Conversion Drive

conversion drive
Image from Steve Bowers
The energy produced by a conversion drive is considerable, and can be sustained for relatively long periods. Click for larger image

Conversion drives are a class of reaction engine that derive at least part of their power from monopole catalyzed baryon decay, and became possible following the creation of stable GUT magnetic monopoles by Second Singularity transapients.

The basic reactions associated with monopole catalyzed baryon decay are conceptually similar to the reactions found in matter-antimatter annihilation (though the mechanism and types of reaction products are quite different). As a result, the standard types of antimatter rocket can be readily modified to become conversion drives. Unlike antimatter drives, the storage and manipulation of GUT monopoles is considerably simpler, with minimal concerns over containment failure as GUT monopoles will not readily interact with un-ionized matter.

History and Background

The first conversion drives were developed almost immediately after the development of magnetic monopole generation technology by early Second Singularity intelligences. It was not until the Late Federation period that such technology was made available to modosophonts. Though S2 conversion drives did not substantially outperform S1 amat drives, they were simpler to construct and vastly easier to fuel. Conversion starships could readily attain higher speeds and operate more safely than their antimatter counterparts.

For a time conversion drives were seen as a sort of 'calling card' of the newly rising S2 'demi-gods'; however as first S1 and then modosophont level minds acquired monopoles and developed the technology to readily 'breed' them in particle accelerators, various forms of the drive became first common and then commonplace. Perhaps the most well-known effect of the drive was to make truly relativistic interstellar travel a practical possibility for organizations smaller than interplanetary governments. Much less well-known, but arguably just as important, the widespread use of conversion drive would break the stranglehold of the great amat megacorps; ending a monopoly extending all the way back to the Interplanetary Age.

Conversion Thermal

Also known as solid core conversion rockets, these systems use a dense metal heat exchanger much like solid core antimatter rockets. Their operation is slightly more complex, as a portion of the heat exchanger must be ionized and monopoles injected into the resulting plasma in order to heat the rest of the engine. Specific impulse is limited to ~1000-2000 seconds, but thrust can be very high making such engines suitable for planetary heavy lift rockets.

Conversion Initiated Fusion

Also called Monopole Catalyzed Fusion, this is the simplest form of conversion reaction and forms the basis of the most common designs of modosophont power generators and rocket engines. It is conceptually very similar to antimatter initiated fission or fusion. Conversion monopoles are injected into a hot, dense plasma of fusion fuel, such as deuterium/helium-3. Some of the monopoles will interact with nuclei in the plasma, and the energy released by the resulting baryon decays is sufficient to induce fusion in the plasma.

Unlike antimatter, it is safe to combine the monopoles and fusion fuel ahead of time. This is typically done by constructing fuel pellets with an iron core to which the conversion monopoles are bound, and an outer layer consisting of deuterium ice and helium-3. Heating and compression of the fuel pellet using standard inertial confinement techniques is enough to ionize the iron core and liberate the conversion monopoles. Though the resulting reaction is no more energetic than standard antimatter initiated fusion techniques, the engine design can be greatly simplified as it removes the need for antiproton beams or antihydrogen plasma emitters. Such fuel pellets can be used in unmodified modosophont ICF rockets, and are very common in ultratech societies throughout the terragen sphere.

Performance and efficiency is identical to conventional beam-driven or antimatter initiated fusion, with a maximum specific impulse of approximately 800,000 seconds. At that performance level, even the best transapient-designed drives do not have thrusts exceeding 5 gees.

Plasma Core Conversion

Similar to antimatter plasma core rockets, using a magnetically confined reaction chamber and magnetic nozzle. Unlike the antimatter versions, the fuel must be ionized by some other means before injection into the reaction chamber, often by using standard plasma generation techniques or pulsed laser systems which complicates the design preventing a straightforward upgrade from antimatter to conversion.

