[ExI] Thermal nuclear rocketry

[Adrian Tymes]

The Department of Energy has finally given permission to discuss this in
public, after they published the award.  (Source:
https://science.osti.gov/sbir/Awards - FY24, Phase I, Release 2, search in
the spreadsheet for "CubeCab".)

Chemical rockets are barely capable of getting payloads into orbit,
requiring discarding the rocket's spent fuel tanks in stages.  It has long
been speculated that a hotter source would provide enough performance (as
measured by thrust and specific impulse) to get payloads to orbit without
needing to discard most of the launch vehicle in the process.  Nuclear
fission has been suggested but, until recently, the designs either did not
have enough thrust to overcome gravity, did not care about spewing
radiation all over the place, or suffered other technical challenges - not
to mention the legal ones.

Note that "until recently".

The DOE is in the process of funding a study, with me as principal
investigator, to modernize its MITEE engine design - originally for
in-space use - in light of recent technical and regulator developments,
including but not limited to:

* Commercial availability of HALEU and other nuclear fuels powerful enough
to give better than 1 G thrust while remaining short of bomb-grade.  (Yes,
even HALEU can be detonated with some work.  As most of the readers of this
list know, there are far cheaper and simpler ways than nuclear bombs to
destroy humanity.  The issue here is the laws: 20% is the legal limit baked
into a bunch of regulations.  Under 20% is far more feasible to use for
commercial purposes than over 20%.  The original MITEE design proposed
93.5%, which is among the reasons it did not see commercial use.)

* "Closed cycle" designs that contain the radioactivity while still heating
the fuel.  The exhaust is no more radioactive than chemical fuels'.

* Regulations that, as a side effect but still well within the laws as
written, provide a clear regulatory environment for licensing of a launch
vehicle with nuclear fuels.  (FAA licenses as normal with a more attention
paid to the environmental review, while NRC licenses handling of nuclear
material intended for use in space under its existing regulations for such
- written contemplating purely in-space use but technically covering this
case too.)

* Commercial feasibility of microlaunch, so as to make it feasible to prove
this out with relatively tiny launch vehicles - and thus, risking
relatively tiny amounts of nuclear fuel, not to mention a relatively small
development budget.  (This is, to certain policy makers, the biggest
development and the reason why my company CubeCab was selected.)

What has been funded for now is just a 6 month Phase I STTR to prove both
the technical and regulatory feasibility of the project.  Of course, the
real objective is to present a viable design, including schedule and budget
to make a prototype.
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