ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 274 of 422
Figure 4-25 illustrates the anticipated time of 6 h EVAs. Given that EVAs are scheduled for
every two days, the minimal time between two scheduled EVAs is 42 h. This time is greater than
the 36 h for which flight protocols exist today. Therefore, as a first cornerstone, the study
assumed that such protocols are valid for mission to Mars. This implies using current technology
and knowledge to assess the needs and issues concerning Martian EVAs.
4.3.3.8 Atmospheric composition
Different aspects drive the atmospheric composition selection. To schedule flexible EVAs and
have the fastest contingency EVA capabilities, the SHM and MAV could use a 100% oxygen
atmosphere at non-toxic pressure. However, an oxygen-rich atmosphere increases the risk of fire,
poses constraints on the material selection and engineering of the MEV, and perhaps an over
pressurisation during reentry and ascent will be necessary. A standard composition atmosphere
of 101.3 kPa would allow flexible on-ground testing and references. Conversely, it would
increase the required pre-breathing time rendering the schedule less flexible and increases the air
losses.
Given a standard atmosphere (101.3 kPa, 21% O 2 ), the pre-breathing time would range between
30 min (Russian protocol, TR=1.8, suit pressure 39.2 kPa) and 4.5 h (American protocol,
TR=1.6, suit pressure 29.6 kPa). However, both protocols assume the possibility to return to
Earth quickly to treat an astronaut if symptoms of decompression sickness (DCS) occur. Such
options will not exist for a mission to Mars. Therefore, more conservative pre-breathing time
estimates must be considered, which would cause a significant increase in EVA preparation time.
However, the scheduled activities for an astronaut on an EVA day would exceed 11 h so there
would be no advantage.
Therefore, additional atmosphere compositions have been investigated that depend on the
allowable tissue ratio. The results are shown in Table 4-6 and are valid for a 26 kPa suit pressure.
TR=1.6 TR=1.4 TR=1.2
101.3kPa, 21Vol% O2 284 341 408
70.14kPa, 26.5Vol% O2 95 153 219
50kPa, 50Vol% O2 0 0 0
28kPa, 100Vol% O2 0 0 0
(suit pressure 26 kPa, 100% oxygen, half time constant 300min)
Table 4-6: Pre-breathing time as a function of suit internal pressure (min)
As shown, an atmosphere composition of 50% nitrogen and 50% oxygen at 50 kPa total pressure
would be a suitable compromise, as it seems to avoid pre-breathing and still provide some
marginal fire hazard reduction. NASA STD 3000 reports that the fire hazard is reduced by 50%
in comparison to a 100% oxygen atmosphere yet still poses an increased fire hazard. As Figure
4-26 shows, the composition is within the zone of unimpaired performance of the astronauts.
Discussions with medical personnel indicated that short and long-term health concerns for the
crew due to the exposure to this atmosphere are not expected.