ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 101 of 422
Table 2-37 shows that the overall assembly time does not alter. The change in the assembly
sequence only affects the amount of cryogenic boil-off. The effect of delaying the assembly of
all habitable modules until the end of the sequence, increases the boil-off by about 7.85 tonnes.
When considering the in-orbit assembly sequence, the determining factors will therefore be the
‘operational life’ of the (habitable volume) equipment for the mission as a trade-off against the
amount of fuel boil-off considered acceptable (mainly in the cyrogenic tanks launched first in the
sequence) and also whether a refuelling launch shall be considered.
2.9 Safety/risk assessment
2.9.1 Mission-specific characteristics
The driving characteristics of this mission are:
• It falls within the category of Human Space Flight, including EVA activities.
• It is an inter-planetary mission with sample return, therefore the Interplanetary Protection
Rules and the UN treaties are applicable.
2.9.2 Definition of “safety and mission success”
The first step in the risk assessment process is to establish the mission success definition and to
set the safety goals of the mission:
• Mission success: to bring a crew of 6 members to Mars and return them safely to Earth.
• Safety goal: to identify all possible safety hazards, to eliminate/control them to an
acceptable level during all the phases of the mission.
• Probabilistic goals (overall safety & mission success risks): Human Spaceflight statistics
show a 5% risk of losing the crew. Any next-generation system for transporting
astronauts to Mars will be probably designed to a risk requirement much lower than that,
e.g 0.5%.
2.9.3 Safety requirements:
• Double & Single Failure/Fault/Operator error tolerance to catastrophic & critical events;
safety margins
• Fail safe: This is the capacity of the system to remain in a safe condition when a failure
occurs or to skip directly into another safe condition
2.9.4 Mission factors/issues:
Throughout the mission design the following factors are important:
• Mission abort/ rescue capabilities. (Acceptable risks can be achieved if abort options are
designed into the mission for all phases except for those for which it is impossible
• Greater reliability and / or redundancy of systems. (e.g. Common Mode/Common Cause
failures)
• Preventive and/or corrective maintenance strategy (e.g. robotics, spares, aged equipment
control, caution and warning system)
• Capability to monitor/ detect and assess effects of slow events such as: metal fatigue,
cracks; dust, corrosion and rust