Okay firstly, how are they getting fusion? You need a lot of energy to overcome the Coulomb barrier for boron - apparently at least 148 keV [1]. For a proton to have that much kinetic energy it needs to be going at 5340 km/s [2]. "The exhaust velocity of a [pulsed plasma thruster] is of the order of tens of km/s" [3]. So what gives? I'm guessing that exhaust velocity figure is for heavy ions, but are protons really that much faster?
Secondly, how are they turning the fusion into thrust? Causing fusion in the exhaust stream after it's left the motor will make it hotter, but how does that heat couple to the vehicle? In a pulsed plasma thruster, thrust is produced from the reaction against the acceleration of the plasma by the magnetic field. How does fusion affect that?
> You need a lot of energy to overcome the Coulomb barrier for boron - apparently at least 148 keV [1]. For a proton to have that much kinetic energy it needs to be going at 5340 km/s [2]. "The exhaust velocity of a [pulsed plasma thruster] is of the order of tens of km/s" [3]. So what gives? I'm guessing that exhaust velocity figure is for heavy ions, but are protons really that much faster?
This isn't necessarily a satisfying answer, but in their patent they mention "laser-assisted fusion":
> In addition, a plurality of pulsed lasers are utilized to focus on focal regions of the exhaust region to catalyze boron fusion events...The pulsed lasers are modulated to create a superposition and constructive interference region where fusion is likely to incur in the reaction region. The plurality of pulsed lasers assists fusion ignition and/or further energizes the protons frame to assist fusion reaction. It is an insight of this invention that acceleration of particles using multiple small lasers would be helpful as only small hot regions of plasma are necessary for measurable fusion results.
You can very much do table-top fusion in your garage if you like. You just won't get net energy out of it -- far from it. But if you're interested in higher ISPs for deep space rocket engines, then the fact that this isn't energy efficient might not matter to you -- getting enough extra delta-v for your propellant and reactor/thruster might well justify the expense.
The real problem is that the electric power needed to run such fusors has to come from somewhere, and in deep space the hardest thing to do is to cool that something, though here I suppose a fusion-enhanced thruster might be able to use propellant as coolant for a fission reactor that generates the electrical power for the fusor that add enough additional heating to the propellant to make this workable.
Although I don't know the specifics of this thruster — the phrase "generates high speed protons through the ionization of water vapor" sounds like the reporter/press release writer didn't understand what the scientists told them — it's very easy to get protons up to 148 keV in a lot of different ways.
148 keV is not too much. As electronics engineer I few times talked with colleagues about fixing X-ray devices, and there very typical to use voltages 60kV..120kV, which are directly converted to electrons kinetic energy, and than it converted to X-rays.
The low exhaust velocity implies the plasma temperature is low. This means the loss of energy from protons to electrons will be very high, so high that even with a purely boron plasma, the fusion yield will be quite small.
Attempts to make p-11B work typically have electron temperature above 100 keV and ion temperatures of maybe 300 keV. For warm or hot electrons, the energy loss of energetic protons to the electrons scales as T_e^(-3/2), so a colder plasma will cause ruinous energy loss from the protons.
From this, I conclude any fusion heating is quite minor compared to the simple plasma heating from the protons slowing down.
Very odd. How could they then reach any fusion if they have a hundredth of the needed velocity (50 km/s instead of 5000 km/s)? If they still detected alpha particles...
EDIT: Eh, maybe not. I see it's describes as the "exhaust plume" elsewhere...which really does suggest the fusion is taking place outside the motor. So I remain confused.
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> Secondly, how are they turning the fusion into thrust? Causing fusion in the exhaust stream after it's left the motor will make it hotter, but how does that heat couple to the vehicle? In a pulsed plasma thruster, thrust is produced from the reaction against the acceleration of the plasma by the magnetic field. How does fusion affect that?
I think (?) the idea is that it's being added to the exhaust stream before it's left the motor and in particular while it's still in the magnetic fields. This would indeed be like an afterburner, where the additional fuel is added to the "exhaust" of the turbine but while still inside the engine.
(The fields accelerate both positive and negative charges in the same direction. It does seem like you'd need a positive-negative bias to the typical output velocities in order to use the Lorentz force here, but maybe I'm confused.)
> By contrast, electromagnetic thruster ions are accelerated by the Lorentz force to accelerate all species (free electrons as well as positive and negative ions) in the same direction whatever their electric charge, and are specifically referred to as plasma propulsion engines, where the electric field is not in the direction of the acceleration.[1][2]
> Adam Hecht, professor of nuclear engineering at the University of New Mexico said, “RocketStar has not just incrementally improved a propulsion system, but taken a leap forward by applying a novel concept, creating a fusion-fission reaction in the exhaust.
