Wednesday, May 14, 2008

Reactor Size

I was rereading Sekora's paper on POPS [pdf] and came across this gem.
It is very difficult to achieve field strength approximately 50-100 kV/cm without causing arcing.The maximum field strength is governed by the Paschen curve.
So let us look at what a decelerator BFR might look like. Reaction volume 1 meter radius. That is a given. Let us assume 200 KV drive voltage. If we assume a voltage of 20KV/cm that gives 10 cm to zero voltage. Then assume 2 MV decelerator voltage. That is 1 meter. So the total radius is 2.1 meter. At 40" per meter (roughly) That is 84" radius or 168" diameter. About 14 ft in diameter for a 100 MWth reactor. Suppose we go to 40KV/cm (about the limit). That would be 1 meter reaction space. About 5 cm to zero voltage. And 50 cm for the decelerator. That would be 1.55 m radius. 3.1 m diameter. About 124" or a little over 10 ft in diameter. So odds are the reactor will be between 10 ft and 15 ft in diameter for a design with direct energy conversion. That would fit on all but the smallest ships.


Rhinobird said...

Would it be possible to perhaps shave 2-3 feet off that diameter. I want these to fit in standard shipping containers (20 ft by 8 ft by 9 ft tall). Then you can start thinking goofy stuff like fusion powered trains. Or the ability to ship them on standard trucks and drop them off in remote locations and provide power.

There's already generators built into shipping containers. Some even provide ship power I believe. If a BFR can be squeazed into one then it's a super powerful drop in replacement for those applications.

M. Simon said...

All I can say is : maybe.

What could be done is shipping the reactor as bolt together segments.

Or how about a special 4 container size container for port to port shipping.

kurt9 said...

Making them small enough for locomotives or airplane engines is a nice idea. However, I think this unlikely. What is far more likely is that the cheap electricity and thermal energy from these fusion plants will be used to make synthetic hydrocarbon fuels which, in turn, will power our cars and planes.

Of course, synthetic hydrocarbons will be more expensive than oil was in the 90's, but could be cheaper than it is today.

Unknown said...

I think kurt is most likely right. We know how jet fuel works pretty well. We'll probably use algae grown biofuels rather than direct electric.

The geek in me wants to point out though that the widebody aircraft in use today (like the Boeing 777 and Airbus A380) have bodies more than large enough to hold a fusion reactor. A plane with one of those could fly pretty much forever (in practical terms) without landing. I would expect the military to do this, since they spend tens of millions of dollars just shipping fuel around the world.

Plus, in-theatre synthetic fuel production would increase operational flexibility and lower costs.

M. Simon said...


I have very, very, good reason to believe it as well.

I can say no more.

kurt9 said...

If a polywell fusion device can be made small enough to power an aircraft, I have no doubt that the aviation industry will try to do it.

Fuel costs make up 40-50% of the variable costs of running an airline. Also, the military has considerable logistics involved in make sure that they have motor fuel where they need it (the U.S. military invented logistics to fight WWII).

Any polywell device will be so large that it has to be located in the fuselage of the plane. The resulting "energy" then must be transmitted to the engines to create propulsion. This will require a radical redesign of aircraft to be fusion powered.

kurt9 said...

A polywell fusion device will generate electricity (Boron reaction) or a combination of electricity and heat (Helium 3 reaction). Perhaps the electricity coming from a polywell device could be used to generate an atmospheric arc discharge that could provide a direct drive for an airplane.

Any thoughts?

M. Simon said...


Think superconducting electrical motors.

As to your arc idea. Very doubtful.

Unknown said...

There's an alternative to micro sized reactors, or charging batteries. Road trains, some much larger than the Australian contemporary, are in common use in many areas of the world. Most are large enough to carry a reactor.

It would take a serious oil crisis to make the infrastructure changes to support such machines practical.