ID : 1325-001 TITLE : Technical Notes on Inter-Stellar Propulsion and Related Technologies TITLE2 : Introduction to Interstellar Propulsion AUTHOR : Imperial Research & Development, Technical Documentation Division AUTHOR2: Edited by Seven Swords Special Service, Division of Documents & Records CLASS : Unclassified SOURCE : Imperial Research & Development, Technical Documentation Division Document 4365-12-rev2/2350-A Last Revision: 15 February 2350 Alternate Listings: Introduction to Interstellar Propulsion Keywords: interstellar propulsion, jumpspace, Jump drive, grav drive, grav gate, grav coordinate, FTL communications, exploration probe, jump detection system
The technology and systems described here are the standard as of 2349, although little has changed in over 150 years. The greatest advance was the Divine Lightning stardrive, and that was merely an engineering refinement. Fundamental breakthroughs have been lacking, and seem unlikely in the modern corporate culture. There are no known corporate research groups investigating jumpspace and Jump drive technology; Imperial Research & Development has a small team of theoreticians, but funding is limited (see Document 1000-2350-A, Itemized Budget, fiscal year 2350, pp 1128-1129).
This overview assumes the reader is familiar with basic physics, particularly gravitation and its influence on surrounding space-time. It also assumes a working knowledge of related high-tech fields, such as power plant types and sub-light communications.
For more depth in a particular area, the reader is referred to any number of papers published in the past few decades. (See also: Document series 4300)
The Shield drive is very simple in theory - create a strong gravitational field in front of the ship, causing it to "fall" forward. All Shield drives (a.k.a. grav drives) are built along the same basic design. A set of Primary Field Generators (PFGs) is located at the front of the ship. Each PFG creates a slight warp in local space-time (i.e. gravitational attraction). The superposition of the entire set of PFGs creates a gravitational field in the shape of a large circular disk parallel to the plane of the PFG array. The disk, known as a primary field (PF), is essentialy 2-dimensional, in that it has no thickness (actually a thickness bordering on Planck's constant). The disk is equivalent to an immense mass in front of the ship, and so the ship "falls" toward it. The result is a propulsion system that consumes no fuel and has the potential for accelerations measured in kilometers per second (equivalent to thousands of time the gravitational acceleration on the surface of Earth).
These high accelerations are achieved by intense primary fields at the front of the ship. Normally, such a large gravitational source would create enormous gradients along the length of the ship. For example, a ship 100m long would have a difference in force of almost 10,000 times. If the acceleration at the front of the ship is 1 km/s2, then at the rear it would be only 0.1 m/s2. Clearly, the stress on the ship's hull would be enormous. In fact, it would rip the ship (and crew) apart almost instantly.
To eliminate these stresses, it is necessary to prevent the rapid decay of the gravitational force. To do this, additional sets of field generators are installed to strengthen the field as it decays with distance. These additional generators, called Secondary Field Generators (SFGs), are located along the length of the hull. As the PF decays, the SFGs create increasing additional fields (known as secondary fields (SFs)). The result is virtually zero stress on the hull, even under large accelerations.
In theory, a ship can be any length. Practically, there are cost and power limits on the number and strength of the SFG arrays that can be installed along the hull. For current power cores and hull materials, the maximum cost-effective length is ~600 m. This length is also influenced by the limitations of the Jump drive (see below).
There are also engineering constraints on the radius of the PF (and hence SFs). Minimum sizes of power cores, field generators, and hull bracing make it impractical to create self-sufficient grav drives with a radius less than 10 m; the advanced Divine Lightning stardrives push this minimum down to 2.5 m. The maximum radius is determined by available power and money. Field generators are expensive and relatively ineffecient. The result is a current practical limit of 105 m. Again, the advances of the Divine Lightning stardrive raise this limit to ~400 m, although no ship is known to have had a PF of that size.
See below for more information on the advances of the Divine Lightning stardrive.
The maximum theoretical speed of a grav drive is the speed of light, c (~300,000 km/s). In practice, relativistic effects become dominant at ~0.5c, and few craft are capable of speeds beyond 0.55c. A few scouts and other specialized craft are capable of achieving 0.8c; the absolute speed record is still at 0.830451c.
Grav Drive Maneuvering
It was determined early in the development of the grav drive that maneuvering with conventional thrusters would be difficult at the expected velocities. The problem was solved by introducing very fine control of the grav drive primary and secondary fields. By introducing slight alterations in the strength of the fields at different areas across the face of the field disk, it is possible to accelerate the ship through an arc, effectively turning the vessel. Essentially, imagine accelerating the upper (or lower, left or right) half of the ship by a slightly greater amount than the opposite half. The ship will gradually turn, at a rate determined by the differential acceleration.
