Post by Machina Haruspex on Dec 17, 2007 16:55:32 GMT -5
Chemical rockets
Chemical rockets typically fall into three broad categories, cryogenic, stable, and hybrid. Chemical cryogenic rockets use fuels that demand active storage measures in almost all star systems if used inside the "snow line." By far the most common such system is the liquid hydrogen/liquid oxygen rocket, which uses plain water as fuel, split into its component elements.Such rockets typically have Isp's of 400 to 500 seconds, a thrust to weight ratio of less than 100 to 1 and a total DeltaV of no more than 10 km/sec.
Chemical stable rockets use fuels that are stable in most star systems up to or beyond the inner region of the habitable zone. Most such are also solid fuel rockets, although not all. One of the most common such fuels is aluminum powder and a non-cryogenic oxygen or fluorine source. Another popular version uses lithium and a non-cryogenic hydrogen source. Such designs typically have Isp's of 200-400 seconds, a thrust to weight ratio of less than 80 to 1, and a total DeltaV of no more than 8 km/sec.
Chemical hybrid rockets use fuels that are cryogenic and non- cryogenic in combination. There are many reasons for such choices. Typically, the solid fuel component is chosen such that it can serve as insulation for the cryogenic one. Some popular combinations are aluminum powder and liquid oxygen, or lithium and fluorine. Such designs typically have Isp's of 500-600 seconds, a thrust to weight ratio of 120 to 1, and a total DeltaV of no more than 12 km/sec.
Stellar Thermal Rockets
These are possibly the simplest space drives of all: Take a mass of any volatile (water and ammonia are both popular), seal it in a balloon, and heat it by putting it inside the local snowline. Add a nozzle, and instant rocket. Such designs have Isp's usually no more than 200, a thrust to weight ratio of 1000 to 1 or better, and a total DeltaV of no more than 5 km'sec or so.
Photo-electric drives
These include the classic "ion drives" of ancient myth. Capture incident photons from a nearby star, convert them to electricity, and use the electricity to fire ionized reaction mass. Such designs have Isp's of 400 to 2000, a thrust to weight ratio of .001 to 1 or less, and a total deltaV of 30 or more km/sec.
The electric or ion or plasma rocket turn the fuel into a plasma (ionized particles), and then uses electricity or ions to eject the fuel.
Basically the Ion rocket uses electrodes to turn the fuel (usually cesium oriodide but other fuels can also be used) into negative or positive ions. Then the positive ions are accelerated and ejected using an accelerating electrode. Ions are particles carrying negative or positive electrical charges that are attracted to each other. It is this characteristic of ions that is used to propel a rocket. Power plants using almost any source of fuel (hydrogen being most common) become possible.
Although ion drives produce high velocity, and due to the high isp it can continue for long periods of time and hence accelerate the ship to great speeds, and the low thrust from the ion jet beam isnt strong enough to give a rapid acceleration or to lift a vehicles off even a moderate sized planet.
Chemical photon rockets
These oddities use chemical lasers as their drive mechanisms. The most common of them use the chemical exhaust as rocket exhaust as well, and have terrible performance, not appreciably better than regular chemical rockets. Such designs usually have Isp's in the 300-400 range, a thrust to weight ratio of 20 to 1, and a total deltaV of 15 km/sec or so. An uncommon variant is the stellar chemical photonic drive, a hybrid that uses lasing reactants that can be regenerated by the light of the local star.
Such designs have a very low thrust to weight ratio but very high deltaV (for the S: level) similar to a lightsail, but have the advantage that the thrust can be directed in any direction. Such designs typically have Isp's of 9000 or more, a thrust to weight ratio of .0001 to 1, and a total deltaV of 100km/sec or more.
Passive Propulsion
The greatest drawback of the reaction driven rocket, whether it derives its energy from chemical reactions or antimatter, is the need to carry its own fuel and propellant on board. To avoid this, various kinds of passive propulsion have been developed, from the basic solar (stellar) sail which uses the light pressure of the local star for propulsion, to the more advanced laser driven concepts, which use powerful stationary lasers based for propulsion. Laser propelled craft can be simple sails, or the laser beam can be used to heat propellant which is expelled to provide thrust. Other concepts such as the magsail use a magnetically constrained cloud of particles as a sail surface.
