Before we begin this rather speculative journey, we need to briefly discuss manned spacecraft. At this point, I should perhaps repeat the caveat from Chapter 2 about the use of the phrase manned spaceflight to mean flights involving both men and women. I know that the phrase may not be quite politically correct, but I dislike the other possibilities, such as "crewed" missions or "peopled" missions.
In earlier chapters we discussed how unmanned satellites are designed, and now we discuss the additional design requirements for manned spacecraft. Space is a hostile environment (Chapter 6), and people are fragile organisms, requiring air to breathe at the right pressure, food to eat and water to drink, as well as an acceptable ambient temperature and a way to get rid of personal waste. These rather obvious requirements translate into the need for a significant mass of hardware and provisions onboard manned vehicles. This trend is readily apparent when we consider current examples of manned space vehicles such as the International Space Station (ISS) (Fig. 10.3), which is
projected to have a mass on the order of 450 metric tonnes when its construction is complete around 2010. Another feature that tends to further increase the mass of manned spacecraft, which is perhaps less obvious, is that of redundancy, which entails having backup systems onboard to make the vehicle safe from failures that would threaten the lives of the occupants. Safety-critical items such as elements of the life support system are doubled-up so that if the primary element fails, the backup system can be brought online to ensure the well-being of the crew. This is a major issue in manned vehicle design; a line has to be drawn by the design engineers to ensure a balance between having an excessively massive and extremely safe spacecraft on the one hand, and having a less massive but potentially unsafe vehicle on the other. From the point of view of launch costs, the less massive option is favored. But this is certainly not the case from the point of view of crew safety!
So manned spacecraft are expensive. Not only is the spacecraft hardware to be lifted to orbit more massive, due to the need for life support systems, but the launcher has to be man-rated. As we saw in Chapter 5, this means that the launch failure rate has to be reduced from the typical 10% for unmanned launchers to something significantly less than 1% for the man-rated launch vehicle. This is done by increasing the amount of redundancy in the launcher itself, which translates into more mass. Good examples of man-rated launchers, which carry manned hardware to orbit, are the Saturn 5, which sent the Apollo hardware and astronauts on their way to the Moon, and the Space Shuttle. As a consequence, these launch vehicles are some of the most complex, massive, and expensive launch systems yet to be employed. As we look to the future, and the projected expansion of manned space exploration and possibly space tourism, this issue of the cost of access to orbit is one of the main stumbling blocks that will ultimately need to be removed. Some insights into the challenges this poses for rocket scientists were discussed in Chapter 5.
The impact of including people in the design of spacecraft entails not only emulating a suitable terrestrial environment onboard to support life, but also dealing with the aspects of the space environment that are hostile to human life, such as radiation, space debris, and microgravity. The following factors influence how manned spacecraft are designed and operated (also see Chapter 6):
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