Nuclear Pulse Drives

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Technology > Application > Transportation > Interplanetary Transport
Technology > Application > Transportation > Interstellar Transport
Technology > Application > Transportation > Propulsion Technology

Propulsion by means of explosive pulses entirely outside the vessel. Includes Orion Drive, Daedalus drive
Image from Steve Bowers
A Daedalus external fusion pulse interstellar vessel. Miniature pellets of deuterium are detonated in the hemisphere behind the ship, providing rapid pulses of thrust

Explosive Pulse Drive
Summary:	The Explosive Pulse Drive uses large numbers of relatively small explosions which are serially detonated behind the spacecraft to produce a more or less smooth thrust. There are two types, the basic lo tech 'dumb' or inert model which uses a physical pusher plate and does not require sentient ai, and the high performance hi tech 'smart' model which requires advanced ai to coordinate the bomblets and the magnetic 'pusher plate'
See also	Fission Drive, Fusion Drive
Basic Propulsion	Reaction
Specific Impulse	lo tech: 10,000 to 35,000 sec hi tech: 10e5 to 10e6 sec
Fuel:	Fission bomblets, Deuterium or He3 pellets, rarely Antimatter bomblets
Reaction Mass:	propellant material may be incorporated as shells around each bomblet
Minimum Technology Required:	lo tech: Microtech, Nanocomposites hi tech: Nanocomposites, Precision Nanotech, Sentient AI
Matter Manipulation:	microscale/nanoscale (precision materials)
Controller required:	lo tech: advanced non-sentient computers hi tech: sentient ai
First Introduced:	lo tech: Early Interplanetary Age hi tech: Late Interplanetary Age
Used by:	lo tech: mostly historical hobbyists, possibly a few isolated ludd or anti-ai groups hi tech: Many polities, clades, individuals and groups away from the Nexus, in low resource solar systems and among members of the Deeper Covenant, out on the periphery, and among some anti-ai luddite groups
Used in:	medium to high speed interplanetary and near-interstellar transport; generally impractical for true interstellar transport, although still used by some deep oort haloist groups
Construction Costs:	lo tech: Autofac: medium (bulky, precision materials); Hylonano: reasonably cheap assuming presence of component materials hi tech: Autofac: high (bulky, precision materials, requires exporting of some parts); Hylonano: somewhat expensive, requires presence of component materials. Requires trained and dedicated ai (generally turingrade or superturingrade)
Running cost:	depends on availability of fuel
Advantages:	reliable (lo tech), good performance (hi tech), reasonably easy fuel storage, can be made with materials in most solar systems, does not require amat (although amat may be used in some pulsed or external drives)
Disadvantages:	insufficient isp for decent interstellar transport, unstealthy, produces radioactive exhaust, use for launches illegal in most polities
Normal Acceleration:	lo tech: 0.01 to 0.1 g hi tech: 0.05 to 1 g or more
Cruising Speed	lo tech: 50 - 100 km/sec hi tech: 50 km/sec to 0.1 c (depending on design and fuel availability)

Image from Steve Bowers
The alien xenosophont race known as the Cybyota continue to use fusion-pulse vessels for interplanetary and interstellar journeys; small fusion devices are detonated behind the shield at the rear of the vessel, and propel the ship during the acceleration phase

Nuclear Pulse Drives

Nuclear Pulse Propulsion is the oldest design of nuclear rocketry, dating back to the end of the Industrial Age even before spaceflight had been developed. In its simplest form, it is the transfer of momentum from the debris of a nuclear explosion to a spacecraft, with repeated explosions being used to provide acceleration and manoevring. The performance of such systems varies greatly depending on the nuclear fuel, the means of initiating an explosion and the mechanism by which the explosion imparts force to the spacecraft.