Internal fission drives
These designs and the external pulse drives are the real workhorses of the S:0 world, assuming a good supply of their rare fuel can be obtained. Fissile and fertile isotopes are very rare in Population 2 stars, but fairly common in Population 1. In very young or very active star-forming regions, many usable isotopes may be common,but in most star systems, only three isotopes are available, U235, U238, and Th232.
Hydrogen or ammonia is the most commonly used reaction mass with these drives. The performance envelope for these drives is very broad, and almost completely determined by the sophistication of the internal shielding available. The lowest performance drives use centrifugally contained solid cores, and need relatively little internal shielding as they are quite cool.
Hotter still are the liquid core designs, usually but not always centrifugally stabilized. In liquid-core designs, the temperature of the fuel increases until it melts. The most common fuel composition is a uranium-iron alloy, U6Fe, which has a very high vapor pressure and boiling point but a fairly low melting point. Liquid core designs use high temperature "normal" or nano materials such as early diamondoid as the containment, actively cooled by blowing reaction mass through the containment.
Even hotter are the vapor core designs, in which the fuel is in the form of a gas, usually uranium fluoride of various compositions such as the U, F, UF4, UF6 family. Chlorine and thorium can also be used with similar performance and chemistries. Such designs are also usually centrifugally stabilized, with complex counterflow gas currents in the core. In early designs, the containment is usually diamondoid in a porous structure, actively cooled by blown reaction mass.
This type of active cooling is more effective in a vapor core design due to the single-phase gaseous core. In more modern designs, the diamondoid is usually supplemented by reflective programmable matter layers. Some vapor core designs also begin the use of electromagnetic shielding to supplement the diamondoid/programmable matter vessel.
Last are the fission plasma core designs, in which the gaseous core becomes hot enough to partially or fully ionize the gas to a plasma, and the use of electromagnetic shielding becomes essential rather than an addition. Plasma core fission drives are the distant ancestors of the much more potent Monopole Catalyzed Conversion Drive, and the work on containment vessels done for these plasma-core devices is critical for early Conversion Drives, as well. Such drives use a complex multi-layer design of electromagnetically cooled plasma, a blown-in gas layer (usually the reaction mass) and a diamondoid/programmable matter last wall.
Internal fission drives have ISP's ranging from 800 seconds for a simple solid core design to 8000 seconds for an advanced plasma core design. The thrust to weight ratio ranges from 10 to 1 to 100 to 1, and the total deltaV varies from 20 km/sec to over 100 km/sec.
External Pulsed Plasma (EPP) drives
EPP's avoid the difficult shielding problems of the Internal drives by moving the "drive chamber" completely outside the ship. Such designs are extremely wasteful of fertile, fissile, and fusion fuels, meaning internal drives have some competitive advantages, but since they do not need to contain the full power of the drive reaction, they can have a very high performance.
Typically, early designs use simple fission devices to generate plasma pulses. More advanced designs use fission/fusion staged pulse units, antimatter catalyzed fusion pulse units, antimatter boosted pulse units, and rarely, antimatter pulse units. The performance of the designs are mainly constrained by the material strengths available. Early ships using fission pulse units and simple steel acceleration disks typically have Isp's around 5000, a thrust to weight ratio of 5 to 1, and a total DeltaV of 60 km/sec.
A middle-ground ship using antimatter-initiated pure fusion pulse units and a primitive diamondoid acceleration disk typically has an Isp around 20,000, a thrust to weight ratio of 20 to 1, and a total DeltaV of 180 km/sec+.
An advanced antimatter-boosted (1 percent) fusion pulse unit design with an electromagnetically shielded nanotech-based programmable matter acceleration disk typically has an Isp around 100,000, a thrust to weight ratio of 100 to 1, and a total DeltaV of 800 km/sec or higher.
External Fission/Fusion Drives
Extremely wasteful and dirty even compared to the external pulse plasma drives, external fission/fusion drives are hybrids between internal fission drives and EPP's. They essentially move the fission drive outside the ship, but do not use pulse units, instead generating a constant fission or fission/fusion reaction.
