When I saw the news on our sister site Sploid that Lockheed Martin announced an aggressive plan to bring compact nuclear fusion into reality, naturally, I was excited. There has been some skepticism about their claim but seeing as how this is Lockheed Martin, the largest defense contractor in the world, they carry a bit more weight than some dude that did this in his garage. No offence to guys who like to tinker in their garages, I like to as well, but when it comes to particle physics, I'm going to leave it to the experts. A physicist I am not, so that's the last I'll talk about whether or not Lockheed can actually do what they're saying they can do.
As an engineer, there are a number of applications that I see as awesome and practical. The first thing that everyone thinks of is solving the world's energy needs. After all, this is pretty much the primary goal of nuclear fusion with it's safe operation, cheap and readily available fuel (hydrogen, the most abundant element in the universe). How they do this is somewhat less clear.
This is a simplification of how current nuclear power generation works. In fact, if you replace "reactor vessel" with "coal furnace", "natural gas burner", or any other fossil fuel method of heat generation, you have the majority of every other method of generating electricity out there. Essentially, the reactor is merely a heat source. You use the heat source to boil water and generate steam, the steam is used to drive a turbine, the turbine powers a generator, and the magic of electricity is born.
Lockheed comes in by replacing the reactor vessel with their compact fusion reactor that has minimal radioactive byproducts, cheap fuel, and inherently safe operation. If there is a reactor breach, the reaction stops. This is unlike traditional nuclear power where the reactor is containing a chain reaction where without input, problems happen, to put it lightly.
Now, aside bringing peace and prosperity to the earth, what else can we do with a heat source that uses barely any fuel? Answer: airplanes. Enter the Convair X-6. Yes, in the 50s, they not only considered but tried nuclear powered airplanes.
In the X-6 experiments, they got as far as flying the plane with the reactor running but not powering the airplane. This is on top of an entire engine development project that proved out the feasibility of the engine design.
These are pictures of the HTRE 3, the proposed engine for the Convair X-6. Thermodynamically, this engine works the same as any other: suck, squeeze, bang, blow. Air is drawn in, it's compressed, heat is added (fuel burnt most commonly), and it's driven through a turbine to power the compressor. The difference with this one is that the rector is designed to be air cooled and the compressed air from the engine is used to cool the reactor. That cooling heats the compressed air, raising the pressure, which is then driven through the turbine on the engine the same as any other, creates thrust, and away you go.
This program was developed with the intention of long range bombers. This was big in the 50s along with many programs like the XB-70. Long range bombers were of great strategic importance to the US because you could take off from the mainland, drop bombs, and return without the need for refueling. Eventually the program was cancelled by President Kennedy with the rise of ICBMs making long range bombing unnecessary.
Now, with the promise of safe, clean power, available for everyone, you could easily take the same idea and testing and research and apply it to commercial airlines. Imagine if you will, a jet the size of a 747 that is able to fly anywhere in the world without refueling.
Fuel is roughly 35 percent of the operating cost for airlines. Granted, this is likely to have a big impact on capital expenditures but fuel expenses are the biggest reason for flying smaller jets and cut backs in operations. It would allow airlines to commonize fleets. This would in turn reduce maintenance expenses from having to have multiple airplanes and staff trained in maintenance. Dare I say it, it would improve flying in general because with aircraft size not being a limitation, row spacing could increase and everyone's bane of flight, no leg room, would no longer be an issue. Embrace the fusion, be happy, fly happy. Ok, that went a little too far.
Anyway, what else can you do? Rockets! Yep, again, in the 50s, they not only tested jet engines with nuclear reactors, they tested rocket engines.
This is the same idea of using nuclear fission (fusion) as a heat source. A propellant (the diagram says hydrogen but it really doesn't matter what it is as long as it's liquid) is run over a reactor, heated, pressurized, and forced through a nozzle to create thrust.
Again, they actually got to the point of testing this and at one point it was considered as an alternative for the Apollo program.
(The test engine was never intended to fly and was named Kiwi as a reference to the flightless bird. See, engineers have a sense of humor)
Anyway, nuclear thermal rockets engines offer a few advantages over traditional chemical rockets. The most notable is their incredibly high specific impulse compared to chemical rockets. Specific impulse is a way of comparing rocket engines to each other by comparing how much thrust they generate with how much fuel they use. A sort of fuel economy for rockets. (If you run the numbers for lift off, it's on the order of feet per gallon.)
Just so everyone knows, ion engines have the highest specific impulse of any rocket engine developed. Chemical rockets, like those currently used by orbital launch providers, have the advantage of large gross thrust. This is where ion engines fall off. They do not have very high gross thrust. However, nuclear thermal rocket engines have both high gross thrust as well as a high specific impulse, which is around 3 times chemical rockets.
There have been a few discussions on whether nuclear thermal engines would be an improvement over chemical rockets but chemical rockets still win out in the gross thrust category. Ideally, you'd like the vehicle to have a thrust to weight ratio between 1:1 and 6.5:1 since the vehicle has to accelerate upward but not too much to be a detriment to the structure. This is why chemical rockets are used as a first stage and will continue to be until something better than rockets comes along. Nuclear thermal rockets just can't seem to get the gross thrust necessary for a first stage of a rocket. This is why the biggest advantage seems to be in the upper stages of a lift vehicle. Once out of the atmosphere (100 km/62 mi... ish), nuclear thermal rockets could provide the acceleration necessary to achieve orbital velocity or exceed it to go beyond. Ultimately, the program was dropped in favor of chemical rockets and it never saw production for the Apollo program.
There is that one pesky drawback of nuclear thermal engines, radiation. This is where our fusion reactor comes in handy. There really isn't any radiation (aside, possibly the containment vessel.) It literally does the same thing, heats the "fuel" (fuel is anything that isn't an oxidizer but lets not get hung up on terminology) to be expelled from the engine at high velocity but it does it using a different process.
So maybe this could be used as a viable upper stage for orbital and beyond rockets. Would this justify the expense of a fusion reactor powered upper stage rocket? I don't know, no one really does yet. However, what gets me excited is what do you do with this fusion reactor once it's in space?
You may break even from a performance standpoint with a [fusion] nuclear thermal rocket engine. However, once it's in space, you have the same thing you have on earth: a nearly unlimited power supply. That thing that was propelling your ass to space, is now powering your ion engine, it's powering your asteroid recovery system, it's powering your asteroid mining operation. The nerd in me is literally freaking out!
Anyway, that's your take away and the exciting thing is that all of these things have been studied already. If Lockheed keeps to their word, we could see a viable space mining operation in our lifetime. Bringing freedom to the solar system. [img:Ronald Regan riding a velociraptor]