“The only way of discovering the limits of the possible is to venture a little way past them into the impossible”.

Project Bussard, is a research project to investigate the physics and engineering issues associated with the Bussard interstellar ramjet. But before we define the scope of the project, let us first revisit history.

Interstellar Ramjet as imagined by Icarus Graphical Engineer Adrian Mann

It was in 1960 that the physicist Robert W Bussard first proposed the interstellar ramjet in his seminal paper. Bussard was born in 1928 and died in 2007. Throughout his spectacular career he had worked on many interesting things, including the nuclear thermal rocket program in the 1950s, called Project Rover. He produced two exceptional monographs based on this research. He is also considered the inventor of the Polywell, a type of inertial electrostatic confinement fusion reactor, funded by the US Navy. The Project Icarus Study Group is also looking into Polywells for potential propulsion applications in deep space missions. But, Project Bussard aims to focus on his interstellar ramjet concept.

The interstellar ramjet method featured in the excellent Poul Anderson book, Tau Zero. The ramjet was a proposed variant of the fusion engine, but rather than carrying along its own fuel, it would use enormous electromagnetic fields to ram scoop hydrogen from the interstellar medium. The high energy protons enter the ram scoop, confined by magnetic field lines and then meeting under the conditions for fusion reactions to occur, producing a high energy exhaust jet. In theory, if the interstellar ramjet can be made to work, then relativistic star travel will be possible. This means trips to the nearest stars in only a matter of years travel time as the spacecraft continues to approach the speed of light barrier, but never quite reaches it. Indeed, trips to the centre of the galaxy, approximately 30,000 light years away, may be possible within a few decades trip time, although a very long time would have past back on Earth due to the special relativity time dilation effect, as first predicted by Albert Einstein in his 1905 paper. In the television series COSMOS starring Carl Sagan, the interstellar ramjet is even discussed (along with Project Orion and Project Daedalus) in episode 8. I am lucky enough to own the original drawing made for this show and it is shown below.

Interstellar Ramjet as imagined by Rick Sterbach for the Cosmos tv show

However, over the years, several physics and engineering problems have been found with the interstellar ramjet making it less credible as a real option for interstellar flight. Here are some of the issues in no particular order:
1. In order to ram scoop sufficient hydrogen fuel for the fusion reactions to take place, the spacecraft must already be travelling very fast, which implies carrying some on-board propellant to start with.
2. Maintaining a constant thrust profile for long period durations, whilst the engine is over heating.
3. Maintaining a very large electromagnetic field configuration over such large distances.
4. Finding areas of the interstellar medium where the density of interstellar hydrogen (or other particles) is sufficiently abundant.
5. In some versions of the ramjet concepts envisaged, the drag force generated as the large scoop passes through the interstellar medium, may exceed the thrust generated by the engine.

Over the years, variations on the interstellar ramjet design have been proposed. This includes the Ram Augmented Interstellar Ramjet, first proposed in 1974 by Daedalus designer Alan Bond, who proposed using the interstellar hydrogen only as a reaction mass. The interstellar hydrogen is not converted to helium in a fusion reaction, but instead it is accelerated by on-board fusion reactions from fuel which the Starship already carries. A modification to this scheme might employ protons hitting a low atomic number target and then converting these to the required burn reactions for jet exhaust. Daniel Whitmire made a proposal in 1975 for a catalytic ramjet, which instead uses the carbon-nitrogen-oxygen cycle producing fusion at a higher rate than in the proton-proton chain associated with the direct collection of hydrogen. Aerospace engineers Dana Andrews and Robert Zubrin have also studied the interstellar ramjet in 1985. One of the first tasks of the Project Bussard Study Group will be to collect all of the references together and work through the previous results. Some of these include:

