Space Travel

It takes about (0.10 ¥ velocity in
yards/second)/(Acceleration in G) sec-
onds to reach a given cruising velocity.
A spacecraft moving at that velocity
takes roughly (0.5 ¥ distance in
miles)/velocity hours to travel a given
distance. For comparison, the moon is
around 0.25 million miles from Earth,
and Mars is 34 million miles away at
its closest approach.
Example: To accelerate to a veloc-
ity of 90,000 yards/second in a space-
craft with an acceleration of 1.5G
would take (0.1 ¥ 90,000)/1.5 = 6,000
seconds, or about 1.7 hours. At a
velocity of 90,000 yards/second, you
would
reach
Mars
in
(0.5
¥
34,000,000)/90,000 = 189 hours.
It is common to give interplanetary
distances in “astronomical units”
(AU). One AU is 93 million miles, the
average distance from the Earth to the
Sun. Interstellar distances are often
given in light-years (5.865 trillion
miles) or parsecs (3.26 light-years).
Earth’s nearest stellar neighbor, Alpha
Centauri, is 4.3 light-years away.
For a spacecraft that uses a
Newtonian reaction drive (e.g., any
real-life rocket), Top Speed is really
“delta-v”: the maximum change of
velocity it can perform before running
out of reaction mass (rocket fuel, etc.).
Each acceleration or deceleration
“costs” a fraction of this delta-v.
To lift into low Earth orbit requires
Move 8,700. To achieve planetary
escape velocity and leave orbit requires
an extra Move 3,600. For other planets,
multiply these velocities by the square
root of (M/R), where M is planetary
mass in Earth masses and R is plane-
tary radius in Earth radii. In addition
to having sufficient delta-v, the space-
craft’s acceleration must exceed the
planet’s gravity (1G, for Earth).
Travel through interplanetary space
requires using up the required delta-v
to achieve the desired velocity, coasting
as described above, then using delta-v
to slow to the velocity needed to enter
orbit at the destination.
Example: A spacecraft in Earth
orbit has a delta-v of 200,000. It uses
3,600 to break orbit and 90,000 to
accelerate to a cruising velocity (Move
90,000). It drifts at that speed for 1.5
hours to reach the moon, and then use
another 88,500 to decelerate to the
moon’s orbital velocity. Its remaining
delta-v is 200,000 - 3,600 - 90,000 -
88,500 = 17,900.
Some superscience space drives
don’t have to worry about delta-v – the
spacecraft can accelerate constantly!
The only requirement for such a
spacecraft to leave a planet is that its
acceleration exceeds the planet’s grav-
ity. When it travels long distances, it
requires time in hours equal to the
square root of (50.8 ¥ distance in mil-
lions of miles/Acceleration in G) to
complete the trip.
If a spacecraft is capable of faster-
than-light travel, its performance
depends on what kind of superscience
exists. The GM should design a drive
to suit his campaign. See the