Research on fusion engine is AIAA Best Paper | Hacker News Reader

Research on fusion engine is AIAA Best Paper

uah.edu

53 pointssconklin19 comments

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

[http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2014001...]

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.

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...

> 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)

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.

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 spaceship, 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!

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

That much Δv at a decent TWR, too...

Stay out of the backwash though :-) The Dyson paper is excellent. Unfortunately, for now at least, it is hard to launch a nuclear RTG without a lot of resistance, imagine how hard it is going to be to launch what is essentially a retriggerable hydrogen bomb into orbit. Does solve a lot of 'time of flight' problems, but adds a few shielding problems of its own.

Project Orion had much more thrust than this proposal but also came attached to a very heavy lifter plate and set of shock absorbers. That meant that the only feasible way to get Orion up to orbit was for it to fly there under its own power. That means that all the radioactive byproducts were going to end up in the atmosphere. Each bomb is tiny but you'll need a lot of them and it adds up.

By contrast this drive is designed to be used in space and once you're outside the Van Allen belt you're pretty much fine to use nuclear drives. The fact that there aren't any critical masses involved also means that you're not violating the Outer Space Treaty.

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

I don't think getting to Mars has ever taken years. The typical numbers given are 6-8 months, depending on where Earth and Mars are in relation to each others' orbits around the Sun.

More than halving the amount of time from 8 to 3 months is still impressive though.

> More than halving the amount of time from 8 to 3 months is still impressive though.

That also suggests lots of spare Δv that could be used for going places.

There is a related paper at:

https://www.nasa.gov/sites/default/files/files/Adams_2013_Ph...

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).

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

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

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.

I wonder if this could be made into a 'blaster' like in science fiction stories, here on Earth.

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

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.

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...

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

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

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