Like many space opera settings, the maneuver drives in Traveller are
capable of enormous amounts of thrust, far beyond the energy content of
the fuel allocated. Conservation of momentum and energy are long gone
unless momentum gets stolen from nearby astronomical bodies, vacuum
energy gets tapped, or some source of energy external to the spaceship
is otherwise drawn upon. Choose a handwave.

Space is big, so ships need to move quickly to move the story along.
But objects travelling at kilometer per second speeds have enormous
destructive potential.

The kinetic energy of an object moving at ~2.9km per second is
equivalent to the object's mass in TNT.

The kinetic energy of an object moving at 5% of the speed of light
(~14,990 km/s) is equivalent to the object's mass undergoing nuclear
fission, or about 20 kilotons TNT per kg.

The kinetic energy of an object moving between 10 and 15% of the speed
of light (~29,980 to 44,870km/s) is equal to the object's mass
undergoing nuclear fusion, or up to 180 kilotons TNT per kg.

Kinetic energy equivalent to rest mass is attained at 86.6% of the speed
of light (~259,620km/s) - the object's mass converted directly to
energy, as per E = mc^2 - a whopping 21.5 megatons TNT per kg.

The canonical workaround to limit the 'near-c rock problem' is that
thrusters lose efficiency away from gravity wells; 10 diameters for
contragravity and 1000 diameters for thruster plates. Beyond these
distances, thrust falls to 1% of the rated level.

The workaround doesn't work as most of the mass of a star system is in
the star(s).
The 1000 diameter limit for a Sun-like star is 9.2 astronomical units
(AU; one AU is the average distance between Earth and the sun or 150
million km).

Even for the smallest M-class dwarf, the thousand diameter limit is 0.92 AU.
So there is plenty of space to zoom about and accumulate a
world-shattering amount of energy.

In any case, no attempt is made to explain where the energy comes from
to give the spaceship the needed velocity.

* Working Towards a Solution
The ideal solution would add minimal complexity to whatever ship design
system one cares to use.

It shouldn't disrupt the official setting too much. So ships should be
capable of at least one week's worth of thrust, so that there is scope
for non-jump space travelling.

If an in-system destination takes more than one week's travel time by
conventional drive, then a microjump is the time-effective solution for
passengers and priority freight.

It would also at least nod in the general direction of conservation of
energy.

In 'High Guard', a ship's power plant and maneuver drive require 1% of
the ship's volume per power plant number for fuel. This gives an
endurance of 30 days.

In T5, 1% of the hull per month of power plant operation in fuel covers
everything except the jump drive.

For the versions of Traveller in between, the fuel requirement is based
entirely on that of the power plant and the maneuver systems have a
notional need for power e.g. 65 or 70MW per displacement ton of
contragrav/thruster unit in Megatraveller.

Now the fuel requirements listed for the power plants are very high.
If we assume proton-proton fusion:
6H -> 2He-3 -> He-4 + 2H + 2 neutrinos yields about 26 MeV or ~7.3
MW-day per gram H, or 7.3 million MW-day per ton H. Alternatively, 1
litre of liquid hydrogen is worth 1.4 megawatt-years of energy, one
cubic metre 1400 megawatt-years.

In Megatraveller, TL 15 fusion has a best case fuel consumption of 9L
per 18MW-hours or 0.5L per MW-hour - an implied efficiency of 1/(35 x
7.3 x 24) or 0.016%.

In Traveller: The New Era TL 15 fusion consumes 100L per 6MW-years or
6/1400 = 0.43%

Efficiency gets worse at lower tech levels.

Improved power plant efficiency means there's some fuel that could be
used for propulsion without dramatically screwing with existing design
systems.

If contragravity or thruster drives could convert fuel energy to kinetic
energy with high levels of efficiency, what would happen?

So kinetic energy of spacecraft = energy from fuel x conversion
efficiency factor

Total conversion of matter to energy yields ~1040.226MW-day per gram,
1040226 per kg, 1.04 x 10^9 per ton.

If we assume thrusters convert fuel to kinetic energy with 75%
efficiency then 720 G-hours of thrust requires a fuel mass fraction of
0.005, a volume fraction of 0.071 and a terminal velocity with 720
G-hours of acceleration of 0.086 of the speed of light - about 25,782 km
per second.

(A G-hour = 1G of thrust for an hour. So a 6G rated drive with 720
G-hours endurance can accelerate for 720/6 = 120 hours).

I chose 720 G-hours as an endurance of 30 days appears frequently in
Megatraveller. There is minimal effect on some of the designs given in
the 'Imperial Encyclopaedia' assuming 75% TL 15 fusion plant efficiency
- in fact there are fuel savings with some of them.

If we look at a wider range of potential accelerations, we get:

(with apologies for potential formatting glitches):

G-hours: acceleration potential in hours (days).

Fuel mass fraction: proportion of vehicle mass in maneuver drive fuel.

Fuel volume fraction: proportion of vehicle volume in maneuver drive fuel.

Terminal velocity: if all the acceleration potential was spent in
building up speed.

G-hours    Fuel mass fraction    Fuel volume fraction Terminal velocity, c

180(7.5)    0.0003    0.004    0.0215
360(15)        0.0012    0.017    0.043
540(22.5)    0.0028    0.040    0.0645
720(30)        0.005    0.071    0.086
900(37.5)    0.0078    0.112    0.108
1080(45)    0.0112    0.160    0.129
1260(52.5)    0.0154    0.220    0.151
1440(60)    0.020    0.290    0.173
1620(67.5)    0.026    0.365    0.194
1800(75)    0.032    0.457    0.216
1980(82.5)    0.039    0.557    0.238
2160(90)    0.047    0.670    0.259

* Conclusions

The values above are consistent with conservation of energy and
relativity. While they still enable the attainment of relativistic
speeds, there is no major impact on travel times etc. except for the
most distant objects in a stellar system.

The power levels required (~hundreds of terawatts) imply that drives
need to be very efficient radiators; neutrino radiation of waste heat
works as a handwave that could be applied to the comparatively tiny
power consumption of other systems.

Agility can be redefined as maximum thrust potential e.g. an agility-6
vessel has 6G rated drives, independent of endurance.

Most starships will have around 720 G-hours endurance, unless built for
military or exploration/rescue applications.

Short-haul vehicles (surface to orbit, out to 100 planetary diameters or
nearby satellites) could have 180 G-hours endurance for bigger payload
capacities.

Rob O'Connor