Interstellar Travel

The Quick Summary

From orbit (which it may have reached by ascending from the ground on pseudogravity repulsors), the starship accelerates on thrusters toward the point where it will enter hyperspace, and decelerates to rest there. This onramp is along the axis of rotation of the local star, about 0.5 to 1 lighthour out (depending on the size of the star and the quality of the ship), so the trip will take a few days. Once in hyperspace, the trip to the edge of the system takes several hours, possibly as much as a day for very slow ships, and the inward trip at the destination system takes an equal amount of time, but the journey between systems rarely occupies more than an hour. At the destination system, the ship emerges from hyperspace and travels to the destination planet, either orbiting or landing once it arrives. The total time elapsed is on average about five days, but varies wildly depending not only on the length of the route but also the quality of the ship and pilot.

The Gory Details

Ground to Orbit

Like aircraft, near-orbit spacecraft use pseudogravity repulsors for propulsion. A repulsor is nothing more than a pseudogravity generator that produces a very large, very diffuse field, which by reacting weakly against the immense mass of a planet produces useful amounts of force on the generator without harmful or even noticeable effects on the surroundings. Although a repulsor field is very large, it is not infinitely so: thrust declines with altitude in a complicated but monotonic way and drops to effectively zero at about one planetary radius. This is well outside any reasonable safety limit for thrusters, however.

A Lengthy But Necessary Digression On The Structure Of Hyperspace

Hyperspace is somewhat complicated. To begin with, it comes in two flavors, bound and free. Bound, or fixed, hyperspace, is that associated with a star. The region of bound hyperspace around a star is roughly spherical, with a radius of approximately 4 lighthours times the square root of the star's mass (in solar masses); outside that limit is free hyperspace.

Free hyperspace is not just the bound hyperspace of the galaxy as a whole; it is qualitatively different. In particular, it is "fluid" enough for ships to enter it fully ("to accept hypermass converted from Einsteinian mass-energy" in physicist-speak).

Where bound and free hyperspace meet is a turbulent region called the hyperpause, which completely surrounds the star. Over the rotational poles of the star, the hyperpause extends inward to the star, in two cones of lesser turbulance called the onramps. The onramps are an intermediate form of hyperspace, which ships can enter, but where they cannot travel faster than light (and in practice must travel slower due to the turbulance. The width of an onramp at any point is between 1/8 and 1/4 the distance from the center of the star, depending on how fast the star spins.

In-System

Fast travel within a solar system is provided by "reactionless" thrusters, which actually react against the mass of the local star through bound hyperspace. (Thrusters also operate in free hyperspace; it is unclear what they are reacting against in that case.) The force provided is very large: modern spacecraft, with their crews protected by compensatory pseudogravity fields, accelerate at tens or hundreds of gravities.

The limitation of the thruster is that it cannot couple to the local hyperspace if their relative velocity differs by too much. The intrinsic velocity of bound hyperspace is very close to that of the star; free hyperspace has something like the average of nearby stars. The critical velocity depends on how well the thruster is tuned, which in turn depends on its basic quality, how well-maintained it is, and how large it is. Smaller thrusters are easier to keep in good tune, so permit higher final velocities, but provide less thrust and therefore lower acceleration for a given payload. Since multiple thrusters automatically have pathetic tuning, smaller ships are in general faster than large ones (given an equivalent drive:payload ratio).

A typical critical velocity, for a small freight or passenger ship with average maintenance, is about 4000 km/s (0.8 light-minutes per hour). Large freighters or dreadnaughts might go as low as 1500 km/s, while small one- or two-person scouts or couriers might reach 6000 km/s. Vessels with lots of manpower dedicated to continuous maintenance (such as military ships) can improve these figures by about 50%. Accelerations for civilian ships are usually on the order of 10-20 gravities (350-700 km/s per hour); military ships reach 50-150g (1800-5400 km/s per hour).

Although thrusters, like repulsors, require nothing but energy (easily provided by a fusor) to operate, they do produce dangerous amounts of high-frequency electromagnetic radiation and conspicuous numbers of neutrinos. This waste is emitted preferentially opposite to the direction of thrust, thus the actinic exhaust ports visible at the stern of most spacecraft. The x-rays produced pose a distinct hazard to anyone behind the ship, so most ports restrict the operation of thrusters near planets and inhabitated or important facilities.

