Traveller: The New Era
House Rules: Astrogation 101
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Date: Sun, 2 Apr 95 18:14:26 -0400
From: Derek Wildstar <wildstar@qrc.com>
Subject: House Rules: Astrogation 101
Here's a House Rules article that's been lurking in my files since
Thanksgiving. I finally finished it off this weekend, and thought
I'd run it by you folks before sending it to Kevin for TTC.
Let me know what you all think. (other than that my shoving 20,000
words of junk into your e-mail box is too much for one weekend).
-----8<-----
House Rules - Astrogation 101
By Guy Garnett
Thanks to John Bogan and Joe Heck for the calculations behind the tables.
"A ship at least 100 diameters out from all massive bodies, ... can make a
safe jump with no chance of mishap." -- T:TNE, pp.226.
"Track us throuh jumpspace? Not this ship, sister." -- anon. smuggler
The Traveller rules are clear on how this relates to a planet like Terra. But
mainworlds in Traveller aren't just found orbiting far from a pleasant main
sequence star. They can also be found circling gas giants, or in an inner
orbit around a dim star, or circling a supergiant star.
Traditionally in Traveller, the 100-diameter limit still applies in these
cases. Locations that are a hundred diameters from a world may well be deep
within the gravity well of a gas giant or large star. Starships must travel
to a point outside of these limits in order to execute a jump. In addition,
no matter where the jump destination is plotted, a starship will always exit
jumpspace outside of or at the 100-diameter limit of any massive body.
In addition, other aspects of the Jump drive make it possible, at least in
theory, to predict destination of a starship, if accurate observations of
its departure are available. While not perfectly reliable, a skilled
astrogator can make educated guesses that are accurate more often than not.
Game Use
Classic Traveller and MegaTraveller uses space vessels that have unlimited,
constant-accelleration drives. With such a drive, even interplanetary
distances are accessable, and given the week-long trip through jumpspace, other
worlds in a star system may be "closer" (in terms of travel-time) than worlds
in other star systems.
In Traveller: The New Era, the unlimited constant-accelleration drive has been
replaced with a drive that features limited accelleration. This has several
subtle game effects, one of which being that other star systems, only a week
away by jump, may be "closer" than other worlds in the star system.
The referee has a choice about enforcing the rule. By making explanations
about "critical density parameter" and "nonlinear gravity gradients", the
100-diameter rule could be waved for stars, gas giants, or both. There are
significant advantages and disadvantages to each approach.
If the 100-diameter rule is enforced, many systems will require long voyages at
sublight speeds before the starship may jump. This is a prime location for
raiders, blocaders, or corsairs, to intercept their prey. In the middle of a
long sublight voyage, the attacker will typically have the advantage of greater
maneuver drive performance, and higher fuel reserves, which allow it to force
an engagement. In addition, any prospective defenders have a large volume of
space to cover, allowing the pirate enough time to capture or loot the victim
and escape before the defenders arrive.
However, long sublight voyages can also be boring for players and referees, and
can considerably slow the pace of interstellar travel. If by slighly bending
the game rules, the referee can run a campaign more to his (or her) players'
liking, this should be encouraged. One referee with whom I played ruled that
a Jump took only three days. While an extreme change like this requires
considerable forethought, it can also help create a unique and enjoyable game
setting.
Stars
To determine the safe distance from a star at which to activate a jump
drive, a table of stellar diamters is required. For convenience, the
following table lists diameters in light-seconds (299,792.5km), and is
followed by a table which lists orbital distances in light-seconds.
Stellar Diameters in Light-Seconds
Spect Luminosity
Class Ia Ib II III IV V VI
B0 243 140 103 75 61 47 --
B5 350 163 93 47 25 21 --
A0 630 233 84 29 21 15 --
A5 696 257 65 21 13 8.4 --
F0 813 276 75 22 13 7.9 --
F5 953 280 84 24 12 6.3 5.3
G0 1392 392 117 33 12 4.8 4.8
G5 2120 598 173 51 13 4.2 2.6
K0 3054 1009 252 75 15 4.2 1.9
K5 4716 1831 579 196 -- 2.6 1.4
M0 6837 4002 1107 294 -- 2.6 1.2
M5 14103 9680 3325 1065 -- 1.7 0.49
M9 16339 13430 4346 1681 -- 0.94 0.25
Dwarf Stars
DB 0.084
DA 0.079
DF 0.061
DG 0.056
DK 0.042
DM 0.028
Orbital Radii in Light-Seconds
Orbit AUs MKm L-S
0 0.2 29.9 99.7
1 0.4 59.8 199
2 0.7 104.7 349
3 1.0 149.6 499
4 1.6 239.3 798
5 2.8 418.9 1397
6 5.2 777.9 2595
7 10.0 1495.9 4990
8 19.6 2932 9780
9 38.8 5804 19360
10 77.2 11548 38520
11 154.0 23038 76846
12 307.6 46016 153493
13 614.8 91972 306786
14 1229.2 183885 613374
15 2458.0 367711 1226552
16 4915.6 735363 2452907
17 9830.8 1470666 4905613
18 19661.2 2941274 9811033
19 39322.0 5882488 19621865
Example: A mainworld orbits a K0 IV star, in the habitable zone, orbit 2.
