Icarus: Reusing Fuel Tanks as Communications Relays
by Pat Galea
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 . 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 . The illustration by Nathan Fowkes, shown in figure 1, describes the Daedalus mission timeline.
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.
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 >
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.
We can now provide an outline of this fuel tank re-purposing scheme:
- 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.
- 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.
- 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.
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  “PROJECT ICARUS: Relays”, P. Galea, Project Icarus internal research note (2010)  “PROJECT ICARUS: Mechanisms for Enhancing the Stability of Gravitationally Lensed Interstellar Communications”, P. Galea, R. Swinney, 61st International Astronomical Congress, Prague, CZ (preprint) (2010)  ”Project Daedalus: The Mission Profile”, A. Bond and A.R. Matrin, JBIS: Project Daedalus Final Report (1978)  ”Project Daedalus: The Vehicle Configuration”, J. Strong and A. Bond, JBIS: Project Daedalus Final Report (1978)  “Project Daedalus: Propellant Acquisition Techniques”, R.C. Parkinson, JBIS: Project Daedalus Final Report (1978)  “NASA Stratospheric Balloons – Pioneers of Space Exploration and Research”, Report of the Scientific Ballooning Planning Team (2005)  “Development of a Preliminary Technology Roadmap for Stratospheric Balloon Platforms Dedicated to Earth Science Applications”, Global Aerospace Corporation, prepared for the JPL/NASA (2002)