When one thinks of a spacecraft, images of star destroyers from Star Wars, the Enterprise from Star Trek, and the Discovery One from 2001: A Space Odyssey may come to mind. A contemporary view includes NASA's space shuttles and the Russian Soyuz series. But early satellites such as Voyager (USA), Sputnik (Russia) and Anik A1 (Canada) were also considered spacecrafts in their day. For the purpose of this article, we'll take a look at the requirements of a modern day manned spacecraft, capable of achieving launch from a planetary body as well as space travel.
In order for a spacecraft to launch, it must be capable of generating enough thrust to overcome the planetary gravity well and achieve escape velocity. Although aerodynamics is important while in the atmosphere, they are meaningless once the spacecraft is in space. Airplanes achieve flight based on the movement of air over their wings; at high altitudes they require more thrust due to air density and every airplane has a maximum ceiling. A rocket can provide the thrust required for a spacecraft, but requires considerable fuel. Early fuels were often dangerous; the Saturn rocket series used an extremely volatile liquid oxygen fuel source. Later advances in the Titan rocket program lead to a binary fuel, which ignited when nitrogen tetroxide and hydrazine were combined. This fuel source was toxic and expensive, and lead to liquid hydrogen and the eventual development of refined petroleum (RP-1).
The main concerns with fuel are threefold: the mass of the propellant, the volume of the propellant, and the specific impulse of the propellant. The complex equation constantly being juggled by rocket scientists is what is the best fuel choice based on mission parameters, payload mass and budget restrictions. A highly effective fuel source such as liquid hydrogen is expensive and requires tremendous amounts of room in booster rockets, while RP-1, although less efficient in terms of specific impulse, is seven times denser.
The choice of fuel affects cost and ultimately available payload. A reusable spacecraft design usually incorporates booster rockets for launch, which separate as exhausted, and rely on the spacecraft being able to reenter the atmosphere by its own means. Propellant needs to be reserved for maneuvering while travelling in space; any design would need to accommodate extra fuel (and its mass) for this task. Launching again from a foreign planetary body (such as Mars) would require establishing a fuel source on that planet.
The payload refers to the mass of the astronauts, all equipment and cargo, and the spacecraft itself. Although weight may disappear in space, mass is a constant. Mass constrains the launch parameters, and dictates fuel requirements. Mass is also critical for all flight path adjustments while underway; and it effects the amount of propellant required for maneuvers. A design principle for payload consideration is to always minimize load based on mission objectives. Therefore, the purpose of the mission (space tourism, orbit, interplanetary travel) is critical for designing the spacecraft accordingly. This is why the goal of a reusable, multi-purpose spacecraft is so elusive. It is often better to design for a specific purpose than for all possible intentions.
Any design will include computer automation, but a spacecraft is mission critical. All systems must be redundant in case of failure. They should be distributed to minimize load and maximize available processor time. They must be able to monitor all hardware systems in real time, and provide reports, updates and alerts as necessary. They must be accessible by both the crew and mission control, remotely updatable, and based on fault tolerant software. Dedicated interfaces should be separate to reduce chance of confusion. Multiple access stations within the ship are required in case a section becomes uninhabitable and control needs to be shunted to a different part.
Life Support Systems
An adequate oxygen supply is mandatory for survival. Oxygen can be retrieved from carbon dioxide emissions; CO2 scrubbers are a requirement for the design. Due to the potential for muscle loss and bone deterioration while on an extended space mission, exercise equipment must be available for reconditioning and medical support systems must be present to monitor crew health. Medical supplies must be abundant to treat common illnesses.
Water from urine? Say it's not so! Well, this is the norm for a space traveler. A spacecraft must have in its design a comprehensive waste reclamation and recycling system. Core materials that can be reused represent less payload. Organic waste can be composted. The spacecraft design should include a comprehensive liquid waste decontamination, purification and distribution system.
Environmental controls ensure the pressurized cabins of the spacecraft can maintain a habitable environment within the hostile confines of space. Temperature and humidity must be regulated to be within safe, comfort zones. A spacecraft design should include lighting control (dimming to night, brightening to day) to help the crew adjust and maintain schedules on extended voyages.
On short trips, such as a jaunt to the International Space Station, renewable food sources aren't required. Food can be vacuum packed to reduce weight and volume, and nutrient supplements ensure a balanced diet. On a long trip, growing one's own food may be the only way to survive. It doesn't take much: space seeds are pre-germinated and minimal in volume and it also allows for a varied diet. Carnivores beware though, for the lack of meat will require alternative protein sources.
Heat insulation and Radiation shielding
Spacecrafts are exposed to the extremes: the frigid coldness of space, the extreme heat of launch and re-entry into the atmosphere, and the constant inundation of cosmic radiation. The spacecraft must be designed to shield and insulate the occupants. Radiation is often unstoppable, but the worst of forms can be effectively shielded or dramatically reduce through the use of various barriers in the insulation of the skin of the spacecraft. Heat must be disbursed to protect the craft, and ablative heat tiles on the outer hull can be used for this purpose.
A spacecraft design must incorporate landing mechanisms for in atmosphere (Earth, Mars) and potentially for out of atmosphere (the Moon). Russian Soyuz capsules plunge through Earth's atmosphere until they can deploy parachutes; American space shuttles glide on predetermined glide-paths to make smooth picturesque runway landings. Landing on Mars, where the atmosphere is much thinner, or on the moon, where there is no atmosphere require different considerations. For the moon, propellant could be used to navigate and land the craft as long as there is enough propellant to reach escape velocity upon launch. For Mars, the lack of pre-existing infrastructure means a wheel landing is out of the question; a combination of propellant and chutes may resolve the problem. The mars rover probe used a unique "bounce" landing approach, where inflated spheres absorbed the impact and cushioned the gear. However a manned spacecraft contains precious cargo: handle with care.
Designing the Spacecraft of the Future
The elements outlined in this article, although critical, are only a subset of all the considerations necessary for a spacecraft design. Scientists and science lovers have been considering the design a challenging problem for decades. With continual advances, demand for space tourism and an inherent yearning to explore the stars, this is an area bound to be realized in the near future.