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The Icarus Challenge

Icarus faces many design challenges, one of which is communications. The Icarus starship will be conducting important measurements of the interstellar medium, and other long baseline measurements, which will necessitate a reliable communications link with Earth [1, 2]. Although communications are of primary importance, perhaps the most difficult design challenge are the propulsion systems.

Using Daedalus as an early baseline, we can estimate the D-He3 pulsed fusion engine requires around 50,000 tonnes of propellant [3]. On Daedalus, 6 fuel tanks were allocated to the first stage and 4 in the second. Each fuel tank was estimated to weigh in at 20 tonnes and so to maximize the total change in velocity (Δv), these tanks would be discarded as they were spent along the way [4].  The illustration by Nathan Fowkes, shown in figure 1, describes the Daedalus mission timeline.

Figure 1. Daedalus Mission profile, by Nathan Fowkes.

In the fifth icon as detailed in figure 2, the fuel tanks are discarded. In Daedalus the spent tanks are jettisoned along with the first stage engine. The second stage tanks are discarded after their fuel reserves are depleted.

Figure 2. Excerpt showing spent fuel tanks being discarded.

It is important to note the last 4 tanks on the second stage can be jettisoned years after the end of the acceleration phase. The 10 fuel tanks on Daedalus weigh 50,000 tonnes, thus the final 4 weigh 20,000 tonnes. When the tanks are depleted they weigh 20*4=80 tonnes. Thus the mass deficit is less than 1% (0.4%) giving us the option to delay ejecting them. Since they provide minimal drag in space and we not trying to accelerate their mass, there’s no intrinsic benefit to kicking them away earlier.

Fuel Tank Reuse Scenarios

So if we do take them along, what would we use them for? Let us explore some options:  

Option 1: Fuel Tanks as Communications Relays: This provides Icarus with a great opportunity – to use the spent fuel tanks as our communications relays along the acceleration path. The acceleration rate could be adjusted to fit a mission profile in which, fuel tank ejection timing fits coincides with the communication relay positions.

If we further correlate the pulsed fusion rate with the Icarus heat dispersion/power generation profile, we will arrive at a best case scenario for fuel tank re-purposing under this scheme. (Establishing the validity of this scenario would have to wait until the Icarus Main Engine is designed and thermally modeled – work planned for 2012.)  

Option 2: Fuel Tanks as Science Probes: Alternatively, the fuel tanks could be retrofitted as scientific stations for the exploration of the interstellar medium. A network of ejected drones could also be used as transmission lines for an interferometric gravitational wave detector, such as the proposed ESA/NASA LISA mission.

Option 3: Fuel Tanks as Drones: Seeing as the fuel tanks would have already been accelerated to velocities of some measurable fraction of the speed of light, they could be directed towards other interstellar targets of interest. With sufficient planning the drones can even be directed towards other target stars, on sub-missions geared for late encounters and mission times of a few hundreds of years.

The enormous 60 meter diameter fuel tanks could be covered in solar panels and be programmed to turn on automatically when they are within a certain solar flux. Gravity would provide some basic trajectory adjustments, pulling the drone towards their target star’s gravitational well. A charged laser communications pulse would signal back to Earth a summary of findings.

Given their size, the drones could be equipped with their own RTGs and be used for a wide variety of interstellar experiments, effectively combining Option 2 and Option 3.

Out of these options, the most advantageous to the Icarus primary mission is Option 1. Icarus’ main mission success outweighs the options for extending the main mission objectives.  We will also need to drop large communications relays anyway and so, since the fuel tanks will be dropped along the way, this scheme fits perfectly with the Icarus baseline mission.

Include the He3 Mining Balloon Storage Tanks

To make the fuel tanks capable of transforming into drones or relays, we would need to have some basic electronics package on each tank [5, 6, 7]. The tank interface with the Icarus could have this built in already. In fact, the electronics could be the same instrument package used for accumulating the fuel in the first place – part of the He3 mining instrument. The fuel will not need to be transferred from the fuel acquisition balloon tanks to the storage tanks onboard the Icarus – we just place them directly onto the spacecraft and consequently, reuse them again as transponders.

Therefore the Icarus fuel tanks are in fact “Transformers” with three phases:

Mining Balloon Storage Tank >

Fuel Tank >

Communications Transponder

Operational Outline

Under these assumptions, fuel tank geometry should be modified to fit in with our aspiring multi-use profile. The most restrictive design parameters are imposed by the communications relays. As such we propose the fuel tanks are constructed out of two parabolic dishes forming a clam-shell.

This would allow them to be used as effective communications dishes, to be deployed after separation from Icarus. One side of the ‘shell’ would face forward (towards Icarus) and the other towards Earth (or then next relay station). A small inertial wheel system for attitude control coupled with a Radioisotope Thermal Generator (RTG) for power, would allow two-way communications, as outlined in figure 3.

Figure 3. Parabolic clam-shell fuel tanks are deployed as communications relays. Note the fuel tank deployment is meant to be circular and out of the page.

