Pulsar Fusion Achieves Initial Pulse Experiments and Targets Orbital Test in 2027 and Production Fusion Rocket in Early 2030s

3 thoughts on “Pulsar Fusion Achieves Initial Pulse Experiments and Targets Orbital Test in 2027 and Production Fusion Rocket in Early 2030s”

1. As Brett sez … the numbers don’t add up, even remotely. Moreover, even though superficially ‘really high Isp’ numbers seem desirous, in truth they too have a dark counter-side: Way, way more wasted power. Ironic, isn’t it?

   Because (do take a look above) conventional chemical propulsion technologies like Methane-Oxygen (Isp = 330, maybe) … or Hydrogen-Oxygen (Isp = 440) have Isp’s in the lowish looking side. The contrary side is that the specific energy of the exhaust stream is a function of ½(9.81 * Isp)² … so 440 Isp is 9.32 megajoules of kinetic energy per kilogram of exhaust gas. In a way “its free” for chemical engines, the Isp being a function of the energy of the chemical reaction and (inverse) density of the reaction gasses. The fusion case though requires a bunch of input energy to accelerate (and magnetically bottle) the reactants long enough to ‘fuse’ in the helpful way, and then be redirected (capturing the momentum i guess) to sluice out the back end. Isp’s of 25,000 (per diagram above) in turn require fusion-power-enhanced-output streams having kinetic energy over 300 megajoules per kilogram.

   And this is where Mr. Brett is spot on: If those 300 megajoules are largely a result of fusion, and near miraculous energy capture (elastic and thus near lossless magnetic fields), and the fusion multiplier is ‘only 1.1’, (figure of 0.1), then 270 megajoules of external energy would be needed externally to accomplish it. And where might that power come from?

   He then — again with crisp logic — goes on to ask, well … what about the cooling fins? Because anything near the power levels required is going to demand production of a lot of electrical energy (which fusion depends exclusively on). And that in turn requires some other energy source (methinks, fission) to go through the whole Carnot cycle heat-to-electricity conversion. 35% tops. LOTS of radiators, heat fins. Lots.

   So, while tantalizing from a Science point of view, I think their proposals are still nearly fictional.

   • Yup. That’s why fusion rockets aren’t actually any practical use until you’ve got a gain high enough that you almost could use them for a power plant if you wanted.

     At a gain of 0.1 it’s just an extra complicated VASMR that becomes radioactive.

     Now, I’m assuming they, or at least their engineering staff, know that. And that they’re really hoping to get a lot higher gain than the above chart shows. They just don’t want to scare the investors away by admitting how far it currently is from practicality.

     By the way, while I’m usually finding that Google Gemini is pretty good, it’s coming across as a bit bad at complex mathematical reasoning for this. I had to go around several times with it before it ‘realized’ that it was reasoning wrong.

     It did have an interesting point, though: Fusion wins earlier than you’d expect because the fraction of the energy produced inside the engine doesn’t carry any Carnot loss and resulting radiator weight penalty.

     So it actually would be better than VASMR at Q=1. Quite a bit earlier than I’d anticipated.

     This won’t work at 0.1, but it would work for a rocket well before it had a Q high enough to use for a power reactor.

2. So, not so much a fusion *powered* rocket, as a fusion *enhanced* rocket.

   Well, it IS true that a fusion rocket doesn’t have to reach engineering breakeven to be useful, the way a fusion reactor does. As long as you’re getting enough fusion to multiply many times over the input power in the exhaust, the fact that you’ve got to supply power to keep it running isn’t a big deal. A 100MW fusion rocket that demands 1MW to run is a good bargain.

   But, at a fusion gain of 0.1, most of the energy in the exhaust will have to come from outside the engine. You’ll be turning 1MW of outside power into 1.1MW worth of exhaust, which is maybe worthwhile, but maybe not if you take into account the added complexity. It’s really just VASMR with a bit of fusion energy juicing the exhaust a bit. But only a bit.

   And that outside power source is going to be really heavy, remember.

   You really either want a fusion gain of 10, or an engineering gain of 0.9, to get the ratio of exhaust power to input power to be high enough to be worth it.

   A second point: Where are the freaking radiators, in those illustrations?

   Now, maybe a fusion rocket operating at engineering breakeven wouldn’t require radiators, because you could use cryogenic propellant to cool the systems, and all the heat would go out the nozzle.

   But a fusion rocket that’s not near engineering breakeven requires an enormous outside source of power, and that power will almost certainly be operating under ordinary Carnot efficiency constraints. The power source to run that beast will require huge radiators. If it didn’t, it would BE the rocket!

   It always bothers me when a tech firm has fancy *unrealistic* illustrations like this. I take it as a bad sign. The rocket in their illustrations has to have enormously better specs than the numbers they’re promising above, to be remotely realistic.

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