Introduction

In order to accomplish its function, exploring a star system in a completely autonomous fashion, a robotic interstellar probe must be able to function in an uncertain environment, despite any possible failure of its composing elements and systems. As it will be operating for over a century, it is inevitable that parts of the vehicle will fail. Some of the designs, such as Firefly, are subject to very high levels of radiation and will also require regular replacement of radiation fragilized parts. So the probe must be able to carry out both maintenance and on board repairs. It must do this while maintaining a maximum overall effectiveness to achieve the desired objectives for velocity and transit times, data communication rates and exploration activities₍₁₎.

A starship will need maintenance and self repair systems for structures, equipment, networks and even software.

The effectiveness of the starship can be evaluated in classical maintenance theory as a factor of following elements: Availability, Reliability, Maintainability and Capability.

To increase its chances of success, the Starship needs to be designed in such a way that no single failure can disable it permanently. Systems must be available when needed; for example, the radiators must be available to cool the drive while it operates. All components must be as reliable as possible, and if there are signs of potential failure, need to be replaced before they fail. In order to be able to do this and keep functioning, the ship needs redundant systems, or must be capable of shutdown and restart, to allow time for maintenance. All elements should be replaceable or repairable. And the ships needs to maintain its capability and avoid degradation of their systems over time.

In order to evaluate the maintenance required, it is essential to identify the threats to the spaceship, both internal and external, that need to be mitigated. A threat analysis is included at the end of this chapter.

Like the Ship of Theseus, much of the starship may be replaced over time. However, unlike the sailing ships of the past, a starship must bring along all of the materials it needs to repair itself, as well as the required tools. Living off the land is not an option. And the capacity of refurbishing components in place will be critical to the mission’s success.

Availability

Systems can remain available through redundancy and the mission design must include allowances for maintenance and system downtime. Secondary power systems are particularly crucial, as the fusion drives are all bootstrap devices that require an external power source to start operating. Rather like a battery and a dynamo in a car. Power must be available at all times for refrigeration of the propellant during boost and coast phases, and to heat parts of the ship that have minimum temperature requirements.

Redundancy

In any complex system redundancy allows for failure and repair, and to this end, many of the spacecraft systems must be designed to have redundant, triple redundant and quad redundant backups. Thus the reaction control system should have multiple redundancy, and the same for the power system, the flight computers and various IT systems. Some design might even go all to way to providing redundant engines.

Reliability

The reliability of space qualified hardware and software is significantly greater than non space qualified equivalents, however, in the case of an interstellar spacecraft, the reliability standards will need to be increased further, because of the lack of serviceability and mission critical failure issues. Ensuring hardware is maintained or replaced before it reaches it probable point of failure is critical. Reducing the number of moving parts and the velocity of rotating equipment or moving fluids at the conceptual design level is also a way of increasing reliability.

Radiation from the drive can transform elements that would seem highly reliable, such as metal structural members, into risks. It can change the atomic characteristics of materials, creating microscopic defects or inducing creep in heated materials under strain. Materials can expand and tight tolerances become contact points. Flexibility can be reduced and leaks appear in formerly fluid tight materials.

To increase reliability instrumentation and software diagnostics can be used to detect changes in materials and systems in order to intervene on the hardware before it fails.

Maintainability

Maintenance on the starship will involve accessing the elements to be maintained, repairing them, and in some cases replacing them from storage.

Wardens and spiderbots

To access and maintain equipment, some form of mobile service device is required. Technology has developed massively in the area of autonomous systems with flexible and capable handling abilities since the Project Daedalus study proposed autonomous robots called Wardens for these tasks. Thus, it is possible that any successor to the Daedalus “Wardens” is more likely to resemble the NASA Robonaut or Valkyrie series of anthropomorphic robots or Canadian DEXTRE, albeit it is likely they will, by then, have been able to be reduced in size and mass through use of new materials and smaller electronics, whilst maintaining useful power to weight ratios for heavy tasks, yet still be able to undertake intricate tasks.

From a mobility perspective, given the antenna farm (see communications section) would be constructed by spiderfabs, it may be possible to combine the features of, say, a Robonaut with a spiderfab unit.

Other options would be specialised evolutions of robots such as Boston Dynamics Atlas, which, even in its current version, is capable of untethered autonomous operations in adverse terrain.