We can now provide an outline of this fuel tank re-purposing scheme:

1. He3 Mining Tank Phase: The He3 mining balloons, will have some instrument package and basic thrusters. It’s essentially a large fuel tank which would be filled with He3, with some attitude thrusters and a balloon on top.
2. Icarus Fuel Tank Phase: Icarus would sweep around Jupiter (or Neptune) collecting these tanks and assemble them on its body, on the way out of the solar system *(Icarus builds his wax wings). The He3 refinery contains supply nozzles from its He3 scrubbers, which collect He and separate out He3. Those same nozzles now connects to the main fusion engine propellant supply.
3. Communications Relay Phase: According to our early mission baseline, the Icarus main engines are ignited outside of our solar system. The Icarus adjusts burn rate so that a fuel tank is depleted at positions where relays are needed. A fuel tank is dropped and is deployed, transforming into a relay.

Figure 4, depicts all three incarnations of the now multi-purposed fuel  tanks, which will primarily provide the necessary communications relays and redundancy for a successful mission.

Figure 4. The three incarnations of the Icarus fuel tanks.

Seeing how novel ideas are being introduced at remarkable rates during this study, it is certain that Project Icarus will have many surprises for us.

Project Icarus is manned by distinguished scientists donating their time and creativity to further the current state of the art in interstellar spacecraft design. If you have found this article to be of value then please consider donating a small amount to Project Icarus to assist us with our ambitions of creating a credible starship design.

Acknowledgements

I would like to thank the Icarus design team, for assisting in the development of this novel idea, amongst the many others.

References

[1] “PROJECT ICARUS: Relays”, P. Galea, Project Icarus internal research note (2010)

[2] “PROJECT ICARUS: Mechanisms for Enhancing the Stability of Gravitationally Lensed Interstellar Communications”, P. Galea, R. Swinney, 61st International Astronomical Congress, Prague, CZ (preprint) (2010)

[3] ”Project Daedalus: The Mission Profile”, A. Bond and A.R. Matrin, JBIS: Project Daedalus Final Report (1978)

[4] ”Project Daedalus: The Vehicle Configuration”, J. Strong and A. Bond, JBIS: Project Daedalus Final Report (1978)

[5] “Project Daedalus: Propellant Acquisition Techniques”, R.C. Parkinson, JBIS: Project Daedalus Final Report (1978)

[6] “NASA Stratospheric Balloons – Pioneers of Space Exploration and Research”, Report of the Scientific Ballooning Planning Team (2005)

[7] “Development of a Preliminary Technology Roadmap for Stratospheric Balloon Platforms Dedicated to Earth Science Applications”, Global Aerospace Corporation, prepared for the JPL/NASA (2002)

In engineering, maintenance is an essential topic not only during operation, but also when planning it during the design process. Maintenance has to be considered as early as possible in order to allow for the correction of errors and for the repair of hardware as easily and as fast as possible. For complex technical devices on Earth, a detailed maintenance plan is typically developed to fulfil this aim. When talking about spacecraft,  the approach to the maintenance plan is somewhat different. While for manned space missions the maintenance of a spacecraft can be carried out by astronauts, this is not possible for unmanned missions. Often the cost of maintenance and service for space missions is actually higher than the cost of losing the functionality of the subsystems or even the entire spacecraft. Regarding the rather short lifetimes of space vehicles, which is in most cases only a couple of years, maintenance simply is not required because the subcomponents have sufficiently long lifetimes. For a spacecraft such as Icarus which would have a flight time of about 60 years, the question is not if on-orbit-servicing and repair is required, but how it shall be implemented in the spacecraft.

The only known spacecrafts which had, and still has a significantly long lifetime without having to be repaired, are the Pioneer and Voyager probes launched in the 1970’s. The maximum lifetime of the Pioneer probes is 31 years (Pioneer 10) and 22 years (Pioneer 11). For the Voyager probes 1 and 2 it is currently 33 years and expected to become up to 48 years until the end of their functionality in the 2020’s. The limiting factor for these extremely long missions is not the fuel, as one could assume, but the electrical power supply. Because they are so far away from the Sun, these probes cannot use solar panels. To be independent from external sources, they draw electrical power from radioisotope thermoelectric generators (RTG) which obtain their power from radioactive decay.

Even though the lifetime of these early interstellar probes comes very close to the projected lifetime of Icarus, they cannot directly be compared. As Icarus aims to fly a distance of about six light-years, it will undergo far greater forces than the Voyager probes which, to date, have ‘only’ covered a distance of about 0.002 light-years (Voyager 1) [1]. For Icarus, these external forces would occur due to the higher acceleration of the vehicle and the unknown environment through which it would fly, e.g. radiation and particle bombardment. Additionally the thermal loads on the mechanical structure must not be ignored. Between the engine and the payload, huge temperature differences will appear which would need to be compensated for throughout the structure. Furthermore, all moving parts on the vehicle, such as steering mechanisms for the propulsion system as well as instruments on the payload side, would be sensitive to external forces. Even more critical than these static loads would be  the dynamic forces due to vibrations from the engine. Summing up these factors, the life of Icarus would  be much harder than for the interstellar Voyager probes and therefore a completely new challenge concerning its design and the way of maintaining the vehicle in an operational state over a lifetime of about 60 years.

