Last night after posting this thread, I sent an email to Dr. Forward asking him for help. He sent a reply to me this morning. Very cool. Below is my letter to him and his response.

My letter:

"Dr. Forward,

I was wondering if you could help me. I'm working on a science fiction short story grounded in hard science fact, and I need a question answered about propulsion.

In my story, the spacecraft is propelled by a beefed up form of nuclear pulse propulsion based on the proposed Orion craft. The pulse units are shape charged thermonuclear devices in the several hundred kt to 1Mt range. This ship weighs in at around 30,000 tonnes and is traveling at
0.2c. The crew of 200 is in suspended animation. I was wondering if such a system could propel a ship to that speed? And if not, what type of system could?

I've seen studies of the Daedalus project that talked of speeds of 10-15%c. Surely a Mt class Orion could beat that. Thanks for your time.

Lee,"

Dr. Forward's reply:

"Dear Brian,

For near-relativistic rockets it isn't the size of the bomblets that counts in determining the final velocity reached by the ship, it is the amount of mass of the fuel that is converted into directed kinetic energy in the fuel exhaust products that counts.

A fission bomb only converts about 0.5% of its mass into energy, and you are lucky to get 1/3rd of it into directed thrust. A fusion (thermonuclear) bomb only converts about 2% of its mass into energy (you will have to check those percentages yourself if you are going to use them) and again you are lucky if you can get 1/3rd of it into directed thrust. This DIRECTED kinetic energy (converted into equivalent mass) of the remainder of the bomb that hasn't been converted into energy, then needs to be compared with the total mass of the fuel to get a conversion efficiency e. This then needs to go into equation 11 in the attached paper to get the equivalent exhaust velocity w of the nuclear bomb fragments. Then that needs to go into the relativistic version of equation 10 to find out the mass ratio of the vehicle needed to get up to a speed of DV=0.2c.

For example, if we use fusion bombs with 0.02 of the mass converted into energy, and we are able to get half of it directed into thrust, then the efficiency is e=0.01, the exhaust velocity is w=0.14c, and the exponent in equation 10 is c/2w=3.54. If we want to reach a speed of DV=0.2c, then we need a mass ratio of 4.25, which is 3.25 tons of bombs for each ton of empty vehicle moving at 0.2c. If you want to stop, then the DV is 0.4c, the mass ratio is 20, and you will need to start with 19 tons of fuel for each ton of empty vehicle raised up to 0.2c speed, then slowed down again at the destination to zero speed.

So that is how you use the equations. Best of luck at finding a good combination that doesn't do much damage to either physics or your story. Remember, however, my mother-in-law's rule for good gossiptelling: "Never let the facts get in the way of a good story." You might want to consider antimatter-energized bomblets.

Bob,"

So it is feasible, but maybe I need to lower my cruise speed to 10-15% c, or go with the antimatter pulse units.

Lee, (I'll say it again, Dr. Forward is a very nice guy.)