Should larger units be required, these could be based on future evolved versions of the NASA MMSEV, only with the crew module replaced by systems more appropriate for an autonomous vehicle, and possibly with embedded 3D printers to enable instant manufacture of components at a repair site.

Mobility

To avoid expending propellant, the maintenance units would, wherever possible, move around the vehicle via a rail based tracks and use their ‘arms and hands’ to access parts. They would operate on electrical power from internal batteries, which could be recharged when they return to a power station following maintenance activity. The energy might be transferred by induction, reducing the needs for connectors and moving parts.

Replacement

There will be some components or subsystems that will be sufficiently mission critical, or complex, or the result of elaborate manufacturing processes, that it will be prudent to carry replacements for. The components would be laid out in an accessible component store in the payload bay, enabling a maintenance robot to acquire them easily via manipulator systems, and transport them to the location where they would be needed.

Fabrication

Rather than try to store multiple copies of every single part of the ship, it should be possible to provide a starship with a fabrication system able to create most parts as required. After all, the ship will be entirely designed using computer aided design and fabrication. The ship can store in its memory a complete set of plans and fabrication procedures for every single element that composes it. 3D printing systems and other machine tools would be installed within the maintenance section of the Payload bay, to print composite and metal parts. The amount of spare material carried would require careful analysis of how regularly new components are required, the increased likelihood of components wearing out over time, and overall mission duration including extended missions at the target.

3D printing has been tested in space on the International Space Station, thus the feasibility of using 3D printing in both an Earth gravity environment and a microgravity environment has been proven.

Current 3D printing is in its infancy compared to the 3D printing capabilities that will be available by the time of the spacecraft construction, let alone the departure time. Printing resolution and range of materials is increasing, and following the development curves of all other new technologies, this trajectory is likely to continue into the foreseeable future.

It is likely that during the mission, it will be possible for onboard 3D printers to print upgraded versions from new developments on Earth being transmitted to the starship.

As the supplies will be limited, it would be useful to design the starship with recyclable parts. A recovery unit would need to be developed that could break down the used parts into their individuals elements, and reprocess these to create new materials. These could then be fed to the 3D printers for fabrication.

The fabrication unit could also find service building and adapting probes for the specific requirements of the target sun’s planetary systems. On site observation might lead the Starship to decide to reconfigure probes for missions that vary from the original plan, replacing landing probes by floating probes for water worlds, for example.

The fab unit might also include a module for in-situ resources utilisation, since this is really a repurposing of the recycling capabilities of the unit. Being able to collect small asteroids or cometary material and transform them into useful products, at the very least into added fuel for the probes, could extent the mission capabilities well beyond the original payload. Depending on the technological level these fabrication systems have achieved by the time the starship is launched, and even as they are upgraded as the ship moves towards its target, the capabilities of the fabrication unit might far exceed their original specifications.

Parts and bulk storage would be stored in an adapter Automated Storage and Retrieval System (ASRS) for maximum density and efficiency.

Containment

One issue to consider is the possibility of components floating away during repair operations, or even due to component failure during the mission. As a result of this, consideration should be given to enclosing structures which house internal equipment. The enclosures could be as simple as mylar blankets rather than heavier sheets of material. Most likely, they would also do double duty as thermal shields.

Capability

During its century+ of operation, the starship must maintain its capabilities, and will even add new ones. In particular it will build a communication antenna at destination, and probably transform/transfer some of its components to exploration probes. It will also need to restart its engines after nearly eighty years of standby mode, performing system checks and diagnostics autonomously.

Mission design and maintenance

It is interesting to note that the mission plan for Icarus was much more flexible than the one that was chosen for Daedalus. Every system on Daedalus had to be perfectly operational for the few hours of the encounter time at Barnard’s Star, or the whole mission would have been in vain. On the other hand, Icarus can spend years at destination, try out different strategies, allow for maintenance and downtime of various systems, and even obtain instruction from Earth, for long term decisions and goals that go beyond the capabilities of the ship.

Threat analysis

Most of the threat to the vehicle are internal. The main exterior threats are interstellar dust and gas, and these are handled by mass shielding, as described in the Shielding chapter. The threat analysis was established by consulting the experts in the fields at Icarus Interstellar and collating the answers in the following table, established for Firefly, but essentially the same for other versions of Icarus.

References

(1)- Philip Reiss, Project Icarus M18:Vehicle Risk&Repair. Repair Warden, Technology, Icarus Interstellar Internal report

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