• R.W.Bussard, “Galactic Matter and Interstellar Spaceflight”, Astronautica Acta, 6, pp.170-194,1960.
• A.R.Martin, “Magnetic Intake Limitations on Interstellar Ramjets”, Astronautica Acta, 18, pp.1-10, 1973.
• A.Bond, “An Analysis of the Potential Performance of the Ram Augmented Interstellar Rocket”, JBIS, 27, pp.674-685, 1974.
• D.Whitmore, “Relativistic Spaceflight and the Catalytic Nuclear Ramjet”, Acta Astronautica, 2, pp.497-509, 1975.
• T.A.Heppenheimer, “On the Infeasibility of Interstellar Ramjets”, JBIS, 31, pp.222-224, 1978.
• D.G.Andrews and R.M.Zubrin, “Magnetic Sails and Interstellar Travel”, JBIS, 1990.
• B.N.Cassenti, “Design Concepts for the Interstellar Ramjet”, AIAA 91-2537, 27th AIAA/ASME Joint Propulsion Conference, Sacremento, CA, June 24-26, 1991.

If you would like to inform us of other papers, please let us know on the comment section below. As the team begins Project Bussard, officially launched on 1st February 2012, it will move forward at a steady pace, over an approximately two year durtion. It is important to note that Project Bussard will not aim to ‘design’ a ramjet vehicle. This is considered too premature right now. Instead, the focus will be on researching the physics and engineering issues that prohibit the method from being apparently workable and providing potential solutions for a future team to research further. The project is to be split into five phases, as follows:

• Phase (I): Construction of project programme plan and assembly of team.
• Phase (II): Literature review, knowledge capture and identification of physics/engineering issues.
• Phase (III): Problem solving.
• Phase (IV): Strawman concept development.
• Phase (V): Write up of final study report.

As the co-founder (with Richard Obousy) of our flagship initiative, Project Icarus, this is the second technical project I have launched under the Icarus Interstellar umbrella, although back in 2009 Icarus Interstellar didn’t exist. I am very excited to see where the research for this most interesting of propulsion schemes leads. It is often said that the interstellar ramjet is the great hope for interstellar travel. Even the pioneering interstellar physicist Robert Forward, a proponent of propellantless propulsion, spent some time looking at interstellar ramjets. The Icarus Project Bussard Study Group hopes at the very least to produce a modern review of all the past literature and identification of the physics and engineering issues. But at the best, we might even stumble across an idea, and show that Robert Bussard was a true pioneer too. The physicist Greg Matloff will be joining the Project Bussard team and it is worth finishing this brief blog article with a quote from his outstanding book ‘The Starflight Handbook”:

“…Even if we never build a proton-proton fusion ramjet, the effort spent investigating it will not have been wasted….As fantastic as these ideas may seem, we should be open-minded about them, not cavalierly rule them out because of their current infeasibility. After all, the march of technology is full of well-known surprises and serendipity. To cite one example: the laser was invented virtually at the same time that the fusion ramjet was proposed. Lasers have since become the most promising way, in theory, to ionize the advancing path of a fusion ramjet and facilitate electromagnetic fuel collection”.

Kelvin Long
Vice President (Europe) Icarus Interstellar

This next version of the model was a wooden version that is advertised on ebay. It is claimed to be a “Daedalus NASA Starship spacecraft Wood Model”. Constructed from kiln-dried Wood Mahogany and hand-painted by artists. The origin of the model is unknown but to be fair to the company selling it, I will state that its claimed to be sold by MyAsianArt, an art and antiques gallery based in Manila, the Philippines:

The last of the models, 1 meter tall, was spotted at a Star Wars exhibit in Alaska by Icarus Designer Andreas Tziolas. Again, the origin of the model is unknown but a caption reads:

“In the 1970s, the British Interplanetary Society challenged its members to design an interstellar spaceship using existing and emerging technologies. The result was Daedalus, a two-stage rocket designed to send an unmanned probe on a fifty year trip to Barnard’s Star, a distance of six light years. Daedalus would use tiny fusion reactions to propel itself. A chemical rocket typically burns 800 pounds of fuel for every pound of payload it carries. A nuclear rocket, like Daedalus would use only 100 pounds of fuel per pound of payload.”

The Icarus team has decided to get our own model built for Daedalus which we can use at exhibitions and conference trade stands. We have recruited the excellent British model builder Terry Regan for this challenge. Terry has got started on this ambitious project and we will update you with photographs as he progresses. Meanwhile below shows an image from his early construction of the second stage model, with a payload bay around 120 mm in diameter and 50 mm tall. Well done Terry, this is sure to be a really exciting initiative as the model evolves into the full spacecraft design.

