Nuclear Rocket Engine Idea

Long time back, I saw a TV show about the tests they were doing
developing technology for the X-30 NASP; one of these tests was taking a
piece of ceramic material and alternately dunking it in molten salt and
liquid nitrogen over and over again every few seconds to see if it could
handle the repeated thermal shock without failing; this hit me as odd at
the time, because I couldn't picture how it would be experiencing that
sort of treatment on the X-30 during a flight into space and back.
Then along came the revelation of the secret Air Force "Timberwind"
project to develop a nuclear rocket engine that was supposed to use
technology superior to the NERVA or Dumbo concepts, by feeding liquid
hydrogen through a particle bed of fuel pellets.
Part of this project was a test called "Project PIPE" which involved
bringing a reactor fuel element to repeated momentary criticality by
using a external neutron source, and seeing if it would survive heating
the propellant and cooling down again when a new batch of LH2 was added.
(it didn't work right - the fuel element began to break up after around
half a minute of operation).
At the time I suspected that this was somehow related to the NASP video
with the ceramic heating and cooling test, but couldn't figure out how
you were going to stick a neutron source on the particle bed reactor to
make it work in a pulsing mode, so there it sat.
Then today I remembered something about the wartime German nuclear
experiments; they were looking for a material to moderate neutrons, and
just like the US first tried graphite...but where we succeeded, the
German test flopped, because their graphite wasn't pure enough and
absorbed the neutrons rather than just slowing them down. So then they
switched to heavy water despite it being very difficult to acquire in
any great quantity.
But they tried something else too, although most people never heard of
it - they were working on a reactor design using dry ice as a neutron
moderator, though how far it got, I've never read.
Why dry ice? Because it's solid CO2, and the carbon in it could moderate
neutrons the same way graphite could.

So here's my rocket idea: You build a particle bed reactor and attach it
to a liquid CO2 supply; the liquid CO2 flows into the reactor and serves
as a neutron moderator, causing the reactor to go temporally
critical...which converts the liquid CO2 into superheated gas, which is
then expelled out of the rocket nozzle... but as soon as the CO2 is gone
the reactor returns to a sub-critical state, because it now lacks a
neutron moderator...until the next batch of liquid CO2 is injected into it.
This would neatly account for those "donut-on-a-rope" contrails that
were observed when everyone was getting interested in whatever "Project
Aurora" was. The nice part about using liquid CO2 as a propellant is
that it's easily and cheaply available, simple to transport and store
(i.e. fire extinguishers) and a lot more compact in volume by weight
than LH2. When Lockheed was working on their canceled LH2 fueled CL-400
"Suntan" recon aircraft design they were having a hard time disguising
deliveries of LH2 to the Skunk Works, and went so far as painting fake
tires on the LH2 trailers so that they wouldn't raise suspicion by
having fewer tires than one would expect for storage tanks that large
and presumably that heavy.

Pat

Pat Flannery wrote:
Long time back, I saw a TV show about the tests they were doing
developing technology for the X-30 NASP; one of these tests was taking a
piece of ceramic material and alternately dunking it in molten salt and
liquid nitrogen over and over again every few seconds to see if it could
handle the repeated thermal shock without failing; this hit me as odd at
the time, because I couldn't picture how it would be experiencing that
sort of treatment on the X-30 during a flight into space and back.
Then along came the revelation of the secret Air Force "Timberwind"
project to develop a nuclear rocket engine that was supposed to use
technology superior to the NERVA or Dumbo concepts, by feeding liquid
hydrogen through a particle bed of fuel pellets.

I'd be happy with just a simple restart of a NERVA type engine with modern
materials. For space applications we don't have to second guess ourselves when
we have something that was not only prototyped but demonstrated.

Project PIPE - NASP - reactor ceramics - wartime Germans
graphite/heavy-water/frozen-CO2? - liquid CO2...

I'd have to study the nuclear cross section of liquid CO2 to see if it really
is truly suitable as a moderator. There are obvious material differences
between both graphite and CO2, without some research I could not say that
liquid CO2 makes a good moderator.

