Simply "piping" hydrogen is much less challenging than heating it in a reactor such as an NTR.

Re. the 2nd question, it depends on how long you need to maintain the steady state temperature : For a commercial NPP, that needs to be 40-plus years. Even with inert helium gas, it will be very challenging to get the operating temperature up to 1000C while avoiding serious component degradation. See http://nuclear.inl.gov/gen4/vhtr.shtml

Hydrogen is much worse. But the NERVA/ ROVER tests showed that average coolant exit temperatures of 2550K are possible. The "Pewee" reactor ran at 514 MW for 40 minutes in 1968, Isp = 845 sec; fuel was UC2 beaded particles with pyrolytic graphite coating, in a graphite matrix with zirconium carbide coating to protect against hydrogen corrosion. At higher temperatures, these particles would crack, due to mismatched thermal conductivities.

Tests on the nuclear furnace (NF-1) showed that composite carbide matrix (U,Zr)C in graphite substrate and ZrC coating could provide fuel good to ~2800 K for 4 to 6 hours of operation. For one hour operation, the carbide fuel could possibly run as high as 3300 K.

A final design in the project, called the "Small Engine," was intended to be coupled to a large hydrogen fuel tank in a complete Shuttle-compatible nuclear stage (see Los Alamos rep't LA-5044-MS). Its overall length of 18.3m (with engine nozzle skirt folded away) and 4.5m tank diameter would fit into the Shuttle cargo bay, providing a reusable nuclear stage that could be supplemented with additional hydrogen fuel tanks in order to support many interesting space missions....

Then the project was canned.

Dusty wrote:

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Multiple NTR’s are not really an option Or Option #3 J   How about a central high temperature reactor heating hydrogen to a high temp and then distributing it to various attitude control nozzles situated in appropriate locations around the periphery of the ship (I envisage some sort of “Ring Main” to minimise thrusters lag. A bit like the hot water distribution system in “Well designed” large buildings)

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I guess that would depend on the size of Orion ship you're talking about. For the minimal versions that were initially conceiver for launch by Saturn V type boosters, the single NTR would certainly work well, and the piping wouldn't be more than a few meters long.

For the big Orions though, I think the piping would be of such great length, that it would cause too much performance loss for a single central NTR.

But NTRs are small things, so putting six or eight of them around the periphery wouldn't be a great detriment to overall ship performance. Certainly piping cold H2 propellant to the NTRs from a central cryo tank would be much easier to do, than trying to do the same thing with superheated exhaust jets !!

Since Dusty was talking about attitude control thrusters, I figured that somewhat less extreme flow conditions could be designed -- comparable to large turbojet engines, with which we have plenty of ducting & thrust vectoring eperience.....

I would guess that one way to lower the risks is to keep the exhaust duct from the NTR at roughly the same diameter as the reactor vessel, to maintain a low flow velocity. Of course at the far end, one would still need the nozzle throat constriction, which is what may cause oscillation trouble, if not designed properly.... And if any radical flow turning is required, that would probably need to be done upstream of the nozzle throat, in the large-diameter pipe.

You may recall that the gas core reactor rocket faces a similar problem, requiring a radical 180-degree exhaust duct turn, to avoid dumping the fuel from the reactor vessel, when the ship starts accelerating. That is a far worse situation, because it's the main ship propulsion system, in this case. It makes the standard GCNR design impractical, IMO, for all the reasons spelled out by Googlenaut.