ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 176 of 422
• external and deployable radiators,
• regenerative regulation controlled via bypass valve
• ammonia will be used as the working fluid (higher figure of merit than alcohols)
Different sinks will be made available to the user (commutation between different heat
exchangers, use of a variable bypass, or a pump with variable mass flow) to accommodate the
vehicle thermal loads.
Appropriate sizing and selection of devices would require better knowledge of the heat load
distribution, pressure losses and in general of the main system architecture features.
On the basis of existing programmes (Columbus, ISS, Soyuz), a preliminary sizing has been
done and a budget estimated (see Section 3.3.3.4) to answer the requirements on the temperature
and on the heat dissipation.
• Figure 3-35 shows the differences between monophasic and diphasic systems:
Radiator area [m2]
NH3 monophasic, 2 faces radiator (eff. 0.8), T outlet=-60C, P=12kW
90
80
70
60
50
40
30
20
10
0
0
-60 -40 -20 0 20
temp. inle t [C]
40 60 80 100
area mass flow
0.2
0.15
0.1
0.05
Mass flow [kg/s]
Radiator area [m2]
Figure 3-35: Comparative monophasic / diphasic
ammonia biphasic, 2 faces radiator (eff. 0.8), P =12kW
90
80
70
60
50
40
30
20
10
0
-60 -10 40
temp. condensation [C]
90
area mass flow
Figure 3-35 shows the advantages of a diphasic system: lower radiator surface and lower mass
flow. Note that, for a same mass flow, equivalence of the two systems is never reached (Tboiling -
Tfreezing < H/Cp).
The parasitic heat loads on the radiators will result from a compromise between the different
pointing constraints of the vehicle (solar arrays, antennas, radiators). It is premature at this stage
to estimate which one would prevail, depending on the mass savings of this trade-off. Optimising
the parasitic heat loads is possible if the Sun, the planet and the radiator are in the same plane,
which is possible with a two-degree freedom or a one-degree freedom plus constraint on the
vehicle attitude. An alternative is to constrain the radiative surface so that a certain level of
absorbed energy can be tolerated, or finally to reduce the rejection to a single face.
0.20
0.15
0.10
0.05
0.00
Mass flow [kg/s]