for this purpose. We propose to find a mast topology that is best suited for the application, then optimize the structure and quantify its performance using impact testing with high-speed photography. Availability of an optimized mast will significantly increase the likelihood that the transportation system will be brought to fruition.

Background
Raw materials, energy, and real estate are available in essentially unlimited quantities within our solar system. However, using these resources is not currently economically justifiable because of the high cost of operating in space, which stems primarily from the cost of launching materials from Earth to orbit. Unless a less expensive means of transport from the Earth’s surface to orbit can be found, the number of people in space at any given time is likely to remain only a handful for the foreseeable future, as it has been for the preceding thirty years.

Considerable effort has been expended toward the goal of reducing the cost of access to space. Ongoing work includes that of Scaled Composites in cooperation with Virgin Galactic, as well as Space Exploration Technologies Corporation (SpaceX). Because these projects are based on conventional rocket technology, albeit with ingenious modifications, they are unlikely to reduce the cost of access to space by more than a factor of two, leaving space projects still too expensive to undertake on a large scale.

More radical technology has also been investigated. The feasibility of using impulsive launchers to achieve orbital altitude was demonstrated during the 1960s, when modified conventional artillery was used in the High Altitude Research Program (HARP), run by McGill University and the US Army Ballistic Research Laboratory. During this project, 100-kilogram projectiles were repeatedly launched to apogees of 180 kilometers [1]. (See Fig. 1.) A related, more refined launch method called a ram accelerator has been demonstrated at laboratory scale at the speeds necessary to reach orbital altitudes, and can be affordably scaled to useful masses. [2]

The benefits of launching a projectile at inclinations near vertical are limited, however. A rocket is always required for orbit insertion, and higher speed projectiles are damaged by heating in the Earth’s atmosphere. If a satellite already in orbit could catch projectiles like those launched by the HARP guns, costs could be drastically reduced — An estimate using the TRANSCOST model predicts that such a method could reduce the cost of transportation to LEO to $260/kg [3]. The method would operate as shown in Fig. 2.

The primary difficulty with this method is performing the catch nondestructively. Technology has recently been demonstrated that can accomplish this task.

Core technology
The technology that can perform a nondestructive capture of an object traveling at orbital velocity relies on a well-known principle of magnetic braking, but with incorporation of state-of-the-art superconducting material. [4] In this magnetic brake, the changing field produced by movement of a superconducting magnet induces an electric current in the wall of a conductive tube. The interaction of the field and current leads to a repulsive force that slows the motion of the magnet.

The magnitude of the braking force depends on the square of the field of the magnet. Small superconducting magnets have been demonstrated that produce fields of over 17 tesla, compared to 0.5 tesla for the best permanent magnets. The improvement in braking force is therefore more than a factor of one thousand. A model of magnetic braking predicts that a 100-kilogram projectile would need to carry a five-kilogram superconducting magnet (including cooling for the magnet) to stop in a distance of tens of meters, which is a reasonable length structure to deploy in space.

Although there is no contact between the projectile and brake tube, the magnetic force can cause failure of the catch tube in the same way as an impact, and, according to classical buckling theory, this strength limit currently sets the lower limit of the system’s mass. Finding a means to increase buckling strength is therefore a crucial aspect of the feasibility of the transportation method.

Role of tensegrity masts
Tensegrity structures can potentially have a very high strength to weight ratio. [5] Tensegrity masts have been developed for deployment in space (Fig. 3), and so are well on their way to applicability to this system. [6] Some work has been performed on analysis of buckling loads, however, buckling in this case would occur under conditions that have not been widely studied in simple columns, much less in tensegrity masts. The load in a hypervelocity magnetic brake is applied for a very short duration. Simple shells can resist short-duration loads larger than their maximum static load; the maximum load is inversely proportional to the duration of the load. [7] This behavior should hold approximately true for tensegrity masts, however, other dynamics are also likely to emerge due to the complex nature of the structure. High-speed photography could be used to study buckling dynamics in tensegrity masts, as it was for the simple columns shown in Fig. 4. [8]

The goal of this research project is therefore to optimize a tensegrity mast for short duration loads. The approach will be to first use numerical modeling, and then to test the resulting structures in conditions similar to those in the magnetic capture system. A high-speed camera will be used to gain understanding of buckling dynamics.

