Project Timberwind

James Dewar, in his book The Nuclear Rocket, doesn't have much good to say about it.  He says they tried to re-invent the wheel.

In case you are wondering, Project Timberwind was an SDI program which was intended to make a nuclear upper stage like the Nerva, only better.  It failed and was canceled early in Clinton's presidency.

According to the pdf final report, the idea was to intercept ballistic missiles.  If that's true, it was a dumb idea.  Good thing it was canceled.  I could examine the pdf  in detail, but first impressions make it look like a foolish gold plated program that was destined to go nowhere.  At the moment, it doesn't seem worth the time to read the pdf report.

Claims were for it to have a "45" version and a "75" version.  Whatever is meant by those designations, I am not sure.  The weight specs are amazingly low and the performance exceedingly high.  Using Dewar's rule of thumb of 1 MW equals 50 lbf of thrust, looks like those were comparable in power to those in the Project Rover/Nerva days during the Apollo Era.  However, these were to have 1000 sec of ISP in a vacuum.  If Dewar is right, it could never achieve that kind of performance from a pebble bed reactor.  They'd have to run it too cold to keep it from sending the nuclear core out the nozzle.  Not good.

My impression is that this program wasn't vetted properly beforehand.