Varieties

Thermonuclear Pulse (Orion)

The earliest designs suggested the use of modified thermonuclear weapons, using plutonium fission to ignite a lithium deuteride fusion reaction. This in turn reduced a propellant disk to plasma, directing as much as 80% of the explosion's energy at a solid flat or hemispherical "pusher plate" mounted to a spacecraft via shock absorbers. Such designs exhibited a high thrust to weight ratio, capable of lifting a spacecraft form the surface of the Earth, as well as high exhaust velocity leading to the possibility of single-stage spacecraft capable of flying from Earth to Luna, Mars, or even a moon of Saturn or Jupiter and returning back to Earth. Some designs were even capable of interstellar travel at over 3% of the speed of light, able to reach Alpha Centauri in a little over 130 years. Political issues surrounding the use of nuclear weapons, nuclear power in space and strategic arms limitations treaties ultimately prevented these spacecraft from being constructed before the advent of even more efficient and much cleaner and safer beam and fusion propulsion systems.

Performance of these primitive designs in some circumstances actually exceeded that of the beam and electric propulsion systems that were in use towards the end of the Information Age over a 100 years later, with specific impulses (Isp) ranging from 2000-4000 seconds (and a thrust to weight ratio suitable for launching a spacecraft from Earth's surface, and hence sustained average accelerations of several gravities), up to 7500-12000 for advanced designs that were still capable of high thrust. The theoretical limit for the design approaches 1.5 million seconds Isp, practical for use on a slow starship but requiring more sophisticated magnetic nozzles instead of material pusher plates. Notably, efficiency of this kind of drive increased with size, but a correspondingly vast spaceship would be required to make use of such power.

This design of drive is vanishingly rare in ultratech and transapient polities, but still finds use in mid- to high-tech space military forces given its power, simplicity and easy combination with other weapon systems such as bomb-pumped lasers.

Z-Pinch Fission

In order to avoid the issues of nuclear weaponry, one later design used unconventional pulse unit designs which could not be used as practical weapons. A powerful electromagnetic z-pinch detonation system would be used instead. The pulsed power system that drove the z-pinch had a considerable mass, making a z-pinch fission thruster more than ten times as heavy as other contemporary nuclear pulse designs, and also required large heatsinks and complex power recovery systems to recharge the capacitors after each drive pulse. Although practical, such designs never saw use within Solsys. Thrust would have been lower than the classic pulse design at a few tenths of a gravity, but the efficiency of the small pulse units fired at a higher rate meant that specific impulses were of the 10000-16000 second range, suitable for driving a manned spacecraft to the moons of Saturn or Jupiter from Earth, and unmanned craft further afield.

Fissile materials with low critical masses are desirable fuel sources for this drive. Curium-245 has both a reasonable half life of several thousand years and a critical mass similar to plutonium but does not occur naturally in any significant amounts and so much be synthesized. The expense and inconvenience of fuel production meant that antimatter-initiated microfission and magneto-intertial fusion drove the early expansion across solsys instead.

Z-pinch fission remains extremely rare, but certain low-tech polities that lack antimatter and the engineering expertise required for fusion drives have been known to operate spacecraft using these engines.

Magneto-Inertial Fusion

Magneto-inertial fusion was the first practical fusion rocket, and also the first nuclear pulse engine to attain significant use in Solsys. A plasmoid of fusion fuel is compressed and heated by a magnetically accelerated lithium jacket. Highly neutronic fusion reactions such as D-D and D-T could be used, as the lithium jacket was an effective means of converting the large neutron flux to hot plasma capable of generating thrust via a magnetic nozzle. Thrust is rather low for simple designs (centigees, at most, though advanced designs can manage as much as 1G), and presence of large amounts of lithium in the exhaust results in lower than ideal exhaust velocities hence an Isp of 5000-10000 seconds for most designs up to a maximum of around 20000 seconds. On the other hand, the principal fuels are easily available throughout most starsystems, Solsys included, which meant that it was economical to use them in the outer system away from convenient sources of fissiles. Other fusion fuels such as Tritium and Helium-3 can be used if available which improves the Isp somewhat, but the former is of course radioactive and the latter is not so easy to come by as deuterium.

Though MIF still finds some use in orbital space and moon-tranfer operations, it has been almost completely been by ICF designs for interplanetary travel throughout terragen space. The ease with which it may be constructed and the ready availability of fuel has ensured it still remains in use with lower-tech societies.