Research plan
The goal of the proposed project will be accomplished as follows:
1. Use existing literature to find a tensegrity mast topology that is appropriate for support of a magnetic brake tube.
2. Perform numerical investigations of the response of this type of tensegrity mast to pulsed buckling loads.
3. Construct subscale masts. (An example of a tensegrity structure built by UH architecture students appears in Fig. 5)
4. Characterize the response of subscale masts by using the UH launcher (Fig. 6) to fire appropriately scaled masses at the masts. The response of the masts will be recorded using a 15,000 frame per second camera (Redlake MotionScope M2).
5. Modify the model as required by test results.
6. Repeat steps 3 through 5 as necessary, and as time and budget allow.
7. Study secondary aspects of tensegrity mast dynamics, such as vibration frequency response.
8. Survey other technology for use in tensegrity masts, including nano-scale materials, smart materials, or inflatable structures.

Follow-up development
Working with researchers at the University of Washington, private investment is being sought to build a ram accelerator that can launch 100 kilograms to an apogee over 100 kilometers. This facility is expected to be profitable based on a survey of the suborbital launch market. Successful completion of the launch facility will lead to development, construction, and deployment of the orbiting magnetic capture system. This phase will require several million dollars of additional investment, followed by about $25 million for launch services. Considering that this system could play a very large role in the space transportation infrastructure, this cost is quite low, and is in fact within the reach of private financing.

References
1. Bull and Murphy, Paris Kanonen — the Paris Guns (Wilhelmgeschultze) and Project HARP, Verlag E.S. Mittler & Sohn, Herford, Germany, 1988.
2. Knowlen et al., “Ram Accelerator as an Impulsive Space Launcher: Assessment of Technical Risks,” Presented at International Space Development Conference, May 25–28, 2007, Dallas, TX.
3. Pearson et al., “Low cost launch systems for the dual-launch concept,” Proceedings of the 51st International Aeronautical Congress, IAA-00-IAA.1.1.06 (2000).
4. Putman, “Skydock: An orbiting magnetic arrest system,” Presented at International Space Development Conference, May 25–28, 2007, Dallas, TX.
5. Fuller, Synergetics, Macmillen Publishing, New York, 1982, p. 404.
6. Tibert and Pellegrino, “Deployable tensegrity masts,” Collection of Technical Papers — AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Vol. 7, pp. 5275–5284 (2003).
7. Lindberg and Florence, Dynamic Pulse Buckling, Martinus Nijhoff Publishers, Dordrecht, 1987, p. 307.
8. Gladden et al., “Dynamic buckling and fragmentation in brittle rods,” Physical Review Letters, Vol. 94, pp. 035503/1-4 (2005).

Principal investigator
Phil Putman has been aware of the large-scale problems facing civilization since reading A Choice of Catastrophes while he was in the fourth grade. After earning Bachelor's and Master's degrees in Electrical Engineering from Cornell University, he considered the application of his educational background to averting the various disasters described in the book. He concluded that lowering the cost of access to space would mitigate most of the dangers, and, furthermore, would be neat. This led him to perform research on the incorporation of superconductors into electromagnetic launchers. Dr. Putman completed a Ph.D. on this topic at the University of Houston, and continues there as a Research Assistant Professor with the Mechanical Engineering Department and the Texas Center for Superconductivity.

In addition to performing traditional research and development, Dr. Putman has competed in events such as the Ford Hybrid Electric Vehicle Challenge and the robot combat tournaments Robot Wars and BattleBots. A car that he and a team of other Cornell students built won the inaugural Ford HEV Challenge. The first robot that he built won the Lightweight Melee event at the first Robot Wars. His second robot was the first to successfully walk during a Robot Wars competition.

Dr. Putman has co-authored more than 20 research papers on materials processing, characterization, and electromechanical applications, and has pending patent applications in the field of electromagnetic launch.