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)

“Project Daedalus” was inspired by advances in nuclear fusion technology as well as the 1960s “Orion” nuclear pulse rocket. The apparently rapid developments in laser and electron beam initiation of fusion in the early 1970s caused many to believe that interstellar travel was now at least possible, no longer merely in principle, but using real near-term technology. Thirty years later nuclear fusion is edging closer to “break-even” – when sufficient energy is generated to power the triggering of the reaction, at least if energy conversion machines were in place – and the possibility of interstellar travel has become more tangible. “Project Icarus” carries the torch of “Daedalus” further forward.

Fusion will give us much more than interstellar travel, however. As the energy source of the Sun and the stars thus, indirectly of all life on Earth, fusion already has given us so much. Modern civilization became possible because of its use of chemical energy stored in the remains of living things once powered by the Sun, and in effect already uses (stored) fusion energy. While plenty of sunshine for our needs falls on planet Earth, it is spread very thinly and requires large areas to collect, and very large batteries or chemical fuels to store. Nuclear fusion can use the relatively abundant fuel of deuterium, found in any body of water in the Solar System, though some advocate rarer elements like boron and lithium, or even rarer isotopes like helium-3. Sufficient amounts of deuterium exist in the oceans to supply an energy-rich civilization on Earth for billions of years. With such abundant energy we could even mine the planet’s “rubbish” mountains for the resources we have discarded as “unusable” waste and begin reversing the damage wrought by 3 centuries of fossil-fuel powered industrial development.

High-performance fusion rockets would also open up the Solar System to speedy access via large space-vehicles, as well as enabling star-probes. Assuming a 1,000 ton space vehicle and constant acceleration at 0.722 m/s², which an exhaust velocity of 10,000 km/s with a mass-flow rate equal to “Daedalus’s” second-stage would produce, we get the following figures for travel times to various Solar System destinations.

Table 1. Travel Times for Mean, Minimum and Maximum distances. Mean distance is the difference between the mean orbital radii of the destination and Earth, minimum uses the distance of the destination at its perihelion and the maximum places the destination at its aphelion on the opposite side of the Sun to the Earth.

As can be easily seen even distant Eris, the former Tenth Planet, can be reached in a little over 3 months for just 960 tons propellant. The most difficult flight to Mars needs a mere 60 tons of propellant to deliver 500 tons of space-vehicle and its 500 ton payload. For comparison, the nuclear-propelled Integrated Manned Interplanetary Spacecraft (ref. Encyclopedia Astronautica), flying to Mars and back on 420 day missions were extensively studied in the 1960s. They typically required 1226 tons of materials in Earth orbit, used 873 tons propellant, and carried only 110 tons of payload, accommodating 6 crew persons. Since the shutdown of the NERVA program in the early 1970s nuclear rocket technology has essentially stagnated. More recent VASIMR plasma-rocket technology requires 476 tons of propellant to deliver 124 tons of space-vehicle in 39 days to Mars, while requiring 200 megawatts of power from a Magnetohydrodynamic generator fed by a gas-core reactor, itself a technology almost as difficult to achieve as the “Daedalus” fusion engine.

Another potential use for high-thrust, high-exhaust velocity fusion-rockets is the deflection and/or orbital management of comets and asteroids. Asteroids and comets more than 200 metres wide pose a long-term threat to human civilization, if not all life, on Earth. A fusion rocket using 100 tons of propellant and a 10,000 km/s exhaust velocity can produce a 0.1 km/s change in the orbital velocity of 200 metre wide asteroids that mass roughly 10 million tons. Even multi-billion ton asteroids, kilometres across, can be steered away from a planetary collision given a year or two of warning. Smaller asteroids can also be shepherded into more useful orbits and mined, their products returned to Earth-orbit via fusion rocket.

“Daedalus”, and its successor “Icarus”, extend our fusion-making abilities by requiring greater energy efficiencies and harder to achieve fusion reactions. Regular deuterium fusion, for example, produces ~40% of its energy as neutrons, which produces large amounts of heat. On Earth this is very useful and we have advanced steam-era technology for turning heat into power, but in space heat must be radiated away and this complicates the energy conversion process. “Daedalus” reduced the neutron heat-load by using the deuterium-helium3 reaction which produces far fewer neutrons and its end products are charged particles that can be steered via electromagnetic fields in useful directions. “Icarus” hopes to do the same, but with a clearer view of what such reactions require from our fusion technology. On Earth such low-neutron fusion-power would reduce the heat being ‘dumped’ to the environment and reduce safety concerns from neutron-activation of containment vessel structure, a source of low-level radioactivity.

