Thursday, February 03, 2011

Making Fusion Rockets Relevant

If you read the literature on fusion rockets you probably have a pretty firm idea in mind of what they're good for and when they'll be relevant - in "the future". No good fusion rocket paper is complete without a superconducting magnet here, and a magnetic nozzle there - in fact, these widgets are a primary ingredient of any fusion propulsion design and the more infeasible or untested they are, the better. This seems obvious: fusion rockets are the future because we don't have fusion yet.. right? Actually, no.

Producing nuclear fusion isn't all that hard. Amateurs regularly cobble together desktop fusion devices like the Farnsworth Fusor and other contraptions. The significant hard problem of fusion is getting more energy out of the device than you put into it. The current government backed effort to achieve this is the ITER project who are building a tokamak style device, but many other schemes are also being tried, with significantly less funding.

One of these is the dense plasma focus of hydrogen/boron fuel, a combination called focus fusion. The technique is relatively easy to understand. You take a metal chamber and put a single tubular electrode in the middle, ringed by a number of solid electrodes. Pump all the air out with a vacuum pump and then add the fuel until it is at a few torr. Dumping about two mega-amps of current into electrodes causes a plasma compression called the "pinch" in which nuclear fusion occurs. The result is a stream of electrons in one direction, a stream of ions in the other direction and a whole lot of x-rays, and virtually no neutrons. These happen to be the perfect products for producing electricity and if that's your goal, it means you can do it very efficiently.

The challenge of focus fusion is getting enough power into the device to burn the fuel - typically done with a big heavy bank of capacitors - and containing that heat in the plasma for long enough. Hydrogen / Boron 11 (or pB11 as it is often called) is the hardest fuel to get fusion going, requiring temperatures over 123 keV. As such, dense plasma focus fusion researchers tend to use deuterium instead, which only requires temperatures of 15 keV. The government program uses deuterium/tritium which only requires 13.6 keV, but tritium is a little hard to come by - it has to be made in nuclear reactors - and is strictly controlled. Deuterium can be picked up in rented bottles from your local gas supplier.

Using a dense plasma focus device to produce deuterium-deuterium fusion is pretty simple and requires minimal startup costs - especially if you do your homework and learn from the mistakes of others. Unlike pB11 fuel, D-D fusion produces neutrons. Shielding fusion researchers from neutron exposure is easily achieved with two things: distance and concrete. Measuring neutron output can be as low tech as looking for bubbles in a contained gel, and as high tech as CCD detection of scintillator stimulation. When you're producing neutrons you know you're achieving fusion.

Getting back to rockets, let's look back up at how I finished my first paragraph describing focus fusion: producing electricity [..] if that's your goal. While nuclear-electric propulsion sure is sexy, what if our goal is just to make a good old nuclear thermal rocket? Back in the 60s the US did a lot of great nuclear-thermal rocket work. They were using highly enriched uranium folded into a solid core with liquid hydrogen running through it. They got specific impulse in the 850 s (vac) range and had plans to achieve higher power before being defunded for obvious political reasons. So what might a nuclear fusion thermal rocket look like?


As our goal is to produce heat, not electricity, it makes more sense to use deuterium as our fuel. We only need to produce pulses of electricity to feed into the electrodes to produce fusion, and the most readily available technology to do that with sufficient power density is a compulsator. Much like an alternator, a compulsator is an electromechanical device that converts mechanical rotation to electrical energy in the form of alternating current. A high power rectifying bridge converts that to direct current to feed into the dense plasma focus. Compulsators have been built for railguns which produce more than enough current (and way more than enough voltage). I haven't read much on reducing pulse width (sometimes called "rise rate") of compulsators, but the ~2 microsecond pulses needed for dense plasma focus does seem challenging.

The rest of the rocket cycle is pretty standard. The expansion nozzle is cooled by cycling the fuel through it, this heats the fuel enough for a state change to occur and the expansion is used to turn a turbine which pumps the fuel, and finally the fuel is used to cool the core. The only difference is that the turbine serves double duty by turning the compulsator. A smart engineer will recognize that the rotors of the compulsator could be the turbine. Similarly, although all three components are shown schematically as being on the same drive, there most likely will be gearing involved to keep the pump constant.

Unlike a device for the production of electricity, the dense plasma focus will probably be made from copper. This will absorb the x-rays and transfer the heat to the "fuel" (aka, the coolant, traditionally the propellant-which-isn't-an-oxidizer of a rocket has been called the fuel). The already slow neutrons will pass right through the copper core and be slowed more by the fuel, hopefully enough that they don't hit the outer chamber with enough velocity to make it irradiated or contribute to wear.

Speaking of fuel, most readers familiar with nuclear thermal rockets have probably been thinking about hydrogen this whole time. Although compulsators are certainly more mass efficient than equivalent capacitors and the means to recharge them, they are not known for being light. As a fusion rocket is incapable of spreading radioactive material into the atmosphere, the traditional safety concerns of launching nuclear thermal rockets from the ground does not apply. As such, propellant density is once again important and a hydrocarbon first stage fusion rocket doesn't need strap-on boosters trumping its inherent safety.

