

Research on fusion engine is AIAA Best Paper - sconklin
http://www.uah.edu/news/research/research-on-fusion-engine-that-could-be-a-mars-hot-rod-is-aiaa-best-paper

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Animats
Non-paywall version, direct from NASA: [1]

[[http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2014001...](http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140012884.pdf)]

It's a neat idea. It's really a pulsed fission reactor, pumped by a fusion
stage that's below breakeven but produces a lot of neutrons that can ignite
fission. It runs on U-238, depleted uranium, which won't fission by itself but
can be pumped into fissioning. This is somewhat similar to the process that
occurs in H-bombs, so it's well known nuclear physics.

Depleted uranium is about as hazardous as lead. Breathing or eating it is bad
for you, but the risk is heavy-metal poisoning, not radiation. It's very
heavy, so in water it will sink to the ocean floor. Launching over the ocean
is indicated.

This looks promising.

~~~
jhayward
Is the proposed use of Tritium practical in terms of supply? It's very
expensive to produce.

Also, there are significant impacts on space-borne science observation
missions from nuclear reactors in space [1]. It would be a shame if the
outcome was to render large sections of space opaque to observatories.

[1]
[http://www.calpoly.edu/~dhafemei/SciAm_June_1991_NuclearPowe...](http://www.calpoly.edu/~dhafemei/SciAm_June_1991_NuclearPowerSpace.pdf)

------
stephengillie
> We have developed an understanding of what propulsion systems enable
> missions to Mars in 90 days or less, and to otherwise reduce the trip times
> to the outer planets and beyond by a factor of three. - Dr. Jason Cassibry,
> Associate professor - Mechanical and aerospace engineering

Wow, this seems tremendous. The article itself is behind a paywall. (Abstract:
[http://arc.aiaa.org/doi/abs/10.2514/6.2014-3520](http://arc.aiaa.org/doi/abs/10.2514/6.2014-3520))

Before this article, I'd only dreamed of using nuclear propulsion for
interplanetary travel. TIL others have been researching how to make real this
technology before I was born. Finding diamonds like this among life's rough
sand dunes and rocky hills makes me wonder how many other gems are out there;
working theories for components that could be combined to build amazing new
devices.

And the idea of reaching Mars in months instead of years is too positive to
coherently express.

This must be what it felt like for travelers settling the American Old West
when the First Transcontinental Railroad was completed. Suddenly trips that
took months now took weeks, or even days. Cities that were out of reach were
now close enough to visit on vacation. Not to mention the benefits from trade
- food and other perishable goods (including viruses) could be transported
much further, and very large equipment went from being untransportable to
being only difficult to transport.

Surely, this will be like exploring the solar system in a covered wagon.

~~~
vilhelm_s
The book "Project Orion: The True Story of the Atomic Spaceship" by George
Dyson is really fascinating.

The Orion project was all done before the Apollo project was planned, so it is
kindof an alternative way the US space program could have gone. Rather than
launching dinky little space capsules, they were planning on launching
thousands of tonnes into orbit. The spaceship would be a literal space _ship_
, build similarly to how you construct submarines. The scientists involved
kindof assumed that they personally would get to go on a voyage to Saturn (why
not, there's plenty of space in the ship...). Compared to what we actually
got, with chemical rockets, it was way more impressive!

~~~
TeMPOraL
The simple thought of having that much Δv in a craft that heavy pretty much
blows my mind...

~~~
TheLoneWolfling
That much Δv at a decent TWR, too...

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curtis
There is a related paper at:

[https://www.nasa.gov/sites/default/files/files/Adams_2013_Ph...](https://www.nasa.gov/sites/default/files/files/Adams_2013_PhI_PuFF_inProgress.pdf)

I found the Executive Summary and Background sections to be pretty easy to
understand, at least for this layman. I haven't read the rest of the paper
yet.

TLDR, assuming I understand correctly: Imagine an Orion-drive spaceship
powered by tiny, tiny H-bombs where the chemical explosives have been replaced
by a magnetic z-pinch (a la the Sandia Z-machine).

~~~
lbearl
I'm guessing you just mis-typed, but I think you meant nuclear instead of
chemical.