The high-speed spaceflight enabled by high-efficiency, low-neutron fusion energy enables another desirable opening of possibilities – the mining of the gas-giants for helium-3. The interdependence of the two might seem a bottleneck, but there is a significant amount of helium-3 closer to Earth embedded in the crystal structure of the Moon’s regolith, deposited by the solar wind. As recently revealed the Moon’s regolith is also periodically coated by a thin layer of water, probably also produced by interaction with the solar wind. This may well provide a useful source of deuterium, thus allowing fusion rocket propellant to be extracted solely from the Moon, at least at first. The estimated helium-3 resource of the Moon is roughly 2.5 million tons, and the deuterium is probably similar. An estimated 4.1 million tons of fusion propellant as close as the Moon will be important in the early days of a developing solar economy. Long before we have disfigured the Moon we will have the means to mine the gas-giants, a resource many billions of times greater than the Moon’s.

“Project Daedalus” was the vanguard of practical fusion propulsion technology, and “Project Icarus” will carry us all closer to that goal. A goal that once achieved will give us far more than the stars, nothing less than security for our planet and access to many more as well. “Icarus” aims for the stars, but will lift up all of us with it as well.

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.

Alternatively take a look around the remainder of the Icarus site.

You will probably have heard of the old tradition (before GPS!) of using the stars to help navigate around the earth especially at sea.  But when you’re actually travelling among the stars, and they are no longer ‘fixed’ in the sky, how would you navigate then?  This was the question asked by the Daedalus team in the 1970s as they planned their flight out of the solar system to Barnard’s star.  Despite noting that the interstellar probe presented problems of navigation far beyond those that had to be solved for the current interplanetary probes,  their original investigation concluded that there were no fundamental problems that could not be overcome through the normal process of technological development.  So one might ask what has happened over the last 30-40 years and what are the implications for the Icarus Team?

Those fundamental issues were discussed in the original Daedalus navigation paper but if you don’t want to search out your historical JBIS here are the main conclusions: they determined that an angular measuring accuracy of a microradian or so (0.2” – arc seconds) and a 5 metre telescope using sensors in the visible to thermal bands (0.5-100 micrometres) would do the job.  But there was a caveat that currently known stellar positions were not accurate enough for interstellar navigation although they supposed that this problem would be resolved by taking astrometric observations from Daedalus flight trails.

Well, in the first instance let’s check out one of the most recent missions that might offer some comparison …The NASA New Horizons Mission to Pluto and the Kuiper Belt.  The guidance and control system on New Horizons is capable of providing attitude knowledge to better than +/-471 microradians.  That relates to how accurately you can point your communications system back to earth; there’s more on this in the earlier blog by Pat Galea.  The Daedalus paper stated 250 microradians would suffice for this during its cruise phase so at least it is getting close to that requirement.  Interestingly, Galileo Avionica, the manufacturer of the star trackers often used for guidance and control in interplanetary missions including New Horizons, states the accuracy of their A-Str star tracker as better than 10” which is about 48 microradians.  So, still not that near the requirements set out by the Daedalus study.  Perhaps that was good enough for navigating around the outer solar system so hence there was no real need to go for greater accuracy.

Suppose we take a look at a mission specifically designed for accurate stellar measurements such as the ESA Hipparcos mission in the 1990s?  Hipparcos was designed to measure stellar parallax (and positions) using a main mirror of only 29cm…In this case the error was given as between +/-0.7 to 0.9” (for star magnitudes down to the 8^th magnitude).   That’s not a whole lot more than a microradian.  The follow-on ESA mission of Gaia promises even greater accuracy albeit with larger mirrors.  So it seems that the Daedalus team were correct in their assumption and sufficiently accurate systems are now possible.

You might recall that the Deadalus mission profile was to carry out a fly-by of the target system and the only complication was for the sub-probes to not fly too close to the targets (remember the fly past velocity was to be 12% of the speed of light – if you get too close the relative motion becomes too great).  Interestingly, it now looks possible that Icarus may include some deceleration or perhaps some elements might even go in to orbit which will remove some of the problems identified by the Daedalus team but will add a significant amount of complexity to a fully automated system.  In the near future and with the plans for exoplanet detection it is likely that there will be another advantage for Icarus: that of having prior knowledge of the planetary system before it sets off.  Indeed, there are some concerns that by the date of a possible Icarus launch/arrival at the target, future solar system based observations may well exceed the capability of the Icarus onboard systems until the probe gets quite close, but at least that will mean there would be a high level of knowledge about the target system.

This all sounds fairly promising for the Icarus study and there is more good news.  Recent studies have been made into using x-ray pulsars to help navigate space.  In effect, the suggestion is that by using 4 x-ray pulsars and their precise timing signals then you can effectively set up an interstellar GPS system.  You can find more information about this at http://www.centauri-dreams.org/?p=136. 

Finally, after all that promise from developments of the last 30-odd years there still seems to be one nagging thought that was also raised by the Daedalus team…how are we going to make the system reliable enough to last the anticipated mission time i.e. perhaps 50-70 years!  Modern star trackers are rated for around 15 years although some spacecraft out there, for example the Voyagers, are already over 30 years old and still under guidance and control and still expected to go on for another 10 years! 

