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Even if we pretended that it's rational to hedge all your bets on not only having a working fusion reacter in the near term future but having the capability to

Even if we pretended that it's rational to hedge all your bets on not only having a working fusion reacter in the near term future but having the capability to miniturise it to a degree where it could be used as an engine for a launch vehicle, you would still be dead wrong. Fusion still shares the same fundumental problem that chemical rockets or even more advanced options like blackhole or antimatter drives face - the rocket equation. We need to use near term reusable launch vehicles like the Starship to help set up the required launch assist structures that would make genuine mass transit of people and cargo into orbit possible. Things like skyhooks, lofstroom loops, and eventually orbital rings that would allow people to ride trains or cable cars from an earth city straight into orbit for the same price range as intercity travel today. Needless to say, that is not going to happen until there is enough of an industry up there to justify the large upfront costs of such megaprojects (especially the orbital ring) so the Starship is our best near term hope of getting us up to that point. Note that none of these projects require advanced materials or new technologies (as opposed to the space elevator that is likely to remain science fiction when it comes to dealing with a body with the gravity well of earth anyway), they're just massive engineering projects.

dchichkov 5y ago
The difference between chemical/fission and fusion are orders of magnitude. It is <10km/s for chemical/fission. And 1000km/s for fusion: "Today’s best chemical rockets produce propellant exhaust velocities up to 4.5 km/s. Fission (nuclear thermal) rocket engines could roughly double that, to about 8.5 km/s. A working nuclear fusion core producing thrust through direct exhaust of hot plasma, could readily generate up to ~1000 km/s exhaust velocities" - https://link.springer.com/article/10.1007/s10894-015-0034-1 https://link.springer.com/article/10.1007/s10894-015-0034-1 Likely necessary for advanced space flight. Likely necessary to allow for "clean energy" on earth. Unlike advances to flamethrowers.
- vimacs2 5y ago
  Having high exhaust velocities does not mean you will have the high thrusts required to get yourself out of the earth's atmosphere. Most current proposals for Nuclear Fusion powered ships involve an exhaust that is orders of magnitude more diffuse than that of chemical rockets. They only come into their own once they are already in space or in orbit which returns us to the problem that we need to have launch assist infrastructure that can deal with that first step without resorting to those "flamethrowers" as you call them. It's also not true at all that fusion is necessary to allow for clean energy (even ignoring that many of the near term fusion reactor designs still need to have radiation shielding to deal with dangerous neutron emissions). We could use a combination of renewable sources and nuclear fission to deal with the energy crisis now and then utilise fusion once it's finally matured to the point of usefulness. Indeed, we could use that orbital infrastructure to help construct a planetary cloud of solar collectors that can operate 24/7 and beam that energy down via microwave rectiner technology that is already capable of achieving greater than 85% energy conversion (keep in mind that we lose far more energy simply from the diffusion of sunlight through the atmosphere). This approach not only can power the earth but any off earth colonisation prospects with line of sight with our planetary cloud - without needing to haul the likely massive early generation fusion reactors. Neither is fusion necessary for advanced space travel. It would be very useful but chiefly for travel to under-developed sectors of the solar system. solar reflectors scaled up enough can be used to speed up and slow down entire fleets of ships with far higher potential top speeds than even antimatter drives permit (since there is no requirement to have onboard propellant). They also double as handy point defense systems for dealing with both debris and ships that are attempting to turn themselves into relativistic kill missiles - remember that there is no such thing as an unarmed space ship, they are all potential weapons of mass destruction, especially once we reach the state where we have fusion drive powered ones.
  - dchichkov 5y ago
    I think, a few more bets in the projects like - https://arpa-e.energy.gov/technologies/projects/next-generation-pfrc https://arpa-e.energy.gov/technologies/projects/next-generat... - may have a chance of yielding solutions. But it is not going to happen by itself. And orbital infrastructure... Sounds impractical. Even if somehow possible, it'd be risky to have a dependency on such infrastructure. Fragile and susceptible to so many risks.
    - vimacs2 5y ago
      They can be made as sturdy and secure as your budget and desire extends. These things would be designed from the ground up to have additional atlas piller supports for redundancy in case a ring gets damaged with many different power supplies to guarantee that the active support system continues to operate. Remember that you do not need to move all of this mass off anywhere once in place so you build it like a building, not a ship that needs to be optimised for low mass and acceleration. You do not build these flimsy since even the modest ones will have a big upfront cost and the better equipped one is, the bigger the payoff. They are incredibly easy to extend and maintain once the initial ring is set into place though since they are in orbit so as long as you match its own orbital plane, you can operate on it just fine. You will have a big superstructure magnetically suspended around the moving ring rotating in the opposite direction to the inner ring at the speed of the earth's own rotation so it stays stationary relative to the ground, and which is both half the reason why we build these things but also makes it significantly difficult to penetrate that inner ring. That superstructure is going to be much heavier than the inner ring by necessity since that inner ring is moving faster than orbital speeds to counteract the momentum generated by the superstructure. So you might as well use that mass for useful things. We should also not think of active support structures as somehow less sturdy than passive ones. In fact they are far more sturdy since they can be used to create arbitrarily high levels of compressive strength - whatever level of juice you are willing to expend - and this is where those fusion reactors would indeed come in handy. You would also in reality use multiple inner rings instead of just one though one would be fine too for an initial construction. Keep in mind though that even considering conventional power sources and no superconductors, these things can regenerate a decent fraction of that expended energy back by using the moving ferromagnetic matter inside to generate electricity through the induction generated by either orbiting along the earth or in the case of the atlas piller, falling all the way down from the mesosphere or even further if you decide that it's worth your investment to extend one all the way to geostationary ala the classic space elevator. Might be a bit overkill though since the orbital ring itself can easily launch you at decent speeds for cislunar and even interplanetary travel. You also probably have a shit tonne of solar panels on the superstructure itself and would probably want to add a fission or fusion reactor too for added redundancy since you always want multiple power sources for very large structures. Either that or we continue to rely on rockets which cannot be scaled up to the point where off world travel is as common as air travel without bringing ecological ruin and gargantuan amounts of noise pollution. It's just about the only real curse of being on a planet that has both a thick atmosphere relative to most rocky planets (in our solar system) and also being the massive of the rocky planets.

