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Comparison of Liąnid Propellant Rocket Engine Feed Systems - 1 - 8

2.2.2. Propellant tank mass

In a similar way, an expression can be developed for these tanks, combining the expressions 2.2.1 to 2.2.5:

=p_tP^AĄ

	^1 P,p 1	'^3VjA'
l^4*- J	2<rJ	)

(2.2.9)

This expression is combined with 2.1.2 and 2.1.3 to express it in function of the adeąuate parameter to make the comparison:

³ A,

m,_n =    - k.k k„cc m_np_r

'    — _ lp P U p pr (

(2.2.10)

where k,_p is a constant that provides a safety margin with respect of the estimated mass.

The suffixes “p” must be replaced by “o” or “f” as correspond to the oxidizer or fuel tank, except in case of k_p and m_p variables.

In the case of the propellant tanks of the systems that use pumps, the tanks walls are thin. The thickness estimated by the Laplace's Law application, might be too thin to withstand the loads that could appear during the vehicle acceleration [3], Therefore, a minimum thickness is established as a limit from which the previous eąuations are valid. In the case where the thickness estimated by the Laplace’s Law, be less than the minimum established, the estimation must be madę directly using the following expression:

(2.2.11)

being e_min the minimum thickness of the propellant tank wali (m).

2.3. Propellant pumps mass

To estimate the pump mass, one should start remembering that the pumping power is proportional to the propellant volume and the pressure raise in the pump, while it is inversely proportional to the operation time [4], This means:

V

Popu = typu f    (2.3.1)

where, P_opu: Pumping power (W)

Appu: Pump imposed pressure raise (Pa). t_b: Engine buming time (s)

In tum, watching the figurę 1 schemes, for the three systems the pressure raise can be write in this way:

(2.3.2)

typu = Pc + ty, -Pp

where Ap, is the pressure fali in the injector (Pa).