If you allocate 10% of your pressure budget to the injector thats 150PSI to
atomize the fuel in a 1500 psi motor and 15psi in a 150 psi motor. Now if
you allowed 150psi to vaporize your propellant in a 150 psi motor that
would probably go a long way to helping.
however I think that one would optimize to a higher Cp and lower injector
pressure drop.
On Sun, May 24, 2015 at 2:20 PM, Lloyd Droppers <ldroppers@xxxxxxxxx> wrote:
Norman,
In practice I believe that vaporization is easier for a high pressure
motor not because of any chemical equilibrium effects but because you have
move pressure drop available across the injector generally leading to lower
droplet sizes.
As far as vaporization is concerned it does matter what temperature the
fuel vaporizes at. While your comment is valid in that 2000-(-20) and 2000-
100 are only 5% off vaporization temperature ofset from ambient also
matters. For a thought experiment if you are injecting a liquid at 10C and
one vaporizes at 20C and the other vaporizes at 100C the propellant that
vaporizes at 100C take 10 times longer to start to "boil" with the same
heat input all other factors being equal.
Lloyd
On Mon, May 25, 2015 at 7:33 AM, Norman Yarvin <yarvin@xxxxxxxxxxxx>
wrote:
Paul Breed wrote:
low andFor a pressure fed vacuum stage the chamber pressure is going to be
anda fuel like RP-1 that is hard to vaporize will probably have stability
performance issues.
... and then in a followup:
A lot easier to get good vaporization in a high pressure motor.
Is this really going to be a problem? Pressure pushes the equilibrium
in the direction of liquid, not of gas. It's temperature that makes
for vaporization, not pressure; and temperature of combustion is about
the same regardless of pressure.
What does change with pressure are the kinetics. In a lower pressure
motor, the rate of heating of the liquid drops will be lower because
the surrounding gas has a lower density. And that's an effect which
is roughly proportional to the pressure, so is important. But there
are ways of changing kinetics: make the fuel injector nozzles
narrower, to get smaller droplets, for starters. Or in the last
resort lengthen the chamber to extend the dwell time there.
Still, when the chamber is at 2000 deg C, does it really matter much
whether the fuel vaporizes at 100 deg C or at -20 deg C?
--
Norman Yarvin http://yarchive.net/blog