9 comments:

  1. I like this idea. I'm going to post it on my Facebook page and blog.

    ReplyDelete
  2. Anonymous11:25 PM

    I love the concept. However, please help me out here, and apologies for the naive question (I am truly trying to understand, no snark intended):

    When you say: "Producing nuclear fusion isn't all that hard." I take it to mean that producing non-self sustaining fusion isn't all that hard. i.e. at our current state of research, it takes more energy to initiate and contain controlled (vice thermonuclear) fusion than is produced from the fusion reaction. If my interpretation of your meaning is correct, and you are proposing non-self sustaining fusion, then where is the extra power coming from to power the fusion during flight? Essentially, your design pulls heat from the nozzle -> converts to mechanical energy -> converts to electrical energy -> which is used to initiate fusion -> which produces heat -> which expands the working fluid (e.g. H2) to produce thrust. Even ignoring losses, I don't understand how is this self-sustaining since the heat pulled from the nozzle is only a smallpercentage of the total heat energy produced? What am I missing? Really, no criticism intended. Thanks.

    ReplyDelete
  3. You're not missing much.. I don't actually know it will work. However there is potential energy in the fuel which you're missing: the fuel is a liquid, which expands into a gas when it passes through the heat exchanger to cool the nozzle. That expansion is what powers the pump and produces the electricity.

    I'm just assuming the fusion is more efficient than some other form of electric heating of the fuel. That may be wrong and most probably is, but it's probably a lot more robust than other techniques and is definitely a lot more reliable than combustion. If you can eliminate cryogenic fuels then you get even more robustness.

    ReplyDelete
  4. I don't mind making a fool of myself. I know I don't know jack squat about rocket science or fusion. What I do know is that if fusion is taking place, then that is adding additional energy into the system. Now if there's a way to use this energy to make thrust, we've got something. I'm still fascinated by the idea. I'm sure somebody has looked at this already and whatever got said has been filed away somewhere and forgotten about. Maybe for good reason. I just wish I knew why it wouldn't work, or how it could be made to work.

    ReplyDelete
  5. I say start with the compulsator.. even if you're "only" successful at making a dense plasma focus without a capacitor bank you'll have saved yourself a small fortune.

    ReplyDelete
  6. superkuh6:37 AM

    I already explained to QuantumG why it is not feasible. The complexity and fragility of the very high power pulse compression system required to shorten the very long compulsator pulses by three orders of magnitude make the design untenable.

    ReplyDelete
  7. Hi QuantumG
    Nice diagram. Power-to-mass ratios are the killer for launch applications via nuclear fission or fusion power sources. NERVA couldn't launch a rocket from the ground because its maximum thrust was less than it's own weight. The only vaguely viable fusion launcher designer I've seen is Robert Bussard's, which is described in his fusion space vehicle application papers at Askmar.com. Well worth a read.

    ReplyDelete
  8. Eventually this was a great post. I don't know if this rocket will eventually work. I am glad you posted this; at least I was informed of how this thing goes.

    ReplyDelete
  9. I down loaded this page on my desk top. It is interesting. I came to this web site from initially looking on the search engine if alien life forms exist. From there I clicked on a few of the web addresses and came to a page that had a drawing simular to mine that I have in my posession, and draw abt. 17 years ago, maybe more then that. Though it was of rotating current to defy gravity using magnetic principles. I understand am alright with everyone versions of different rockets, i find it interesting myself so on the the site i clicked on I wasn't sure of the definition of a fusion rocket, yet there is a drawing on this web page close toone a drew long ago, close to it only. I had a drawing of where I drew a chamber that had condensed fuel in it then had a release valve that released the two fuel mixures into a heat chamber, sorta like heated wire coils, and that extreme heat fused the two gases and created presssure to force the gases out of a smaller opeing, and then it gets reheated again for more efficentcy before going to the exhanst nozzel, of which on my drawing the nzzel would be able to open and close for different speeds. The illustrationon this page, I know of some of the schmatic symbols as my Dad is a TV repair man, you have an IC a resistor and a couple switches and a diode next to the other conponents. I like your illustration and find the article inspiring and neto. so I am going to study this stuff for the next few months a little at a time. Though, if you can get a device to rotate magnatism in a circled loop, you might be able to defy gravitiy. Proof is in the pudding according tothis letter I had my drawing pencile sketch already downloaded onto my computer. I wish I could learn from the guy that wrote this article. I used to make my own rockets whne I was a boy, either bought from the store with the soild rocket motors that need the fuse to ignite yet made my own propultion systems as well. And I like this science stuff, always had, and love aircraft too. Have you ever looked into scram jet techno or collecting magnetic fields for an engine in space to have propultion? Gravitational constant ='s X times force squared. One of my own equations. I had a formof PTSD and trama long ago and am trying to reremember. I built my own magnetic flashlight when Iwas in third grade from my Dad's shop in the garage. It was made of a copper coil when we made crystalk radio sets there wasa a coil leftover and I put a woffer magnet in it and maybe out of sheerluck I connected a couplke other parts, one being a tiny bulb and it was made so I could read my Dad's letters he wrote to my Mom in the Viet Nam area. I was told not to go into the bag that had all of Dad's letters in it, and like a kid I did anyway, and there was no light so I made this flash light that did work. I really am impresssed with this article and am going to study itmore. The guy that wrote this is bright.
    Patrick B. Rasmusson

    ReplyDelete