Guest contributor Kelvin Long describes the design considerations for the Icarus starship. Part 2 of 2.

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 2 of this two-part article, Kelvin Long, Design Lead for the Project Icarus Vehicle Configuration, describes the design constraints that will be placed on the future Project Icarus interstellar spacecraft.

After outlining some of the elements of the Daedalus spaceship configuration in Part 1, we can now consider the future possibilities for the Icarus vehicle.

The design team is not yet at the stage of making such decisions, but we can at least speculate on what changes may be likely.

The obvious change is the large mass of the Daedalus vehicle; at nearly 53,000 tons this presents problems for manufacturing, assembly and launch. The mass of the vehicle will need to be reduced.

Also, the requirement to mine the Helium-3 fuel from the gas giant Jupiter is also suggestive of a solar system-wide economy, perhaps supporting inertial fusion reactors on Earth for commercial energy production. Arguably, this places the launch of such a probe in the 23rd-24th Century timescale. To do it sooner, we need to have an alternative source of fuel and more compact, efficient engines.

Technological Leaps

The engine masses for the Daedalus first and second stage was approximately 988 tons and 318 tons, respectively. These are massive numbers and reducing them may require some radical technological leaps. But we only have to look at the last 100 years of scientific progress to know that this is entirely possible.

The additional requirement to slow the spacecraft down when approaching its interstellar destination will require a deceleration mechanism. This will have a dramatic effect on the appearance of the vehicle.

The deceleration mechanism could come in the form of a solar sail, magnetic sail or something more complex like a "Medusa sail" or the use of multiple laser-lens beaming systems.

In addition, the use of interesting experiments on board the flight may add features not associated with the Daedalus design. These could include gravitational wave experiments or even instruments that search for extraterrestrial intelligences.

There is also the possibility of splitting the final vehicle up into multiple sub-probes, in a so-called 'Icarus constellation' set-up. Each sub-probe would investigate a different part of the target star system.

Vehicle Configuration

Currently, all options for the Icarus vehicle configuration (and performance) are open and the design team is creatively speculating on what the final appearance may be.

The most obvious change is to employ further engine staging, as shown in the figure above.

Another idea is to have something similar to the Discovery vehicle layout used in the Stanley Kubrick film 2001: A Space Odyssey based on the novel of the same name by Arthur C Clarke. The main motivation for the shape is to separate the science probe from the nuclear propulsion component.

However, because the vehicle is traveling a large distance and needs to attain a high cruise velocity, multiple tanks may need to be stacked.

Evolving Daedalus

A key thing to keep in mind is that the Icarus team has chosen to build on the Daedalus design, evolving the Icarus vehicle from there. The alternative would have been to have started from a fresh idea but then continuity and redesign of Daedalus would have been difficult to justify.

The evolved design may have several characteristics when compared to Daedalus. These are listed within the team as options 1-6 and defined as follows:

(1) Daedalus configuration with improved calculations.

(2) Daedalus configuration with minor subsystem changes.

(3) Daedalus configuration with major system changes.

(4) New design with major system attributes to Daedalus.

(5) New design with minor subsystem attributes to Daedalus.

(6) New and radically different design.

In order for the Project Icarus Study Group to claim a 'Daedalus re-design' the level (1) definition would be the minimum required from the teams final output.

However, with the addition of a deceleration requirement, any mechanisms are likely to push the Icarus design towards the higher options.

Summary of Vehicle Configuration

The Project Icarus Study Group has one thing on its side: time.

The volunteer design team is working to a program plan, but has the option of changing the schedule at any point to ensure we can deliver a design that others will judge to be 'good' and qualifies as an acceptable Daedalus redesign.

The final design may look just like the 1970s Daedalus configuration, or it could be very different, perhaps similar to one of the creations often seen in science fiction films. However it turns out, what should drive that configuration are the engineering calculations.

The main vehicle will not be flying into any atmospheres, so wings won't be needed. Similarly, pointed noses are not seen as an engineering necessity. The criticism the Icarus team will always face is the same as someone watching the sequel of a movie or reading the sequel of a book by another author: "I prefer the original."

But does this really matter? As long as the Project Icarus Study Group applies rigorous analysis in their work, follows the numbers and adequately considers all of the Daedalus systems, we can be proud of our vehicle, and hopefully inspire the next generation to evolve the interstellar vehicle further still.