Craig Berry wrote:
 > Reminds me of the drives in Iain Banks' Culture novels, which are FTL
 > but have similar speed limits based on a preferred rest frame.

IIRC, speed was determined by how well the ship could push against The
Grid that underlies 4-d spacetime.

Grid manipulation also enabled the enormously destructive Gridfire and
may have had something to do with effector field weapons.

 > But it can result in e.g. a ship going too fast to be able to engage
 > its maneuver drive successfully, which has some interesting plot seed
 > possibilities...

Yep.
Needing to borrow and offload momentum from massive bodies also adds
some quirks to reactionless drives.

Rupert Boleyn wrote:
 > As Traveller's FTL doesn't seem to break causality in the Trav
 > universe, I suspect there's a privaleged frame of reference out
 > there somewhere, and for now background radiation is the stand-in
 > IMTU.

It is a good candidate:
CMB temp (K) = 2.725(1 + z), where z is the redshift.

One attempt to limit/prevent near-c-rocks is in the rules from
Megatraveller(IIRC) onwards: M-drives get less efficient beyond 1000
stellar diameters.

Limiting the amount of thrust available has a number of effects:
- the distance at which it is quicker to microjump increases with
increased delta-v;
- the differential performance between 1G and 6G drives increases with
increased delta-v due to decreased coasting time;
- the power required increases steeply with increased delta-v.

Examples:
G-hours     delta-v,km/s     1 week distance, AU      6G/1G     MW/kg
                              1G      6G               [1]       [2]
5           176.53           0.352   0.356            1.01      0.216
10          353.05           0.692   0.710            1.03      0.433
20          706.10           1.342   1.413            1.05      0.866
40          1412.21          2.515   2.798            1.11      1.731
60          2118.31          3.517   4.154            1.18      2.597
80          2824.42          4.35    5.483            1.26      3.462
100         3530.52          5.013   6.783            1.35      4.328
160         5648.83          5.981   10.512           1.758     6.925
168         5931             5.995   10.99            1.83      7.271
340         12004            5.995   20.172           3.365     14.715
420         14828            5.995   23.729           3.958     18.178
1008        35588.45         5.995   35.97            6*        **

* The current situation in the rules, ignoring relativity-
delta-v is ~12% of c.
You could round up to 6AU per G thrust per week.
[1] - ratio of 6G to 1G one week travel distances
[2] - for 1G thrust, 100% efficiency at converting power to kinetic
energy, per kilo of vehicle mass.
** 43.626MW/kg for 1G thrust at 100% efficiency

Rob O'Connor