ESA Document - Emits - ESA
ESA Document - Emits - ESA ESA Document - Emits - ESA
s 2.7.13.3 Baseline design HMM Assessment Study Report: CDF-20(A) February 2004 page 84 of 422 From the analyses presented above, it is evident that it is not possible to meet all of the aerobraking constraints with the current vehicle. If the duration is constrained the loads are unacceptably high. Conversely, if the loads are constrained, the duration is so long that the additional ∆V that would be required to reduce it to meet the constraint would make the aerobraking mass savings negligible. Therefore, an aerobraking manoeuvre was not chosen for the baseline design of this mission case. 2.7.13.4 Manoeuvre budget A typical manoeuvre budget for an aerobraking phase that reduces the apocentre altitude from initially 96 000 km to 500 km is 115 m/s, 15 m/s for pericentre control (initial lowering and subsequent adjustment manoeuvres) and 100 m/s for the perigee raise from the final altitude 2.7.13.5 Options In addition to the more conventional aerobraking manoeuvre considered, “deep aerobraking” is a possible option. This would involve deploying an inflatable heat shield (or using an ablative heat shield), storing the solar arrays, and going deep in the atmosphere to shed the orbital energy in a limited number, of passes. This option has not been analysed in this study. 2.7.14 Artificial gravity One of the biggest problems that must be overcome is the harmful effects of weightlessness on the human body. These effects include loss of bone and muscle mass, loss of red blood cells, fluid shifting from the lower to the upper body, cardiovascular and neurosensory deconditioning, and changes in the immune system. The physiological systems start to change immediately upon launch into microgravity and the time courses of change is different for each of them. For example, the fluid shift and cardiovascular system start immediately within hours while the muscle and bone need some time to adjust to microgravity; deconditioning starts after days (muscle) or weeks (bone). Body fluid and cardiovascular system adapt to new environments in less than 2 weeks. However, cardiac arrhythmia might be a problem in-flight for some individuals during elevated workloads. Muscle loss (muscle volume and power) is maximum within the first 4 weeks, but afterwards the loss rate is reduced (strongly dependent on in-flight countermeasures). Bone loss, however, continues progressively (1% per month) throughout the mission in free space. Figure 2-41 summarizes the reactions to microgravity of each of the physiological systems:
s % of loss from baseline 20 18 16 14 12 10 8 6 4 2 0 % of loss from 1g baseline during flight 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 weeks in microgravity HMM Assessment Study Report: CDF-20(A) February 2004 page 85 of 422 Figure 2-41: Effect of microgravity on physiological systems Currently, some countermeasures for weightlessness such as physical exercise, lower body negative pressure and drugs are used in human spaceflight. However, these countermeasures prevail for the permanent ISS crew but have not proven to be sufficient for longer duration missions on-board MIR. Moreover, these countermeasures currently adopted focus only on stimulating a particular physiological system. Artificial gravity, however, represents a different approach to the problem of microgravity effects because it simulates our natural 1g environment. 2.7.14.1 Trade-offs There are two options for how to approach the implementation of artificial gravity as a way to fight against the negative effects of long exposures to weightlessness. One is by providing artificial gravity through continuous rotation of the entire spacecraft with a large radius of rotation and low angular velocities. The second option is intermittent exposure to artificial gravity enough to overcome the damaging effects of microgravity, using on-board centrifuges with a small radius of rotation and high angular velocities to simulate gravity for certain amounts of time. 2.7.14.1.1 Continuous artificial gravity This option requires that the comfort level for the astronauts is respected (see Figure 2-42 from NASA – Habitability data handbook, Volume 1, MSC-03909, 1971). It implies that the minimum rotation radius must be 17 m, achieving 0.3 g at the maximum spin rate of 4 rpm. bone muscle fluid cardio
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s<br />
% of loss from baseline<br />
20<br />
18<br />
16<br />
14<br />
12<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
% of loss from 1g baseline during flight<br />
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34<br />
weeks in microgravity<br />
HMM<br />
Assessment Study<br />
Report: CDF-20(A)<br />
February 2004<br />
page 85 of 422<br />
Figure 2-41: Effect of microgravity on physiological systems<br />
Currently, some countermeasures for weightlessness such as physical exercise, lower body<br />
negative pressure and drugs are used in human spaceflight. However, these countermeasures<br />
prevail for the permanent ISS crew but have not proven to be sufficient for longer duration<br />
missions on-board MIR. Moreover, these countermeasures currently adopted focus only on<br />
stimulating a particular physiological system.<br />
Artificial gravity, however, represents a different approach to the problem of microgravity<br />
effects because it simulates our natural 1g environment.<br />
2.7.14.1 Trade-offs<br />
There are two options for how to approach the implementation of artificial gravity as a way to<br />
fight against the negative effects of long exposures to weightlessness. One is by providing<br />
artificial gravity through continuous rotation of the entire spacecraft with a large radius of<br />
rotation and low angular velocities.<br />
The second option is intermittent exposure to artificial gravity enough to overcome the damaging<br />
effects of microgravity, using on-board centrifuges with a small radius of rotation and high<br />
angular velocities to simulate gravity for certain amounts of time.<br />
2.7.14.1.1 Continuous artificial gravity<br />
This option requires that the comfort level for the astronauts is respected (see Figure 2-42 from<br />
NASA – Habitability data handbook, Volume 1, MSC-03909, 1971). It implies that the<br />
minimum rotation radius must be 17 m, achieving 0.3 g at the maximum spin rate of 4 rpm.<br />
bone<br />
muscle<br />
fluid<br />
cardio
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