Orbit 2 is 349 light-seconds from the star; the safe jump distance is 1500
light-seconds, or just beyond orbit 5. Assuming the ship spends 30 G-Hours
of fuel, the journey to the jump-point will take nearly 5 days.
Example: Terra orbits Sol in orbit 3. Sol a G2 V star with a diameter of
4.67 light-seconds. The safe jump distance is 467 light-seconds, or slightly
inside orbit 3.
The following table lists the first orbit outside the 10-diameter limit (at
which a dangerous jump may be attempted), as well as the first orbit outside
of the 100-diameter limit (at which a safe jump may be performed). If a
world's orbit is inside (less than) the listed limits, consult the tables above
to determine the distance a starship must travel in order to attempt a jump.
Dangerous/Safe Orbit Table
Spect Luminosity
Class Ia Ib II III IV V VI
B0 6/10 5/9 5/9 4/8 4/8 3/7 ---
B5 7/10 6/9 5/8 3/7 2/6 2/6 ---
A0 8/11 6/10 5/8 2/7 2/6 1/6 ---
A5 8/11 6/10 4/8 2/6 1/5 0/4 ---
F0 8/12 7/10 4/8 2/6 1/5 0/4 ---
F5 8/12 7/10 5/8 2/6 1/5 0/4 0/4
G0 9/12 7/11 5/9 2/7 1/5 0/3 0/3
G5 10/13 8/11 6/9 3/7 1/5 0/3 0/2
K0 10/13 9/12 6/10 4/8 1/6 0/3 0/1
K5 11/14 9/13 8/11 6/10 --- 0/2 0/1
M0 11/15 11/14 9/12 7/10 --- 0/2 0/1
M5 12/16 12/15 10/14 9/12 --- 0/1 0/0
M9 13/19 12/16 11/14 9/13 --- 0/0 0/0
Dwarf Stars
DB 0/0
DA 0/0
DF 0/0
DG 0/0
DK 0/0
DM 0/0
Example: A bulk ore carrier calls on a gas mining station orbiting far from a
M0 II supergiant star. The mining station is way out at orbit 10, orbiting the
system's gas giant. Unfortunately, the safe limit lies between orbits 11 and
12. Referring to the tables above, the exact distance can be computed.
Example: A tramp freighter visits a frontier outpost on a world that orbits a
dim M5 V star in orbit 1. Checking the table above, the referee determines
that orbit one is more than 100 diameters from the star (in fact, the 100
diameter limit falls somewhere between orbits 0 and 1, while the 10-dimaeter
dangerous jump limit is inside of orbit 0).
Planets
A starship may be departing from, or bound for, the moon of another planet.
This is particularly the case in Traveller, where some habitable worlds are
the moons of gas giants. In such cases, the 100-diameter limit of the larger
world must be determined. Luckily for armchair astrogators, this is easier than
it is for stars, because of the IISS convention to number satellite orbits in
terms of the main body's radius.
In other words, a world orbiting a gas giant in orbit 35 is, by definition, 35
planetary radii (or 17.5 planetary diameters) out from the central world.
Therefore, the dangerous jump limit falls at orbit 20, and the safe jump limit
is at orbit 200. The following table lists the minimum, maximum, and average
planetary radii in kilometers, the diameter in light-seconds, and the jump
limits in kilometers and light-seconds, for all Traveller world sizes.
World Size and Distance Table
Size Diameter (Km) Avg Diameter Dangerous Safe
Code Min Max Km L-S Km L-S Km L-S
R 0 <1 0 0.0 10 0.0 100 0.0
0 1 199 100 0.0 1000 0.0 10000 0.03
S 200 799 400 0.0 4000 0.01 40000 0.13
1 800 2399 1600 0.0 16000 0.05 160000 0.5
2 2400 3999 3200 0.01 32000 0.11 320000 1.1
3 4000 5599 4800 0.02 48000 0.16 480000 1.6
4 5600 7199 6400 0.02 64000 0.21 640000 2.1
5 7200 8799 8000 0.03 80000 0.27 800000 2.7
6 8800 10399 9600 0.03 96000 0.32 960000 3.2
7 10400 11999 11200 0.04 112000 0.37 1120000 3.7
8 12000 13599 12800 0.04 128000 0.43 1280000 4.3
9 13600 15199 14400 0.05 144000 0.48 1440000 4.8
A 15200 16799 16000 0.05 160000 0.53 1600000 5.3
SGG 20000 59999 40000 0.13 400000 1.33 4000000 13.3
LGG 60000 119999 90000 0.30 900000 3.00 9000000 30.0
Note: The average sizes given for small and large gas giants may vary slightly
from that assumed by the Traveller: The New Era rulebook (average sizes were
not given). The averages above are the middle of the size range, and are
generally larger than corresponding worlds in Earth's solar system.
