Pulsar Fusion Developing 2 Megawatt Direct Fusion Drive for 2027

66 thoughts on “Pulsar Fusion Developing 2 Megawatt Direct Fusion Drive for 2027”

1. I would think if these guys got fusion to work, they’d concentrate on powering the world first, and space travel later.

   • Exactly, this is one big factor that indicates this is a scam. Once the world is powered by fusion, I’ll be a little more willing to listen to other application pitches.

   • Different engineering challenges.

     Scale of Power Generation: A fusion reaction for spacecraft propulsion may not need to generate as much power as a commercial power station on Earth, which needs to produce significant amounts of power continuously. A spacecraft can potentially utilize intermittent, controlled fusion reactions to generate thrust, making the engineering challenges different and, in some ways, less daunting.

     Containment and Control: Fusion reactions require containment of extremely hot plasma. In an Earth-based reactor, this is a major challenge. The plasma must be kept from touching the walls of the reactor, which requires complex magnetic containment systems. In contrast, in a spacecraft, some proposed designs for fusion engines (like a pulsed propulsion system) would allow the plasma to expand freely in one direction (the “exhaust”), reducing the requirements for containment.

     Radiation and Safety: Earth-based fusion power plants have stringent safety requirements because any failure can have catastrophic consequences, including significant radiation leaks. This places high demands on the containment systems and materials used. In space, while safety is still crucial, the potential for a catastrophic failure to cause damage is less because the spacecraft is not in a populated area.

     Efficiency and Energy Recovery: For an Earth-based power station, it’s critical that a large proportion of the energy produced by the fusion reaction is captured and turned into electricity. In a spacecraft, on the other hand, it’s the momentum from the fusion reaction that’s most important for propulsion, not the capture and conversion of energy.

     • Once again you have explained beautifully the differences between fusion for power generation and fusion for propulsion. Personally I think fusion for power generation is the crackpot scheme that will never work, while fusion for propulsion is far more straightforward for all the reasons you have enumerated. besides, the power generation problem is already thoroughly solved by fission and does not need fusion even if it were practical, whereas propulsion badly needs better solutions than chemical energy.

     • Good points.
       Though the pulsed fusion rockets rely on creating charged particles which can be directed by magnets, rather than neutrons which slam into the containment vessel.
       That then makes power conversion relatively easy: the “exhaust” is channeled through as solenoid. It’s also highly efficient.
       So it’s a good basis for electricity generation. But, a lot of electricity is needed to accelerate and energise the plasmas in the first place, which is a major cost driver.
       I would guess this is the most promising path to power generation from fusion, but the issues you point out are real and not cheap to overcome.

   • Fusion is the name of the company, not the type of reaction. It’s a fission reaction, built by a company named Fusion.

     • According to them it IS fusion.
       You should probably review the article.

2. I’ll put my trust in the author’s deeming this worthy of reporting, and Angry Astronaut’s comment here. The physics nerds making rude comments raising issues with what’s being presented should probably keep their slide-rules in check until static testing starts.

   • Perhaps you should listen to a Nuclear Engineer who worked at LLNL in Y-Div (lasers) and M-Div (magnetic confinement), not that I mentioned that until now. Or listen to GoatGuy, who has been here a very long time and understands how hard fusion is, and can do the math far better than me.

     Brian also spend years and years reporting on the Rossi ECAT scam, though pretty much everyone with some Engineering background knew it was a scam from the start. Brian eventually became convinced after enough ‘laughable’ demonstrations of power generation.

     I have no idea on Angrys qualifications, and I generally like most of his YT stuff, but I’m guessing its not in NE.

3. Do they intend to assemble this thing on ascent? If so, then this DFD is supposed to deliver a great deal of thrust. If not, then it looks like they have found the Holy Grail of rocketry: a metalox SSTO. Now, which one is more probable? Right.

   • It’s an extremely low amount of thrust, like a big ion engine. It would only be useful after you
     achieve low earth orbit (LEO), assuming it could operate with a 100% duty cycle (until the fusion fuel runs out). There are no hazardous bits, so full assembly on the ground is OK. Even space based fission reactors are only slightly hazardous during the trip to orbit, as there are no fission products before you start it up (assuming U235/U238 based, not Pu239). Enriched reactor fuel (generally UO2) is safe enough to hold in your hand before being used in a reactor core, though licking it is unwise.