What fission is taking place? Is this just referring to the fission of the excited isomer Carbon-12 (into three alpha particles)? Isn't that like, extremely short-lived? Aren't all fusion reactions "fission-fusion" in this trivial sense?
(My brother is a plasma physics postdoc at LLNL and he says he's never heard of anyone refer to p-B as a "fusion-fission" reaction. So it still seems weird to me.)
I don't think this is inconsistent though? The goal is to optimize the Isp you get from your fuel, not to get more energy out of the process then you put in necessarily - i.e. in space you can harvest solar power all you want, but if you don't have reaction mass then you can't give yourself momentum which is what you need to get around.
So efficiently using the reaction mass you have is worth it, even if you put in more energy then you get out.
Indeed, but my surprise is due to how far away Farnsworth fusors seemed to be from useful for anything besides being a switchable source of (ironically given this article is about aneutronic p-B11) neutron radiation.
The only thing I can figure is that this beam-target fusion except that the proton beam is their primary thruster so that any protons that don't fuse are not wasted. Still not sure how they get a 50% increase from fusion with boron but I assume that is merely optimistic.
Not sure if a comparison to a Farnsworth fusor is relevant. This is proton boron fusion. Presumably the higher ISP comes from the increased exhaust temperature.
They are using Coulomb forces to produce fusion. That's what a Fansworth-Hirsch fusor does, as well as a line of related fusors like the Polywell which I'm guessing will never produce net energy. For this application net energy is not necessary, just enough additional heating to get enough additional ISP to increase the amount of delta-v per-kg of propellant -- the additional weight of the power plant and fusor must not make this so inefficient/expensive as to lose that ISP advantage.
I doubt they have any additional power plant or a fusor. They’re just injecting boronated water into the proton exhaust, and presumably getting enough fusion reactions to be useful.
“The fusion process was first devised during a R&D program for the US Air Force’s AFWERX (Air Force Work Project) initiative, where boronated water was introduced into the pulsed plasma thruster’s exhaust plume. This created alpha particles and gamma rays, clear indications of nuclear fusion.”
This sounds exciting, but I don't have a practical example in my head to understand the potential value here. 50% increase in ISP is great but is fuel mass the limiting factor on this type of engine or is it energy production? It would be great to get a simple example of utility here by possibly comparing existing uses to something with this modification. Anyone have a good example?
Fuel mass is very much the limiting factor. "Electric" or "ion" propulsion of this general type are used on satellites for things like orbit raising or orbit maintenance. They don't provide very much thrust, but they provide a very high ISP (often 10x what would be possible for a chemical reaction propellant), which typically translates to a longer lifespan for a vehicle or the ability to carry less reaction mass and more payload.
The problem with this, I suppose, must be the power plant needed to produce the electric power needed to drive the fusor. My guess is that for a deep space ship one would need a propellant-cooled fission reactor, while for a thruster for orbit management a solar/battery/capacitor system might suffice (but with very short run times).
For a deep space system there would be challenges. Shutting down the fission reactor would require continuing to use propellant as coolant long after the fusor stops working -that or a massive radiator-, and either way the net ISP would be driven down. And then the fission reactor would have to stay shut down for quite some time. Though if you jettison the fission reactor when propellant runs out then there are no such problems!
For orbit management thrusting though this might yield a very weight-efficient system!
They don't increase ISP, they increase thrust. This is also something you can do with an ordinary afterburner: Trade ISP for thrust. For space electric propulsion this seems reasonable, since ISP is usually extremely high, but thrust is ridiculously low. It's ok for keeping satellites in orbit, but even a 50% increase means we're not going to send humans anywhere with that any time soon. Additionally, their fusion idea works a bit differently by not directly using the energy created from fusion, which makes the 50% claim seem rather wild. I would wait for a peer reviewed paper or an actual flight test before getting excited about any of this. Small companies promising revolutionary space drives that end up as vaporware are quite common nowadays, because there are lots of simple minded venture capitalists out there who dream of funding the next SpaceX despite having no clue about physics.
Thanks for the correction! Yeah, thrust != Isp. In this case though, wouldn't this increase the ISP? This type of propulsion gets its energy external to the mass ejected so Isp here, with the energy source not in the factor, should just be based on total propellant mass and the max velocity it is ejected at right? So this -should- increase ISP. I am 100% not a rocket scientist so I am probably way off here. From Wikipedia: 'For engines like cold gas thrusters whose reaction mass is only the fuel they carry, specific impulse is exactly proportional to the effective exhaust gas velocity.'
