Three different configurations of the Icarus spacecraft. Top: A Image courtesy Adrian Mann
Project Icarus is an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination. Headed by the Tau Zero Foundation and British Interplanetary Society, a non-profit group of scientists dedicated to interstellar spaceflight, Icarus is working to develop a spacecraft that can travel to a nearby star.
In Part 1 of this two-part article, Kelvin Long, Design Lead for the Project Icarus Vehicle Configuration, describes the design constraints that were placed on the original Project Daedalus interstellar spacecraft.
One of the most exciting parts of Project Icarus which is on everyone's mind with an interest in the project, is what will the spacecraft look like? Will it be very similar to the Daedalus design or radically different?
Before we address the design of the Icarus spacecraft, let us remind ourselves about the Project Daedalus spacecraft configuration.
The core section of the Daedalus vehicle was the use of a slender structural spine, from which all the other components could be attached. The use of this spine was possible due to the low vehicle acceleration levels and assembly in space.
During the design work, several spine types were investigated. With maximum torsional (twisting) stiffness and minimal buckling being one of the major engineering requirements, a triangulated structure was chosen for the Daedalus vehicle.
Along the spine were several core parts. For the first stage, it was divided into two sections, the engine bay and the service bay, which included all facilities and propellant tanks. The second stage also had these sections but included the science payload bay.
Daedalus has a structure mass of 2,670 tons being propelled by 50,000 tons of deuterium and helium-3. In total, the Daedalus spacecraft would begin its interstellar journey the approximate mass of a medium-sized oil tanker. At a length of nearly 200 meters it is nearly twice the height of the Saturn V rocket that took men to the moon in the 1960s.
On the second engine stage, it carries a 450 ton scientific payload — a little more than the mass of the completed International Space Station.
The payload was divided into a 'layered cake'-like structure so that different sub-systems could be included. This included 18 sub-probes which were each powered by a nuclear electric engine. It also included astronomical instruments, communications, warden probes and ancillary spare parts for repair.
The Daedalus spacecraft is a two-stage engine design. This means that after the first period of accelerating — lasting for just over two years — the first section drops away and then the second stage engine is ignited.
Propellant tanks are also discarded en route, the first two when the spacecraft reaches 0.004 light-years from Earth followed by another when the spacecraft reaches 0.018 light-years. The remaining three first stage tanks are separated at the end of the first boost phase when the spacecraft reaches a distance of 0.05 light-years.
The first two second stage propellant tanks are then separated when the spacecraft reaches a distance of 0.12 light-years and the remaining tanks separated when the spacecraft is at a distance of 0.21 light-years, 3.8 years into the mission.
The choice of ignition system played an important role in determining the vehicle configuration layout. Daedalus used relativistic electron beams to compress and heat the fuel pellets. Significant technological developments were assumed to make the design of the ignition system possible.
All fuel pellets were to be stored in the fuel tanks as pre-formed pellets and then moved via a fuel transfer system into the reaction chamber (i.e. where the fuel undergoes fusion, boosting the spacecraft on its way).
The choice of materials for the main vehicle structure was always going to be a challenge, where they must be capable of maintaining its structural integrity for minimum mass while being subjected to various environmental conditions. This will be discussed further in the later article on the Materials Module.
However, it is worth noting that molybdenum was selected as the main material for Daedalus being low density, abundant and strong. Molybdenum is commonly used in steel alloys and it has high-temperature, high-pressure applications in industry.
The structural integrity of the Daedalus vehicle would also have to be maintained through the boost, mid-course correction burns and pre-encounter sub-probe dispersion burn phases. All loads from the pulse detonation would be carried from the reaction chamber to the aft end via a system of struts and ties.
And at the front of the spacecraft is a large 9 millimeter-thick, 32 meter (105 ft) radius beryllium particle erosion shield. Even in interstellar space, it is expected that particles will hit the shield, slowly eroding it over the years of travel. The shield was thought to be adequate for repelling even high energy protons. Calculations show that an estimated 50 tons of the shield would be lost during flight.
In discussions with members of the original Project Daedalus Study Group, it was revealed that one member kept adding a pointy nose to the spacecraft, but with no proper justification from the engineering calculations. From a human perspective, aesthetic qualities in an engineering design can be important but the Project Icarus Study Group has adopted a design philosophy where the vehicle configuration is to be derived solely from engineering and physics calculations where possible.
In Part 2 of "What Would an Interstellar Spaceship Look Like?" Kelvin Long will describe the design considerations for the Project Icarus interstellar spacecraft.