RocketStar tests fusion-enhanced in-space propulsion process
aerospacetestinginternational.com
aerospacetestinginternational.com
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
I get 1600 km/s from a naive calculation, still a factor of 30 off: https://www.wolframalpha.com/input?i=sqrt%282*150+kev+%2F+%2...
There's a specific kind of fusion reactor which gets done as a highschool science-faire project every so often, similar voltage level: https://www.popularmechanics.com/science/energy/a34312754/12...
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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]
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.
https://en.wikipedia.org/wiki/Polywell and related
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.
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.
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?
Do you mean “smaller”? I thought the defining characteristic was that the atomic number was increasing.
> B-P fusion results in 3 alphas, so it's fusion-fission.
You’re saying the “fission” part comes because one of the inputs, Boron-11, is larger than the outputs, the alphas?
A 50% boost is both impressive and, given my memory of Farnsworth fusor discussions, surprising.
So efficiently using the reaction mass you have is worth it, even if you put in more energy then you get out.
They don't, and didn't claim that.
“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.”
It sounds fairly crude, but if it works it works.
https://thedebrief.org/rocketstar-successfully-demonstrates-...
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!
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.
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.
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.
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”
I assume this is for interplanetary (or perhaps lunar) travel.