Jake Anderson wrote:
the energy that goes into pressuring the propellants isnt waisted
that energy is converted to KE too (through the injectors and as pressure
drop across the nozzle)
Yeah, between you and Gregory you basically have it.
The rotary rocket tip is in much the same class of propulsion system as a jet engine- the propellent is scooped up from zero velocity and accelerated to the velocity of the tip. The energy that this takes isn't lost- it goes into heating and pressurizing the propellent; much, but not all, of which can be recovered by the De Laval nozzle.
So, the idea that the ISP is fixed is flawed; it's positively related to the rotation rate- the faster the rotation the more thermal energy in the combustion chamber, hence a higher combustion temperature, and the higher the ISP.
The other flaw easily missed is that the nozzle presumably has a fixed expansion ratio; or atleast, a fixed maximum expansion ratio.
This determines how much of the pressure and temperature that went into accelerating the propellent gets turned back into kinetic energy and how much ends up as thermal and other waste heat (and disassociation). Eventually when the fraction of the the energy that is unused equals the chemical energy in the propellent; whatever the ISP is at that point, determines a maximum rotation rate (since the only other major source of energy is the 'ram effect' of the rotation, so the rotation has to be slowing past that point- it's not a perpetual motion machine!)
Of course, in theory you could increase the propellent flow; but in practice the pipes have a fixed diameter, and cannot supply ever-increasing fuel flow.
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