Antimatter Initiated Microfission

Also frequently referred to as "Antimatter Catalyzed Microfission", the burnup of antimatter in the drive reaction means that it is not a true catalyst. This is first pulse system that really made travel throughout Solsys practical. Even smaller pulse units were used than the z-pinch design, with a few grams of uranium as the ignition source for a few grams of deuterium fusion fuel. A beam of antiprotons is fired at the uranium to trigger the reaction, making this also the very first practical antimatter-burning rocket, though the energy of the antimatter annihilation makes a negligible contribution to the total energy output of the drive. When an antiproton hits a uranium nucleus, it annihilates one of the nucleons which releases enough energy to cause the nucleus to undergo fission. The hot fission fragments in turn heat up the deuterium until it undergoes fusion.

The simplest design uses an ablative nozzle, often made of a ceramic like silicon carbide, which ablates under the neutron and EM-radiation flux released by the exploding fuel pellet to generate plasma which may be directed by a simple magnetic nozzle to generate thrust. The ablative nozzle liner must be replaced at the end of the journey; a slightly more inconvient operation than replacing fuel pellets or refilling tanks, but it shields the rest of the drive assembly from much of the pulse radiation. More sophisticated designs use propellant jackets around the pulse units and more powerful magnetic nozzles to confine the fireball.

Thrust tends to be a little lower again than z-pinch fission designs (usually a few centigees), but drive efficiency started at 14000 seconds and the thruster assemblies are substantially lighter and simpler. Drive sizes tended to be limited by the limited range of antiproton guns, so no giant starships were envisaged as there were for the original pulse drives. The requirement for Uranium-235 fuel is less onerous than the synthetic fissiles used in z-pinch fission, but still required significant infrastructure in order to be used in the outer parts of star systems away from any heavy metal rich inner planets.

The combination of high thrust and high efficiency means that AM-fission still finds limited use across terragen space where fissiles are plentiful and antimatter harvesting and production is at least one microgram per year but hydrogen or water is harder to come by making fusion rockets less practical. Some early independent vec settlements were in such regions. High tech and ultratech societies tend to use MIF and ICF instead, as they can use more conveniently available fuels.

Z-Pinch Fusion

The second true fusion rocket design was the Z-pinch using D-D or D-T fuel. Using lithium plasma as the cathode of the z-pinch circuit absorbs neutron radiation from the main fusion reaction reaction and eliminates the problem of cathode wear that had plagued earlier Z-pinch fusion designs. The use of a z-pinch significantly increases fusion confinement times and hence the amount of energy that can be extracted from the fuel. With early designs having an Isp of 20000 seconds and several milligees of thrust and no requirement to use limited supplies of antimatter, these drives could operate throughout Solsys and allowed much more frequent flights than the AM-initiated microfission rockets. They were also extremely useful in the aftermath of the disasters of the Sundering, when both AM harvesting and synthesis and He3 harvesting operations were severely curtailed. Thrust is low, generally in the milligee range for most vessels, but it can be sustained for extended periods of time making these ships capable of flying continuous thrust trajectories to destinations within a few AU.

Although much harder to construct, requiring large and powerful magnetic nozzles, a pure deuterium z-pinch freed of the need to use lithium as a propellant can develop exhaust velocities of approximately 4% of the speed of light. This gives rise to a 1.3 million second Isp, which combined with the stability of the deuterium fuel makes it a practical platform for deep space travel. Sustained centigee thrusts for years at a time make this a true torchship. Some of the earliest interstellar probes used this technology, and could reach Alpha Centauri in a century. D-He3 can be used in a z-pinch rocket for a maximum exhaust velocity of 8.9% of lightspeed, and a much lower level of neutron radiation.

ICF has largely surpassed z-pinch for high power, high thrust drive systems (Torch Ships) as it simplifies some aspects of reaction chamber design, shielding and cooling. Z-pinch fusion is also much less easy to ignite or boost using antimatter or conversion monopoles, but some high tech civilizations which have limited access to such technologies may still use this kind of rocket in preference to ICF. The lack of high power drive beams make z-pinch ships less readily used for warfare, which helps their popularity in more pacifistic societies.