The turning radius for modern starships is directly dependant on the velocity of the ship. Modern hulls (and crews) are not able to withstand more than a few g's of differential acceleration. Hence, a ship at significant speed (i.e. 0.2c or greater) often cannot perform a full reversal in anything less than a few billion km of space. Ships at high speed are essentially fixed in a straight-line course. For this reason, such speeds are rarely attained except as a prescursor to Jump.
Although grav drives are capable of turning a ship that is not accelerating (very small fields of only a g or so), fine maneuvering with the grav drive is exceptionally difficult. Moreover, the grav drive is unable to turn the ship in place. Hence, all modern ships also have secondary thruster systems for fine maneuvering for docking, etc. These thrusters are also sometimes used for stationkeeping in very deep space.
Grav drives are predominantly a uni-directional propulsion system. The PFGs create an intense field that draws the ship forward. It is possible to decelerate with the grav drive, although it is less efficient. By greatly reducing the primary field and increasing the secondary fields, it is possible to essentially reverse the grav drive, decelerating the ship. Since the primary and secondary field generators are virtually identical, it is possible to decelerate with up to ~80% of the maximum forward acceleration of the drive. The loss of 20% is due to the reduced power feeds to the SFGs and the slightly lower rating of the SFGs. Some warships are known to have all field generators identical, allowing deceleration at the same rates as acceleration. This is very unusual, and expensive.
It should be noted that when under deceleration, the Shield Effect applies to the rear of the craft, not the front.
Similar to deceleration is the idea of stationkeeping with the grav drive. By altering the relative strengths of the primary and secondary fields, it is possible to give the ship an acceleration (forward or backwards) that exactly counteracts that due to the local gravity field. The ship then remains perfectly still. This does require the ship to be heading parallel to the local gravity field, but that is rarely a problem in space. This technique is particularly useful for warships in orbit defending a planet. The field will be strongest towards the planet, requiring the drives to be energized away from the planet, shielding the ship from invader's attacks. It is far less effective against planetary defenses. This technique also allows a vessel to achieve geostationary orbit at any altitude.
The Shield Effect
Grav drives acquired the name Shield drives because of the effect the operating PFGs have on the space in front of the vessel. Briefly, the operating grav drive creates an impenetrable shield in front of the ship. Matter and energy in the path of the ship do not strike the ship at all. Some is absorbed, some is warped around the ship itself.
The effect arises as a consequence of the rather large distortion of space-time created by the drive. When operating, the PF is essentially a 2-D gravity source, creating infinite gradients at the front of the disk. The side of the disk towards the ship has essentially zero gradient, due to the SFGs. This infinite gradient creates an event horizon, identical to the one of a black hole. Anything crossing the event horizon is trapped in the energy well of the source disk. Matter (which is reduced to atoms or less by the gradients) and energy become trapped with essentially zero velocity relative to the ship. When the grav drive shuts down, the energy and particles dissipate in random directions with low velocities. Radiation shielding of the ship's hull absorbs any radiation that comes towards the ship.
Note that there are no event horizons created along the length of the ship - the SFGs prevent them. The SFGs create zero gradient along the length (and breadth) of the ship. Because of this, the energy well remains finite and below the threshold of an event horizon. Note that for matter and energy perpendicular to the ship, there is no shield effect; energy (and matter) is drawn to the PF, but along a curved path that often intersects the ship's hull. Hence, starships have relatively thick shielding near the front, despite the shield effect.
Divine Lightning
The Seven Worlds are the last group to have made significant improvements to the design of a grav drive. In particular, advances in materials allowed for 3 critical advances:
The results of these advances are simple:
With the complete annihilation of the Seven Worlds system, it is doubtful that anyone recovered the technology behind the Divine Lightning. No corporation, not even TRIDENT, has released any stardrive designs that indicate familiarity with the technology of the Divine Lightning. There is no indication that Imperial Research & Development is pursuing any practical research on grav drive design.
Shortly after the development of the first grav drives, a ship was attempting to test the pratical limits of the drive. Achieving a speed of 0.4c, the ship pulsed an enormous burst of power into the field generators, attempting to achieve higher speeds for a short time. The ship vanished from known space. It had unknowingly performed the first Jump. The ship and its crew was lost, but sensor recordings of the event allowed scientists to work out what had happened. Thus was the Jump drive discovered.
Jump drives are really not a completely separate drive system. Rather, they are an extension of the grav drive. A jump drive consists of several major systems:
The jump capacitor is sized to the vessel it is mounted on. It also occupies a significant fraction of the vessels mass and is the majority of the cost. Superconducting capacitors capable of holding 500,000 TJ are large, expensive, and very dangerous. If the energy were improperly discharged, it could easily melt the entire hull and crew. If the power connections have even a slight resistance, they will overheat and melt, possibly resulting in mis-Jump and the destruction of the ship. For these reasons, the capacitor is one of the best protected systems on any vessel.