But as John S. points out CO2 is not a liquid unless under pressure, so you
are minimally talking about a fairly sophisticated design that must take into
account a phase change in its normal operational cycle. Maybe it'd be easier
to use that pressurized CO2 in a heat exchanger and keep it pumping through
the core? OTOH we know how to do this already with normal water and even
liquid sodium!

My first paragraph not withstanding...

For a propulsion system, I think it might be better rather than cycle the
reactor during moderator phase change, to instead keep the flow continuous and
let the liquid CO2 expend its energy and phase change in an expansion chamber
just after the reactor core and before the nozzle. So essentially you have a
liquid CO2 heat pump, with the heater being the reactor and the cooler being
the expansion chamber/nozzle. This would be an open cycle concept, thus it
consumes CO2 as its propellant. OTOH why not do the same with just H2O and
steam, far easier to deal with than liquid CO2. Cannot liquid water also act
as a moderator? I seem to recall reading that with the proper mixture of
elements such as Boron, it can...

Speaking of modern materials, how well would ceramics work in this type of
set-up? I know steel suffers from neutron activation, but what about ceramics?
How does the half-life of irradiated ceramics compare with steel?

So here's my rocket idea: You build a particle bed reactor and attach it
to a liquid CO2 supply; the liquid CO2 flows into the reactor and serves
as a neutron moderator, causing the reactor to go temporally
critical...which converts the liquid CO2 into superheated gas, which is
then expelled out of the rocket nozzle... but as soon as the CO2 is gone
the reactor returns to a sub-critical state, because it now lacks a
neutron moderator...until the next batch of liquid CO2 is injected into it.
This would neatly account for those "donut-on-a-rope" contrails that
were observed when everyone was getting interested in whatever "Project
Aurora" was.

Whoa Pat, going a bit off the reservation there. I think that famous
"donuts-on-a-rope" contrails photograph that appeared in AvLeak can be
contributed to a study of a novel internal combustion engine with very few
moving parts, known as a Pulse Wave Detonation Engine or PWDE for short.

http://en.wikipedia.org/wiki/Pulse_detonation_engine

http://www.staynehoff.net/strange_stuff.htm

Probably a prototype that was flown and tried. Lack of further photographic
evidence suggests the trials were somewhat disappointing. I would suspect the
performance profile of such an engine leaves a lot to be desired. Maybe ok for
constant cruise at a constant altitude, but otherwise, seems iffy and
temperamental to me....

The nice part about using liquid CO2 as a propellant is
that it's easily and cheaply available, simple to transport and store
(i.e. fire extinguishers) and a lot more compact in volume by weight
than LH2.

And water even more so... If I were deviating from NERVA, I'd probably go that
route... But why deviate from NERVA?

Dave

On 7/19/2011 7:42 AM, David Spain wrote:

Pat Flannery wrote:
Long time back, I saw a TV show about the tests they were doing
developing technology for the X-30 NASP; one of these tests was taking
a piece of ceramic material and alternately dunking it in molten salt
and liquid nitrogen over and over again every few seconds to see if it
could handle the repeated thermal shock without failing; this hit me
as odd at the time, because I couldn't picture how it would be
experiencing that sort of treatment on the X-30 during a flight into
space and back.
Then along came the revelation of the secret Air Force "Timberwind"
project to develop a nuclear rocket engine that was supposed to use
technology superior to the NERVA or Dumbo concepts, by feeding liquid
hydrogen through a particle bed of fuel pellets.

I'd be happy with just a simple restart of a NERVA type engine with
modern materials. For space applications we don't have to second guess
ourselves when we have something that was not only prototyped but
demonstrated.

Both Timberwind and the little-known Dumbo engine design probably
deserve a second look; Dumbo was going to use a monolithic fuel element
pierced by a multitude of channels so small in diameter that the LH2
would get low-drag laminar flow as it passed through it; unfortunately
at the time the only way to make the channels was to run a thin sheet of
uranium through a pair of gears that would corrugate it. This led to too
big of channels to get it to work optimally, but today you could cut the
channels via lasers or even photoetch them with chemicals.



