I am having trouble understanding the in-system space flight mechanics in the Union/Alliance universe.

It seems that the spaceships come from Jump at a moderate high fraction of lightspeed, which they get rid of via "dumps". They then use their main engines to do the rest of the work. So far, so good.

Unless I am very much mistaken, the ships do not have any compensation for acceleration, so they are restricted to a few tens of m/s2.

So, the velocities the ships can reach in a few hours or days are quite limited - around 0.6% of lightspeed in two days for 10 m/s2, reaching around 1 AU from a dead stop. Yet, the stories seem to indicate that events happen much faster than that.

Am I missing something here? Or did Cherryh play loose with her internal physics?


I've read all or nearly all of the Alliance-Union Universe books but I do not recall any detailed explanation for how all of it works. Therefore, I can't really cite support for my thoughts. However, I'll share what I think.

Cherryh may have worked out all of it and it may be internally consistent but I believe (like you) that she played fast & loose with her physics.

From the descriptions of in-flight mechanics and their affects on the crew:
1. Agree, there is no inertial compensation, crews feel all of the accelerations
Crews must be in acceleration couches during dumps, main burns, and docking maneuvers.
2. There is no artificial gravity
Ships rotate for artificial gravity but "down" switches to the thrust axis during main thruster maneuvers.
3. Thruster accelerations (STL drive) limited by ship thrust to mass ratio and human tolerances.
4. FTL accelerations are NOT felt by the crew as the FTL drive uses gravitational forces for acceleration and the ship and crew all fall towards the projected gravitational mass.
5. FTL drive can be used for some STL accelerations/decelerations.
6. I don't recall any discussion of the required distance from a star's mass that is required to use the FTL drive.
7. I do recall battles occurring a distance of many light seconds.
8. I recall it taking days between jump in and a ship's arrival at a station.

Using this information, I assume 30 m/s^2 acceleration (3 g) for 2 days and I get a distance of 447,897,600 km (about 1/2 the distance between Jupiter and the Sun) between break out point and a station. The delta V for this maneuver would be 5,184 km/s.

This calculated distance does not align with my mental image of the solar systems and ship breakouts. My mental image (I can't remember whether it says this in the books or just my imagination) is that breakout occurs around the distance of Uranus / Neptune which is about 10x the distance calculated above.

Only military vessels or unloaded freighters would be capable of 3g acceleration.


Human or Compact?

The two civilizations exist in the same universe (i.e. same fictional physics) and have some contact, but shipbuilding traditions differ. Hani ships have half a dozen crew, human ships have hundreds, Kif have thousands. They also differ in sustainable acceleration, armaments, ...

Early or Late?

Faded Sun is in the same setting as the historically earlier books. FS has a slightly different look-and-feel. (Note: Foreigner doesn't fit into the history, Morgaine probably does but it doesn't have ships.)

Probable Answer:

The vanes can not just "pulse" to slow an incoming vessel down, they can also "pulse" to accelerate a ship. There are some clues in Hellburner, early riders have no vanes so they cannot outrun their own ordnance, while a carrier can. Late model riders, as in Downbelow Station, obviously can keep up with a maneuvering carrier.

Problem with that: The braking pulses are mentioned all the time, the acceleration pulses are not clearly mentioned. Explanation: Incoming ships are faster than outgoing ships, and their pulses are more time-critical. An incoming ship rushes towards the inner system, and outgoing ship rushes towards the void. The stations don't worry if that pulse isn't on schedule. Also a case of Early Installment Weirdness (tvtropes link), compare again DS and HB.

Alternative Answer:

Ships do not have to reach very high velocities to jump, they have to clear the vincinity of the station. The first clue is the threat of the Pride to cycle her vanes while docked, thereby blowing herself and the station. The second clue is how ships jumping in "dirty" areas take along debris. Since we all know that even asteroid belts are quite empty, they must grab matter over a very large radius.

Problem with that: The Pride wouldn't have taken the station into FTL. Explanation: You need some outgoing velocity and some distance from the gravity well to go interstellar, otherwise the ship is sucked into the sun of the starting system, with messy results.

  • I suppose with some math and creative thinking an internally consistent set of rules could be established for the workings of these engines - and perhaps Cherryh has done this. However, IMO she just doesn't provide enough technical details to establish the whole set of rules. e.g. perhaps the speed one needs to reach is solar escape velocity. Not too high a velocity and it changes as you move away from the sun.
    – Jim2B
    Mar 23 '15 at 20:23

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