So there is real hope and the Icarus team will be taking on the challenge of all these issues for their new interstellar design.  The review of the navigation problem will consider all the relevant assumptions of the Daedalus team and will be taking a closer look at the technology and the real missions that can be related to it.  Overall, it seems possible that rather than leaving it to any future technological development Icarus may be able to use current technology and meet the requirements for interstellar navigation.  But, as my partner, Sue always says, if all else fails we could always stop and ask for directions!

Next time I shall take a look at the scientific benefits of sending a probe to the nearest stars with specific reference to a related navigation issue raised by Project Daedalus: improving the accuracy of measuring stellar positions and distances and how it could help calibrate the scale of the universe.

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.

Alternatively take a look around the remainder of the Icarus site.

References:

Fountain, Glen H., et al, ‘The New Horizons Spacecraft’, arXiv: 0709.4288v1 (26 Sep 07)

Perryman, M.A.C. et al, ‘The Hipparcos Catalogue’, Astron. Astrophysics, 323, L49-L52, (1997)

Richards, G.R.  ‘Daedalus Project: The Navigation Problem’, JBIS, ppS143-148, (1978)

When people read about The British Interplanetary Society Project Daedalus they usually marvel at the boldness of the idea and the amount of thought that went into the engineering calculations. Indeed, the major objective of Project Daedalus was to carry out a feasibility study for a simple interstellar mission but using only present day technology and with reasonable extrapolation to near future technology. However, it is not immediately apparent why Project Daedalus was undertaken in the first place.

One of the reasons was to investigate the Fermi Paradox first postulated by the Italian Physicist Enrico Fermi in the 1940s. This supposes that there has been plenty of time for intelligent civilizations to interact within our galaxy when one examines the age and number of stars, as well as the distances between them. Yet, the fact that extra-terrestrial intelligence has never been observed leads to a logical paradox where our observations are inconsistent with our theoretical expectation. This original question from Fermi seemed to also reinforce the prevailing paradigm at the time that interstellar travel was impossible.

Project Daedalus was a bold way to examine the Fermi Paradox head on and gave a partial answer – interstellar travel was possible. The basis of this belief was the demonstration of a credible engineering design just at the outset of the space age that could in theory, cross the interstellar distances. In the future scientific advancement would lead to a refined and more efficient design. The absence of alien visitors would therefore require a different explanation because Project Daedalus demonstrated that with current, and near future, technology, interstellar travel was feasible. Therefore, another solution to the absence of extra-terrestrial visitation was necessary. Although Project Daedalus was ostensibly focused on designing an interstellar flyby probe, the underlying motives were to frame discussions about the Fermi paradox.

In the years after Project Daedalus, a member of the Daedalus team Alan Bond plunged himself into researching biology to understand how species evolve from a single celled organism to something as complex as human beings. He argued that historically pure guess work had been used to determine the number of intelligent civilizations in our galaxy. The culmination of his research resulted in the publication of a paper ‘On the improbability of intelligent extraterrestrials‘ in 1982 in the Journal of the British Interplanetary Society. He concluded that organisms with the complexity of human beings may be rare and only occur with a probability below much less than once per galaxy.

This startling conclusion was based upon the development of a biological model assuming an observed exponential growth in the complexity of biological life in the fossil records over time. Bond had addressed the probability of Earth-like planets with Carbon based biology existing in our galaxy. Proteins, the complete set of genes which pass from a parent to an offspring, were seen as the fundamental mechanism of biochemistry on all worlds. The genome would increase in size over time and the establishment of intelligence would require a certain level of intelligence and therefore a minimum size of genome; the more complex an organism then the larger the genome required for its specification. Attainment of an intelligence level like us would take merely a few million years, for species which had a similar genome size.

Bond himself stated in the paper that a lot of assumptions had been made with inaccurately known parameters and that more work needed to be done. He said further that there was clear potential for an order of magnitude variation around the estimates that were derived. His actual numbers suggested that a planet with the development level of the Earth only occurs once in 50,000 galaxies.

He concluded that “whilst we are sufficiently rare to inhibit contact, at least with the Galaxy at its present age, we are not so rare as to defy phenomenological explanation”. The conclusions of this paper are a disappointment for those who believe intelligent life to be prolific. But it is interesting that in the Cosmos television series Carl Sagan also expresses the view that although life may occur purely as a function of chemistry and on most worlds where the environment is suitable, intelligent life in complex beings like us may be rare. When two great thinkers share a similar vision this requires contemplation. The rare intelligence hypothesis paints a very different picture of intelligent life in the universe to that of shows like Star Trek.

As a literature search on the internet shows, there are many potential answers to the Fermi Paradox and it may remain unresolved for some time. Although, advances in observational techniques for looking at distant extra-solar planets may lead us to an answer sooner than we think. One thing is for certain though, the contribution of Project Daedalus to the debate was first rate, demonstrating the possibility of star travel and forcing us to consider other more profound answers. It was no surprise that Alan Bond would also be one of the pioneers in searching for alternative explanations. There is one way we will know for sure, and that it to build something like Daedalus or Icarus in the coming decades, and then go see for ourselves.

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.

Alternatively take a look around the remainder of the Icarus site.

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