The thrust is transferred to the ship via an electromagnetically shielded programmable matter acceleration disk. Due to the constant burn, a very good shielding system is needed, at least as good as the best used for the internal drives, but since the reaction is not contained, much higher power levels can be tolerated, with similar high performance.
Indeed, such designs have higher power levels than either EPP's or internal fission drives, as the cost of enormous fuel consumption. The one outstanding feature of such a design is the tremendous thrust and acceleration levels possible, making such drives popular with militaries.
The most primitive of these use pure fission reactions and electromagnetic/diamondoid acceleration disks. More advanced versions use electromagnetic/nanotech acceleration disks and fission/fusion reactions. The most primitive of these designs has an Isp of around 30,000 seconds, a thrust to weight ratio of 200 to 1, and a total deltaV of 200 km/sec. The most advanced, using fission-fired D-D fusion reactions, has an Isp of around 1,000,000 seconds, a thrust to weight ratio of 300 to 1, and a total deltaV of 5000 km/sec.
Internal fusion drives
Such devices are the follow-on drives of the internal fission drives, assuming once again that a good supply of their rare fuel can be found. They also serve as the ancestors of the first antimatter drives. There are many fusion reactions possible with technology, but only one is really well suited for space travel, that being the fusion of He3 to He3.
Other candidate reactions such as p-B11, He3-D, D-T, and D-D all have large levels of neutron production. While nanotech self-healing shields with programmable matter elements can effectively shield against these neutrons, such shields are very heavy indeed. He3-He3 is the least neutron-producing reaction available.
Sadly, He3 is rare even when it can be found at all, and the best sources, large gas giants, have very large gravity wells. Fissile fuels are also rare, but can be found on small rocky worlds, planetoids, and asteroids.
The ignition conditions for fusion reactions are also much, much higher than for fission reactions, so it is common for internal fusion drives to carry small amounts of fissile material to use as "seeds" to ignite the more potent fusion reactions. Such ignitors are called "fissile pits" for ancient reasons. (Indeed, even the super advanced Conversion Drives still carry such fissile pits, and for the same reason.) These fissile pits serve the same use as matches used by modosophonts.
Even the mightiest fire needs the first spark to ignite it, and for most drives, that spark comes from fissile pits. Fusion drives always use very advanced nanotech based internal shielding, with a layer of "cool" plasma closest to the active plasma, a layer of gas vapor outside the cool plasma, and a nanotech solid outer wall, with electromagnetic shields running on the solid outer wall and plasma inner "wall."
Such internal shields are similar to those used by advanced fission internal drives. Internal fusion drives have an Isp usually between 1,000,000 and 3,000,000 seconds, a thrust to weight ratio of 30 to 1 or better, and a total Delta V of 30,000 km/sec or maybe more, if the extreme mass fraction required can be tolerated. As a note, 30,000 km/sec is roughly equal to .1C, and thus internal fusion drives are the first drives to even potentially have use as interstellar drives.
Antimatter Drives
All practical antimatter drives are internal drives, as antimatter is far, far too expensive to use in a wasteful manner. Antimatter is rarely found in nature, and always very thinly spread in the vacuum of space as a byproduct of high-energy processes. The harvesting of such space-borne amat is rarely economically viable. For most purposes antimatter has to be manufactured from pure energy, a difficult process indeed.
Antimatter is also difficult and extraordinarily dangerous to store. The only reason early modosophonts were compelled to endure the extreme difficulties with it at all was the truly incredible performance it offered compared to fusion, and the fact that antimatter drives are the first practical interstellar drives. An antimatter drive is always used to convert matter directly to energy, and then that energy is used either as direct reaction mass, or to heat another reaction mass.
In general, direct antimatter drives are by far the best performers and the only ones referred to as true antimatter drives, while intermediary antimatter drives are usually referred to as boosted fusion or antimatter catalysed fusion drives.
As a matter of fact, almost any boosted fusion drive can be run in a pure antimatter mode, and almost any antimatter drive can be augmented with some fusion fuel or other reaction mass. In general, boosting or not boosting a drive involves trading Isp for thrust. Drives used in gravity wells are usually heavily boosted, while drives used for deep space travel are usually run as pure antimatter drives.
