ESA Document - Emits - ESA

s
Number of stacks
18
17
16
15
14
13
12
Number of stacks required
3 months commissioning time
1 month between launches
11
10
3 months commissioning time
3 months between launches
0 500 1000
Boil-off rate (kg/month)
1500 2000
Figure 2-26: Number of stacks required as a function of launcher rate and boil-off rate
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 58 of 422
The analysis led to a boil-off requirement rate of 500 kg/month. Figure 3-31 shows that the
maximum number of stacks in the worst case is 13.
Cryogenic propulsion systems could be extended to other mission phases, i.e. MOI, if we take
into account that the time from departure to the MOI is around 6 months and the assembly time
is already more than 54 months, and also that the interplanetary environment is more benign than
the LEO environment from the thermal point of view. However, for the design case, the
conservative case of only cryogenic TMI has been taken.
2.7.6.4 Return approach
Several strategies can be adopted for the return to Earth. The crew will return from Mars in the
THM and ingress in the ERC to perform the atmospheric entry descent and landing. You could
insert the THM into Earth orbit so it can be reused in a next mission. If not, it has to be discarded
so that it does not impact the Earth. Besides, the ERC with the crew inside can also be inserted
into an Earth orbit allowing the crew transfer to an orbital space station (e.g. ISS) before landing,
or it can perform a direct entry. Table 3-16 shows a preliminary analysis of these possibilities:
THM and THM discarded, THM discarded,
ERC inserted ERC inserted ERC direct entry
Mass to LEO (tonne) 4892 1813 1336
Table 2-15: Mass to LEO for the different strategies on Earth return
As shown in Table 3-16, parking the THM and/ or ERC in Earth orbit is too expensive in terms
of mass to LEO, as the propellant to perform the manoeuvre has to be taken from Earth to Mars
and back (other techniques as aerocapture/aerobraking have not been analysed for this
manoeuvre).
Planetary protection issues imply, no contaminated vehicle should remain in Earth orbit, and this
is the case of the THM, as its exterior will be contaminated by the Mars Ascent Vehicle (MAV).
In the case of the ERC, which will be also contaminated, the problem is overcome by the
sterilization arising from high temperatures during reentry.