so there might still be a net gain over the energy expended in triggering the fusion reaction.

The mechanical, heat, and friction losses in a conventional nuclear steam generation plant are pretty substantial, but they are feasible because no external energy source is required to start a fission reaction. The laser fusion prototype requires a huge input of energy to start the reaction.

Plus, with this sort of pulse fusion reactor there's the problem of containment of the explosion as this thing gets scaled up. How do you safely contain a hydrogen bomb, even a small one? How many cycles before the container vessel has to be replaced? What do you do with the radioactive container vessel? Some of the same waste disposal issues as a conventional plant.

We're still a long way from economically feasible commercial applications, and I know what's in the back of everyone's mind - we're still a long way from a pulse fusion reactor for space travel, although it may actually be a better application than electrical generation, as one could theoretically just hang a reactor way, way back behind lots of shielding with a rear-directed outlet (magnetic nozzle), to get a pulse reaction drive. A starship with such a propulsion system have been termed "torchships."


Theoretically, they are capable of sustained 1 g acceleration for many, many years. Long enough to get to a near-light speed velocity with a practical possibility of human flight to nearby star systems. Because time "slows down" according to the relativistic constant the closer one gets to the speed of light, one could travel enormous distances in a perceived period of only a few years, as illustrated by the following equation. 6.58 years (crew perceived time) to get to Vega (27 light years away):

From The Relativistic Rocket in the Usenet Physics FAQ
In the following equations, note that a*T/c = (Ve / c) * ln(R)
Time elapsed (in Terra's frame of reference)

t = (c/a) * Sinh (given acceleration and proper time)

t = (c/a) * Sinh<(Ve / c) * ln(R)> (to expend all propellant, given exhaust velocity and mass ratio)

t = sqrt<(d/c)2 + (2*d/a)> (given acceleration and distance)
Distanced traveled (in Terra's frame of reference)

d = (c2/a) * (Cosh - 1) (given acceleration and proper time)

d = (c2/a) * (Cosh<(Ve / c) * ln(R)> - 1) (when all propellant is expended, given exhaust velocity and mass ratio)

d = (c2/a) (Sqrt<1 + (a*t/c)2> - 1) (given acceleration and Terra time)
Final Velocity (in Terra's frame of reference)

v = c * Tanh (given acceleration and proper time)

Δv = c * Tanh<(Ve / c) * ln(R)> (given exhaust velocity and mass ratio)

v = (a*t) / Sqrt<1 + (a*t/c)2> (given acceleration and Terra time)
Time elapsed (in starship's frame of reference, "Proper time")

T = (c/a) * ArcSinh (given acceleration and Terra time)

T = (c/a) * ArcCosh (given acceleration and distance)
Gamma factor

γ = Cosh (given acceleration and proper time)

γ = Cosh<(Ve / c) * ln(R)> (given exhaust velocity and mass ratio)

γ = Sqrt<1 + (a*t/c)2> (given acceleration and Terra time)

γ = a*d/(c2) + 1 (given acceleration and distance)
where
a = acceleration (m/s2) remember that 1 g = 9.81 m/s2
T = Proper Time, the slowed down time experienced by the crew of the rocket (s)
t = time experienced non-accelerating frame of reference in which they started (e.g., Terra) (s)
d = distance covered as measured in Terra's frame of reference (m)
v = final speed as measured in Terra's frame of reference (m/s)
c = speed of light in a vacuum = 3e8 m/s
Δv = rocket's total deltaV (m/s)
Ve = propulsion system's exhaust velocity (m/s)
R = rocket's mass ratio (dimensionless number)
γ = gamma, the time dilation factor (dimensionless number)
Sqrt = square root of x
ln = natural logarithm of x
Sinh = hyperbolic Sine of x
Cosh = hyperbolic Cosine of x
Tanh = hyperbolic Tangent of x
The hyperbolic trigonometric functions should be present on a scientific calculator and available as functions in a spreadsheet.

In many cases it will be more convenient to have T and t in years, distance in light-years, c = 1 lyr/yr, and 1 g = 1.03 lyr/yr2.

Here are some typical results with a starship accelerating at one gravity.
T Proper time elapsed t Terra time elapsed d Distance v Final velocity γ Gamma
1 year 1.19 years 0.56 lyrs 0.77c 1.58
2 3.75 2.90 0.97 3.99
5 83.7 82.7 0.99993 86.2
8 1,840 1,839 0.9999998 1,895
12 113,243 113,242 0.99999999996 116,641

Of course, as a general rule starships want to slow down and stop at their destinations, not zip past them at 0.9999 of the speed of light. You need a standard torchship brachistochrone flight plan: accelerate to halfway, skew flip, then decelerate to the destination (which makes sense, since such starships will have to be torchships). To use the above equations, instead of using the full distance for d, divide the distance in half and use that instead. Run that through the equations, then take the resulting T or t and double it.

Example: The good scout starship Peek-A-Boo is doing a 1 g brachistochrone for Vega, which is 27 light-years away. Half of that is 13.5 light-years. How long will the journey be from the crew's standpoint (the proper time) ?
T = (c/a) * ArcCosh
T = (1/1.03) * ArcCosh<1.03 * 13.5 / (12) + 1>
T = 0.971 * ArcCosh<13.9 / 1 + 1>
T = 0.971 * ArcCosh<13.9 + 1>
T = 0.971 * ArcCosh<14.9>
T = 0.971 * 3.39
T = 3.29 years
That's the crew time to the skew flip. The total time is twice this
T = 3.29 * 2
T = 6.58 years