If you know of any other Daedalus models ever built, please let the Icarus team know.

Kelvin
Vice President (Europe) Icarus Interstellar

During my stay in New York I also visited my good friend and interstellar mentor Greg Matloff. He wrote the foreword for my recent book, “Deep Space Propulsion: A Roadmap to Interstellar Flight”, published by Springer (http://www.amazon.com/Deep-Space-Propulsion-Interstellar-Astronomers/dp/1461406064). Greg is also one of the world authorities on solar sail propulsion. We discussed interstellar flight and the future of research in this most unique of fields. An important point Greg has made to me before and re-iterated this at our recent meeting was that the interstellar community was very small, and it was very important that we didn’t fracture that community by advocating specific propulsion options against others in a hard line way, at least until a clear front runner emerges. I absolutely agree with this and have adopted this as part of my own philosophy, given I am usually a nuclear pulse advocate. Icarus Interstellar has now launched many new projects and whilst the team members are working on those specific propulsion schemes, it is important they keep the spirit of this message alive.

During my visit to New York I also met up with Icarus Board of Director Bill Cress and we visited the Hayden Planetarium together, a wonderful show. At one point I found myself hurtling into the edge of the galaxy in a mind spinning display of stars. I also found time to visit the American Museum of Natural History which had a ‘Beyond Planet Earth’ space exhibit. This featured a lunar space elevator for the purpose of getting materials to and from the Moon’s surface. Although the Moon is only one-sixth the gravity of Earth, it still requires a lot of fuel to get a payload into orbit. The space elevator could reduce the cost and effect significantly. As an aside I note that in the press today the US Republican Presidential candidate Newt Gingrich is promising a lunar base by 2020. Perhaps a space elevator might be more possible after all.

The American Museum of Natural History also featured a Vostok capsule. The spherical capsule was designed by Soviet engineers to be as simple as possible. Used in the 1960s to carry Yuri Gagarin into space. The capsule had just four switches and 35 indicators on the control panel inside, meaning it was almost fully automated and the cosmonaut had very little do. I feel there is a lesson for the Icarii here from our Russian colleagues when doing spacecraft design – simple is often best.

As a final comment from my trip to New York, I walked around the site of the World Trade Centres and Battery Park; a moving experience. It’s a terrible thing that happened back then in 9/11 which has scarred humanity for the decades and centuries ahead. One has to wonder what ET would think of humanity looking down on us, not quite deciding whether we were the most beautiful creatures they had ever seen or the most horrible. Our ability to overcome our own self-generated problems has wide ramifications for whether or not we have what it takes to travel to the stars. This constant battle between our good and bad side is what the English philosopher Olaf Stapledon referred to as the dark and the light. I leave you with a quote from this most eccentric of science fiction writers:

“Is it credible that our world should have two futures? I have seen them. Two entirely distinct futures lie before mankind, one dark, one bright; one the defeat of all man’s hopes, the betrayal of all his ideals, the other their hard-won triumph”. Stapledon, O, “Darkness & the Light”, Methuen, 1942.

Kelvin F. Long
Vice President (Europe) Icarus Interstellar

Today we are surrounded by technology, mainly through the medium of a computer. Even engineers and physicists will use computational programs, numerical codes and visualization software to address problems in science. But do we lose some essence of ourselves in this process? An ability to be creative straight from the mind to the board. Leonardo said “A true understanding of all the forms found in the works of nature…is the only way to understand the maker of so many wonderful things and the way to love so great an inventor“. He also said “The divinity which is the science of painting transmutes the painters mind into a resemblance of the divine mind“. These are profound statements but one lesson we could take from the Master is that to understand nature you must be connected to it. Understanding an equation gives you that contact, so that you can as Richard Feynman once said understand a flower on many levels giving you a greater appreciation for its aesthetic beauty. But perhaps also in attempting to draw or paint engineering designs we begin to have an instinct for the rules of nature, as Leonardo did.

In an earlier post, I referred to the new art form that Richard Obousy and J R Flemming had created which they called ‘interstellar steam punk’. In the same vein, I wish to advocate another form of art, which I shall call ‘da Vincian design’. This can be defined as a fusion between engineering and physics (or biology) expressed in a creative way, leading to design concepts or explanations which elucidate principles of operations. It is essentially a sketch of something you wish to create, no corrections to mistakes are permitted (to encourage perfection in the drafting process) and it is usually accompanied by equations or descriptions. An example I have produced is shown below, for something that is similar to the Daedalus second stage engine design, surrounded by doodles, thoughts, problem solving and equations. To an outsider it has the appearance of a mad man, but to the person behind it, it is merely the manifestation of their creativity depicted by the pencil and paper, unafraid to sketch away to new innovative designs. Like Leonardo himself, perhaps one day, designs of brilliance will be born and the Leonardo tradition of studying how things work, or could work, can continue.

Leonardo designed tanks, parachutes, even studied the wings of birds. One has to wonder what he would have made of the concept of Star travel. Had he realized there were many solar systems in the galaxy, would he have speculated as to the appearance of other life forms (their anatomy) and what varieties of nature they may have enjoyed. Indeed, what would he have thought of designs for a starship? what principles of operations would he had based the engine upon? Steam? gun powder? Cannons? We will never know the answers to these questions of course. Although his art was produced five centuries ago, his tradition of creating engineering machines through the process of art, is as relevant today as it ever was. Leonardo was a polymath on many levels, seeing designs for machines that were centuries ahead of his time. In some ways, we in Icarus are trying to conceive of engineering mechanisms for a Starship design that may not be built for several centuries. In this regard, we are all students of Leonardo da Vinci. For those of you that like to doodle whilst working on engineering problems, sketch away, you never know what creation may spring from that simple sketch born from your minds eye.

Kelvin F.Long

Vice President (Europe) Icarus Interstellar

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Steampunk

Steampunk is a sub-genre of science fiction, fantasy, alternate history, and speculative fiction that came into prominence during the 1980s and early 1990s.[1] Steampunk involves a setting where steam power is still widely used—usually Victorian era Britain or “Wild West”-era United States—that incorporates elements of either science fiction or fantasy. Works of steampunk often feature anachronistic technology, or futuristic innovations as Victorians might have envisioned them, based on a Victorian perspective on fashion, culture, architectural style, art, etc. This technology includes such fictional machines as those found in the works of H. G. Wells and Jules Verne, or the contemporary authors Philip Pullman, Scott Westerfeld and China Mieville.
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Thanks Richard, sweet revenge is on its way. The other Board of Directors may or may not share their images, but they are awesome.

Kelvin Long

Vice President (Europe) Icarus Interstellar

[artwork by JR Flemming]

When I reflect on the Icarus team, I see this spirit in abundance. All of the members of the Icarus team are giving up their personal time dedicated to the vision of interstellar flight. They each do this for a variety of reasons, but for most it is a combination of shared interest and the knowledge that humankind must think long term. This may be because of the risk of asteroid strikes, global conflict, natural disasters, over population and lack of resources or merely the knowledge that someday our Sun will have exhausted its energy.

For members of the public watching on, it may appear the Icarus team are just a bunch of people having a lot of fun in the pursuit of an unusual hobby. Whilst partly true, its also true that we all share a common bond, to work towards a peaceful and productive future for humankind on Earth and in Space. The dedication of the team working towards the 100 Year Starship Study grant in 2011 certainly required a lot of time and energy for many of us. We did that because we recognized its importance, not just to us individually but for our entire species. It was a moment of opportunity that had to be grabbed whilst it was there, provided by DARPA. An opportunity like this may not come again for many years and now was the time to raise our game and seize the day. – Carpe diem. The team responded magnificently to the challenge that was set before them.

Many Icarii spend many of their hours working volunteer into the late hours, working towards a point in the future when we can clearly see before us a blue print for a real Starship and imagination can finally translate to reality. At Christmas time in particular, I can’t think of a better gift for the members of Icarus to give to humanity than their personal time and energy to make the dream of starflight happen, someday. This is human kindness at the deepest level.

