Diametric drive

The Breakthrough Propulsion Physics Project (BPP) was a research project funded by NASA from 1996-2002 to study various proposals for revolutionary methods of spacecraft propulsion that would require breakthroughs in physics before they could be realized . During its six years of operational funding, this program received a total investment of $1.2 million.

The Breakthrough Propulsion Physics project addressed a selection of “incremental and affordable” research questions towards the overall goal of propellantless propulsion, hyperfast travel, and breakthrough propulsion methods. By the end of the project, sixteen of these investigations were completed. The results found “about a third were found not to be viable, a quarter have clear opportunities for sequels, and the rest remain unresolved.”

By disjunction is meant the separation of the source of a field from the matter with which it would otherwise interact; viz., under normal physical states. According to a summary of speculative propulsion ideas on NASA's website:

This concept entertains the possibility that the source of a field and that which reacts to a field can be separated. By displacing them in space, the reactant is shifted to a point where the field has a slope, thus producing reaction forces between the source and the reactant. Although existing evidence strongly suggests that the source, reactant, and inertial mass properties are inseparable, any future evidence to the contrary would have revolutionary implication to this propulsion application.

The concept is expressed mathematically as:

One proposed method of achieving a diametric drive, or possibly a disjunction drive, which was studied in the BPP was called the pitch drive. This has been described as involving a hypothetical disjoint field which would eliminate the need for the field to be generated on the spacecraft itself.

One specific proposal for such a pitch drive was called the bias drive. According to this proposal, if it were possible to locally alter the value of the gravitational constant G in front of and behind the craft, one could create a bias drive. While the gravitational constant is a fundamental physical constant in general relativity, the Brans–Dicke theory of gravitation does in a sense allow for a locally varying gravitational constant, so the notion of a locally varying gravitational constant has been seriously discussed in mainstream physics. It has been claimed that one problem with the concept of a bias drive was that it might create a singularity in the field's gradient located inside the vehicle.

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