General Design Principles of a Spacecraft

Spacecraft are the most complex transportation systems ever built by humans – and the International Space Station, the most successful semi-permanent abode in space yet, is also the most expensive structure ever built. For such technically complex devices, then, obviously there are a very large number of sophisticated design principles, well beyond the scope of a single brief article. However, the most important general design principles of a spacecraft for an interested observer of the space program to understand are: limiting mass, incorporating crew quarters (if necessary), building in safety and redunancy systems, and incorporating landing or return functions (again, if necessary).

These four principles are the most important general design principles of a spacecraft today, but at most two of them (landing and safety systems) are on the list solely because it is absolutely critical. In practice, the most important guiding imperative in spacecraft design today is cost: spacecraft mass must be limited because takeoff is expensive, crew quarters must be limited (and preferably absent altogether, i.e. an unmanned probe) because they are horrendously expensive, and it is easier to build a one-way probe than a two-way return mission because landing and re-entry equipment is expensive, too.

– The Mass Paradox –

First and foremost, the guiding design principle of a spacecraft is to keep weight down. (This feeds into why the following problems are so expensive, as well.) There are a number of different fuels and rocket designs available for launching spacecraft today, allowing launch engineers to pick the most efficient option in terms of balancing thrust (power) and engine efficiency. Overall, however, right now launching a vehicle into orbit costs about $10,000 per pound – give or take, not least because it obviously costs more to boost an object into a higher orbit than into a lower one. The aging Space Shuttles weigh about 2000 tons each, and while most vehicles are far smaller, even a minor adjustment in mass can quickly result in enormous differences in cost.

However, the issue here is not only one of pure economics, because design engineers must also grapple with another paradox: providing fuel for the mass which is to be launched. Rocket fuel itself is heavy – and therefore expensive. Moreover, if one adds rocket fuel to the rocket, then one has to provide additional thrust in order to lift the fuel itself. Adding more fuel to lift the original fuel then requires adding even more fuel to lift that supplemental fuel – and so on. The fuel in the Space Shuttle’s super-lightweight fuel tank weighs 1.5 million pounds not just because that amount is needed to lift the Space Shuttle into orbit, but so that a large proportion of the fuel can be carried up to an altitude where it will be burned.

Incidentally, in addition to the problem of fuel, spacecraft design must also incorporate one other vital principle: the diameter of the launch vehicle. Launch rockets are by necessity cylindrical and of limited size; if a spacecraft is going to be wider than the available launch vehicles, either it must fold up (as is the case with solar panels) or it must be launched in modular fashion (as is the case in International Space Station), then pieced together in orbit.

The mass-fuel paradox crops up again with respect to the other general design principles of a spacecraft, so we will return to this point repeatedly. Unless and until we can build spacecraft in orbit, the most important, overriding principle in spaceflight will always be covering the costs and fuel requirements of the launch from Earth into orbit. If we ever do mine the Moon, asteroids, or even Mars, then the launch costs would fall tremendously, though they would never be eliminated entirely.

– Crew Quarters –

Manned missions have always captured the public imagination far more than unmanned ones: we need only look at the horrendously expensive Space Shuttle (which despite its technical complexity never goes beyond low-Earth orbit) and the primacy given to manned lunar missions during the Space Race to understand this. Moreover, if we are to eventually move beyond the fragility of our current biosphere, we will need to do so via manned space flight.

For the moment, however, manned spaceflight is also extremely difficult. Incorporating a crew means incorporating all of the features they need for survival and a minimum level of comfort, as well as added reduncancy systems so that fatal accidents are less likely. In spite of all of the reduncancy systems, tragic accidents do still occur: less often than in the 1960s, certainly, but from time to time, as with the loss of the space shuttles Challenger and Columbia. Nevertheless, spacecraft engineers in all countries, even Soviet Russia, were understandably less reluctant to take brazen risks with manned spacecraft than when the worst-case scenario was merely the unexplained loss of a small, unmanned space probe. Moreover, for a long-duration space mission, more equipment must be provided: more water, more oxygen, more exercise and living space, and so on.

In practice, all of this costs money and is far more technically complex than an unmanned space probe – and so, for the most part, the simplest solution is simply not to send humans into space at all when an unmanned probe can do a roughly equivalent job. Plus, the problem of incorporating crew quarters and equipment returns us to the fuel paradox: more mass on the spacecraft means it must carry more fuel, more fuel means carrying fuel to lift the fuel, and so on.

– Safety and Redunancy –

Space is a hostile environment, even for electronics. One must consider the potential damage inflicted by micrometeorites and space junk, by higher levels of radiation, and so on. This is particularly important in the case of manned spacecraft: the loss of an unmanned probe is financially expensive and extremely frustrating for mission participants, but it is not tragic. Nevertheless, even unmanned probes are a long way from the nearest repair service. Once the Space Shuttle retires, even Earth orbit satellites like the Hubble Space Telescope will essentially be beyond any real hope of repair.

For this reason, spacecraft must be built with sufficient redundant systems that will be able to carry out all or at least the lion’s share of their mission objectives even in the event of a few mechanical or computer failures. In practice, once again, the level of redunancy is limited by the cost: adding backup systems increases the spacecraft’s weight, which requires fuel, which in turn requires still more fuel, and so on. Nevertheless, the cost of losing a spacecraft to a single minor equipment problem – or, even more embarrassingly, a software glitch, like the metric-imperial switch that doomed the Mars Climate Orbiter in 1999 – is much greater than the cost of building in just a few backup systems.

– Destination: One-Way or Return? –

Sometimes when travelling on Earth, it is cheaper to book return tickets. In space, however, this is never true. First, of course, is the fuel paradox: the fuel required to travel in space is much less than that required to boost a vessel into orbit, but it is not insignificant. Travelling to a destination and back requires more fuel than simply travelling there and remaining, or heading off on a one-way trip out of the solar system (as the Voyager probes are doing); this fuel then requires more fuel during launch, and so on.

Second, however, any complex activity at the other end requires a great deal of sophisticated equipment. Landing on any planet is difficult, though for different reasons. For example, landing on Earth requires the presence of a large and heavy heat-shield to prevent the spacecraft from burning up during re-entry; even a minor problem with the heatshield can have fatal consequences (as occurred with the Space Shuttle Columbia). Landing on Mars, however, requires a different set of equipment: the Martian atmosphere is too small to effectively brake a spacecraft, so on Mars, probes must have special landing equipment. Right now one of the preferred methods is to land probes in a sort of giant “airbag,” allowing them to bounce and skid along the surface while the protective shell breaks the force of their fall.

All of this equipment is complex and expensive – but it becomes even more so if one begins to build in the equipment necessary to launch from that other space object and return back to Earth. Not only is this yet another system which must be custom-designed and built, but it also must be loaded with its own separate volatile fuel supplies, and then this entire mass must be launched into orbit from Earth – returning us to the fuel paradox. In practice, all of these difficulties mean that most interplanetary probes are unmanned, and that most of those unmanned probes are on one-way missions only. Sample returns from beyond the Moon are very rare. So far, only a handful of sample return missions have been attempted: America’s Apollo (the Moon), Genesis (solar wind; crashed), and Stardust (comet Wild 2); the Soviet Luna (also the Moon), and Japan’s Hayabusa (comet Itokawa). Martian sample return missions have been on the drawing boards in America and Russia for several years, but so far have not progressed beyond the planning stage.