Boosted drives usually have Isp's between 1,000,000 and 10,000,000 seconds, a thrust to weight ratio of 20 to 1 or better, and a total deltaV of 10,000 to 80,000 km/sec. Pure antimatter drives usually have Isp's close to the maximum physics allows, 30,000,000 seconds, a thrust to weight ratio of 100 to 1 or more, and a total DeltaV of 150,000 km/sec. These last figures lead to the classic specifications for the ancient "AMAT Clippers", namely, for a launch mass that was one quarter antimatter and one quarter reaction matter, you could reach .25C on both legs of a round-trip journey without refuelling.
Adding a ramscoop to such a design improves the range and performance considerably, and such designs are the earliest ones to break out of the Low Speed flight regime and into the High Speed flight regime.
Conversion Drive
A matter furnace uses magnetic monopoles to convert any matter directly to energy, usually in a catalyzed fusion plasma. Those magnetic monopoles which can be created, are massless and have short lifetimes, meaning more must be constantly generated while the matter furnace is running, requiring large particle accelerators.
While expensive, heavy, and complicated, such manufacturing equipment was still much lighter and cheaper than an equal mass of antimatter, as well as being much, much safer. Mounting a matter furnace on a ship creates the Conversion Drive, and such drives immediately began to dominate space travel. These drives convert matter to energy without the extreme expense and danger of antimatter.
Early conversion drives are very large, yet have low power outputs compared to their more developed performance, mainly due to the need for monopole creation mass drivers and the continued use of plasma/vapor/solid/EM shielding systems.
The ease of refueling and the existence of useful ramscoop technology means that DeltaV calculations no longer have much meaning. Similarly, reaction drives have essentially the same Isp, the maximum allowed by physics, namely 30,000,000. In general, Conversion-ships have a maximum speed of .7C and a maximum range of 100 to 200 lightyears, mainly limited by the simple passive ablative shielding they employ.
The Monopole Catalyzed Conversion Drive is such a huge advance, it almost completely displaces all previous technologies.
Chemical rockets typically fall into three broad categories, cryogenic, stable, and hybrid. Chemical cryogenic rockets use fuels that demand active storage measures in almost all star systems if used inside the "snow line." By far the most common such system is the liquid hydrogen/liquid oxygen rocket, which uses plain water as fuel, split into its component elements.Such rockets typically have Isp's of 400 to 500 seconds, a thrust to weight ratio of less than 100 to 1 and a total DeltaV of no more than 10 km/sec.
Chemical stable rockets use fuels that are stable in most star systems up to or beyond the inner region of the habitable zone. Most such are also solid fuel rockets, although not all. One of the most common such fuels is aluminum powder and a non-cryogenic oxygen or fluorine source. Another popular version uses lithium and a non-cryogenic hydrogen source. Such designs typically have Isp's of 200-400 seconds, a thrust to weight ratio of less than 80 to 1, and a total DeltaV of no more than 8 km/sec.
Chemical hybrid rockets use fuels that are cryogenic and non- cryogenic in combination. There are many reasons for such choices. Typically, the solid fuel component is chosen such that it can serve as insulation for the cryogenic one. Some popular combinations are aluminum powder and liquid oxygen, or lithium and fluorine. Such designs typically have Isp's of 500-600 seconds, a thrust to weight ratio of 120 to 1, and a total DeltaV of no more than 12 km/sec.
Stellar Thermal Rockets
These are possibly the simplest space drives of all: Take a mass of any volatile (water and ammonia are both popular), seal it in a balloon, and heat it by putting it inside the local snowline. Add a nozzle, and instant rocket. Such designs have Isp's usually no more than 200, a thrust to weight ratio of 1000 to 1 or better, and a total DeltaV of no more than 5 km'sec or so.
Photo-electric drives
These include the classic "ion drives" of ancient myth. Capture incident photons from a nearby star, convert them to electricity, and use the electricity to fire ionized reaction mass. Such designs have Isp's of 400 to 2000, a thrust to weight ratio of .001 to 1 or less, and a total deltaV of 30 or more km/sec.
