On Sun, 2004-06-13 at 10:05, Ian Woollard wrote: > 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. > That wouldn't increase ISP (exhaust velocity/g) and thus wouldn't increase the speed of the engine. > _______________________________________________ > ERPS-list mailing list > [EMAIL PROTECTED] > http://lists.erps.org/mailman/listinfo/erps-list > Pretty close there, there will be a maximum possible ISP for any particular fuel mix and it will be well less than the velocity equivalent to the total chemical energy in the fuel mix, due to disassociation and other inefficiencies. Even if we design the engine to be most efficient at the stable velocity, we have a limit.
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