Example: A world orbits an average large gas giant in orbit 35. The
100-diameter limit is out at orbit 200, 30 light-seconds from the gas giant.
Working from the figures above, orbit 35 is 5.25 light-seconds from the planet,
and ships must make a 24.75 light-second journey away from the gas giant before
they may jump.
Example: Luna orbits the Earth in orbit 60, 1.3 light-seconds out. The safe
jump point (orbit 200) is 4.3 light-seconds away from Earth. Starships
departing Copernicus Warren must travel an 3 light-seconds farther out, before
they may jump safely.
When computing the distance a given orbit is away from the central world, it is
handy to use the safe distance in the following formula: dist=safe*(orbit/200).
This formula will work with either kilometers or light-seconds, and the answer
will be in the same units. In the Earth example above, the distance from Luna
to Earth is 4.3*(60/200), which works out to 1.29 (and compares quite favorably
to the measured average of 1.2741 light-seconds).
Other Objects
Objects such as nebulae, gas clouds, and other rare astronomical oddities will
only be encountered if the referee specifically creates them. As such, their
effects are entirely at the discretion of the referee. In general nebulae and
gas clouds will be sufficiently dense to prevent a ship from jumping safely
from within them, but will not affect a ship that is outside of the cloud.
Unusual and special cases could, of course, be an exception.
Relative Motion
The jump drive preserves a starships kinetic energy through jump. When a ship
emerges from jumpspace, it has the same absolute vector that it had when the
jump drive was activated. However, while the speed of the ship may remain
constant, the motion of the destination world will almost certainly not be the
same as the origin. The relative motion of the origin and destination star
systems will add to the aparrent residual velocity of the ship.
Each world will orbit its star at an orbital velocity determined by its orbit
and the central star's mass. Each star in the galaxy orbits the center of the
galaxy with a different velocity. A skilled astrogator will be able to
compute the most efficient jump entry and exit points, allowing a ship to
enter the destination system with whatever relative motion favors the ship's
flight plan. Rather than attempt to compute these velocities, an abstract
system will be used to simulate the effects.
The relative motion is 1d6 g-TURNS per parsec jumped (one g-turn is half a
g-hour). The total may be adjusted by the astrogator. The starship's
astrogator may subtract (or add, if for some reason a higher residual
velocity is desired) all or part of his skill level (not asset) from the
total of the dice. This represents the effect of plotting efficient jump
entry and exit points.
This roll can be made by the astrogator once the jump coordinates have been
computed (step 7 on page 225 of T:TNE). The final result (which may be
positive or negative) is added to the ship's residual velocity, if any (step
10 on page 226 of T:TNE).
Example: A free trader is jumping one parsec from Regina to Ruie. The
astrogator has skill 6 (an asset of 13). The one-parsec jump causes a
residulal velocity of 1d6. The die is rolled, resulting in 4. The
navigator uses his full skill level to reduce the relative motion.
4-6 is -2: the relative motion and good astrogation reduces the ship's
residual velocity by two g-turns.
Example: an ImperialLines type TJ courier is carrying a member of the
Imperial household on an urgent mission. The astrogator's skill level
is 8 (an asset 15 crack crew operates this ship), and is executing a
maximum-range jump of 6 parsecs. 6d6 rolls 21, and the astrogator
elects to use her skill to reduce the relative motion to 13 g-turns.
Even so, this ship will be arriving fast and hot out of jumpspace.
Tracking Ships through Jump
Relative motion and residual velocity considerations make the proper placement
of jump points important. In any given star system, there will be preferred
jump points for ships arriving from and departing to other star systems.
Predicting the destination of a starship is not an exact science, a good
deal of educated guesswork is required as well.
In order to attempt to predict the destination of another starship, an
astrogator must have available a record of the ship's velocity vector when
it jumped out, as well as the exact time and location of the jump. A sensor
lock-on will provide this information, and the required data can be easily
recorded for later analysis. In theory, the origin of a starship that jumps
insystem can also be determined by the same method. However, since the
exact time and location of an inbound ship's appearance can't be accurately
predicted, there is no practical way of determining the required data.