The main limitation on the rate of Jumps is the charging of the capacitor. A typical capacitor stores enough charge for 2 transitions; one from normal space to jumpspace, and then back. This allows for one complete Jump without recharging. Typically, a ship is equipped with enough power reactors to charge the capacitor in 2 weeks. This is in addition to normal STL maneuvering and operations. If all the power is devoted to charging, the time can often be reduced by a factor of 2 or more. Some military vessels, especially scouts, have extra-large capacitors to allow for 2 or more full Jumps before recharging. This way, a ship can enter an unknown system and be able to quickly leave should it prove hostile. Such capacitors, while possible, are exceedingly expensive. Civilian transports never have such items for this reason.
Jump computers, while powerful at number-crunching, and equipped with some of the best timing circuits ever devised, are actually very cheap. Computing power is cheap, and the calculations are relatively simple; there are simply a lot of them. The timers are based around simple counters, so they are also very cheap. After the field generators and the jump capacitor, the jump computer is the most valuable system on the ship. Without it, the ship cannot safely jump - it is likely that the ship will emerge on a collision course with a star or planet, if it even comes close to the target system at all.
The computations required for Jump are very complicated, and are based on empirically derived coordinates and path integrals. This makes the accuracy of the computations, and hence the Jump, highly dependant on the reliability and accuracy of the coordinates of the target destination, jumpspace entry point, and intervening gravitational influences (large masses). Since the jumpspace entry point can be directly scanned by a ship before commiting to Jump, the accuracy of those coordinates is very high. Accuracy of the intervening masses and the target coordinates can vary widely, depending on how frequently the region is travelled, and the conditions of the destination system. This one of the reasons grav gates are so valuable; grav gates can create stable, well-known grav coordinates for any system.
During a Jump, the ship is completely under computer control. The computer is responsible for initiating transitions, timing the jumpspace transit, and beginning deceleration after the return to normal space. During jumpspace transit, the computer continously adjusts the grav drive to maintain jumpspace.
A single Jump can be thought of as having several phases:
The approach is only begun after calculations are completed. Transition velocities are typically 0.2c or higher, and so it takes several hours to acheive the necessary speed. For a ship with an acceleration of 19.6 km/s2 and starting from rest, it takes 51 minutes and 91,823,265.8 km or ~0.5 AU. To compute the acceleration time and distance, use the following formulas:
v
t = - or t = (0.2*3x105 km/s) / (19.6 km/s2) = 3061 s or ~51 minutes
a
a*t2
x = --- or x = 0.5*(19.6 km/s2)*(30612) = 91,823,265.8 km
2
Note that the acceleration must be in km/s2 (thousands of g * 9.8 m/s2) and the time is initially calculated in seconds.
When a ship "activates" its Jump drive, it is really dumping energy from the capacitor into the ship's field generators. The pulse of energy vastly increases the strength of the PF, causing it to actually rip space-time. This rip has geometry identical to the PF of the ship. The ship then passes through the rip, transitioning from normal space into jumpspace. The rip, without external energy input, will quickly collapse. The lifetime is proportional to the energy and size of the rip. Jump drives are designed to provide a lifetime of 10 µs or longer. For a ship travelling 0.2c, a lifetime of 10 µs allows for 600 m to pass into the rip. If the ship is less than 600 m in length, it is fine. Otherwise, the ship must increase the lifetime of the rip or the transition velocity of the ship. Since acceleration of a ship is often more limited than the power of the capacitor, the normal solution is to increase the lifetime while maintaining a transition speed of 0.2c.
During the transit of jumpspace, the ship itself does not gain or lose speed - the operating grav drive maintains the jumpspace, and the ship continues at the same velocity as at transition. Hence, the distance travelled can be determined by simply varying the time spent in jumpspace. No jump computer will make a jump that requires more than 10 s in jumpspace. Ships that have spent longer have not come back.
Due to slight irregularities in the compression ratio of jumpspace, inaccuracies in timing, and inaccuracies in computations, the maximum reliable distance of a single Jump is ~100 ly. Beyond that, the chances of the ship and crew surviving the Jump and arriving at the target destination quickly fall off. Jump computers will not accept coordinates for jumps over the maximum listed range of a ship. This is a fundamental safety feature that is very difficult to override or bypass, either intentionally or accidently.