Military ships often have extra equipment to redirect the emissions to where they hope enemy sensors aren't, but there is always some neutrino leakage.

Interstellar

Entering hyperspace is easier further out on the onramp, as measured as a fraction of the distance to the hyperpause. Modern ships can make the transition at somewhere between 1/8 and 1/4 the hyperpause radius, depending solely on the quality of their hyperdrive: ship size is irrelevent. For a Sol-sized star, that's 30 to 60 lightminutes, which takes 40-75 hours to cover at a nominal 4000 km/s (plus acceleration and deceleration).

Travelling along the onramp is always bumpy, but is much much worse and actively dangerous if the ship entered hyperspace with anything other than zero relative velocity; deceleration is not optional. A ship leaving hyperspace gets whatever velocity relative to local hyperspace it had with respect to hyperspace where it entered, which by the above is almost always very close to zero.

Regardless of the ship's normal-space velocity, travel along the onramp is limited absolutely by the speed of light and practically by the pilot's ability to use her sensors to detect impending turbulance. As a rule of thumb, a ship with good sensors and two competent pilots can safely make 0.5c, taking 6-7 hours to reach free hyperspace. Flying the onramp takes a great deal of concentration, so standard procedure is to have two pilots switch off. A ship with not-so-good sensors, or only one pilot, should stick to 1/3 or even 1/4 c, while a top-of-the line ship can reach 0.75c or even more with a sufficiently crazed pilot.

Once in free hyperspace, travel is very quick: modern ships can move at half a light-year to two or three lightyears per minute. Inhabited systems would be only a few minutes apart on straight-line courses. Sadly, such courses are rarely feasible.

Free hyperspace is decidedly non-homogenous. Large sections of it are filled with the same sort of turbulance that characterizes a system's hyperpause, and this turbulance is not even necessarily fixed in position. However, it does tend to stay within certain areas, or more importantly, out of certain areas, permitting safe passage along mapped routes. Unfortunately, modern technology does not permit safe or efficient mapping of hyperspace: the range of hyperfield sensors is not enough to detect rough hyperspace from farther away than it can expand to engulf a ship. With few exceptions, hyperspace maps are left over from pre-Imperial times, and generally consist only of known safe routes, rather than complete surveys of flat and rough areas. Even this fragmentary knowledge is not evenly distributed: although public knowledge includes routes between any two systems, there are many shortcuts and alternate routes known only to a few.

The prominent exception to this is the core of the Empire, a region approximately 80-100 lightyears across which is, with the exception of a few well-known and well-mapped obstacles such as Murcher's Wall and The Muyin Pit, is completely flat. Within the Middle Provinces, interstellar travel can almost always take the fastest and cheapest route, a fact not completely unrelated to the region's prosperity.

In Hyperspace

A hyperdrive requires a closed shell with certain properties, and affects everything inside that shell and nothing outside it; even objects in direct physical contact with the outside of the shell are not taken into hyperspace. In modern ships, the necessary shell is provided by the hullfield, so a hullfield failure will prevent hyperspace travel.

If the shell becomes nonclosed while in hyperspace, the ship is never seen again. Anything that leaves the shell with the hyperdrive (even though that requires a shell of its own to avoid falling under the previous sentence) is likewise gone for good. Physicists have come to blows over exactly what happens, but that it's not anything survivable is unquestioned.

Ships in hyperspace can detect each other, at a range of up to several light-minutes, and can communicate at low bandwidth by varying the characteristics of their hyperdrive fields. Since nothing can leave a hyperdrive field and continue to exist, hyperspace combat is effectively impossible. However, contact between two hyperdrive fields causes them both to fail. Safety mechanisms will usually cause the ships to fall back into normal space (probably with damaged hyperdrives), but a malfunction can cause one or both to vanish forever. Some warships carry missiles with individual hyperdrives to disable other ships, but this is very expensive.

The peculiar nature of hyperspace signalling means that it is ineffectual beyond a few lightminutes; interstellar radio is not possible by any techniques currently known, or even known to the Republic. Dedicated couriers that travel from onramp to onramp, delivering and receiving messages by laser to avoid having to travel to the local planet, can cut out most or all of the overhead time, but the speed of hyperspace travel is still the upper limit on the speed of information between systems.


This file was last modified at 1635 on 22Jun99 by trip@idiom.com.