2949775323

Comparison of Liąuid Propellant Rocket Engine Feed Systems -1-9

At this point, is convenient to refer the above defined pressure fali, to the combustion chamber pressure. For this purpose the following constant is introduced:

k

Pi

4p,

Pc

(2.3.3)

Now, a pump merit factor named power density, is defined by relating the pumping power with the total mass:

(2.3.4)

where, d_pu: Propellant power density (W/kg). m_pu: Propellant mass (kg).

From eąuations 2.3.1 to 2.3.4 and 2.1.10 two expressions are derived for the fuel and oxidizer pumps masses, both valid for the electric-pump and the turbo-pump systems:

a _rp_rm_n

»W=(1+ **-*,) '    .    (2.3.5)

°puf^tb

^anPr^mn

m_puo=(\ + k_pi-k_p) °J^Ct ^p    (2.3.6)

2.4. Electric engine mass

It is begun by relating the inlet pump power (taken from the electric engine) with the output pumping power. This is necessary because the pumps dissipates energy as heat, due to the frictions on their moving parts, and not all the electric motor power is transformed in pumping power. These losses are taken into acount defming a merit factor called efficiency, as:

(2.4.1)

where, Poee- Electric engine mechanical output power (W). rj_pu: Propellant pump efficiency.

The power to be supplied by the electrical motor will be the sum of the power that each pump reąuires to do their work. Furthermore, a motor merit factor (similar to the one defined for the pumps) is introduced for convenience, the motor power density:

m_ee

With the prior expressions the engine mass can be obtained: