Small Interstellar Probes, Riding Laser Beams – The Project Dragonfly Design Competition Workshop

Today we look beyond Pluto/Charon toward possible ways of getting a payload to another star. Centauri Dreams readers are familiar with the pioneering work of Robert Forward in developing concepts for large-scale laser-beamed missions to Alpha Centauri and other destinations. But what if we go smaller, much smaller? Project Dragonfly, in progress at the Initiative for Interstellar Studies, proposes to explore this space, and as Andreas Hein explains below, it was recently examined in a workshop giving student teams a chance to present their ideas. A familiar figure in these pages, Andreas received his master’s degree in aerospace engineering from the Technical University of Munich and is now working on a PhD there in the area of space systems engineering, having conducted part of his research at MIT.

by Andreas M. Hein

The Project Dragonfly Design Competition, organized by the Initiative for Interstellar Studies (i4is) was concluded on the 3rd of July in the rooms of the British Interplanetary Society (BIS). To choose the rooms of the society is no coincidence. The BIS conducted the Lunar Lander study in the 1930s, which foreshadowed in an almost uncannily precise way the Apollo mission. Forty years later, in 1978, the BIS presented the first design of an interstellar probe: Daedalus. And in 2015, it was a natural choice to choose the BIS’ rooms for what might again be the stage for imagining things to come: Four international teams, almost exclusively consisting of students, are going to present their design for an interstellar probe. And this time, things get small.

Background

The field of interstellar studies can be divided into two categories. First, the field of interstellar studies is teaming with huge spacecraft, often as large and heavy as today’s largest skyscrapers. However, there is a second stream of concepts for interstellar spacecraft, beginning with Robert Forwards’ Starwisp probe and Freeman Dysons’ Astrochicken [1, 2]. These concepts stimulated thinking about the opposite: How small can we get? A small interstellar probe has a fundamental advantage. It needs less energy to accelerate to the same velocity. This is particularly relevant for interstellar missions, as we usually talk about required energies that often surpass current global energy consumption. Hence, any reduction in size might considerably increase the feasibility of an interstellar mission. Figure 2 shows an overview of some of the most relevant interstellar concepts and designs. The columns indicate the mass of a spacecraft from a particular concept or design in orders of magnitude. The rows indicate the level of detail. High-level concepts often consist of a basic feasibility analysis. System-level designs go deeper and describe key spacecraft systems in considerable detail. Subsystem-level designs add additional detail to all relevant spacecraft systems. The objective of Project Dragonfly is to address the gap in the lower left: To design an interstellar spacecraft down to subsystem level, ideally with a mass below 10 tons.

Figure 2: Some of the most relevant interstellar concepts and designs

The idea behind Project Dragonfly emerged in early 2013 when I visited Professor Gregory Matloff in New York. Greg is one of the key figures in interstellar research. That night we talked about different propulsion methods for going to the stars. We realized that nobody had yet done a design for a small interstellar laser-propelled mission. Soon after this conversation Project Dragonfly was officially announced by i4is. The name “Dragonfly” was chosen in order to pay credit to Robert Forward, who wrote the novel The Flight of the Dragonfly in the early 1980s, featuring a laser sail spacecraft. Later in 2013, i4is organized the “Philosophy of the Starship” Symposium at the BIS, where first presentations on laser-propelled interstellar probes were given by Kelvin F. Long and Martin Ciupa. Further vital preparatory work was done by Kelvin that year that fed into defining the competition requirements. With the first set of requirements defined, we finally got to the point where we were able to organize an international design competition in 2014. The purpose of the competition would be to speed up our search for a feasible mission to another star, based on technologies of the near future.

The Project Dragonfly Design Competition focused on small, laser-sail-propelled interstellar probes. Why small and laser-sail-propelled? In getting small, we are following a trend which started in the last decade. With the emergence of the CubeSat Standard first universities and then companies started to develop satellites, often not larger than a shoebox. Today, NASA and ESA are even thinking about sending small satellites to the Asteroids and Mars [3, 4]. However, the spacecraft still needs to be big enough to get enough science data back, setting a lower limit to spacecraft size, sufficient to host and supply power to the communication subsystem.

Why laser-sail-propelled? Laser sails are similar to solar sails. They both use light pressure for accelerating the spacecraft. Solar-sail-based spacecraft are today developed by various space agencies and organizations, from JAXA’s Ikaros mission to The Planetary Society’s LightSail 1. The elegance of solar sails is that they are scalable and use an abundant energy source: the Sun. Most types of solar sails could be used as a laser sail and vice versa. Hence, using a potential laser sail on a solar sail precursor would be possible in most cases. This would lower the barrier for testing a new type of sail, as operating a solar sail does not depend on a laser infrastructure.

Project Dragonfly leverages these two technology trends, as they seem to be promising to realize an interstellar mission in a scalable way.

The Project Dragonfly Design Competition

In August 2014, international university teams were invited to participate in the competition. All candidate teams had to submit solutions to a problem set first. This problem set included a range of small problems that were intended to train the teams in the key areas relevant for the competition, such as the basics of laser sail propulsion, laser systems, and in-space communication systems. The objective of this initial problem set was two-fold. First, it was intended as an entry barrier for all teams that do not have a serious intention or the capabilities to participate in the competition. Eliminating teams that would not make the cut later on also had the purpose of avoiding overburdening the reviewers. The reviewers we invited are very busy individuals. We wanted to use their time as effective as possible, giving them the opportunity to focus on the best teams. Second, the successful teams would be able to develop or strengthen their capabilities to solve the main competition task of designing a laser-sail-propelled interstellar probe. They would also get familiar with the existing literature on the topic and get a “feeling” for the subject.

The teams that were able to pass this hurdle were then confronted with the mission requirements. These requirements used the requirements developed during Project Icarus as a starting point. Project Icarus is an ongoing collaborative interstellar study between the BIS and Icarus Interstellar, in which I am participating [5]. However, the requirements were adapted and extended to the laser sail case. The requirements are depicted in Figure 3 in a graphical fashion.

Figure 3: Graphical representation of the Project Dragonfly requirements

Written out, the mission requirements are:

1. To design an unmanned interstellar mission that is capable of delivering useful scientific data about the Alpha Centauri System, associated planetary bodies, solar environment and the interstellar medium.

2. The spacecraft will use current or near-future technology.

3. The Alpha Centauri system shall be reached within a century of its launch.

4. The spacecraft propulsion for acceleration must be mainly light sail-based.

5. The mission shall maximize encounter time at the destination.

6. The laser beam power shall not exceed 100 gigawatts

7. The laser infrastructure shall be based on existing concepts for solar power satellites

These requirements were deliberately fine-tuned in order to be challenging. The 100 GW beam power requirement constrains the design space considerably. The particular value was selected as it constrains the mass of the spacecraft to tens of tons. Furthermore, it is a beam power that is very challenging to generate with an in-space infrastructure within the 21st century but not completely out of reach. The 100 year time constraint sets a theoretical minimum average trip velocity of 4.3% of speed of light in order to reach the Alpha Centauri star system. With the power constraint only the spacecraft mass, its sail system parameters, and the duration of acceleration / deceleration are left as key variables. A long acceleration duration allows for reaching a high velocity. However, a long acceleration duration means that the laser beam has to be steered over long distances. This in turn makes pointing and focusing the beam challenging.

The science data requirement is also challenging to fulfill. If the teams decide to reduce the spacecraft mass, they need to shrink their communication system as well. However, communication over interstellar distances requires large amounts of power, if useful science data is to be collected and sent back.

The teams needed to navigate in this design space, making careful trade-offs between different parameters. The competition included two intermediate stage gates and a final review of the reports. Each stage gate required a different set of deliverables that are commonly required for concept studies in the space domain, such as an initial feasibility analysis, a technology readiness assessment, and detailed calculations for key aspects of the mission. The stage gate process allowed us to check and adjust our expectations for the next stage of the competition and provide targeted support if teams were struggling in a particular area. Furthermore, each of the deliverables covered a vital aspect that is commonly needed for a concept study. The staged approach enabled the teams to work on a limited set of deliverables at each stage, reducing the difficulty of the overall task.

The competition wouldn’t have been possible without our reviewers and advisors. Fortunately, we were able to recruit experts with considerable experience in solar and laser sailing studies, such as Les Johnson, who is working as the Deputy Director of the Advanced Concepts Team at NASA and Bernd Dachwald, a German professor.

Four international teams, out of initially six contestants were able to take all hurdles and submit a final design report:

Technical University of Munich

University of Cairo

University of California Santa Barbara

CranSEDS, consisting of students of Cranfield University, UK, the Skolkovo Institute of Science and Technology in Russia, and the Université Paul Sabatier in France.

These reports were this time graded by the reviewers. Furthermore, the teams were invited to present their designs at the final workshop in London, on the 3rd of July 2015.

The workshop

The main purpose of the workshop was to mimic a typical design review in the space sector. The teams would give a presentation, covering all vital aspects of their design and would then answer questions from the audience and review panel. The review panel consisted mostly of aerospace engineers. Notable members were the Executive Director of i4is, Kelvin F. Long, and Chris Welch, who is working as a professor at the International Space University.

The first team to present was the team from the Technical University of Munich. Their spacecraft would be accelerated up to a distance of 2.2 light years and a velocity of 9% of the speed of light. The laser infrastructure would be placed on the Moon. Their laser sail would consist of a graphene sandwich material. Deceleration is enabled by a staged magnetic / electric sail. The team chose this staged approach, as the magnetic sail is very efficient at high velocities but gets increasingly less efficient at lower velocities. The electric sail is capable of decelerating at lower velocities. The spacecraft reaches the Alpha Centauri system after 100 years. Communication is enabled by a laser communication system. Power is supplied, either by solar cells that generate energy from the laser beam or the electric sail, which generates electric power when flying through the interstellar medium.

Figure 4: The spacecraft of the Technical University of Munich. Sail is not to scale.

The overall spacecraft is relatively heavy, compared to the other designs. It’s mass is 14t. Part of the reason is that the team aimed at maximizing the payload mass. A higher payload mass leads to a higher scientific yield but also leads to a heavier communication system, due to the higher data rate. Another effect is that a heavier spacecraft needs a longer duration to accelerate, imposing pointing requirements on the laser optics that are difficult to meet. Another difficulty with the overall architecture of the mission is that the laser system is located on the lunar surface. Although in principle feasible, installing such a system is very costly, unless a large amount of in-situ materials are used.

Figure 5: UCSB team wafer spacecraft design

The second presentation was given by the UCSB team. This team’s design combined the highest number of innovative technologies. It distinguished itself in numerous ways from the other teams. First, the concept of the spacecraft was a “wafer-based” design. This means that the spacecraft is basically imprinted onto a chip with all spacecraft subsystem integrated into it. The sail would consist of a dielectric material with an extremely high reflectivity, in order to withstand the enormous power density of several gigawatts per square meter. Note that sunlight in Earth orbit has a power density of about 1.4kW per square meter. Hence, the power density of the laser is roughly a million times higher than what today’s spacecraft are usually facing.

A highly reflecting surface avoids that part of the energy that is absorbed by the spacecraft, immediately melting it. The spacecraft is also accelerated rapidly, within a distance of three astronomical units, up to a velocity of 25%c. The team was able to consider such high velocities, as the spacecraft does not contain any deceleration system. Using deceleration systems such as a magnetic sail or an electric sail would lead to significant deceleration durations that may nullify any decrease in trip duration as a result of the high cruise velocity. However, the lack of a deceleration system does not comply with the mission requirements. The laser is a phased-array fiber-fed laser.

Figure 6: Samar Eldiary presenting the Cairo Team’s spacecraft

The third presentation was given by the Cairo University Team. The team’s mission design is based on an initial acceleration via laser beam, based on the DE-STAR system developed by the UCSB team. The spacecraft is decelerated via magnetic sail, and then separates into two sub-probes. One probe will collect data from Proxima Centauri, the other data from the Alpha Centauri A and B system. The laser sail is made out of aluminum. A laser communication system is used for sending back data to the Solar System. Power is provided by three Radioisotope Thermoelectric Generators (RTGs). The team presented an innovative approach for attitude control during the acceleration phase by changing the shape of the sail.

Figure 7: CranSEDS spacecraft design. The left image depicts the spacecraft bus with payload. The three cylindrical objects are the RTGs.

The last presentation was given by the CranSEDS Team. The interesting thing about their mission architecture is the use of a staged approach. A total of three spacecraft are launched within 33 year intervals. The rationale behind these intervals is to use each subsequent spacecraft as a communication relay station as well as exploiting technological advances that have occurred in the meantime.

First, the spacecraft is accelerated up to a velocity of 5%c at a distance of about 5,800 astronomical units. This phase takes about 3.7 years. The laser sail, used during acceleration, is then jettisoned. The subsequent cruise phase takes up to 77.5 years. Deceleration then starts via magnetic sail and the spacecraft enters the Alpha Centauri system after about 98 years of flight. 33 years after launch, a second spacecraft is launched with a similar configuration, but mission phases shifted by 33 years. The third spacecraft is launched after a similar interval of 33 years.

The team’s spacecraft is propelled by a Silicon Carbide sail. Power is provided by three RTGs. Data is sent back via laser communication. The overall mass of one of the spacecraft is about 4.5 tons. Each spacecraft hosts a scientific payload of 93kg, consisting of various instruments such as spectrometers, a magnetometer, and a cosmic dust analyzer.

The team presented detailed trade-off analyses for each of the critical aspects of the mission such as how many spacecraft to send and each of the spacecraft subsystems. The reviewers, however, remarked that the mission architecture, consisting of three separate spacecraft might induce programmatic risks, as the mission would need to be sustained over a period longer than a century. Furthermore, the so-called waiting paradox might come into play: A spacecraft launched later could overtake an earlier one due to a more sophisticated technology or laser infrastructure.

After the teams’ presentations, the review panel retreated for ranking the teams. As mentioned earlier, the team reports already contributed to the overall grading with two-third of the points. One-third would consist of the presentation and the teams’ performance during the Q&A session. After a few discussions, the review panel reached a conclusion and got back to the teams, waiting eagerly to hear the results.

Figure 8: The teams and members of the i4is leadership

We then announced the winners:

4. Cairo University

3. UCSB

2. CranSEDS

1. Technical University of Munich

The first prize, which went to the team of the Technical University of Munich, went along with one of the Alpha Centauri Prizes, which i4is awards to contributions advancing the field of interstellar travel.

Figure 9: Alpha Centauri Prize logo

Figure 10: The team from the Technical University of Munich (TUM) is awarded the Project Dragonfly Alpha Centauri Prize (left to right: Kelvin Long (i4is), Johannes Gutsmiedl (TUM), Nikolas Perakis (TUM), Andreas Hein (i4is))

After the ranking was announced, the next steps for Project Dragonfly were presented. First, the teams are requested to submit a summary of their report to a peer-reviewed international journal. The purpose is to receive another independent validation of the designs. Furthermore, the teams would gain experience in writing scientific publications. Another step is a technology roadmap, based on the technologies that were selected by the teams. Some of the technologies were common, such as laser communication and a magnetic sail for deceleration. However, the teams diverged in other technologies such as the laser sail material and power supply. With the teams, we will select key technologies and think about what steps are needed for developing them, along with prototypes and demonstration missions.

Later in the afternoon, the workshop participants gathered at the local bar and restaurant, the Riverside: A traditional gathering place after BIS events. Here, new friendships were forged between the participants and the future of Project Dragonfly was hotly debated.

Conclusions

The main conclusion is that a small, laser-sail-propelled interstellar mission is in principle feasible by using a laser infrastructure providing a 100GW laser beam. The Alpha Centauri system could be reached within 100 years. The spacecraft mass would be somewhere between 15 and a few tons. With the use of innovative technologies, even masses below one ton could be achieved.

The following conclusions can be drawn from the presented spacecraft designs:

Laser communication seems to be a promising approach for achieving communication over interstellar distances.

Magnetic sails seem to be the currently most promising way to achieve deceleration from velocities of a few percent of the speed of light.

The trade-offs for the best laser sail material are non-trivial and there seem to be several promising materials.

Most teams have used RTGs as power supply.

More research needs to be done on the laser infrastructure. In particular, where to place it and how to leverage on potential future solar power satellite infrastructures.

However, there are several other feasibility issues that need to be addressed, such as beam pointing requirements over distances of several to thousands of astronomical units. Manufacturing and deployment of kilometer-sized solar sails is also an issue. Furthermore, spacecraft autonomy during the mission is a huge challenge as well. Deployment of magnetic sails with several kilometers in radius remains another feasibility issue.

Despite these challenges, let us not forget where we came from: Missions using the whole energy consumption of humankind. We were able to decrease that by two orders of magnitude or more. Yes, building such an infrastructure is an immense challenge but it is less a challenge than for example mining Jupiter for Helium 3 for two decades, as proposed for Project Daedalus, or harvesting large quantities of antimatter.

Maybe, and just maybe, some of the ideas presented during the workshop might one day open up the pathway to the stars. Until then, a lot of work remains to be done.

Let’s get started!

References

[1] Forward, R. L. (1985). Starwisp-An ultra-light interstellar probe. Journal of Spacecraft and Rockets, 22(3), 345-350.

[2] Matloff, G. L. (2005). The incredible shrinking spaceprobe. Deep-Space Probes: To the Outer Solar System and Beyond, pp.61-69.

[3] https://www.nasa.gov/press-release/nasa-prepares-for-first-interplanetary-cubesats-on-agency-s-next-mission-to-mars

[4] Asteroid Impact Mission (ESA).

[5] Long, K. F., Fogg, M., Obousy, R., Tziolas, A., Mann, A., Osborne, R., & Presby, A. (2009). Project Icarus-Son of Daedalus-Flying Closer to Another Star. Journal of the British Interplanetary Society, 62, 403-414.