questionable concept for space propulsion as the use of fast reactors has more to do with waste reduction/recycling and fuel efficiency than any raw performance characteristics.
So to summarize the tech changes I would make:
- Radioisotope Thermal Generator + Nuclear Radioisotope Engine
- Pressurised Water Reactor + Nuclear Thermal Engine
- Pebble Bed Reactor + Nuclear Pulse Engine
- Gaseous Fission Reactor + Nuclear Gas-Core Engine
- Magnetic Mirror Fusion Reactor + Ion Drive
- Stellarator Fusion Reactor + Magneto-Plasma Drive
- Tokamak Fusion Reactor + Magnetic Confinement Fusion Drive
- Inertial Confinement Fusion Reactor + Inertial Confinement Fusion Drive
This goes along with the proposed shifting of the higher-tier techs down one slot and the addition of the new top-end tech.
Side Note: On the subject of inertial confinement fusion (ICF), nearly all approaches thus far have been laser-driven (whether direct or indirect), thus we
could rename the ICF techs as "Laser Fusion Reactor" and "Laser Fusion Drive" (probably shortening the MCF Drive to "Magnetic Fusion Drive"). I would recommend this, except for the fact that TN laser technology is a separate tech line and I think it could be confusing to have a laser drive type that is not related to the laser weapon type.