3 ms·

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

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?

cryptonector 3y ago
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!
sjburt 3y ago
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.
sigmoid10 3y ago
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.
- jmward01 3y ago
  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.' https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse
  - sigmoid10 3y ago
    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.
    - cryptonector 3y ago
      They must be getting heat from the fusion process, thus increasing ISP. Nothing else would make sense here.
      - sigmoid10 3y ago
        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: https://thedebrief.org/rocketstar-successfully-demonstrates-firestar-nuclear-fusion-enhanced-pulsed-plasma-propulsion-drive/ https://thedebrief.org/rocketstar-successfully-demonstrates-...

cryptonector 3y ago

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!

sjburt 3y ago

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.

sigmoid10 3y ago

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.

jmward01 3y ago
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.' https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse
- sigmoid10 3y ago
  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.
  - cryptonector 3y ago
    They must be getting heat from the fusion process, thus increasing ISP. Nothing else would make sense here.
    - sigmoid10 3y ago
      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: https://thedebrief.org/rocketstar-successfully-demonstrates-firestar-nuclear-fusion-enhanced-pulsed-plasma-propulsion-drive/ https://thedebrief.org/rocketstar-successfully-demonstrates-...