Tracking a ship requires an astrogation computer and 1d6x1d6x10 minutes of
calculations. Interpreting the results is an Uncertain, Difficult task using
Astrogation. The result is a two or more probable destination worlds. On a
result of No Truth, the destinations should be reasonable but entirely
spurious. For Some Truth, the actual destionation should be listed, while
for Total Truth, the actual destination should head the list.
The referee should adjust the difficulty of the tracking task to account for
additional circumstances: If the tracker has an astrogation computer at
least one tech level above that of the ship being tracked, reduce the
difficulty level by one. If the distance of the jump performed is known for
certain, reduce the difficulty by one level.
If a starship's astrogator believes that a tracking attempt will be made,
deliberately using an inefficient or unusual departure vector or jump point
can confuse trackers. The astrogator may add dice to the relative velocity
roll, up to a maximum of his skill level (not asset). Every two dice added
to the roll increases the difficulty of the tracking task by one level.
Example: A smuggler with an Imperial patrol cruiser hot on his tail jumps
his modified free trader one parsec from Regina to Ruie. His astrogation
skill level is 6 (asset 13). He elects to add all six dice, and thereby
increasing the Imperial's difficulty by three levels. Ordinarily, a free
trader's astrogation computer would be no match for the high-tech Imperial
warship, but one of this ships modifications is a new, military-surplus
astrogation system. The patrol cruiser does know that the free trader can
only jump one parsec, reducing the difficulty level by one. The final
difficulty level of the tracking task is increased two levels to Impossible.
However, the smuggler must roll 7d6 (which in this case, results in 30)
reduced by his astrogation skill level (to 24) for a whopping residual
vector when he arrives in Ruie. On the other hand, the patrol cruiser will
likely be in Jenghe at the time.
Accuracy
As was noted above, each system will tend to have preferred jump points that
are used by the majority of traffic bound to or from other nearby systems.
Starships using these points will have similar approach and departure
vectors as well. Both pirates and law-enforcement vessels find this
convenient - by patrolling near the jump points of a system, they can
intercept the majority of vessels using those points.
For purposes of space combat, particularly Brilliant Lances combat, the
referee should determine the world's standard jump points, designating the
hexes for each. These jump points will move with time, but their location
can be predicted by any astrogator and should be made known to the players
if they run the computations.
Ships exiting from jumpspace with an Outstanding Success
(step 10 on page 226 of T:TNE) appear in the jump point hex, their residual
velocity vector directly towards the planet, and facing as determined by that
ship's astrogator. Ships with ordinary success appear in that hex on a 1d6
roll of 1-3, or in a randomly-selected adjecent hex on 4-6. Ships whose
astrogators fail the roll have their jump exit points plotted at elsewhere
on the map (randomly or at the referee's discretion) outside the
100-diameter limit. The ship's facing is random on entry, and in neither
case should the ship's velocity vector point towards the world. In the case
of catastrophic failure, the vector should point very nearly away from the
destination.
Cagey captains can elect to use a less-efficient jump point, if they
can afford the maneuver fuel required to adjust their velocity vector after
arrival. An astrogator may select non-standard jump points when calculating
the coordinates for a jump. If non-standard jump points are used, the
astrogator's skill level may not be used to reduce the effects of relative
motion, and a course correction of 50% of this ship's current velocity
(including any relative motion) will automatically be required, exactly as
if the astrogration roll on exit from jumpspace was failed (step 10 on page
226 of T:TNE). If the jump exit point was plotted correctly, the ship then
emerges at any location outside the 100-diameter limit desired by the
astrogator.
Example: Terra's jump points would be 43 Brilliant Lances hexes from the
planet, and the normal location for arriving ships to appear would be a
seven hex area (the hex of the jump point, and the surrounding six hexes).
Most of the patrol ships (or pirate ships) would be able to bring the entire
lane, from Terra to the jump point, under fire. Even low-tech designs would
require only about three patrol ships per lane to cover the entire lane.
Many worlds, however, will not be able to to maintain even this many ships
on station, particularly in the New Era.
Time Spent in Jumpspace
When involved in a chase, the time spent in jumpspace can be important to
both ships. Because there is variation of up to two days, a ship which
jumps later than its quarry may arrive a couple of days earlier than the
ship it was following.
The table in Step 9 on page 226 of T:TNE can be used to determine the time
spent, or the following formula can be used where mere hours could make a
difference.
The total time spent in jumpspace is 140 hours, plus 8d6 hours.
This results in a transit time from 148 hours (6 days and 4 hours) to 188
hours (7 days 20 hours), with an average of 168 hours (7 days exactly).
-----8<-----
wildstar@quark.qrc.com
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"It's Science Fiction, if, presuming technical
competence on the part of the writer, he genuinely
believes it could happen." --- John W. Campell, Jr.