Project PIPE - NASP - reactor ceramics - wartime Germans
graphite/heavy-water/frozen-CO2? - liquid CO2...

I'd have to study the nuclear cross section of liquid CO2 to see if it
really is truly suitable as a moderator. There are obvious material
differences between both graphite and CO2, without some research I could
not say that liquid CO2 makes a good moderator.


Well, for starters you have oxygen involved, but that's the case with
heavy water also. Also, other carbon containing substances, like propane
and butane should be looked at for possible neutron moderation also.
Butane needs only a thin containment to remain liquid at room
temperature, and is also very low priced.

But as John S. points out CO2 is not a liquid unless under pressure, so
you are minimally talking about a fairly sophisticated design that must
take into account a phase change in its normal operational cycle. Maybe
it'd be easier to use that pressurized CO2 in a heat exchanger and keep
it pumping through the core? OTOH we know how to do this already with
normal water and even liquid sodium!


I was basing the pulse concept on the fact that when the reactor goes
critical, it's going to go super-critical for a second or two, dumping a
large amount of heat into the cold CO2, vaporizing it and forcing it out
the nozzle. This makes the reaction self-governing; the hotter the gas
gets, the sooner and faster it comes out of the nozzle...so you don't
have to worry about a meltdown and don't need rotating neutron
reflectors/control rods like in NERVA. It's a very simple engine design
that's throttled entirely by the new CO2 being injected into the
reactor, cooling down as it's released and expands.


My first paragraph not withstanding...
For a propulsion system, I think it might be better rather than cycle
the reactor during moderator phase change, to instead keep the flow
continuous and let the liquid CO2 expend its energy and phase change in
an expansion chamber
just after the reactor core and before the nozzle. So essentially you
have a liquid CO2 heat pump, with the heater being the reactor and the
cooler being the expansion chamber/nozzle. This would be an open cycle
concept, thus it consumes CO2 as its propellant. OTOH why not do the
same with just H2O and steam, far easier to deal with than liquid CO2.


Unless you're using heavy water as the propellant mass, water wouldn't
cause the reactor to go critical by moderating the speed of the neutrons
like a carbon-containing substance would.
Picture the thing like a graphite-moderated reactor where you can take
all the graphite out in a split second, leaving just the uranium fuel
rods with only high speed neutrons passing through them, unable to be
captured and start a chain reaction.


Cannot liquid water also act as a moderator? I seem to recall reading
that with the proper mixture of elements such as Boron, it can...

Boron will absorb neutrons, but I don't think it will moderate them.
We were looking at boron-containing plastic sheets as anti-radiation
armor on tanks in the early 1960's, as well as a lighter alternative to
lead for shielding reactors for tanks and aircraft (yes, they were
designing atomic-powered tanks in the late 1950's-early 1960's - the
light one used a reactor around the size of a oil drum that superheated
air to drive a turbine which made electrical power to drive it; the
heavy one was one of the most hilarius-looking thing you ever saw, like
a giant loaf of bread on treads that used either a pressurized water or
sodium cooled reactor to make steam to drive a turbine, like a miniature
atomic power plant. It had a separate reactor engineer walking around
inside of it, and probably would have stood around fifteen feet tall to
the top of the turret. I'll say one thing for the concept; it would have
been nearly invulnerable to attack, as no Soviet soldier in his right
mind would dare shoot a RPG-7 at it, for fear of what would happen next
if he hit it. :-D


Speaking of modern materials, how well would ceramics work in this type
of set-up? I know steel suffers from neutron activation, but what about
ceramics? How does the half-life of irradiated ceramics compare with steel?


If my speculation about those severe repeated thermal shock tests
regarding ceramics video I saw in regards to the X-30 NASP are correct,
that's just what they had in mind.. Timberwind was going to use fuel
pellets sealed in ceramic shells to power its reactor to prevent
radioactive particles being released into the air as it fired.