Typically, systems with the projected complexity of the Icarus spacecraft receive regular maintenance and repair services where the most stressed components are checked and replaced. For such systems on Earth we have a comprehensive support of repair facilities for each type of hardware. If we look for comparable analogues to spacecrafts such as Icarus on Earth we find that aircrafts have very similar level of complexity. The current operational lifetime of civil aircrafts is about 15-20 years, but only if they are regularly maintained. The mean time between minor checks and maintenance for civil aircrafts is about 300-600 flight hours (A-Check), which is about 1-2 months. Additionally they are checked daily after each cycle of take off and landing (Trip Check) [2]. Of course one has to consider that these systems are manned, opposite to the unmanned spacecraft Icarus, which necessarily leads to more stringent safety regulations. However, the challenge to design a spacecraft with a lifetime of 60 years without access to human repair services, compared to designing an aircraft with a lifetime of 20 years including monthly checks is obvious.

Concluding these thoughts, it seems inevitable to implement on-board-servicing devices on Icarus. But not only the repair devices themselves have to be considered, but also the spare parts which have to be transported additionally. As a rule of thumb it can be assumed that one-sixth of the total components should be capable of being replaced (spareable items) and one-sixth of the spareable items should be repairable off-line (repairable items). These numbers depend on the equipment which has to be removed from service and the depth of repair which has to be performed.  By modifying the rule of thumb for the application to Icarus, we can assume one-tenth of the items being spareable and, as before, one-sixth of the spareable items being repairable. Because these items can not be removed from service on Icarus, they have to be repairable in-situ [3].

To carry 10% spare parts on an interstellar mission is a huge additional effort, due to the problems of storing these items, and the additional mass of the spacecraft which further implies higher fuel requirements.  The spacecraft would have to carry its own “repair shop” including a large “warehouse” of spare parts. The “workers” then have to be considered separately: In order to be available at any position of the vehicle and to transport the spare parts from the “warehouse” to the respective repair site, mobile service robots are required. Partly this task could be fulfilled using robotic arms mounted on the outside of the spacecraft as already done on the International Space Station and on the Space Shuttle. They could be installed near critical components, such as the payload section or the propulsion system of Icarus. With the help of rails these robotic arms could slide along the structure of the spacecraft and reach various spots of the vehicle. Making the entire vehicle accessible for these arms would require a complex system of rails and arms which leads to the need for autonomous mobile repair vehicles. These could be small service robots which have their own propulsion and power system and carry a universal adapter, e.g. a robotic arm, which connects to the respective interfaces at the repair site. In this scenario the spacecraft would then contain an entire self-service and self-repair system where any defective device could be fixed automatically with on-board resources. To be capable of repairing a large variety of components, the design of the spacecraft subsystems should therefore be modular and generic where mechanical and electrical components are designed using common basic structures which could easily be replaced. This modularity is also required for the pre-flight assembly of the spacecraft in orbit, so that this design aspect pays off twice.

In order to cover all features of on-orbit-servicing, the service robots would  have to perform the following tasks: Remote and on-site inspection, docking, maintenance and repair. For the remote and on-site inspection, accurate manoeuvring and therefore a sophisticated attitude control system would be required. For the next step of servicing, an additional technology would  have to be used by the robots: Rendezvous and Docking. Docking is necessary wherever remote inspection is not sufficient and where the robot locally has to provide the spacecraft with resources or repair its components. For this case, universal mechanical and electrical interfaces have to be implemented, where the robot can dock, receive diagnostic data and provide resources such as power, lubricant or propellant. In addition to designing the interfaces universally, the easy external accessibility to potential repair sites has to be ensured. If, for instance, an insulation foil has to be cut first to access a repair site, the service robots would have to be capable of repairing the insulation foil additionally, which is not necessarily what they would have  been designed for in the first place.

In the past and present space missions we already find a couple of such on-orbit-servicing satellites and robotic systems. Although this technology is still young and not yet really established in common space missions (Spacecrafts are rather thrown away or left dead instead of getting repaired or reactivated), there have already been some successful precursor missions on this field and more are to come within the next years. The following missions are noteworthy in this regard:

• SNAP-1
• DART
• ETS-VII
• ORBITAL EXPRESS
• FREND (SUMO)
• TECSAS & DEOS
• OLEV

Icarus can directly benefit from these missions, as they are important technology drivers in robotics, Rendezvous and Docking and Attitude Control. The on-orbit-servicing technology is a logical step in the development of future unmanned spaceflight and moreover it is inevitable for interstellar spaceflight and hence for Icarus.

References:

[1] Wikipedia, Voyager 1 – Current Status, http://de.wikipedia.org/wiki/Voyager_1

[2] K. Engmann, “Technologie des Flugzeugs”, Vogel Verlag, 3^rd Edition, 2006.

[3] T. J. Grant, “Project Daedalus: The Need for On-Board Repair”, JBIS, 1978.

[4] W. Hallmann/W. Ley/K. Wittmann, “Handbuch der Raumfahrttechnik”, Hanser Fachbuch, 3^rd revised Edition, 2007