dchichkov 5y ago

The difference between chemical/fission and fusion are orders of magnitude. It is <10km/s for chemical/fission. And 1000km/s for fusion: "Today’s best chemical rockets produce propellant exhaust velocities up to 4.5 km/s. Fission (nuclear thermal) rocket engines could roughly double that, to about 8.5 km/s. A working nuclear fusion core producing thrust through direct exhaust of hot plasma, could readily generate up to ~1000 km/s exhaust velocities" - https://link.springer.com/article/10.1007/s10894-015-0034-1 https://link.springer.com/article/10.1007/s10894-015-0034-1 Likely necessary for advanced space flight. Likely necessary to allow for "clean energy" on earth. Unlike advances to flamethrowers.

vimacs2 5y ago
Having high exhaust velocities does not mean you will have the high thrusts required to get yourself out of the earth's atmosphere. Most current proposals for Nuclear Fusion powered ships involve an exhaust that is orders of magnitude more diffuse than that of chemical rockets. They only come into their own once they are already in space or in orbit which returns us to the problem that we need to have launch assist infrastructure that can deal with that first step without resorting to those "flamethrowers" as you call them. It's also not true at all that fusion is necessary to allow for clean energy (even ignoring that many of the near term fusion reactor designs still need to have radiation shielding to deal with dangerous neutron emissions). We could use a combination of renewable sources and nuclear fission to deal with the energy crisis now and then utilise fusion once it's finally matured to the point of usefulness. Indeed, we could use that orbital infrastructure to help construct a planetary cloud of solar collectors that can operate 24/7 and beam that energy down via microwave rectiner technology that is already capable of achieving greater than 85% energy conversion (keep in mind that we lose far more energy simply from the diffusion of sunlight through the atmosphere). This approach not only can power the earth but any off earth colonisation prospects with line of sight with our planetary cloud - without needing to haul the likely massive early generation fusion reactors. Neither is fusion necessary for advanced space travel. It would be very useful but chiefly for travel to under-developed sectors of the solar system. solar reflectors scaled up enough can be used to speed up and slow down entire fleets of ships with far higher potential top speeds than even antimatter drives permit (since there is no requirement to have onboard propellant). They also double as handy point defense systems for dealing with both debris and ships that are attempting to turn themselves into relativistic kill missiles - remember that there is no such thing as an unarmed space ship, they are all potential weapons of mass destruction, especially once we reach the state where we have fusion drive powered ones.
- dchichkov 5y ago
  I think, a few more bets in the projects like - https://arpa-e.energy.gov/technologies/projects/next-generation-pfrc https://arpa-e.energy.gov/technologies/projects/next-generat... - may have a chance of yielding solutions. But it is not going to happen by itself. And orbital infrastructure... Sounds impractical. Even if somehow possible, it'd be risky to have a dependency on such infrastructure. Fragile and susceptible to so many risks.
  - vimacs2 5y ago
    They can be made as sturdy and secure as your budget and desire extends. These things would be designed from the ground up to have additional atlas piller supports for redundancy in case a ring gets damaged with many different power supplies to guarantee that the active support system continues to operate. Remember that you do not need to move all of this mass off anywhere once in place so you build it like a building, not a ship that needs to be optimised for low mass and acceleration. You do not build these flimsy since even the modest ones will have a big upfront cost and the better equipped one is, the bigger the payoff. They are incredibly easy to extend and maintain once the initial ring is set into place though since they are in orbit so as long as you match its own orbital plane, you can operate on it just fine. You will have a big superstructure magnetically suspended around the moving ring rotating in the opposite direction to the inner ring at the speed of the earth's own rotation so it stays stationary relative to the ground, and which is both half the reason why we build these things but also makes it significantly difficult to penetrate that inner ring. That superstructure is going to be much heavier than the inner ring by necessity since that inner ring is moving faster than orbital speeds to counteract the momentum generated by the superstructure. So you might as well use that mass for useful things. We should also not think of active support structures as somehow less sturdy than passive ones. In fact they are far more sturdy since they can be used to create arbitrarily high levels of compressive strength - whatever level of juice you are willing to expend - and this is where those fusion reactors would indeed come in handy. You would also in reality use multiple inner rings instead of just one though one would be fine too for an initial construction. Keep in mind though that even considering conventional power sources and no superconductors, these things can regenerate a decent fraction of that expended energy back by using the moving ferromagnetic matter inside to generate electricity through the induction generated by either orbiting along the earth or in the case of the atlas piller, falling all the way down from the mesosphere or even further if you decide that it's worth your investment to extend one all the way to geostationary ala the classic space elevator. Might be a bit overkill though since the orbital ring itself can easily launch you at decent speeds for cislunar and even interplanetary travel. You also probably have a shit tonne of solar panels on the superstructure itself and would probably want to add a fission or fusion reactor too for added redundancy since you always want multiple power sources for very large structures. Either that or we continue to rely on rockets which cannot be scaled up to the point where off world travel is as common as air travel without bringing ecological ruin and gargantuan amounts of noise pollution. It's just about the only real curse of being on a planet that has both a thick atmosphere relative to most rocky planets (in our solar system) and also being the massive of the rocky planets.