In the most simple scenario where you inject something inert (literally anything for that matter) into the exhaust stream, you increase its momentum but decrease its velocity. That means higher thrust but at the same time lower ISP. For this fusion enhanced engine here they don't seem to get anything from the fusion process energetically at all, which makes the numbers extra weird. We'll have to wait and see if this whole concept even works.
No they aren't. It seems their design only really reduces the amount of unpaired electric charges expelled, which otherwise would get pushed back to the cathode because of the plume's total charge density. They don't even use the energy released by fusion itself, because for that you would need to fuse the particles before they reach the exhaust nozzle (which is super hard and also the reason why noone managed to build a real fusion rocket so far). That's why they call it a "fusion-enhanced" instead of "fusion-powered" engine. They talk about it in more detail in another article:
I’ve seen a few papers on using electrostatic fusion (Farnsworth fusor type designs) for propulsion.
Electrostatic confinement is not a method that’s ever come close to overunity but that’s not the point. The power source would be fission or solar. The idea is to use a driven fusion reaction to accelerate propellant to achieve even higher iSP than pure ion drives.
"During the tests the process was seen to create the ionizing radiation and improve the base propulsion unit’s thrust by 50%, said RocketStar."
It does not say that the fusion increased the unit's thrust by 50%.
If you shoot protons into a gas containing boron, you will see some fusion. But the problem is not achieving fusion, it's achieving significant fusion. In this case, I'm betting almost all the extra heating is just due to the injected protons depositing energy, not due to the fusion they might be inducing.
If the gas being hit is only partially ionized then energy loss of the protons to additional ionization will slow them down very rapidly, and very little fusion will occur. This is why just slamming nuclei into a solid target can't possibly reach breakeven.
> I’m betting almost all the extra heating is just due to the injected protons depositing energy
And their team over at HPEPL / Georgia Institute of Technology wouldn’t have noticed that basic fact because…?
If simply injecting tiny amounts of boron (or anything else) into the exhaust to get burnt improved performance by 50% this would have been ubiquitous by now. Afterburners were invented in the 1940s after all.
> And their team over at HPEPL / Georgia Institute of Technology wouldn’t have noticed that basic fact because…?
Who says they didn't notice? Nowhere did they say the heating was due to fusion. Read carefully what they wrote, not what they left unstated and allowed you to wishfully assume.
The heating would not be due to tiny amounts of boron, but rather due to the energetic protons slowing down, depositing energy. This would happen even in the absence of boron or of any nuclear reactions.
“This fusion process, like an afterburner in a jet engine, transforms boron into high-energy carbon, which rapidly decays into three alpha particles. The result? A remarkable 50% improvement in thrust compared to our FireStar™ Foundation thruster.”
“We've witnessed fusion reactions occur in our lab and the result is a 50% jump in thrust performance”
Indeed, I suspect one could get much more than a 50% boost from fusion, so maybe they're not getting much heating from fusion. It's a nice idea though, a Fansworth-style fusor to heat propellant.
For about the last year or so I've been getting pushed fishy articles on another nuclear propulsion company, Pulsar Fusion, via Google Now and elsewhere. AI-generated-reading articles on nobody websites. Something strange about all of this.
Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.
I assume this is for interplanetary (or perhaps lunar) travel.
Research missions up to Jupiter will benefit, perhaps Saturn with a really big ass solar array. Other than that, station keeping for Earth orbit, so longer lived birds translate to perhaps cheaper sat services.
You could run a propellant-cooled fission reactor to power the fusor to get the extra ISP. When you get to your destination and finish decelerating you toss the fission reactor overboard so you don't have to deal with its remaining heat.
No. It's not energy-positive, fusion provides less powe than is put in. Instead it's a way to produce very energetic exhaust, resulting in better fuel economy.
No, this would be heating the propellant to higher temperatures than chemical reactions can get -- in principle one could get much more than 50% extra ISP for the same propellant mass. In rocketry/satellites/deep space propellant mass is a big deal -- anything you can do to reduce the amount you need without having to gain even more mass in other ways is a big win.
Secondly, how are they turning the fusion into thrust? Causing fusion in the exhaust stream after it's left the motor will make it hotter, but how does that heat couple to the vehicle? In a pulsed plasma thruster, thrust is produced from the reaction against the acceleration of the plasma by the magnetic field. How does fusion affect that?
[1] https://www.nature.com/articles/s42005-023-01135-x
[2] https://www.omnicalculator.com/physics/relativistic-ke
[3] https://en.wikipedia.org/wiki/Pulsed_plasma_thruster