Research on fusion engine is AIAA Best Paper

> 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.

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

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.

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!