The electric or ion or plasma rocket turn the fuel into a plasma (ionized particles), and then uses electricity or ions to eject the fuel.
Basically the Ion rocket uses electrodes to turn the fuel (usually cesium oriodide but other fuels can also be used) into negative or positive ions. Then the positive ions are accelerated and ejected using an accelerating electrode. Ions are particles carrying negative or positive electrical charges that are attracted to each other. It is this characteristic of ions that is used to propel a rocket. Power plants using almost any source of fuel (hydrogen being most common) become possible.
Although ion drives produce high velocity, and due to the high isp it can continue for long periods of time and hence accelerate the ship to great speeds, and the low thrust from the ion jet beam isnt strong enough to give a rapid acceleration or to lift a vehicles off even a moderate sized planet.
Chemical photon rockets
These oddities use chemical lasers as their drive mechanisms. The most common of them use the chemical exhaust as rocket exhaust as well, and have terrible performance, not appreciably better than regular chemical rockets. Such designs usually have Isp's in the 300-400 range, a thrust to weight ratio of 20 to 1, and a total deltaV of 15 km/sec or so. An uncommon variant is the stellar chemical photonic drive, a hybrid that uses lasing reactants that can be regenerated by the light of the local star.
Such designs have a very low thrust to weight ratio but very high deltaV (for the S: level) similar to a lightsail, but have the advantage that the thrust can be directed in any direction. Such designs typically have Isp's of 9000 or more, a thrust to weight ratio of .0001 to 1, and a total deltaV of 100km/sec or more.
Passive Propulsion
The greatest drawback of the reaction driven rocket, whether it derives its energy from chemical reactions or antimatter, is the need to carry its own fuel and propellant on board. To avoid this, various kinds of passive propulsion have been developed, from the basic solar (stellar) sail which uses the light pressure of the local star for propulsion, to the more advanced laser driven concepts, which use powerful stationary lasers based for propulsion. Laser propelled craft can be simple sails, or the laser beam can be used to heat propellant which is expelled to provide thrust. Other concepts such as the magsail use a magnetically constrained cloud of particles as a sail surface.
Internal fission drives
These designs and the external pulse drives are the real workhorses of the S:0 world, assuming a good supply of their rare fuel can be obtained. Fissile and fertile isotopes are very rare in Population 2 stars, but fairly common in Population 1. In very young or very active star-forming regions, many usable isotopes may be common,but in most star systems, only three isotopes are available, U235, U238, and Th232.
Hydrogen or ammonia is the most commonly used reaction mass with these drives. The performance envelope for these drives is very broad, and almost completely determined by the sophistication of the internal shielding available. The lowest performance drives use centrifugally contained solid cores, and need relatively little internal shielding as they are quite cool.
Hotter still are the liquid core designs, usually but not always centrifugally stabilized. In liquid-core designs, the temperature of the fuel increases until it melts. The most common fuel composition is a uranium-iron alloy, U6Fe, which has a very high vapor pressure and boiling point but a fairly low melting point. Liquid core designs use high temperature "normal" or nano materials such as early diamondoid as the containment, actively cooled by blowing reaction mass through the containment.
Even hotter are the vapor core designs, in which the fuel is in the form of a gas, usually uranium fluoride of various compositions such as the U, F, UF4, UF6 family. Chlorine and thorium can also be used with similar performance and chemistries. Such designs are also usually centrifugally stabilized, with complex counterflow gas currents in the core. In early designs, the containment is usually diamondoid in a porous structure, actively cooled by blown reaction mass.
This type of active cooling is more effective in a vapor core design due to the single-phase gaseous core. In more modern designs, the diamondoid is usually supplemented by reflective programmable matter layers. Some vapor core designs also begin the use of electromagnetic shielding to supplement the diamondoid/programmable matter vessel.
Last are the fission plasma core designs, in which the gaseous core becomes hot enough to partially or fully ionize the gas to a plasma, and the use of electromagnetic shielding becomes essential rather than an addition. Plasma core fission drives are the distant ancestors of the much more potent Monopole Catalyzed Conversion Drive, and the work on containment vessels done for these plasma-core devices is critical for early Conversion Drives, as well. Such drives use a complex multi-layer design of electromagnetically cooled plasma, a blown-in gas layer (usually the reaction mass) and a diamondoid/programmable matter last wall.