Inertial Confinement Fusion

The ultimate nuclear pulse drive system is inertial confinement fusion, where a pellet of fusion fuel is compressed and heated by bombarding it with laser or ion beams on all sides until it undergoes fusion. Although initially highly promising, various plasma instabilities were found which sharply limited confinement times making net energy production impractical, leading to the later development and deployment of MIF and z-pinch designs instead.

One possible solution involved the use of antimatter to heat a fuel pellet to fusion temperatures, but the amount of antimatter required was prohibitive during the early Interplanetary Age. Following the substantial increases in antimatter supply following the development of Saturn and Jupiter orbital space, antimatter-initiated ICF became a practical design and its use opened up the outer solar system. Experience gained with these designs eventually allowed the use of "pure" beam driven ICF that did not need antimatter at all.

The energy required for the driver beams is typically between 1% and 0.1% of the energy liberated per pulse, depending on the sophistication of the heating mechanism. Handling the waste heat created by the beam generator can be a major engineering issue, and may require large radiators. Common driver beams are lasers operating at visible, near ultraviolet, or vacuum ultraviolet wavelengths, or heavy ion beams. One consequence of this design is that some ICF-driven spacecraft can re-direct their beams for other purposes at the expense of propulsive power, leading to a large population of heavily armed spacecraft.

D-T Inertial Confinement Fusion (ICF)

This is one of the earliest torch drive designs, almost as old as fission and thermonuclear pulse drives. The fuel and propellant are plastic-clad pellets of deuterium/tritium ice. The ignition method is rather primitive - either the shock heating of the primary driver beams ignites the fuel (direct drive) or a second, ultra-short laser pulse in the petawatt range impinges on the fuel pellet at maximum compression (fast ignition). The D-T reaction is the easiest fusion reaction to ignite, allowing these crude ignition methods to work. However, 80% of the fusion energy is lost as neutron radiation. In addition, tritium is radioactive, with a 388 megasecond half life - consequently, D-T fuel cannot be stored for long periods of time.

Direct drive ICF is quite inefficient, driver beams can only produce pulses of about 10 times their beam energy. Due to the neutron energy losses and the high beam energy, pulse drives of this design usually need an additional power source for the driver beams (although some designs surrounded the reaction volume with a blanket of molten lithium to recover the neutron energy and breed additional tritium. Needless to say, these lithium-blanketed designs could only run at relatively low power to avoid melting the fusion chamber). Fast ignition D-T ICF can deliver drive pulses of around 100 times the driver beam energy. This allows some energy to be directly harvested from the drive pulse plasma to power the driver and igniter beams, while simultaneously allowing higher thrusts.

At 10% burnup and 85% nozzle efficiency, pure D-T fuel delivers an exhaust velocity of 0.9% c or 2,700 km/s (~275000 seconds Isp). However, because of the platic cladding necessary for driving the implosion, typical exhaust velocities are around 0.5% c or 1,500 km/s (~152000 seconds Isp). The exhaust velocity scales as the square root of the burnup, so early designs achieving 1% burnup had exhaust velocities as low as 500 km/s (~51000 seconds Isp). Centigee thrusts are commonly acheivable, but high neutron flux discourages more powerful engines of this type.

D-3He Inertial Confinement Fusion

Advanced fast ignition systems can be used to detonate pellets of deuterium/helium-3 ice. Although this requires a relatively high beam energy to pulse energy ratio (thus decreasing the total drive power), it allows operation using only readily available fuel that can be extracted from any gas giant or star without radioactive isotopes or exotic forms of matter. Approximately 1/20 of the pulse energy is lost as neutrons from D-D fusion side reactions, and 1/5 of the pulse energy escapes as bremsstrahlung x-rays. The remaining 3/4 of the pulse energy is available for propulsion.

Because D-3He fusion is more difficult to ignite, even with fast ignition the drive pulse energy is only around 10 times the beam energy. Typically, this energy is extracted from the plasma, resulting in only about half of the total pulse energy remaining for propulsion.