4. Visited their website. Several things going on. Ion thrusters which is a proven technology. The DHe3 fusion concept using the FRC as depicted will never work in the timeframe claimed. Whomever is funding this is getting ripped off. I suspect that Dinan (ceo) is flushing his old money trust fund down the loo. No one with even the most rudimentary knowledge of the subject would back this. Dinan is and interesting character: friend of prince Harry, reality TV star. Pretty funny.

   • Never say never …oops

   • Step 1. Get D-T fusion to work on the ground (recent NIF result a good ‘scientific’ step, but just a very tiny step towards a workable power plant). Clearly not flight weight.
     Step 2. Update design until Q=20 to 1000, with high power output, sell and make lots of money
     Step 3. Get D-D fusion to work if you find some reason that makes sense (needs higher temps than D-T), though T no longer needs to be bred from a Li liner with neutrons
     Step 4. Jump on the He3 lunacy train and claim mining the moon for it as a ‘solution’.
     Step 5. Pretend to build prototypes, cash paychecks from the foolish investors for 10-15 yrs
     Step 6. Ignore the fact that D-He3 needs MUCH higher temperatures than D-T or D-D, and
     that D-He3 STILL emits 2.5MeV neutrons from all the D-D reactions (though useful to make He3).
     Step 7. Imagine some non-powerplant application to fool space nerds and sci-fi fans. Pretend it can be scaled down to the size of a large ion engine.

     • Spot on Mike

5. Nice, This is all old technology but used in a new way!
   Your taking the current fusion reactors that work but are not profitable on earth. Due to cooling and atmosphere and input electric cost.
   A earth fusion reactor wants to run a lot longer than 100 second bursts. But that perfect for space. On earth the heat builds up cooling is expensive. In space cooling is free the heat is appreciated and you can even make power from the difference in temperature.
   On earth venting the plasma not good. In space venting the super hot expanding gas is called thrust!!!
   Id bring a standard uranium heater as a back up heat source. The pallot sized ones work great in Antarctica the coldest days make the most electric, and heck space is a lot colder lol.
   P.s. keeping the sun from killing everyone on board is the hard part. Way too much radiation from the sun. Your magnetic confinement rings should help.

6. Superheating a gas into plasma then expelling it at high velocity. Got it. So in other words they’re taking VASIMR and marketing it with the buzz word “fusion”

   Dr Change-Diaz actually has a working space ready device ready to test with a NASA contract in hand but hasn’t done it yet due to the little matter of powerful magnetic fields screwing with the electronics if any host craft.

   This fake fusion engine is nothing more than a cash grab. Borders on fraud

   • Most spacecraft propulsion works by superheating gas and expelling it at high velocity. Including Saturn V.

     So no, it’s not VASIMR marketed with the buzzword “fusion”

     VASIMR is electric. You need 200 megawatts of gererated electrical energy (how do you generate that with low weight?) to accelerate the ionized gas and reach Mars in 30 days. Except that you can´t produce 200 megawatts of electricity with a low mass of powerplants to achieve that.

     Direct Fusion achieves the same thrust with much less energy produced “directly” by fusion reactions.

     The gas acceleration is provided by fusion. The energy for the magnetic fields just DIRECTS the ionized gas, instead of accelerating it.

     • “Direct Fusion achieves the same thrust with much less energy produced “directly” by fusion reactions.

       The gas acceleration is provided by fusion. The energy for the magnetic fields just DIRECTS the ionized gas, instead of accelerating it.”

       Thank you for explaining it so clearly and concisely!

7. I seriously doubt this will come even close to working out. Yeah, it sounds nice, but the engineering difficulties have proven to be massive, and I’m highly skeptical that fusion will be involved in any sort of energy production (electric, propulsion, etc). I Would love to be proven wrong, but won’t be holding my breath for this.

   • Your sceptic view is justifiable.
     There is not much to go by aside from the picture.
     But one of the big problems with fusion had been dealing with the explosion of pressure and gas from the heat of fusion.
     It looks like they will use that ventilation as the fast thrust. plasma ejection then cooled by the vacuum of space and infrared absorption to cool the reactor..
     On earth cooling is a huge problem.

     Unfortunately they dont mention how they might get the 2mw electric so i would guess a sterling/steam engine ammonia as the refrigerant that boils near the reactor and then cools/condenses furthest from the reactor.
     This could also bring warmth to the rest of the ship if plumed correctly for it.