Internal fission drives have ISP's ranging from 800 seconds for a simple solid core design to 8000 seconds for an advanced plasma core design. The thrust to weight ratio ranges from 10 to 1 to 100 to 1, and the total deltaV varies from 20 km/sec to over 100 km/sec.
External Pulsed Plasma (EPP) drives
EPP's avoid the difficult shielding problems of the Internal drives by moving the "drive chamber" completely outside the ship. Such designs are extremely wasteful of fertile, fissile, and fusion fuels, meaning internal drives have some competitive advantages, but since they do not need to contain the full power of the drive reaction, they can have a very high performance.
Typically, early designs use simple fission devices to generate plasma pulses. More advanced designs use fission/fusion staged pulse units, antimatter catalyzed fusion pulse units, antimatter boosted pulse units, and rarely, antimatter pulse units. The performance of the designs are mainly constrained by the material strengths available. Early ships using fission pulse units and simple steel acceleration disks typically have Isp's around 5000, a thrust to weight ratio of 5 to 1, and a total DeltaV of 60 km/sec.
A middle-ground ship using antimatter-initiated pure fusion pulse units and a primitive diamondoid acceleration disk typically has an Isp around 20,000, a thrust to weight ratio of 20 to 1, and a total DeltaV of 180 km/sec+.
An advanced antimatter-boosted (1 percent) fusion pulse unit design with an electromagnetically shielded nanotech-based programmable matter acceleration disk typically has an Isp around 100,000, a thrust to weight ratio of 100 to 1, and a total DeltaV of 800 km/sec or higher.
External Fission/Fusion Drives
Extremely wasteful and dirty even compared to the external pulse plasma drives, external fission/fusion drives are hybrids between internal fission drives and EPP's. They essentially move the fission drive outside the ship, but do not use pulse units, instead generating a constant fission or fission/fusion reaction.
The thrust is transferred to the ship via an electromagnetically shielded programmable matter acceleration disk. Due to the constant burn, a very good shielding system is needed, at least as good as the best used for the internal drives, but since the reaction is not contained, much higher power levels can be tolerated, with similar high performance.
Indeed, such designs have higher power levels than either EPP's or internal fission drives, as the cost of enormous fuel consumption. The one outstanding feature of such a design is the tremendous thrust and acceleration levels possible, making such drives popular with militaries.
The most primitive of these use pure fission reactions and electromagnetic/diamondoid acceleration disks. More advanced versions use electromagnetic/nanotech acceleration disks and fission/fusion reactions. The most primitive of these designs has an Isp of around 30,000 seconds, a thrust to weight ratio of 200 to 1, and a total deltaV of 200 km/sec. The most advanced, using fission-fired D-D fusion reactions, has an Isp of around 1,000,000 seconds, a thrust to weight ratio of 300 to 1, and a total deltaV of 5000 km/sec.
Internal fusion drives
Such devices are the follow-on drives of the internal fission drives, assuming once again that a good supply of their rare fuel can be found. They also serve as the ancestors of the first antimatter drives. There are many fusion reactions possible with technology, but only one is really well suited for space travel, that being the fusion of He3 to He3.
Other candidate reactions such as p-B11, He3-D, D-T, and D-D all have large levels of neutron production. While nanotech self-healing shields with programmable matter elements can effectively shield against these neutrons, such shields are very heavy indeed. He3-He3 is the least neutron-producing reaction available.
Sadly, He3 is rare even when it can be found at all, and the best sources, large gas giants, have very large gravity wells. Fissile fuels are also rare, but can be found on small rocky worlds, planetoids, and asteroids.
The ignition conditions for fusion reactions are also much, much higher than for fission reactions, so it is common for internal fusion drives to carry small amounts of fissile material to use as "seeds" to ignite the more potent fusion reactions. Such ignitors are called "fissile pits" for ancient reasons. (Indeed, even the super advanced Conversion Drives still carry such fissile pits, and for the same reason.) These fissile pits serve the same use as matches used by modosophonts.
