ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 55 of 422
This opportunity minimises the total mission duration as well as the time spent in deep-space
(inbound and outbound trips) and maximizes the time spent around Mars. Therefore, it
maximizes also the ratio time around Mars / time in deep-space. Finally, it offers one of the
lowest entry velocities on return to Earth, although the approach followed for the ERC design
reduces the influence of this parameter.
2.7.6.2 Surface stay duration
One of the objectives of the study is to select the simplest mission case.
A long stay duration on the surface would imply a much higher complexity of the mission, as
more resources and infrastructure would be required to support the astronauts while on the
surface, typically more complex life support systems, more consumables, more habitable volume
and higher power demands in general.
This increment in the mass of equipment required for a long stay would imply the definition of a
cargo mission to take all the extra infrastructure. This would lead to the new requirement for the
lander, of high precision landing, as the astronauts will have to rendezvous with the
infrastructure on the surface.
To avoid this complication, a short stay duration on the surface of Mars has been selected.
After landing, one week is required by the astronauts to recover from the deconditioning, and it
is assumed another week for the launch preparation. Taking into account the recommendations
for the surface operations, seven EVAs are required as minimum. A surface stay of 30 days is
therefore a minimum reasonable time.
2.7.6.3 Propulsion
The propulsion technologies for a human mission to Mars can be reduced to the following:
• Chemical propulsion
o Cryogenic
o Storable
• Solar electric propulsion
• Nuclear electric propulsion
• Nuclear thermal propulsion
According to the criteria defined for the mission case selection, electric propulsion has not been
studied, to keep the complexity low. For the same reason and because it is not a mature
technology (reduced knowledge which leads to not reliable estimations) nuclear propulsion has
been also discarded.
Therefore the choice remains between cryogenic and storable propulsion systems, as they are
well known technologies and therefore offer a good starting point for analysis.
The benefits of cryogenic propulsion is high Isp, which allows a significant reduction in the
propellant mass required for a given ∆V and payload. But the drawbacks are the boil-off of the
cryogenic propellants and the volume of the tanks due to the low density of the propellants.
The propulsion module design chosen follows a modular approach, combining both the
cryogenic and the storable system in the same mission. Three cases have been defined; all
storable, all cryogenic, and cryogenic for the first manoeuvre (TMI) and storable for the
remaining two (MOI and TEI).