So here's my rocket idea: You build a particle bed reactor and attach
it to a liquid CO2 supply; the liquid CO2 flows into the reactor and
serves as a neutron moderator, causing the reactor to go temporally
critical...which converts the liquid CO2 into superheated gas, which
is then expelled out of the rocket nozzle... but as soon as the CO2 is
gone the reactor returns to a sub-critical state, because it now lacks
a neutron moderator...until the next batch of liquid CO2 is injected
into it.
This would neatly account for those "donut-on-a-rope" contrails that
were observed when everyone was getting interested in whatever
"Project Aurora" was.


Whoa Pat, going a bit off the reservation there. I think that famous
"donuts-on-a-rope" contrails photograph that appeared in AvLeak can be
contributed to a study of a novel internal combustion engine with very
few moving parts, known as a Pulse Wave Detonation Engine or PWDE for
short.



The guy who posted the photo went outside to see what was going on after
his radio monitor picked up traffic involving two aircraft calling
themselves "Gas Pipe" and "Dark Star"
A PDW aircraft would be able to take off under its own power, and not
need a carrier aircraft to ride on. A TSTO vehicle would involve two
aircraft; the big carrier being "Gas Pipe" and the orbital upper stage
being "Dark Star". What powers "Gas Pipe" is open to speculation; maybe
two or four SR-71 type turboramjet engines plus a tail-mounted rocket of
some kind to get it up as high as possible and around Mach 4-5.
Dark Star then comes off the back and heads to orbit using some sort of
pulsed engine like I described, the "donuts on a rope" being it on the
way up. PDW engines are suited to use in the atmosphere, unless they
carry an internal oxidizer supply.
The V-1's pulse jet was a very early PDW concept, and in that case the
lower it flew the more thrust it put out, because the thicker atmosphere
gave it more oxygen to explode with its fuel...it fired at a resonant
rate of around 45-50 pulses per second at full speed.
The contrail suggests something firing at a lot lower rate than that,
say around 1/2-2 seconds per pulse, which would be awfully bumpy for the
pilot if it was a PDW engine, but about what you would expect for my
concept of a nuclear pulse rocket engine where you have to heat the
propellant up in the reactor and eject it, and end the criticality
before injecting more CO2 or whatever.


http://www.staynehoff.net/strange_stuff.htm
Probably a prototype that was flown and tried. Lack of further
photographic evidence suggests the trials were somewhat disappointing. I
would suspect the performance profile of such an engine leaves a lot to
be desired. Maybe ok for constant cruise at a constant altitude, but
otherwise, seems iffy and temperamental to me....


Why you would be flying it over a town in Texas in daylight is a very
good question, and I would suggest a hoax if it weren't for the fact the
photo looks real, and PC's weren't all that good at photo manipulation
when it was taken.
The ideal place to test something like this is out over the ocean on a
overcast night. Note that the photographer didn't say anything about a
sonic boom associated with it, which means it wasn't flying
supersonically or was at one hell of a high altitude when the photo was
taken.
One criticism of the photo was that the "donuts" would have been huge in
size if it was flying at airline to SR-71 type altitudes, but if it was
way, way, up, the thin atmosphere would have allowed the backblast of
the engine to expand to great size almost instantly due to the terrific
pressure difference between it and the surrounding air.
At those altitudes (say 100-200 thousand feet) it would be beyond
conventional contrail formation height, but the ejected CO2 gas would
form into dry ice crystals as it underwent a huge pressure drop and
supercooled because of that.


The nice part about using liquid CO2 as a propellant is that it's
easily and cheaply available, simple to transport and store (i.e. fire
extinguishers) and a lot more compact in volume by weight than LH2.

And water even more so... If I were deviating from NERVA, I'd probably
go that route... But why deviate from NERVA?


It's clunky and heavy by today's standards; Timberwind was shooting for
a fully developed reactor about the size of an oil drum, with the thrust
output of a Saturn V F-1 engine.
You can do that, and the era of the rocketships has just arrived.
Single Stage To Orbit?
How about Single Stage To The Moon And Back?
Zowie!

Pat