I think he's rather clear on pg. 53 under Assumption 2: The four Waves of Engine Development.

"I have made a solid case for resuming development of the B-4 core and not reinventing the wheel. My second assumption follows from this that a small engine based on the B-4 will be developed first, going through four generations before developing a large engine. I further assume this development will center first on its use to reach and return from LEO-breaking the taboo-and once achieved, then a small engine is developed for uses beyond LEO."

You can also follow the interview with Dave Livingstone THE SPACE SHOW


I personally discussed scenarios of launching of nuclear rockets from secure USAF-KSC sites for civilian purposes under the supervision of DOE-USAF the same people that launch RTG birds. None of the personal ever express to me this would be an impossibility. They said it was doable like any RTG launch and they would welcome the task. Knowing full well risks involved that's how confident they were at launching nuclear rocket boosters. Judging from the record they have for launching nukes to LEO they can have the job. 

If I were a decision maker they would be one of go-to organizations to launch any nuclear rocket to LEO the other organization would be the RSA and Russian Air Force secure facilities.

I also agree with Dr. Dewar you should be able to launch nuclear rockets from the launch pad or mid flight launch (ASPEN, DARPA RASCAL) and also perform in-orbit re-core and re-entry hot spent cores for study and quality assurance inspections on rocket engine reactor cores. Placing restrictions on a nuclear rocketry program is counter productive and places the program in jeopardy.

Of course I'm not the USAF or DOE LANL LLNL ORNL or NASA most engineers and scientist from these agencies in the field of study would opt for launch pad launches.

But for me it's stratagy and marketing any free civilian space program where investment is granted access you need to sell the concept first so the first two launches should be for a non-activated (at launch) mini fast reactor concept like the LANL one and the other concept a cis Luna in-orbit nuclear engine lunar shuttle.
After the public sees the value of  these nuclear space operations then ground launching programs have a easier sell toward the public.

Most of the public's knowledge about ground launches of nukes is FEAR.
Akin to vaccines. You mention the word 'vaccines' against disease to families and they react with fear about the vaccine even-though statistically the vaccine in 98% of cases is going to work to reduce the chances of serious life threating sickness.

Ive been thinking quite a bit on Jim Dewars concepts on using nuclear rockets to boost payloads into low earth orbit (LEO).  He certainly has an interesting proposal.  Im not so sure about dropping nuclear powered boosters out of C-5s.  It would seem to be that what Bruce suggests, launching them with somewhat larger SRBs or as an upper stage of rocket from the ground, seem to make more sense.  The re-core concept is really good and I like Dewars idea of what to do with spend nuclear engines. 

I dont agree that nuclear propulsion is the only way to advance putting payloads into LEO.  While the NASP was cancelled as was the X-33, I dont see any evidence that infeasibility was the reason.  In the first case the NASP was basically a victim of end of the cold war.  It was always more of a military space project than a shuttle vehicle.  The X-33 has some materials issues that resulted in a budget overrun a time in which sponsor of the program was having financial problems and was in the middle of a reorganization impacting the division in question.

I think that the basic idea of combined ramjet/scramjet/rocket in a SSTO vehicle is a clear competitor.  These vehicles would use air as an oxidizer and reaction mass through Mach 12 and then use internal LOX on into orbit.  These concepts are discussed in detail in the Czysz and Bruno book I mentioned in a previous post.   It seems to me that the practical problems with nuclear not to mention the irrational public fear militate against the ASPEN concept.

Dr. Dewar admits his cost numbers are not really substantiated but I think he tends to be very optimistic on costs.  There is more to a launch vehicle than just the engine.  His pre-ASPEN concepts are basically throw-away boosters save for the engines.  You still have the cost of the RVs to bring them back.  I think that NASP-like vehicles could ultimately be a low cost, reusable, and faster turn around systems.  That would leave nuclear for orbit onward missions not because of a taboo but mainly because of practical factors.