     • There is no ‘explosion’ of pressure for magnetically confined fusion plasmas. Their pressure is only slightly above a vacuum. Ideally, they will run steady state, with the fusion reactions energy being partially deposited in the plasma to keep it at temperature (100M C or more) and compensate for all the losses. Whatever gets spit out the nozzle needs to be replenished at the same rate, plus some heating system to keep the plasma toasty (since everything done to date has Q<<1, and we don't even know if ITAR will reach Q=1, that's a lot of energy being added, with NO net energy).
       RFC or compact toroid plasmas tend to dissipate fairly quickly, and they seem to claim a steady state operation mode. My skepticism is extreme when anyone claims an application FAR beyond what is actually needed on Earth, a Fusion power plant.

     • The 2MW is obtained directly as electricity (NOT as heat) by passing the fast charged exhaust past a coil which very slightly decelerates it.No conversion to/from heat. No boiling, no turbines. It’s induction. That’s the beauty of it. No part of this is a heat engine even if it is admittedly hot.

8. I think a fusion pulse drive, powered by something like a fission reactor, might be possible, but power and propulsion in one, without making a working reactor first? I smell a grift

   • I was thinking the same. They need more than q>1 like q>10

9. [Earth-Pluto ~4.24-7.5*10^12m, 4yrs ~31.5*10^6s, v_const ~200000m/s – v_escape (~12000m/s), ~100d accelerating, decelerating?, W_decelerating=0.5*m*v^2, payload ~1t, propellant=~20%=~1.2t, vehicle mass at start ~2.2 to possibly ~6t(?), W_dec=0.5*2200kg(avg)*(200000m/s)^2 kg*m^2/s^2=4.4*10^13Nm, W_dec=~12GWh(~1.2 Superheavy Booster methane energy ~717t?) heat to v=0m/s]

   • Well, that’s refreshing! B R A V O!!! Good to see people using good old computational physics to (confirm) estimate how thing ought to be.

     I usually just use ‘40 AU’ as Pluto’s distance. 6e12 m. Divide by ( 4 y * 365 d * 24 h * 60 min * 60 sec ) and you get 47,565 m/s. At 100 d acceleration/deceleration (each) the dv/dt needs be 0.0055 m/s^2. Remembering F = ma, and using 2.2e3 kg as m, then F need only be 12 N. Hey!

     If the Isp really is 100,000 sec or dv/dt of 1,000,000 m/s for exhaust and only 100 kg of fuel reaction mass is budgeted for accel or decel, then let’s see. Dv = 1e6 ln( Mbegin/Mend )
     = exp(47565 / 1000000) = 1.0487 = Mbegin / Mend. 1-(1/over) = 4.64% of original mass. 2,200 kg of becomes 100 kg! Well isn’t that encouraging! It all works out.

     As you say though, the kinetic energy of the exhaust plume … divided by the fusion energy coupling to kinetic energy output of what, 10% being generous? Gives 5e14 J of fusion energies. For accel, or for decel. 139 GWh of electrical equivalent. Working with the 5e14 J and 100 kg of DHe fuel at what, 14 Mev per successful fusion. Of which DD fusions are more likely (70:30), and not that many protons emit. Oh well …

     Much luck to this approach. And much GOOD luck in getting a multi-megawatt magnetically bottled fusion reaction to remain stable for years of operation largely without intervention by us hairless monkeys.

     GoatGuy

     • Whether D-D or D-He3 are more likely depends on temperature. Helion is considering using separate reactors, one to breed He3 from D-D and power reactors mainly doing D-He3.

     • Very nice assessment.

       I enjoy the hairless monkey with rockets to clean.

       Lucky the fusion dose not have to be sustainable more than a few seconds at a time. Many reactors can run for minutes so… Burn and dump that H-3.

     • [Thanks for the inspiring correction, sorry me did miss several times factor 4 for seconds in 4yrs, not one year.
       Verifying Isp (specific impulse in seconds) would be between DS4G and ideal photonic rocket 21k-30M (and seems fusion has an efficiency advantage of 4x compared to fission). How does an Isp for 100d accel (8.6M) suit for Tsiolkovsky’s equation (extended with v_e=g_0*I_sp) and why is there always standard gravity ~10m/s^2 on interplanetary missions? (thx)]

10. I smell a Rossi …

11. Stop the pessimism!! I’ve seen this company’s facilities and technology in person. What they’re trying to achieve is far easier than fusion power, and the implications for Spaceflight are mind boggling.
    I’ll be living a short drive away from Pulsar Fusion starting in September and look forward to covering this remarkable story as it develops.

