Re: Re: Tip nozzle paradox

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.