Jumpspace, unlike the early Science Fiction accounts of "hyperspace", is not a separate reality, dimension, or space. It is, instead, a region of highly distorted normal space. The region is so distorted as to be imperceptible to anyone outside it, and in fact nothing passes the boundary of the region; that which is inside stays inside and that which is outside stays outside. To "cross between" jumpspace and normal space requires the intense fields of a Jump drive, which appear to observers to create a rift in normal space-time (realspace), which the ship crosses into jumpspace. However, the rift analogy is limited as there is no external, independant jumpspace; jump drives create a jumpspace when activated.
A jumpspace is created by the enormous field generated by the pulse of a Jump drive. When the ship leaves jumpspace at the end of a Jump, the jumpspace collapses and ceases to exist. As anyone who has seen a ship transition will state, the creation and annihilation of jumpspace is a spectacular event. Note that simply turning off a jumpspace without a correct transition back to realspace will lead to jumpspace collapse with the ship still inside, which destroys the ship.
The creation of a rift requires large amounts of energy, which is dissipated in 2 ways: a gravity wave and an electromagnetic pulse. The gravity wave is a single large transient ripple in the gravitational field. It is omni-directional, and propogates at the speed of light. The intensity decays as 1/r2, so it is difficult to detect beyond a relatively short range. Within a few meters, it is intense enough to be visible as a slight bending of light. The electromagnetic pulse is also omni-directional, and is broad-spectrum; uniform power is dissipated at all frequencies from DC to cosmic rays. The intensity also falls off as 1/r2. For both the gravity wave and electromagnetic pulse, the intensity is below detectable levels by 30 AU.
The rift to transition between normal and jumpspace has a very short lifetime, typically around 10 µs (10 millionths of a second). Because of this, a ship must be moving quickly for the entire ship to transition to jumpspace before it collapses. Any part of a ship still in normal space when the rift collapses will be left behind, typically resulting in destruction of the ship.
Given a rip lifetime of 10 µs, a ship travelling 0.2c can move 600 m before collapse. Hence, the maximum length of a ship with a transition velocity of 0.2c is 600 m. Longer lengths require higher transition velocities or longer-lived rips. For safety reasons, it is preferred to increase the lifetime of the rip, rather than increase the transition velocity. Rift lifetime is dependant on 2 factors: radius and energy.
The relation between energy, radius, and lifetime is:
E1/3
T = C * ---
r3
where T is the lifetime in seconds, E is the energy in Joules, r the
radius in meters, and C is a constant. The constant, which can be
empirically measured, is taken as:
C = 4.03x10-5 m3*s*J-1
Using this relation, a lifetime of 10 µs for a rip 105 m in radius (the
maximum practical limit) requires 2.37x1016 J = 23,700 TJ.
This is equivalent to the energy produced by a 39.19 GW power plant
running continously for a week.
Note that the lifetime scales as the cube root of energy and inversely with the cube of the radius. This means that assuming a fixed lifetime, a small increase in radius requires an imense increase in power. For example, for a radius of 105 m, but an energy input of 24.2 TJ (almost 1000 times less than above), the lifetime is 1 µs, only 1/10 of the above value. So an increase by a factor of 10 in the lifetime raises the power requirement by a factor of 1000 = 103.
There is a minimum energy required to create a rift and jumpspace. This means there is a minimum lifetime for a rift, typically on the order of a few nanoseconds, that is set by the radius of the PF, and the minimum energy to create a rift. This energy is determined by the local gravity gradient. High gradients reduce the energy required to rip normal space. Hence, it is desireable to transition in areas of high gradients. The highest gradients in a system are next to planets and stars. Since there is an unavoidable inaccuracy in the final transition point, and transitioning into a star or planet is instant death, these locations are virtually never used as destinations.
Rather than attempting to arrive near planets and stars, targets that are well out of the planetary orbital plane and still near the system primary are chosen. By convention, two areas are defined that are directly in line with the orbital axis of the planets, and several million (or billion) km from the center itself. These areas, called the "zenith" and "nadir" Jump points, are typically empty of major debris (which is confined to orbital planes) and yet still in areas of high gradient. Common agreement reserves the "zenith" Jump point for arrivals and the "nadir" Jump point for departures. Well-developed systems will often have space stations near the Jump points to service and monitor traffic.
Other Jump points are certainly possible, and military and exploration cratf frequently use them. Military vessels wishing to avoid detection will often Jump into the very edges of a system, well beyond planetary orbits. Jump signatures are more difficult to detect at such distances, as are stray emissions. Exploration vessels are often operating with very crude grav coordinates, and hence are exceptionally careful in their choice of targets. As both types of vessels typically have powerful Jump drives, the low gradients are not a significant problem.
For civilian vessels, low gradients can be very limiting. Many civilian vessels have relatively weak Jump drives, to keep the construction cost lower. Also, high gradients require less energy from the Jump capacitor to transition; hence recharge times are reduced, and the number of cargo runs can be increased, increasing profits. Grav gates can greatly increase the local gradient, vastly reducing the energy (and hence fuel and time) cost of performing a Jump.
During the rifting and creation of jumpspace, enormous gravitational forces are in effect in a very small region of space. Any matter or energy nearby the ship entering jumpspace will often be pulled along. Typically, anything within 1000 m of the Jumping ship is at risk for being pulled along. Anything pulled along that doesn't fully transition before rift collapse is cut into pieces, and typically destroyed. Anything which fully transitions can tag along with the ship through jumpspace, theoretically remerging at the second transition. The kidnapped vessel need not maintain its own jumpspace, so long as it remains near the Jumping vessel; the grav drive of the Jumping ship maintains the jumpspace. Any kidnapped vessel that strays more than a few jumpspace radii from the maintaining grav drive risks jumpspace collapse (unless equipped with a suitably powerful grav drive itself). Even if the kidnapped ship has a grav drive capable of maintaining jumpspace, if it lacks a Jump drive, it may not be able to transition back to normal space; it would be stranded in jumpspace.
Some fighter carriers have been known to take advantage of this fact when entering hostile systems - the fighters are deployed in tight formation just before acceleration, closing within meters of the main hull. The grav drive then accelerates the carrier and nearby fighters uniformly, performing the Jump, and returning to normal space in the hostile system with fighters already deployed and ready to fight. During the few seconds of jumpspace transit, fighter pilots are exceptionally careful to maintain proper spacing so as not to be left behind in jumpspace.
Each jumpspace is unique to the time and place of the initiating ship. For two ships to meet in jumpspace requires their paths to cross almost exactly. Such a meeting would be disasterous; assuming the drives could maintain jumpspace, the two ships would collide at a significant fraction of the speed of light, with no shielding effect to absorb the impact. The ships would be obliterated, followed shorly by the collapse of the jumpspace itself.
Jumpspace collapse is the great fear and horror of crews of starships. There is no known case of a ship surviving jumpspace collapse. In fact, it is currently unknown where a ship in jumpspace goes when it collapses; all that is known is no part of the ship returns to normal space. For this reason, there are numerous safeguards to insure the existance of jumpspace while the ship occupies it. These include the safety limits in the Jump computer, and procedures for the actual Jump process itself.
Jumpspace is maintained by the ship that creates it. Upon entering jumpspace, a ship (or rather, the Jump computer) diverts power to the normal STL grav drive. The grav drive maintains the distortion induced by the Jump drive, allowing the ship to continue travelling at its transition velocity. As the energy of the drive is consumed in maintaining the local jumpspace, the ship does not accelerate; it maintains the same velocity as at transition. This is part of the reason for such high transition velocities. Moreover, the Shield effect of a grav drive does not occur in jumpspace; the warping of space entirely encompasses the ship, and anything inside jumpspace is unaffected by the primary field.
Without the operating grav drive, jumpspace collapses at about the same rate as the space-time rip used to create and enter jumpspace. This gives a ship with a failing drive approximately 10 µs to fix it before total collapse. Failsafe measures in the Jump computer would activate the Jump drive at this point in a last-ditch effort to regain normal space before total collapse.
Maintaining jumpspace, while relatively simple for a functional grav drive under normal conditions, can be very challenging. Even though the boundaries of jumpspace (which are the boundaries of the ship's PF) are impassable, jumpspace is still affected by nearby large masses. With the exception of black holes and neutron stars, there are no natrually occuring masses which are sufficient to noticeably alter a jumpspace. However, if two jumpspaces intersect, the energy required to maintain separate jumpspaces approaches infinity. The two jumpspaces will merge, becoming a single region. For two ships, this is as catastrophic as jumpspace collapse. The two ships would collide at a significant fraction of the speed of light, with no shielding to absorb the impact. However, the chances of two jumpspaces intersecting are effectively zero in open space. At grav gates and system Jump points, the chances are far greater. Various communication protocols and standards have emerged to minimize the chances to 2 ships intersecting while in jumpspace, but the exceptionally rare accidents have occured.
Although affected by immense mass nearby, a jumpspace is not detectable by any known means. In fact, normal space is not affected by a passing jumpspace in any known way. A ship in jumpspace can pass through planets and stars without noticing. Black holes are likely to cause jumpspace collapse, although no ship has been brave enough to test this theory. Neutron stars, while requiring more energy to maintain jumpspace, are ignored by any ship with fully operational drives.
The utility of jumpspace comes from the compression of time and distance that occurs. The distortion of space-time is sufficient to allow a ship travelling well below c in its own reference frame to significantly exceed the speed of light in normal space. Surprisingly, the compression ratio is fairly constant. Slight anomolies occur due to large masses (stars, gas giants) along the path of travel. These anomalies are rarely more than 1-5% of the "constant" value of 1.57788x109.
A compression ratio of 1.57788x109 means that a ship with a transition velocity of 0.2c will travel 100 ly in 10 s. As 10 s is taken as the upper limit for safe transit of jumpspace, the maximum range of a typical jump is 100 ly. Ships with higher transition velocities can travel farther, up to a maximum of ~400 ly with a transition velocity of 0.8c. The practical upper limit is set by the maximum practical velocity. For most ships, this is at about 0.5c in an emergency, for a maximum single-Jump range of 250 ly.
Ships engaging in such long jumps are taking a tremendous risk; finite limitations on the precision of course, timing, and Jump path computations limit the precision of the final arrival position. Transitioning from jumpspace into a planet or star is a one-way ticket to death. The destination precision falls off with distance, from less than 1000 m at 100 ly (for very well known grav coordinates) to several hundred km at 400 ly. This are best-case estimates; when Jumping to systems with poorly known grav coordinates, the final position error could be off by a factor of 10 or more. In some cases, a final position error of millions of km has been encountered.
Jump accuracy is determined mostly by the accuracy of the grav coordinates used the path computations. If the coordinates of the destination and major gravity sources along the path are well known, exceptional accuracy can be achieved. For well-traveled areas of space, where grav gates maintain precise and stable destination coordinates, and large masses are continously tracked, position errors are rarely greater than a few meters for a 100 ly Jump. On the fringes of known space, where grav coordinates shift over time and large mass positions are rarely updated, position errors on the order of tens of km are common. For unknown space, position errors can range from meters to millions of km. This is the main reason exploration probes are used to gather initial grav coordinates of a system.
For those wishing to determine the time it takes to Jump from one point to another, it can be computed from the following formula if the distance is known. Also note that this is only the time of the Jump itself, not the acceleration to transition velocity. Standard practice calls for a transition velocity of 0.2c and a maximum range of 100 ly.
x = 50 * v * twhere v is the velocity as a fraction of light (e.g. 0.2 for 0.2c), t is in seconds, and x is in light-years (ly).
Computed grav coordinates, while accurate enough for the most cautious of Jumps, can be highly inaccurate. The only way to improve the accuracy of the grav coordinates is to directly measure the local gradients. Unfortunately, this requires a ship to Jump into the system and then map the gradients. This is precisely why exploration probes were developed.
With a map of grav coordinates (or a set of computed ones) and a simple geometric coordinate system (typically spherical), a Jump computer can make all the calculations necessary for Jump. Most of the processing power and memory of a Jump computer is devoted to computing and storing grav coordinates.
A map of grav coordinates can also be used to estimate the power requirement to Jump to a particular location. For safety reasons, a ship normally uses a full power pulse for transition, but those wishing to cut costs (or avoid detection) may choose to reduce the pulse. Accurate grav coordinates allow the computer to determine the minimum energy level that will achieve safe transition.
As a long standing convention, all ships, military and civilian, continually submit grav coordinate refinements to the central coordinate repository maintained by the Imperial Bureau of Standards. This repository is open to all friendly ships, and has little or no security features. Accessing the repository is free, and there is dedicated bandwidth on all FTL communication links for ships to update their local databases. Standard procedure for all ships entering a system with FTL communication capacity is to upload any new coordinates, and download the latest updates.
Accessibility of the database to unfriendly ships (e.g. rebels, aliens, etc.) is less certain, although it is believed that some rebel groups (Neemis Enterprises, Seven Worlds) have had or do have access to the Imperial database.
The inner edge of the structure is lined with field generators. These generators locally increase and stabilize the gravity gradient, allowing jump drives to transition to jumpspace with far less energy. The generators also stabilize the gradient, creating a set of grav coordinates that do not change with time. Hence, the calculations required to accurately reach the grav gate are greatly simplified.
Travel from one grav gate to another is far easier than between other systems. Despite the enormous cost to construct and run a one, grav gates typically pay for themselves in a few months of increased commercial traffic. Ships designed solely for gate-to- gate travel can be fitted with smaller jump drives, significantly reducing the cost of the ship. Moreover, the lowered energy requirement for the jumps lowers the fuel consumption of a ship, saving yet more money.
Grav gates are a common sight in the more developed systems. The most developed systems have 2 grav gates - one at each system Jump point. A gate also includes a control station that is physically separate from the gate itself. The station is in the plane of the gate, to minimize the chance of collision with a ship. The station has its own power and communication facilities, and has absolute authority over gate traffic. The gate itself is powered by large solar arrays or fusion plants. All grav gates include a full FTL communications relay.
Given the relatively small amount of space the grav gate occupies, controlling arrival and departure is critical to preventing lost ships due to collisions ore jumpspace collapse. For systems with 2 grav gates, this is simple - departures are allowed from one gate, and arrivals at the others. Arriving ships are pre-allocated an arrival time and position, and are expected to hold to it. Systems with only one grav gate need to pre-allocate both arrivals and departures, significantly lengthening the time required for clearance to jump.
Any ship that violates the traffic control around a grav gate is not only fined sums of money that make Guild corporations quiver, but the crew is often charged with felony criminal offenses. Unauthorized use of a grav gate is viewed as only slightly better than attempted murder.
Note that a grav gate is symmetrical about its axis, and that a ship can use the gate from either side. The approach lanes are actually hemispheres, with a small disk cut out along the plane of the gate itself. Using this system, a jump in any direction is possible, with the exception of jumps directly along the plane of the gate. Because of this, gates are oriented with the plane in the direction of the least travel.
The probes are programmed with the computed grav coordinates of likely Jump points (based on remote sensing via stellar wobble, etc.) and then sent into the system. The probes have full Jump drives, with capacitors sufficient for 2 complete Jumps into systems with essentially zero gradient. Upon arrival, the probe scans the surrounding space, chooses the most likely high gradient region, and heads for it. After a short mapping flight where it records the local gradients of its path near the system, planetary positions, stellar type, and other useful information, the probe Jumps back to the (known) grav coordinates near the manned ship.
The ship then picks up the probes, analyzes the recorded grav coordinates and other information, and chooses a Jump point for itself. The ship can then Jump to the new system with greater confidence in the accuracy and safety of the Jump. Once in the new system, the ship can make an extensive survey of the local gravity gradients, scan planets (if any), and so forth. The ship then uploads the new grav coordinates, finishes charging the exploration probes, and does it all over again. Thus is known space expanded.
Occasionally, a probe is damaged or fails. Assuming such failure occurs in normal space, the probe will engage an emergency beacon, which the ship can use to locate the broken probe. Because of the beacons, the loss of probes is extremely small; typically less than 1% of those deployed.
A typical exploration ship will carry up to 15 or 20 probes, often deplying them to multiple systems at once. As it takes up to several weeks to fully recharge probes, a ship can examine multiple systems before a probe is ready to be deployed. Military craft typically have closer to 10 probes, and civilian craft may only carry 1 or 2. Virtually every starship carries at least one probe, as they can also be used for sub-light exploration.
Exploration probes carry complete sensor suites, making them valuable for exploring any region deemed hazardous to a manned vessel. They can also be used as FTL emergency beacons, as they can reach any system within several hundred light years. As the probe maintains exact grav coordinates of the deploying ship, any vessel happening across a probe can immediately Jump to the troubled ship.
FTL communications are based around a network of FTL transceiver stations. These stations form the backbone that carries all FTL traffic. Ships, planets, and systems send and receive all FTL traffic from the local transceiver station. These stations are capable of handling immense bandwidths, and incorporate conventional STL communication technologies (radio, laser, etc.) as well.
Each inhabited system has one or more transceiver stations. In stations with grav gates, the transceivers are part of the grav gate structure itself. There are also pure relay stations, which are in un-inhabited systems and are used as signal repeaters to increase the transmission range. These stations are virtually autonomous, with only occasional visits by technicians. All transceiver stations incorporate diagnostic and self-repair capabilities.
A typical station for a single system is capable of badnwidths in excess of 100 Tbps for each FTL transceiver. A typical station has at least 10 transceivers. Each transceiver is dedicated to a particular link, so a typical station can directly contact the 10 nearest other stations. Data travelling to other stations is forwarded with the destination address, and the other stations continue routing the information. In this way, any point on the network can contact any other point, albeit potentially slowly. The routing tables are automatically maintained, and can handle the loss of intermediate stations, so long as another possible route exists. For this reason, the network is designed to have at least 2 routes to every point. Every station is equipped to route data at full speed for all connections simultaneously; that is, a typical station can route >1000 Tbps.
In addition to the FTL transceivers, the station will also have standard STL communication gear. This includes numerous radio and laser antennas. These systems are used to gather the local system data for FTL transmission. Similarly, data addressed to the local system is broadcast via the STL gear. Typically, the laser equipment is used for long-distance or high-bandwidth dedicated connections, such as from the main commsat from a local planet. The radio gear is used for short-haul transmission (i.e. to the nearby grav gate control station) or temporary connections, such as to a ship. Note that a typical station will always try to keep at least one FTL transceiver idle, to allow for emergency communications from starships.
Virtually every starship contains a single FTL transceiver. Although not capable of real-time FTL relay, the ship can at least communicate with nearby FTL stations. The transceiver has full bandwidth, so large amounts of data can be transmitted in a very short time. Ships that need to transfer massive data sets (such as returning exploration vessels) will often use the FTL transceivers in-system, just to increase the data rate. Highly-developed star systems also maintain extra laser communication facilities for high bandwidth communications with ships.
FTL transceivers are modified Jump drives. Rather than creating a jumpspace for a ship to transit, the transceiver creates a sustained, stable jumpspace. When two transceivers are closely aligned, their jumpspaces merge, forming a single point-to-point jumpspace tunnel. Lasers are then fired down the jumpspace, emerging on the other side to be received and interpreted. With multiplexing of single frequencies, and the use of multiple frequencies, the bandwidth of such a laser connection exceeds 100 Tbps.
The compression effect of jumpspace allows the laser beam to traverse 100 ly in ~2 seconds. This introduces a fixed, but relatively large, lag for FTL communications. Hence, interactive communications (i.e. video conferencing) are difficult to achieve beyond a few ly.
The two transceivers must remained very closely aligned, as any deviation threatens to separate the jumpspace corridor, breaking communications. The tranceiver antennas are continually adjusted to maintain the connection. This is termed maintaining lock. Since the antennas are simply very small PFG sets, adjustments are performed by 3-axis motion-control systems with very tight tolerances.
The antenna of a FTL transceiver is, as noted above, a very small set of PFGs. Because the station is not transiting the jumpspace, the minimum size restriction of ships does not apply. Hence, to reduce power requirements, FTL transceivers use very small radius jumpspaces; typically 2 cm.
A radius of 2 cm and a lifetime of 1 s require a power input of 7.8 mW. This means that even for large stations, the power requirements are relatively tiny - <80 mW for 10 transceivers. The major difficulty in mounting numerous transceivers is that each requires a stable gravity gradient to maintain lock. Placing large number so transceivers nearby one another creates enormous gradients that can quickly shift, causing frequent loss of lock. To reduce the minimum spacing of transceivers, gravity "shielding" can be installed; these are FGs that smooth the local gradient independant of the transceiver. These generators require a lot of power. It is not practical on typical stations to mount more than 10 transceivers, as the power and space requirements grow too large.
Stations are powered by solar arrays or fusion plants. Solar arrays are preferred, as they do not require fuel or much maintainance. However, some relay stations are in systems with small stars where solar arrays are not practical. These require fusion reactors and very large fuel supplies.
The maximum range for a FTL transceiver is approximately 100 ly. Longer distance require smoother local gradients to maintain lock, as any deviation can be greatly magnified by the distance. Hence, it requires more power to transmit farther. There are some cases where a ship with an operating grav drive (to smooth the local gradient) was able to maintain lock over 300 ly away, but this is uncommon. Stations on grav gates are typically able to maintain lock for at least 150 ly when contacting another gate-mounted station. Stations in systems with numerous large planets are often only able to maintain lock for 80 ly due to the highly variable gradients.
A Jump detection system is a network of automated listening posts that are scattered throughout a system. Each post has an extensive set of gravity and electromagnetic radiation sensors. Each post continously reports its sensor readings to a central command station. This station is responsible for processing the sensor readings and determining which are the results of nearby Jumps.
When a ship undergoes transition within detection range, the posts of the JDS will record the coupled gravity and EM pulse. The central station can then use the differences in arrival times at each post to compute the transition point of the ship. Note that this only works for transition points close enough that the arrival times at different posts are not simultaneous. Automated software in the command station rejects simulatneous sources, as these are assumed to be distant nova and other natural sources. It is possible for a transition to be mistakenly rejected, but only if the transition is extremely far away. The required distance is typically well beyond the rated detection range of the JDS.
Due to limits in the precision and accuracy of timing, gravity wave and EM pulse detection, and listening post positions, the maximum detection range for a JDS is ~30 AU. This is for the largest transitions; small transitions produce a smaller signal, and the maximum range is decreased. The very smallest Jump signatures have detection ranges more like 8 AU.
Because of the limited range of a single listening post, and the immense distances of space, a full system-wide JDS is exceptionally expensive. Only the most developed of systems (e.g. Sol) have full JDSs. Most systems have a limited JDS, with listening posts near the zenith and nadir Jump points, and a few scattered around the edge of the system. This allows for detection of most large Jump transitions, although the computed position is poor.
End Document 4365-12-rev2/2350-A Inquiries should be directed to Reprints, Technical Documents Division, Imperial Research & Development