At 10% burnup and 85% nozzle efficiency, pure D-3He provides exhaust velocities of 2.5% c or 7,800 km/s (~795000 seconds Isp), although the cladding reduces this to about 4,300 km/s (~438000 seconds Isp). All other designs listed below that make use of D-3He fusion achieve similar exhaust velocities. Thrusts are typically at the centigee level for ease of cooling and shielding, though advanced designs can exceed 1g.

Antimatter Initiated Microfusion (AIM)

The fuel for AIM torch drives is a pellet of plastic-clad deuterium/helium-3 ice with an inner core of a heavy element, commonly uranium or lead. The driver beams impinge on the pellet, and at the moment of maximum compression a beam of neutral anti-hydrogen is shot into the pellet at low-relativistic velocities.

The antiprotons penetrate the low atomic weight cladding and fuel and slow to a stop in the dense core. On encountering a nucleus, they annihilate - a process that produces one or two pions and smashes the nucleus into fragments. The hot nuclear fragments rapidly dump their energy into the fuel mixture, heating it to the point of ignition. This method allows ignition with drive pulse energies on the order of 100 to 10,000 times the beam energy, depending on the amount of antimatter used. Consequently, for a given beam power the thrust power can be much higher than with un-boosted D-3He ICF, and more energy is available for propulsion since less needs to be siphoned off to power the driver beams. Like all D-3He fusion, 1/4 of the pulse energy is lost as neutrons and bremsstrahlung x-rays.

Conversion Initiated Microfusion (CIM)

Once GUT monopoles become commonly available, they can be used to ignite fusion in the D-3He fuel. The fuel pellet is a plastic-cladded ball of deuterium/helium-3 ice with a core of iron infused with monopoles and anti-monopoles. When the shock wave from the driver beams ionize the core electrons of the iron, the monopoles can begin to convert nucleons in the iron nucleus into pions and leptons. From there, the reaction proceeds much as happens in AIM, with the iron nuclei fragmenting when struck by the conversion pions into hot nuclear fragments, which in turn heat the deuterium/helium-3 fuel mixture. Since the iron core can be heavily loaded with monopoles, the CIM driver beams can produce drive pulses with about 10,000 times the energy of the driver beam pulses.

Antimatter Thermal

In antimatter-thermal drives, a pellet of a heavy metal such as uranium or lead encloses a quantity of antimatter suspended in vacuum. This is shot into the magnetic nozzle and imploded with the driver beams. Since the driver beams do not need to provide any significant compression, but rather must merely collapse the antimatter containment, the driver beam pulse energy is essentially negligible. The power available to torch drives of this design depends only on the design of the field coils. Due to the high cost of antimatter, this is not commonly used for long duration thrusting. Instead, AIM drives may have a store of antimatter-thermal pellets for limited duration burns at high thrust.

The high thrust of AT fuel pellets (which can be 10 or 100 times as much as the thrust provided by standard fuel) comes in exchange for efficiency, which will usually reduce specific impulse to below 10000 seconds.

Conversion Thermal

A conversion-thermal torch drive uses pellets of iron infused with monopoles. The driver beams ionize the iron and allow the monopoles to begin baryon conversion. As the hot nuclear fragments heat the iron further, the amount of inner-core ionization increases and with it the rate of conversion. This requires a relatively small driver beam pulse energy. Depending on the local industrial base, conversion monopoles can be potentially quite rare or expensive, and so this fuel type is less common outside of transapient or ultratech societies and their trading partners. CT pellets have the advantage that they are entirely stable and safe to handle until ionized.

As with AT fuel, the high thrust of CT fuel pellets (which can be 10 or 100 times as much as the thrust provided by standard fuel) comes in exchange for efficiency, which will usually reduce specific impulse to below 10000 seconds.

Image from Steve Bowers, Juan Ochoa, Anders Sandberg, Aaron Hamilton, John M. Dollan
Size comparison chart of spaceships used at various times in the Terragen Sphere (including a Daedalus flyby mission, and the much larger Double Daedalus probe capable of decelerating at the destination system)