This is one of those patents that absolutely in no way signifies that the company who files it is working on the thing that the patent is patenting. Because we can assure you, Boeing is not*actively developing a laser-triggered fusion powered jet engine. But that doesn’t mean we can’t get excited about it anyway, right?
Here’s the basic idea: you’ve got a cavity that’s a sort of hemisphere shape, kind of like the business end of a rocket engine. You toss a pellet of fuel into that cavity, and then lasers blast the fuel pellet, causing it to release a bunch of energy (by exploding, fissioning, fusing, or whatever). That energy pushes against the walls of the cavity, and the cavity moves forward. At the same time, the explosion heats the walls of the cavity, and this heat is harvested to drive the lasers. Pretty awesome, right? Here’s a taste of technical bewonderment from the patent (which I’ve edited down to remove some of the needlessly redundant patent-babble):
The laser system comprising one or more free-electron lasers for providing pulsed laser beams to vaporize pellets comprising the propellant [deuterium and tritium]. As a result of the compression of the deuterium and tritium, the gas mixture reaches sufficiently high temperatures to cause a release of energy… beyond the ‘breakeven’ level… increasing the overall thrust and exhaust velocity. …A specific impulse of 100,000 – 250,000 seconds may be provided.
Let’s just go through this really quick. First, free-electron lasers. That's a thing. We have those. They're like regular lasers, except they use the wiggling of electrons to generate light, which is nice because you can tune a free-electron laser through a huge range of wavelengths, all the way from microwaves to X-rays. In what is almost certainly not a coincidence, Boeing has been working on one of these since at least 2009, with the goal of putting it on a boat, because you can’t fit the required electron accelerator (plus a whole bunch of amplifiers) into anything much smaller than that. (I’m looking at you, jet engine.)
Now, about this idea of using lasers to cause fusion inside a pellet of fuel: we’re working on that, too. At the National Ignition Facility, they’ve managed to fuse small pellets of deuterium and tritium using 192 lasers, reaching breakeven in terms of the amount of energy deposited into the fuel and the amount of energy that the fuel releases. Note that “the amount of energy deposited into the fuel” is only a tiny fraction of the amount of energy sucked down by the lasers, but in principle, it works. It doesn’t work very well (yet), and it’s a far cry from being able to get stuffed into a jet engine (the NIF combustion chamber starred as the warp core and most of the engine room in the most recent Star Trek movie, for scale), but it’s technically feasible. Eventually.
As for the specific impulse that this hypothetical laser fusion engine would produce, that’s an absolutely bananas number for efficiency. I discuss what specific impulse means in this article about Hall effect thrusters, but essentially, it’s an efficiency measurement, and the most efficient engine that we've ever produced (even as a prototype) has a specific impulse of just under 20,000 seconds. So we're talking an entire order of magnitude more efficient.
So is this idea completely crazy? Nope. Is it mostly crazy? Probably, yeah. It’s not crazy in the sense that the fundamental technologies almost (almost) exist to make an engine like this work, but it is crazy in the sense that by the time someone manages to make those fundamental technologies actually work in the context of a functioning engine, my guess is that either Boeing or the USPTO will be obsolete.
Before I put this article to bed, I would be remiss if I didn’t acknowledge the fact that we have tested nuclear powered jet aircraft before. Sort of. The Aircraft Nuclear Propulsion Program ran from 1946 to 1951, and a Convair B-36 spent a total of 89 hours in the air with a fully operational nuclear reactor chugging along in the back. The reactor wasn’t hooked up to anything (it was mostly a test of the shielding), but eventually, the idea was that you could hook up a compact nuclear reactor to a jet engine, replacing the combustion cycle with air that’s heated and compressed by the reactor. This combination of technologies never made it onto an airplane (as far as we know), but it did result in this:
Image: United States Department of Energy
This is Heat Transfer Reactor Experiment-3, which is a 35 megawatt air-cooled reactor hooked up to a pair of General Electric J47 turbojet engines. The engines produced just under 53,000 newtons of thrust in total, and the system operated continuously for 126 hours during an endurance test on the ground. The picture above shows the unshielded reactor (which was designed specifically to fit into an airplane), and you can still see it on its test stand if you visit Idaho National Laboratory. Still seems futuristic, doesn’t it?
*Note: I actually have no way of knowing whether or not Boeing is really working on this thing, and I’d be overjoyed to be wrong.