In its simplest form, a beam of conversion monopoles is fired down the long axis of the drive chamber. Each monopole can convert more than one nucleon, which increases the efficiency of this design over its antimatter equivalent. More sophisticated designs use transverse monopole beams and collector mechanisms which allow recycling of monopoles and consequently lowering fuel requirements and operating costs. Such monopole recycling systems require extremely complicated electrical and magnetic field control systems in order to avoid warping the magnetic confinement of the reaction mass plasma, and to prevent stray monopoles from contaminating the confinement magnets and reducing engine power. Such engineering feats are at the limits of modosophont abilities, and are generally the preserve of transapient drive systems. The decay products interact with "unburnt" reaction mass to form a hot plasma which may then be ejected out of a magnetic nozzle to generate thrust.

PCC rockets share the same fundamental flaw as their antimatter cousins, namely the significant amount of shielding and cooling required by the reaction chamber magnets. As a result they are generally not used for large, high-power high-thrust "torch drives", but instead for high efficiency, low thrust engines such as those used by unmanned cargo flights and probes. Specific impulse of such rockets can reach as high as one million seconds, making them suitable for deep space and slow interstellar work. High thrust, low specific impulse variants find a home in interplanetary or heavy lift rockets. Though not necessarily as powerful as ICF drives, PCC drives can run effectively on water or plain hydrogen, and can function on even simpler fuels in an emergency, such as regolith dust.

Maximum specific impulse tends to be limited to 800,000 seconds, much like CIF, but maximum thrust is unlikely to reach a single gravity even for S1 drives.

Beam Core Conversion

The equivalent of antimatter beam core, this rocket engine is intended to induce baryon decay in the entirety of the reaction mass. This is a difficult proposition even for second singularity minds, as it either requires very long reaction chambers to ensure than all baryons react with a monopole (which has serious heat rejection, shielding and mass requirements) or a very high monopole density in the reaction chamber. This in turn has its own problems, as like-charged monopoles will repel limiting the density and opposite-charged monopoles will attract forming neutral monopolium bound states that will shoot out of the reaction chamber without contributing much more useful work.

As with plasma core drives, some designs attempt to recycle the monopoles to reduce wastage. Such designs are exceedingly complex, to the point where the control systems simply cannot be developed by modosophonts or first singularity transapients, though higher transapients may sometimes gift this technology to lesser minds. Unlike plasma core drives, the heating and shielding issues are not much more problematic than conventional antimatter beam core rockets.

Performance of basic rockets of this type is very similar to antimatter beam core, with a starship-grade maximum specific impulse of 10 million seconds. Fuel requirements are much simpler, with total conversion drives accepting literally any kind of matter. Baryon decay reactions produce slightly more neutral pions than annihilation reactions, which makes fuel efficiency slightly lower and shielding requirements slightly higher.

The use of conversion monopoles in a ramjet starship is where the design really shows its advantages. Basic engine power and thrust are equivalent to antimatter fuelled Ram-Augmented Interstellar Rockets, but the ability to recycle the monopoles vastly increases efficiency. Much longer journeys can be undertaken before the monopole fuel reserves are exhausted.

Specific impulse is no higher than 10,000,000 seconds. Transapient designed drives can have thrusts of as many as 10 gravities, but shielding and heat rejection issues tend to limit this to only a few gravities of sustained thrust especially for modosophont designs.

Lossless Conversion

With the advent of third singularity minds and their discovery of magmatter, it becomes possible to bind conversion monopoles to a magmatter mesh preventing their loss and greatly simplifying engine design. Unlike S2 monopole recyclers, there is no chance for monopoles to be lost in the exhaust stream or for them to bind to magnets causing "magnetic poisoning" which reduces drive performance. In a ramjet, the need to refuel is entirely removed, finally realizing the potential of Bussard's design. The removal of the need for complex recycling fields also removes the requirement for high transapient-level control systems, allowing S0 and S1 shipwrights to construct a working drive system given just the magmatter components. Salvaging such components from wrecked spacecraft is extremely lucrative... in some cases, the value of the drive mesh exceeds the value of the rest of the ship, crew and cargo altogether, and its extreme resilience to damage beams that deliberately destroying such ships to recover the drives is a practical technique for piracy.