    • Oh hi, Angry

    • First time I see you here.

      Although I don’t like your recent alien videos and I still don’t buy Avi Loeb Occam’s Razor twisted logic where extraterrestrials are always the simplest explanation, your channel still rocks

    • Hi Angry Astronaut, your channel is great!

      The article says: “Since DFD provides power as well as propulsion in one integrated device, it would also provide as much as 2 MW of power to the payloads upon arrival.”

      So this device is supposed to produce 2 MW of electricity from its fusion, and also be able to produce thrust. How is that easier than making a device to produce 2 MWe without the thrust?

      Creating fusion power plus thrust is actually harder than producing fusion power alone. That easier, power-only problem is not yet solved. Though I’m hopeful about several projects currently working on it.

      • We are used to nuclear rocket concepts like NERVA that convert heat to thrust. Or power generating reactors that convert neutron energy to heat, boiling water to convert to mechanical energy, then turbines to convert to electricity. In contrast an ANEUTRONIC reactor generates its energy directly as very fast charged particles (i.e. thrust) rather than heat. Converting some of the charged particle kinetic energy to electricity does not add nearly as much hardware.

        • The funny part is that you seem to think D-3He is aneutronic… Look at p-B for that, but it takes over 1 BILLION C temperatures…

          Current fission plants get almost all of their energy from fission products, not neutrons. The neutrons are used to make more fission products and sustain the neutron flux.

          • The He3 reaction that is truly aneutronic is He3 + He3 yields 4He3 + 2protons + 12.8 MEV. In other words keep all deuterium the heIl out of there. I know helion and others think in terms of breeding He3 in situ, but then you get all the curses of neutrons and heat. He3 can be made separately in fission reactors.

            Okay yes, fission fragments carry energy and are charged, (some ions are +40!) But all that momentum is wasted as heat rather than channeled directly to thrust, unless you are thinking of dust reactors.

            The temperatures quoted for various fusion reactions only apply to reactions achieved at thermal equilibrium by brute force heat. Field reversed configuration is much more complex, I dont pretend to understand it only that it is TOTALLY about thermal DISequilibrium, so not clear that temperature even means the same thing as in “conventional” confined plasma fusion.

            • Damn another typo. Correct reaction is 2He3 to He4 + 2p.

12. The slide talks about 10 metric tons to Titan in less than a year

13. They’re certainly optimistic. But how are they proposing to dispose of waste heat? I don’t see any radiators…

    • Yep, as Brett sez … remarkably optimistic.

      Using their glossies (from above), several things stand out as fairly magic-wand wishful.

      ²H + ³He fuel (and reaction mass?) The deuterium is almost as cheap as it gets. Small percentage of all terran water. The ³He though is obscenely precious stuff. No more than 100 kg of it exists in all the laboratories of our present day physics research firms. And it doesn’t grown on trees. Potentially, it is ‘breed-able’ by neutron spallation of lithium, so might not be as precious as it presently is, but still … Big Unicorn Horn Wand stuff.

      If the fuel also is the reaction mass, then an enormous amount of it (proportionate to its energy potential) is blown out the back end and gone for good. This certainly impacts the economics of operation.

      ISP 10³ to 10⁵ — doesn’t sound all that unreasonable. Since the exhaust velocity is effectively G₀*Isp, then that corresponds to about 10⁴ to 10⁶ m/s. Using basic energy physics (E = ½mv²), that in turn means that the invested kinetic energy ranges from 0.5×10⁸ to 0.5×10¹² joules per kilogram of reaction mass. Lot of energy. But Léts go with it.

      Since the thrust is supposedly 10 to 100 newtons, conversely the reaction mass must be somewhere between (100 N ÷ 10⁶ Ns/kg ) = 10⁻⁴ kg/s or 0.1 gram/second at the high end down to milligrams at the low thrust end.

      That kind of asks “well, how long is the probe supposed to be under thrust?” I can see for reaction-mass conservation, a peaked efficiency 10 N thrust regime consuming 10 mg/second or 860 kg/day might be preferred.

      But wait … that’s almost a ton a day! I thought the probe was to weigh in at 1000 kg itself. Something must be missing from the glossies.

