Air launched fusion craft?

[KerikBalm]

In that loose definition, an Orion drive is nuclear thermal as well.

It's mainly about how you define the term.

I don't think that the way you use it is the way it is commonly understood/used.

I would define it as using a fuel in a nuclear reactor (fission or fusion) to heat a separate propellant.

If there is no added propellant, then it's just a direct fission/fusion drive -still even that criteria has Orion drives in, as the shaped charges have added reaction mass, they aren't just pure cores of fissionable material with practically 100%  fission efficiency.

But it does seem hard to make a definition that includes a nuclear salt water rocket, but excludes an Orion drive

[SciMan]

Well, generally true fusion drives are incredibly efficient, but also quite low thrust things. Normally, you want to use an "afterburner" type arrangement to inject some additional reaction mass into the exhaust stream, the most commonly studied of which is Hydrogen, but Water is also quite good because while the performance is lower, thrust is higher, and it's a lot easier to get Water when you're constrained to landing on rocky bodies.

Also, Orion drive charges are usually not just atomic explosives, instead they're full-on Thermonuclear (aka fusion) explosives, with a chunk of additional mass intended to be accleerated towards the pusher plate by the explosion. For this purpose, a slug of Tungsten is often used in the section of the Orion drive charge facing the craft being propelled by said Orion drive, as it is excellent at transferring momentum and also helps a tiny bit to shield the Orion drive craft from the intense gamma ray pulse from the various nuclear reactions that make the Orion drive charge function. Of course, there's also a large amount of Tungsten in the pusher plate as well, for the reason of radiation protection and resistance to being eroded by repeated close proximity to a thermonuclear detonation.

EDIT: In my opinion, because of the rather direct means of action of the Orion drive, I prefer to call it a "(thermo)Nuclear Kinetic propulsion", meaning that the thermonuclear explosions are of a nature that while they do indeed heat the Tungsten "reaction mass" to a temperature sufficient to vaporize it into plasma, the method of transferring the energy to the spacecraft is not thermodynamic, but Newtonian. So in that way, Orion drives (and other nuclear pulse propulsion drives such as Dadelus and MiniMag Orion) are in their own class separate from Nuclear Thermal, Fusion Thermal, and Pure Fusion rockets.

As a matter of fact, the MiniMag Orion, and other nuclear pulse propulsion drives using magnetic fields to transmit the force to the spacecraft (essentially using a magnetic nozzle but with Orion drive charges) are of a sub-category of "thermonuclear kinetic propulsion", that being called "thermonuclear magnetokinetic propulsion", because non-magnetokinetic drives use a physical pusher plate but drives such as MiniMag Orion have no pusher plate, instead replacing it with a magnetic field.

The only way I can make it make sense in my head is by adding those extra "bins" to sort all these interesting propulsion methods into. Otherwise you end up with conflicting expectations from some entries.

Edited March 5, 2022 by SciMan