~~~
bcmit
No, I think he's talking about the initial, chemical explosion that sets off
the fission + fusion part, at least in the old bombs.

~~~
curtis
That's right. Note that there are other Z-pinch fusion designs which use the
Z-pinch to directly trigger fusion reaction without going through a fission
intermediary.

------
Symmetry
In one sense that's pretty cool. In another it's hard to see where they're
going to get the energy to run the fusion generator from.

First some background. One of the main ways that you could get to Mars or
elsewhere fast is by burning more fuel than you need to to get where you're
going. If you're going to do that you don't want to have a humongous ship with
lots of stages so you want something with a higher exhaust velocity than a
normal chemical reaction.

But not too much higher. The energy required to power your engine is half the
product of the exhaust velocity and the thrust it produces. So if you're
constrained by energy because you're using electricity or the sunlight from a
concentrating mirror or something then you've got a tradeoff between thrust
and how much fuel you have to use. Too high an exhaust velocity and it takes
you forever to get up to speed. Too low and your ship gets too big and
expensive. The further you're going the less it matters that you take a long
time getting up to speed and the higher the power to weight ratio of your
power system the more you want to lean towards high exhaust velocity too.

For mars an engine with an exhaust velocity of 40 km/s like NASA's NEXT ion
drive will let you do a Holman transfer there and back for just 1/3 of your
mass as fuel. With a combination of drives and the most efficient modern solar
cells that'll take weeks to get up to speed but if you're going to carry a bit
more fuel or use aerocapture when you arrive you can you can make the trip
pretty fast.

Since this fusion drive isn't producing much more energy than it takes in, I
don't see why it would be better than an Ion drive. And you'd have to deal
with the heavy depleted uranium nozzle and all the shielding to protect the
crew from the radiation the drive gives off.

The fusion-fission thing is really clever though. D-T fusion gives off most of
it's energy in the form of a high energy neutron and it's hard to turn those
into useful thrust since their velocity is way too high. Using them to trigger
U238 fission gives you some decay products which are easier to use to heat the
greater mass of your propellant to a temperature that'll give you an exhaust
velocity in the sweet spot.

EDIT:

Oh, someone found an ungated version of the paper and it's cleverer than I
assumed:

 _Our phase 1 proposal discussed a pulsed fission-fusion propulsion system
that injected gaseous deuterium (D) and tritium (T) as a mixture in a column,
surrounded concentrically by gaseous uranium fluoride (UF6) and then an outer
shell of liquid lithium. A high power current would flow down the liquid
lithium and the resulting Lorentz force would compress the column by roughly a
factor of 10. The compressed column would reach criticality and a combination
of fission and fusion reactions would occur. The fission reactions would
further energize the fusion center, and the fusion reactions would generate
neutrons that promote more complete burnup of the fission fuel. The lithium
liner provides some help as a neutron reflector but also acts as a propulsive
medium, being converted to plasma which is then expanded against a magnetic
nozzle for thrust. The expansion of the (primarily) lithium plasma against the
nozzle’s magnetic field inducts a current that is used to charge the system
for the next pulse_

So they've got a story for how they're getting the electricity they need. This
could actually be really cool

~~~
Symmetry
Hmm, so they've simulated an ISP of 6,500. That's pretty close to what you'd
want. Man, it's really cool to see all those years of research into fusion
power create these technologies for use by something else. I wonder what the
potential for U238/D-T power reactors would be? Since you've still got fission
byproducts the main advantage is that you can convert a lot of the energy
produced directly into electricity via charged particle/magnetic field
interactions without having to go through a heat engine.

~~~
mng2
A fusion/fission rocket engine using a pinch seems pretty blue-sky at this
point, to be honest. Their paper[0] only briefly mentions the main problem
with cylindrical geometries: "With a successful Phase II award the team will
test at these varying power levels z-pinch processes to drive out the
progression of plasma instabilities in a z-pinch process." Pinches are not
very good at being stable, and they are what people tried before going to
toroidal geometry (tokamaks et al). Of course, for pulsed power you don't need
your pinch to be stable for very long, but at our current state of the art
it's breathtakingly inefficient.

Fusion/fission hybrids have been mooted for power production, but there are
several issues. First, why make your neutrons via fusion when that's
essentially the most difficult way to do it? Second, once you put uranium into
your reactor, you've got the same nuclear waste problem that you've got with
fission, mucking up your arguably-much-cleaner fusion process. So there are
significant downsides to a hybrid system.

0:
[http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2014001...](http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140012884.pdf)

~~~
Symmetry
Well, for a space drive it's really attractive because combining the fuel and
propellant lets you get around a lot of worries regarding things melting. The
big limiting factor around conventional nuclear thermal drives is that the
fuel wants to melt before it gets hot enough to give you the sort of ISPs you
really want.

You can have pure fission designs where your fuel is mixed with or only in
contact with your propellant like the gas core[1] or the nuclear saltwater[2]
rockets but those are in constant danger of exploding.

[1][http://en.wikipedia.org/wiki/Gas_core_reactor_rocket](http://en.wikipedia.org/wiki/Gas_core_reactor_rocket)

[2][http://en.wikipedia.org/wiki/Nuclear_salt-
water_rocket](http://en.wikipedia.org/wiki/Nuclear_salt-water_rocket)