      To get to Pluto in 4 years requires a ΔV of about 50 km/sec. That’s one helluva ΔV. Working backward from Tsiolkovsky’s Rocket Equation, (ΔV = G₀ • Isp • ln( Mstart / Mend )) then one can see that

      G₀ = 9.81 m/s²
      Isp = 10⁵ seconds
      ΔV = 50,000 m/s

      50,000 = 9.81 × 100,000 × ln( Ms/Me ); … rearrange a bit
      0.050 = ln( Ms/Me ); … exponentiate
      1.053 = Mstart/Mend

      So the lost mass is only about 5.3% of the start mass. THAT is seemingly quite reasonable! Whew! I’ll have to read-and-see if the rest of it makes more sense.

      • 10mg/sec = 864 g/day. Off by only 1000x…

        • in 4 years, ~1200kg

        • I know… so close, right? LOL

      • Can I follow you on Twitter or something? Where are you? Every comment you’ve made here has been worth reading. Would love to read more.

      • I guess I could be wildly optimistic and assume practical d-d reactors. In that case there would be lots of He3 byproduct.

        • Seems to me that rather like the parallel (but entirely different) problem of bootstrapping a so-called ‘thorium fission’ reactor (which requires some fissile material like 233U to begin with), maybe the same goes for the D-3He reactor. Suppose a large majority of D; yes-yes-yes, of course the rate of D-D and D-He fusion are both substantially dependent on the fusion ‘temperature’ or internal kinetic energy.

          But the thing is, D-D doesn’t exactly ‘fall off’ as T rises (to where D-He becomes more likely). It’s a statistics thing. Temperature, reactants, time, compression and recycling rate. (‘Catching’ the massively fast out-flying products or kinetically ping-ponged bystander ions, back into the reaction area).

          With a D majority, D-D fusion becomes much higher relative to a 50:50 reaction. D-D makes 3He 50% of the time, so it becomes a breeder. So long, of course, that the 3He+ ions are captured well by the magnetic bottle. Captured, and REFLECTED back in, to keep the ‘temperature’ and reaction rates up. It’s our ‘only hope’, actually.

          So, then a very smallish amount of 3He suffices, and it is bred I would think rather copiously as the enriched D mixture does its fusing. Now, where it gets is fusing energies from … well as another author said, what about a high energy intensity fission reactor too? Hmmm… not a bad idea.

          GoatGuy

          • Why does anyone consider breeding 3He from D-D? Tritium, which decays all by itself to He3 at the rate of 5% per year, was produced in quantity for years for nuclear weapons boosting. Specialized reactors for breeding tritium could scale to any desired amount, or just replacing some control rods in conventional reactors with lithium has been shown to make plenty of tritium. Tritium is also an annoying waste product of Candu reactors.

            • There are not enough Candu reactors for the T or 3He needs of fusion reactors. I’m quite sure they already have customers for all the T they create.

              • Lithium 6 exposed to flux of thoroughly understood FISSION reactor: N+Li6->He4 + He3. You even get 17MEV energy out that could be harnessed. Of course reactors would have to be designed and built specifically for the purpose, but these would be vastly simpler and cheaper than any D-D fusion reactor could ever be, and the tech has already been perfected, (https://www.nrc.gov/docs/ML0325/ML032521359.pdf). My point was not that unintentionally produced waste tritium would ever be sufficient, but that tritium is not so difficult to make that it justifies extreme tech such as fusion reactors. And helium 3 follows from just waiting.

                • SORRY! First sentence has TYPO: its He4+H3 (tritium). He3 is obtained by decay at 5%/year.

      • So, yes … I was off by 1,000x in estimating fuel-reactionmass loss rate on a daily basis. 10 mg/sec * 60 * 60 * 24 = 864,000 mg/day. 864 g/d. 0.864 kg/day. 315 kg per year. Really … I ought to use a spreadsheet for doing these calcs.

      • I thought deuterium was expensive and worth more than it’s own weight in gold presently?

        https://physicsworld.com/a/quantum-sieve-could-make-deuterium-much-cheaper/

        “Pricier than gold

        Deuterium has a range of uses from nuclear fusion to medical imaging and drug discovery. Today, chemical and physical processes are used to extract heavy water molecules – which contain two deuterium nuclei – from water. Then deuterium gas is created by electrolysis. The process is very expensive, with one gram of deuterium costing more than a gram of gold.

