"It's a bit dubious whether the standard mode of TOS shuttle sublight propulsion would involve rockets at all."
My mistake, I was using the colloquialism they used back in the 70's where "rocket" stood in general for "propulsion engine" (in contrast to power engine or power generator). Of course a "fusion rocket" has little in common with our fuel burning rockets IRL as the fuel is not burned but rather released through the exhaust nozzle by its own pressure.
"Not quite so strongly agreed. The Commander doesn't really have a choice: the ship grows visible against his will as power wanes. But the two systems competing for the power are not specified to be drive and cloak. Rather, it seems that it's the weapon that eats power and consumes fuel so that any other systems are compromised, both temporarily and in the long term... Use of weapon results in dropping of cloak and cessation of movement, actions that the Commander would not willingly take unless the shortcomings of the technology thus dictated. But combined invisibility and movement are possible throughout the episode."
Another misunderstanding? The Romulan Bird of Prey's fuel is
a) essential to feed the sublight fusion drive to get them home after their raid on the Federation outposts
b) essential to create the (fusion) plasma torpedo and to draw energy from the fusion reactor to accelerate it to warp speed (in a ideal scenario the Bird of Prey would turn 180° degrees to fire at a pursuing vessel and thus utilize the exhaust momentum of the torpedo to further propel it forward in the direction of its course home. Enterprise
's optimal attack strategy would therefore be to approach the Bird of Prey from the side...).
c) essential to provide the cloaking device with energy the fusion reactor creates.
Therefore b) and c) consume the fuel calculated necessary for the vessel to make it back to a rendezvous point with another Romulan vessel in Romulan territory.
I should add, that I'm a strong supporter that the Romulan ship according to Scotty indeed only has sublight impulse drive (before the vessels were retrofitted in "The Deadly Years" with true warp engines). That the vessel is about to pass a sublight speed comet's trail is a hint, IMHO.
In this particular case the Enterprise's
warp speed capability (other than reverse warp to avoid being hit by the plasma torpedo) would serve it as much as the Messerschmitt's 262 jet engines in WW II when attacking enemy fighters. The Russians knew that the Me 262 was too fast to score a hit, so Russian planes simply dispersed into all directions. The allied bombers didn't have that kind of maneuverability as smaller planes so it was easier for the Me 262 to bring them down as the bombers' formations stayed rather rigid.
Sorry, this should be a new thread (Romulan Bird of Prey capabilities)