One common design of a lossless total conversion system is the so-called "pac-man" drive. The reaction chamber is a hollow magmatter mesh (referred to as the "mantle"), with conversion monopoles bound to around 1/3 of the mesh intersection points (the great mass of GUT conversion monopoles places an upper limit on the practical number that may be employed without degrading drive performance).

The mesh is carefully woven such that ~30% of any baryons passing through the reaction chamber are guaranteed to meet a conversion monopole and hence decay. The drive is operated by pulling the reaction chamber open with a grid of magmatter wires (sometimes called the "rigging"), and injecting a pellet of fuel. Rigging is then adjusted to close the mantle, forcing its contents through the mesh and inducing every nucleon in the fuel pellet to decay. The reaction products fly out through the mesh unimpeded, and may be directed via a magnetic nozzle much like any other beam core drive. A secondary magmatter shield protects the tip of the fuel pellet inject system from the intense gamma radiation that results.

A typical drive of this sort accepts 1mm fuel pellets, with the mantle and shield massing approximately 160,000 tonnes (the vast majority of that number consisting of the GUT conversion monopoles supported by the mesh). The fuel delivery system, also incorporating magmatter, oscillates back and forth at 60MHz. Each pulse liberates approximately 2.8 GJ, produces plasma with an exhaust velocity of 0.3 c, and delivers an impulse of 17 kg m/s. At the 60 MHz repetition rate, this results in a total drive power of 100 PW and a thrust of 1 GN.

The construction of nuclear-scale meshes is complex, even for third toposophic minds, and is an example of femtoscale engineering. Ideally, the mesh (or mesh panels) are formed from single macromagmolecular structures; a sparse 2-dimensional crystal with gaps no wider than ~0.85 femtometres (the approximate diameter of a proton). Much denser meshes (such as pure mag-graphene) and much sparser meshes are easier to fabricate, but the former can be several orders of magnitude heavier than the ideal mesh and the latter do not guarantee total conversion. The lightest meshes and most efficient designs are hallmarks of S4 femtoengineering.

Performance is equivalent to any other beam-core rocket, though fuel efficiencies are slightly higher (or in the case of ramjet starships, fuel efficiency is simply no longer a problem).

Gamma Reflectors

Conversion reactions tend to release more of their energy as gamma rays than annihilation reactions, though they are not amenable to gamma ray laser techniques intended to direct gamma radiation out of the back of a spacecraft to generate useful thrust. For this reason, S2 minds have been known to use antimatter powered starships for long or fast journeys through regions where the interstellar medium is too sparse to support conversion ramjets.

With the advent of magmatter engineering it becomes possible to make gamma ray mirrors using a sparse magmatter net with a grid spacing somewhat less than the wavelength of the gamma rays emitted by neutral pion decay, the principle source of conversion reaction (and indeed antimatter annihilation) gamma rays. This is approximately 17.8fm. The construction of lightweight gamma mirrors is difficult, and so most gift drives use simpler and much smaller solid magmatter shields. Replacing the shield with a parabolic magmatter reflector (variously called the nozzle, the bell or the thimble in different designs) channels a substantial amount of the early gamma rays out of the back of the engine, contributing extra thrust, making greater use of the liberated energy and shielding the rest of the drive more effectively. Energy efficiency can approach 80%, and specific impulse will exceed that of standard conversion and conventional antimatter though it will not reach that of a pure photon drive.

It would be possible to make a much larger gamma reflector in the form of a paraboloid at least 10 m long and 5 m radius that would reflect and channel and even greater proportion of the reaction energy. A magnetic nozzle is still required (as the decay products of a conversion reaction will fly straight through magmatter) but this may be integrated into the fabric of the reflector itself. This is even more energy efficient and hence higher power, though such a drive assembly would weigh many million tonnes.

Performance can approach 20 million seconds for the very best designs, with modest increases in thrust over the standard beam-core design.

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Development Notes
Text by Adam Getchell, Todd Drashner, Mark Mcamuk, and Chris Shaeffer, updated by Ithuriel
Initially published on 26 April 2006.

updated january 2016 by ithuriel