[AR] Re: Dual-thrust solid motors: their history?
- From: "Troy Prideaux" <troy@xxxxxxxxxxxxxxxxxxxxx>
- To: <arocket@xxxxxxxxxxxxx>
- Date: Fri, 26 Feb 2021 10:54:43 +1100
And something I do not get to this day, is how is the nozzle throat area
configured? If it is optimum for the ~5-sec boost phase, the ~20-sec sustainer
phase would necessarily be way off optimum, no?
And if the sustainer runs at nearly optimum At and Pc, how does that affect
the boost burn?
It depends a lot on what altitude the sustainer is operating at. If the average
operating altitude of the booster is say 5000ft (= 0.83atm) and the average
operating altitude of the sustainer is 22,500ft (=0.41atm) then you can
theoretically operate your chamber at half the pressure during sustainer
operation at no loss of thrust coefficient as the momentum thrust part of that
equation only includes the *pressure ratio*.
The c* of typical propellants stays pretty constant across a wide pressure
range (unless heavily metallised) so we can basically say in many cases that
efficiency will primarily be determined by the altitude specific thrust
coefficient and nominal c*.
That is to say, running the chamber at half the pressure shouldn't affect c*
much, so keeping the same throat area (in fact, the same nozzle) at half the
operating mass flow rate shouldn't affect performance if the pressure ratio
(Pc:Pa) is maintained
Troy
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