        Some researchers believe that quantum sieves could do the job at much lower cost. A quantum sieve is a material that is porous on the molecular scale. The sieve is designed so that some types of molecules interact with the pores and pass through, while others do not.”

      • Bravo!
        Napokon neko da napiše nešto ovako.
        Moglo je i ovako:
        P=m*a*w/2 => a= 2*P/m*w
        P= 2MW
        w=Isp*g
        a- acceleration
        But m Is not const. m Is mass of craft.

        • [2MW for 4yrs is ~70GWh energy output for distance Earth-Pluto ~6*10^9km, ~2kWh/100mi (?), until ~2499 (since ~1600) closest approach to Pluto is ~May2238 at ~4.28*10^9km, ‘now’ light needs ~4h41m from Pluto to Earth at ~5*10^9km distance]

      • He3 is unobtainium ONLY because there are hardly any facilities devoted to making it. Tritium decays to He3 all by itself at the rate of 5%/year. Li6 + N->H3+H4 can be used to make as much tritium as desired, in well-understood fission reactors, in a proven process long used to produce tritium for nuclear weapon boosting. Many reactors might be required but they would be much much cheaper and simpler than any D-D fusion reactor could ever be. I am mystified that anyone would think of moon mining or exotic unproven fusion reactors to obtain such a straightforward product.

    • Heat exchanges and some sort of new advanced cooling system. They have it figured out, but have not publicly disclosed it.

14. So we are going to skip fusion power stations because they are too hard and instead go to fusion powered craft that act as power stations? Maybe we can land one carefully and run an extension cord into it. 🙂 😀

    • Not so far off the mark. One of the actions of the Halliburton Administration when it came to power in 2001 was to cut all NASA fusion drive research to the bone for just thar reason.

      “The oil must flow…”

      • It’s a fun story.

        Billions of private capital have gone into fusion. So far, they’ve produced good press releases.

        • Oh sure billions of private capital has gone in to fusion but we all know that private capital isn’t as effective at producing results as NASA spending billions on a project.

    • We should skip fusion power stations because fission power will always give far more bang for the buck for power generation over thermal-equilibrium type (brute force heat) fusion reactors, which really need to be the size of a star or at least a medium sized french province. This kind of fusion is really bad for power generation both on earth and in space; and completely unnecessary as it has no advantage over 4th generation nuclear. On the other hand, aneutronic fusion proposed here (where energy is carried by fast charged particles rather than neutrons) is a natural for propulsion. power generation is an unavoidable side benefit of aneutronic fusion, but this is not yet a good approach for terrestrial power generation. Radically different approaches to fusion.

      • Again D-3He is NOT aneutronic. The D-D reaction is still very prevalent and cannot be turned off. Lots of 2.5 MeV neutrons hitting the walls. The idiots that propose mining the moon for 3He like its some magic ingredient that will make fusion work are just that. It makes fusion MUCH harder while only lowering the neutron count. p-B is fully aneutronic, but needs over a Billion degrees to work.

        • Tritium decays to He3 all by itself at the rate of 5%/year. World production of tritium is tens of kilograms even without any facilities dedicated to producing it. Li6 + N->H3+H4 can be used to make as much tritium as desired, in well-understood fission reactors, in a proven process long used to produce tritium for nuclear weapon boosting. Many reactors might be required but they would be much much cheaper and simpler than any D-D fusion reactor could ever be. I am mystified that anyone ever considered moon mining or exotic fusion reactors for such an easily produced product.

          Thanks for pointing out that D+He3 – or ANY reaction with deuterons in it – has the same problems as DT, although to a lesser extent. The reaction to use is 2He3 ->He4+He2. Keep all deuterons OUT. Until He3 becomes abundant, there may be a need for temporary alternatives.

        • No need for 100% aneutronic, even the d-d reaction produces charged products (ionized gas) that can be steered to use their momentum directly as thrust. Its just that all momentum in neutrons, while great if all you want is heat, is wasted for direct thrust because cannot be steered. Any application aiming to use product momentum directly for thrust and electricity (rather than indirectly as heat) will have to minimize but not necessarily eliminate neutrons (a waste byproduct). (p+B) and (He3 + He3) would completely eliminate that waste, but as you point out pB is too hard and He3 is not yet abundant. For now, generation of maximum stearable momentum may require tolerating some parasitic neutrons. Feels odd since the reverse is true for all other types of nuclear reactors, where neutrons are desirable.

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