Thorium vehicle will run 100 years on 8 grams of fuel

[PamW]

44. Let's examine TOTAL mass..

muriel,

So let's examine the Total mass needed for a given amount of momentum.

Let m = reaction mass. Since p = mv; the reaction mass needed for momentum p is m = p/v

Let M = fuel mass. Let "e" = energy delivered to reaction mass per unit mass of fuel.

So for momentum p; the energy needed is p / ( 2/v ) = pv / 2

So the amount of fuel mass is pv / (2e); where "e" is the specific energy of the fuel ( including efficiency )

So the total amount of mass is the sum of reaction mass and fuel mass = p/v + pv/(2e)

So total mass per unit momentum is 1/v + v/(2e)

Put some realistic numbers in for "e" to see which of the two terms dominates; and unless you use something like nuclear power with high specific energy as your energy source; the fuel mass will dominate.

Let's also see how much momentum we can get from firing a large laser. The world's most powerful laser is the one at the National Ignition Facility in Livermore. The "footprint" of the laser is the size of about 3 football fields. A shot on NIF can produce up to 1.8 megajoules.

So with E = pc or p = E/c we have 1.8 MJ / c = 1.8e+06 km-m^2/s^2 / 3.0e+08 m/s = 6.0e-03 km-m/s

So there really isn't much danger that NIF is going to break loose from its footings with a puny reaction like that.

PamW