Happy Christmas World, let’s keep giving to make our dreams come true.

Kelvin F .Long
Vice President (Europe) Icarus Interstellar

Introduction

In today’s society, everybody is witness to the enormous advances in computer technology from ENIAC’s day until today. Home computers routinely implement features which a few decades ago were found only in mainframe systems. Virtualization, multi-core processing and even the existence of personal computers are but a few examples.

To briefly quantify these advances, let consider IBM’s classic mainframe computer S/360 which was a great commercial success, since its introduction in 1964. After 10 year of continuous technological evolution, its benchmarks peaked at approximately 1.7 MIPS (Million Instructions Per Second). Compare this to a modern commercial home CPU, such as Intel’s I7 processor, capable of 147,000 MIPS and we see five orders of magnitude improvement! Note we are comparing the performance of a mainframe with a commercial CPU.

The main drive behind this evolution is the integrated circuit. The continuing miniaturization of ICs led to the coining of various terms that quantify transistor density, such as Small Scale Integration, Medium SI, Large SI, Very Large SI and finally Ultra Large SI.

An empirical observation, which turned into a trend and was finally branded as a Law, is Moore’s Law [1]: “The number of transistors that can be placed inexpensively on an integrated circuit has doubled approximately every two years”. Now, let’s make clear that the term “Law” is indeed a law in a sense: it has become an industry standard, dictating the targets for microprocessor development. To that end, Moore’s Law serves as the most credible tool that will help us predict the advancement of complex electronics.

In the original Project Daedalus papers [2], factors such as processor speed, memory and storage are taken into account using the standards of that time. A cursory comparison reveals the limited computing resources they had available. In turn, these led to significant concerns relating to issues such as (a) problems arising from the time needed to compile a program, (b) the need for working memory and (c) the cost of software development to mention but a few.

Do those problems still apply today? Are they relevant? Well, the answer seems to be both yes and no.

For example, the same code that used to put a computer in an endless loop 50 years ago can still be used today to produce the same situation. The only difference is the processing speed. An endless recursion will produce a stack overflow error all the same. The difference is that in modern computing environments the error will surface much faster.

On the other hand, higher processing speed, larger and faster working memory and huge amounts of storage provided the flexibility to write progressively complex software using various programming techniques like object oriented programming and adding more and more levels of abstraction.

It seems that with modern computers, many problems that were present in Daedalus (but still faced with ingenuity) are of no relevance today. Still, the task of creating a mostly autonomous system that is faced with decades of space travel in the hostile environment of space, reaching a target star, gathering and transmitting back information, is not solved by raw processing speed and huge storage.

Icarus is not a simple communication satellite. The complexity and the duration of its mission raises the bar for the computing needs.

In other words, we need to pack in a so far unprecedented amount of computing resources.

Hardware considerations – The Radiation Hazard

Of all the hazards a computer may face in space, the biggest threat is radiation. Radiation which comes in various flavors as well (take your pick): protons, electrons, heavy ions and cosmic rays, encountered in a variety of different space environments, from Low Earth Orbit to Interstellar Space. There are two main categories of effects on electronics (a) Total Ionizing Radiations and (b) Single Event Effects [3].

As an example, let’s take the Single Event Effect (SEE). A common error is the bit flip (the change of state of a single bit from 0 to 1 or vice versa). A SEE can be caused by a single charged particle passing through a circuit. Now, a bit flip in allocated memory space can be catastrophic for the execution of a program, or could significantly distort data. There are several techniques of handling this issue, such as cyclic redundancy checks, cross referencing memory and any number of “safe” digital encoding/decoding methods which monitor data integrity. All of which have limitations, which must be scrutinized by the Icarus Computer and Data Management design team.

One solution for this and other kinds of problems is the technique of Radiation Hardening (RH). RH is simply this: use circuits with a scale of integration closer to the low end rather on the high end, thus reducing the possibility of the particle flipping a bit.

So, the general principle so far is that the more low-tech we go, the safer we are.