Even the mightiest fire needs the first spark to ignite it, and for most drives, that spark comes from fissile pits. Fusion drives always use very advanced nanotech based internal shielding, with a layer of "cool" plasma closest to the active plasma, a layer of gas vapor outside the cool plasma, and a nanotech solid outer wall, with electromagnetic shields running on the solid outer wall and plasma inner "wall."
Such internal shields are similar to those used by advanced fission internal drives. Internal fusion drives have an Isp usually between 1,000,000 and 3,000,000 seconds, a thrust to weight ratio of 30 to 1 or better, and a total Delta V of 30,000 km/sec or maybe more, if the extreme mass fraction required can be tolerated. As a note, 30,000 km/sec is roughly equal to .1C, and thus internal fusion drives are the first drives to even potentially have use as interstellar drives.
Antimatter Drives
All practical antimatter drives are internal drives, as antimatter is far, far too expensive to use in a wasteful manner. Antimatter is rarely found in nature, and always very thinly spread in the vacuum of space as a byproduct of high-energy processes. The harvesting of such space-borne amat is rarely economically viable. For most purposes antimatter has to be manufactured from pure energy, a difficult process indeed.
Antimatter is also difficult and extraordinarily dangerous to store. The only reason early modosophonts were compelled to endure the extreme difficulties with it at all was the truly incredible performance it offered compared to fusion, and the fact that antimatter drives are the first practical interstellar drives. An antimatter drive is always used to convert matter directly to energy, and then that energy is used either as direct reaction mass, or to heat another reaction mass.
In general, direct antimatter drives are by far the best performers and the only ones referred to as true antimatter drives, while intermediary antimatter drives are usually referred to as boosted fusion or antimatter catalysed fusion drives.
As a matter of fact, almost any boosted fusion drive can be run in a pure antimatter mode, and almost any antimatter drive can be augmented with some fusion fuel or other reaction mass. In general, boosting or not boosting a drive involves trading Isp for thrust. Drives used in gravity wells are usually heavily boosted, while drives used for deep space travel are usually run as pure antimatter drives.
Boosted drives usually have Isp's between 1,000,000 and 10,000,000 seconds, a thrust to weight ratio of 20 to 1 or better, and a total deltaV of 10,000 to 80,000 km/sec. Pure antimatter drives usually have Isp's close to the maximum physics allows, 30,000,000 seconds, a thrust to weight ratio of 100 to 1 or more, and a total DeltaV of 150,000 km/sec. These last figures lead to the classic specifications for the ancient "AMAT Clippers", namely, for a launch mass that was one quarter antimatter and one quarter reaction matter, you could reach .25C on both legs of a round-trip journey without refuelling.
Adding a ramscoop to such a design improves the range and performance considerably, and such designs are the earliest ones to break out of the Low Speed flight regime and into the High Speed flight regime.
Conversion Drive
A matter furnace uses magnetic monopoles to convert any matter directly to energy, usually in a catalyzed fusion plasma. Those magnetic monopoles which can be created, are massless and have short lifetimes, meaning more must be constantly generated while the matter furnace is running, requiring large particle accelerators.
While expensive, heavy, and complicated, such manufacturing equipment was still much lighter and cheaper than an equal mass of antimatter, as well as being much, much safer. Mounting a matter furnace on a ship creates the Conversion Drive, and such drives immediately began to dominate space travel. These drives convert matter to energy without the extreme expense and danger of antimatter.
Early conversion drives are very large, yet have low power outputs compared to their more developed performance, mainly due to the need for monopole creation mass drivers and the continued use of plasma/vapor/solid/EM shielding systems.
The ease of refueling and the existence of useful ramscoop technology means that DeltaV calculations no longer have much meaning. Similarly, reaction drives have essentially the same Isp, the maximum allowed by physics, namely 30,000,000. In general, Conversion-ships have a maximum speed of .7C and a maximum range of 100 to 200 lightyears, mainly limited by the simple passive ablative shielding they employ.
The Monopole Catalyzed Conversion Drive is such a huge advance, it almost completely displaces all previous technologies.