100 Year Starships

Centauri Dreams covered the 2013 100 year Starship Symposium

At the first track session for “Factors in Time and Distance Solutions,” Terry Kammash (University of Michigan) ran through the basics of the rocket equation to show why chemical rockets were inadequate for deep space travel. Kammash is interested in a fusion hybrid reactor whose neutron flux induces fission, a system that could eventually enable interstellar missions. It is based on Gas Dynamic Mirror (GDM) methods that surround a plasma-bearing vacuum chamber with a long, slender, current-bearing coil of wire. The plasma is trapped within magnetic fields that control the instability of the plasma. Here it’s worth mentioning that a Gas Dynamic Mirror propulsion experiment in 1998 produced plasma during a NASA test of the plasma injector system, injecting a gas into the GDM and heating it with microwaves in a method called Electronic Cyclotron Resonance Heating.

Gas Dynamic Mirror (GDM) Fusion Propulsion system has an engine. It is a long, slender, current-carrying coil of wire that acts like a magnet surrounds a vacuum chamber that contains plasma. The plasma is trapped within the magnetic fields created in the central section of the system. At each end of the engine are mirror magnets that prevent the plasma from escaping out the ends of the engine too quickly.

In 1998, the GDM Fusion Propulsion Experiment at NASA produced plasma during a test of the plasma injector system, which works similar to the forward cell of the VASIMR. It injects a gas into the GDM and heats it with Electronic Cyclotron Resonance Heating (ECRH) induced by a microwave antenna operating at 2.45 gigahertz. Researchers have continued experiments and theoretical work.

2010 lecture- Meeting the World’s Energy Needs with the Fusion Hybrid Reactor by Terry Kammash

Dissertation related to Gas Dynamic Fusion Propulsion

Here is a 2011 dissertation (119 pages) – A Computational Magnetohydrodynamic Model of a Gas Dynamic Fusion space propulsion System

The goal of this body of work was to advance our understanding of gas dynamic mirror (GDM) fusion propulsion systems. Kammash’s analytical model suggested that deuterium−tritium (D−T) and deuterium−3 helium (D−3He) GDMs were feasible, but they were large at 250 to 100,000 metric tons with up to 75% of the mass accounted for by radiators rather than confinement magnets. Starting from that point, this effort has explored alternate GDM concepts, identified the challenges for modeling GDMs using computational MHD approaches, and found solutions to a number of those challenges.

Aneutronic fuel proton−11 boron is not practical in a self-sustaining GDM

D − T and D−3He GDMs as proposed by Kammash are limited by neutron and radiation energy losses from the contained fusion plasma before the plasma leaves the magnetic nozzles for propulsion. More exotic aneutronic fusion fuels are often suggested as potential solutions to this problem, but they had never been modeled in a GDM system.

Even with advantageous assumptions about synchrotron radiation reflection and positing futuristic non-destructive magnetic technology capable of producing 800 Tesla magnetic fields, the best possible P−11B GDM was over 23 kilometers long, almost 1.3 million metric tonnes, and took over a year and half to reach Mars. The size was driven by the GDM design of a self-sustaining nuclear reactor with more fusion energy out than in, Q greater than 1, combined with the overwhelming bremsstrahlung radiation energy losses from operating at 300 keV .

Driven GDMs offer some benefits

The new driven-GDM parametric model was also applied to the more viable D−T GDM. In this case, the specific power of the D − T GDM is higher than that of the nuclear electric reactors. Increasing the fraction of power generated by the nuclear electric reactors decreases system performance in terms of both greater dry mass and longer trip times. When the nuclear electric reactors are producing as much power as the fusion GDM, the system is four times heavier and requires more than twice the time to reach Mars. While the driven systems are not preferable to the original D − T GDM concept, it does demonstrate that there is a viable development path from non-fusion plasma rockets to breakeven self-sustaining GDM rockets.

Nuclear thermal and nuclear electric hybrid

Pauli Erik Laine (University of Jyväskylä, Finland) described himself as a computer scientist rather than a rocket scientist, but the two designs he presented were based, like Kammash’s, on hybrid propulsion systems. In Laine’s case, the method is nuclear thermal propulsion, as developed through the NERVA program coupled with nuclear-electric propulsion and a gravitational slingshot maneuver for initial acceleration.

Fission fragment propulsion with solar sails to get to 5% of lightspeed

Laine also discussed fission fragment rocket technologies in which heavy and light fission fragments, rather than being dissipated as heat through conventional reactor methods, are used directly for thrust. A hybrid fission fragment mission added a solar sail and used multiple staging to achieve five percent of lightspeed.

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