This principle seems to be in conflict with the Icarus design philosophy, which calls for advanced solutions resulting from projections of the current state-of-the-art. We really do want all the high-tech goodies we can fit in. For example we could easily design computers that use cheap, commercially available Error Correcting Code (ECC) Memory. The situation is different regarding processors, but there are solutions there as well. For instance, three identical systems running the same programs concurrently and voting for the accepted result. Or even better a distributed design with no single point of failure [4]. By the way, both have their uses – smaller voting systems could be placed at robotic equipment, while the main computing facility would work using a distributed model.

We can count several other examples of possible failures due to different kinds of radiation and the effect on electronics, such as channel degradation and transistor over-doping, to name a few.

So how might the Icarus protect itself during its almost century-long journey?

Some early thoughts point towards the Icarus employing some type of magnetic shielding [5]. Much like the Earth’s or the Sun’s magnetosphere, such a system would be able to deflect dangerous cosmic rays from inflicting damage to the electronics of Icarus.

Would this be enough?

Well, perhaps it is sufficient for the duration of the travel. But what happens when Icarus reaches the target star and possibly deploys multiple probes, orbiters or even landers? Suddenly the task of radiation shielding becomes more crucial, since several new factors make their appearance.

For example the target star’s solar wind. Is it sufficient to deflect cosmic rays?

Or an orbiter going through a planet’s Van Allen belt.

It seems that radiation shielding in one form or another for the electronics is inevitable.

NASA has performed some important research on “Commercial Microelectronics Technologies for Applications in the Satellite Radiation Environment” [6], essentially looking for ways to use commercial off-the-shelf (COTS) technologies for satellite applications. A notable example is the 1997 Mars Pathfinder mission which used an COTS Motorola radio modem for its communications with Earth [7].

The NASA taskforce proposed a methodology for addressing design specifications regarding radiation hazards for hardware:

1. Define the hazard
2. Evaluate the hazard
3. Define requirements
4. Evaluate device usage
5. Engineer with designers
6. Iterate process as necessary

The methodology in itself is pretty solid. In fact, it shares some modern software development methodologies.

The main problem with applying this methodology to Icarus comes with the first step: Define the hazard. Can we do this accurately? To what extent can we safely ascertain the characteristics of a radiation environment that is light years away?

These are all considerations that are being addressed as part of Project Icarus – an extremely complex and multi-faceted spacecraft systems engineering challenge.

Our objective is to rigorously inform the Icarus designers, allowing for the latest available state of the art technology that may be available through reasonable projections. However, having current computing technology on-board Icarus is a goal on its own: so far most spacecrafts use technology much older than any of us have at home. As the Mission’s 2nd Term of Reference [8] states: “The spacecraft must use current or near future technology and be designed to be launched as soon as is credibly determined”. It would be an irony if we had to use older technology for Icarus. After several modifications (some small, some not so small) we should be able to give Icarus as much computing power as needed in order to provide the facilities required by every mission module.

The fact that this technology will be obsolete upon arrival, should be considered as irrelevant

References

1. Moore, Gordon E. “Cramming more components onto integrated circuits”, 1965.
2. T.J. Grant. “Project Daedalus: The Computers”, 1978.
3. Kenneth A. LaBel, Allan H. Johnston, Janet L. Barth, Robert A. Reed, Charles E. Barnes: “Emerging Radiation Hardness Assurance (RHA) issues: A NASA approach for space flight programs”, 1998.
4. Andreas C. Tziolas, “Distributive Computing Architectures for Icarus Daedalus-Like Interstellar Missions”, Presentation at BIS Symposium: Daedalus 30 years later, September 2009.
5. Adam Crowl, “Shields for Icarus: Part 2 – Navigational Deflectors for Real“, Project Icarus Blog Article, 2010.
6. Kenneth A. LaBel, Michele M. Gates, Amy K. Moran, Paul W. Marshall, Janet Barth, E.G. Stassinopoulos, Christina M. Seidleck, Cheryl J. Dale: “Commercial Microelectronics Technologies for Applications in the Satellite Radiation Environment”, 1997.
7. Scot Stride, Microrover Radio Modem, NASA-JPL, 2007.
8. Project Icarus: Terms Of Reference, 2009.

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)

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