Project Icarus is an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination. Initiated by the British Interplanetary Society, and managed by Icarus Interstellar Inc., a non-profit group of scientists dedicated to interstellar spaceflight, Project Icarus is working to develop a spacecraft that can travel to a nearby star.
Philipp Reiss, who is the Project Leader for the research module "Mechanisms," gives an introduction to spacecraft mechanisms, and their potential application on the Icarus starship.
A "mechanism" is a system of parts that is specifically designed to convert forces and movement for a certain purpose. Icarus wouldn't be able to reach the stars without them just as Columbus' ship Santa Maria wouldn’t have reached America without their simple yet fundamental support.
It might sound self-evident, as almost every machine relies on some kind of mechanism, but their operation in space -- more precisely, unknown space -- is somewhat uncertain. In an environment consisting of extreme temperature changes, radiation and vibrations, permanent exposure to the vacuum of space and varying acceleration forces, spacecraft mechanisms need to be extremely robust to survive any mission.
The vacuum environment, for example, is a problem for common mechanisms since any device containing some kind of liquid or gas involves a high risk of outgassing, leakage or explosion.
But what kind of mechanisms does a spacecraft need? Let's start by taking the above example and looking at the mechanisms that were essential for Columbus on his expedition.
Sailing the High (Interstellar) Seas
First of all: steering. While a ship uses a paddle to control the direction of its propelling force, the Icarus spacecraft would need some mechanism to control the thrust force, too. This could be done by using thrust vectoring mechanisms, similar to the ones used in military jets or rockets.
One way to actually construct such a mechanism is to put a simple movable plate into the exhaust stream of the engine which deflects the airflow opposite to the desired steering direction. Another common method is to gimbal-mount parts of the entire engine so that it can be tilted, with the help of hydraulic or electric actuators. One can imagine that for an interstellar spacecraft which has the potential to be extremely massive, such mechanisms must be designed to be large, robust and, very likely, power consuming.
So, new approaches are necessary, which depend on the type of engine used for such a mission. While hydraulics are a good solution on Earth, it is generally not a good approach for space applications, due to the aforementioned risks involved with the surrounding vacuum.
As an alternative, "thrust vectoring" could also be driven by electrical or magnetic mechanisms, especially if the spacecraft uses electrical propulsion. The ejection of ions could then be manipulated by exerting an additional electromagnetic force on them.
Whatever mechanism is designed to generate the thrust vectoring, it has to be able to withstand lifelong vibration, radiation and high thermal loads, since the engine will likely be operating for a large fraction of the mission.
Another pertinent issue for consideration is the spacecraft communication systems. At his time, Columbus had very limited tools for communication, such as flag semaphore and homing pigeons, which covered only very small distances. Nowadays we use radio and satellite communication, which covers the entire planet.
On its journey through space, the starship needs to maintain a communication link to our solar system even if it is several light-years away. A number of ideas exist as to how this can be accomplished, but when it comes to the communication devices itself, again, mechanisms are a driving factor.
The accurate pointing of a communication dish is of vital importance to the success of maintaining the link to the spacecraft -- everybody who ever tried to adjust a satellite dish for their home television is familiar with this problem.
Considering that both the transmitter and receiver are moving, the complexities associated with communication mechanisms for accurate pointing are immense. Today such extremely accurate pointing mechanisms already exist. The achieved accuracy ranges from a few fractions of a degree, for high gain antenna applications, to micro-degrees for optical inter-satellite links and even descend to the nano-degree range for optical instruments.
Deploy the Main(solar)sail
Another application for mechanisms is probably more obvious: deployable structures and attachable devices. Columbus, on his ship for example, had mechanisms for the deployment of the sails and for launching the longboat.
Similar to that, Icarus needs mechanisms for deploying solar panels or solar sails and also for releasing or docking planetary probes or other payloads.
Structures such as solar sails are large and, therefore, need to be realised with a minimum amount of material and mass. Research in this field is currently investigating a solution using reinforced composite materials or so-called gossamer structures. The latter are often deployed using inflatable or rigidizable structures.
These kinds of deployment mechanisms are quite new and still need to be investigated and tested further. However, Icarus could make use of these approaches for the deployment of larger structures such as a solar sail.
What If Something Breaks?
Another familiar field is the ejection of payloads which is achieved by using different types of release mechanisms. Today these mechanisms are often found on rockets as separation devices for the tanks and boosters or on the International Space Station (ISS) as docking mechanisms for other spacecraft or modules.
Different to the more simple separation mechanisms, docking adapters also provide an exchange of resources between the docked vehicles, such as power, gas and liquids. These mechanisms are often used only once during their lifetime which holds another risk: If a release mechanism is damaged through the decade-long journey, it might not be able to eject the probe to the target star. The failure of such a mechanism therefore clearly puts a high risk on the scientific success of the mission.
But there is a solution to ensure that these mechanisms are operating reliably, despite the environmental conditions: repair devices.
These could possibly be robotic arms, flexibly mounted on the spacecraft, such as the ones used on the ISS. If any onboard device fails -- the pointing mechanisms of the communication devices for example -- these arms could be used for inspection and repair directly on site.
Development of such robotic arms has undergone significant progress over the last few years, and provides an interesting option for onboard repair. Currently, these arms are used to assist with repairs on the ISS and, until recently, they have been used to load and unload cargo from the space shuttle. The most challenging problem for their application on Icarus might be to design them to be all-purpose with multi-functional adapters, so that they can fulfil a variety of tasks on the starship.
In this "computer age," mechanical hardware is continually being replaced by electrically-driven devices. It may seem old fashioned, but complex moving machinery can’t be built without using some mechanical mechanisms including bearings, hinges or levers. To build these components using new materials, and in a way that they can withstand the ravages of deep space, is a big challenge for Icarus, as several environmental factors are still unknown.
Determination of these factors and realising a redundant design are the only way to fulfil this requirement.