5 ms·

Spoiler: the focal point is 3.5x the distance to Voyager 1.

trhway 2y ago
if to use existing nuclear reactor tech and already existing, as tested by NASA (and drives Starlink satellites), ionic drive - about 3500 ISP - that focal point would take about 10 years to reach. I hope that SpaceX flights to Mars will, after the probably first chemical ones, be done using ionic drive with solar as it is just faster, thus getting tech developed and with adding nuclear for beyond Mars - so in 10-20 years we'll have the stuff flying. (note that "small" reactors - 100MW - we have for submarines, and with MS, ORCL, GOOG, AMZN getting into nuclear we'll have such small reactors productized into normal commercial use which will simplify space use too as commercial use require higher reliability/etc. compare to military)
- kibwen 2y ago
  > that focal point would take about 10 years to reach Is this taking into account the time needed to slow down?
  - trhway 2y ago
    It is the napkin scale, not precise mission calculation. Doing 2 stages you can get faster, doing higher voltage you can get faster, etc. Slow down would of course take time and delta-v, changing observation station would also take them, etc. What interesting is that increasing Isp 10x seems to be doable with the today's/near-future tech, and that would even allow 1000 year mission to the closest star using 3 stages (unfortunately even my napkin breaks though when trying to stretch to the 100 years mission to the star using the today's/near-future tech).
    - kibwen 2y ago
      Right, but assuming constant acceleration, there's an enormous difference between accelerating all the way to the target and only accelerating halfway to the target, and then decelerating the rest of the way.
      - trhway 2y ago
        >but assuming constant acceleration it isn't realistic assumption. Until you're talking pure solar, the amount of acceleration is limited by the reaction mass available. Actually to get there in 10 years with the Isp 3500 3 stages are necessary, or better the Isp should be increased 2x-4x - still seems doable - to get with like 2 stages with realistic [today] parameters of the reactors/etc.
        oneshtein 2y ago
        We can use solar sail for initial acceleration, then we can attract surrounding ion/particles/dust using long wires with negative charge, to have more mass for further acceleration.
        trhway 2y ago
        i think solar sail is red herring kind of. Really slow and very big, and hardly working beyond Mars. Compare to nuclear powered ion thruster. Say we get reactor plus generation at 5KW/kg total - takes some engineering, yet nothing unrealistic for current tech (even 10KW/kg seems pretty reachable). Reactor is on a long pole with only small protection wall directly between reactor and payload. Say 5 ton reactor, 25MW. 100 ton whole rocket, 80 ton of it reaction mass. At current NASA 40km/s ion trusters we get delta-v 80km/s in 60 days. If we get thrusters with 80km/s - wikipedia mentions that current ones reach 50km/s, so don't see why we can't increase voltage and thus ejection speed further - then it would take 240 days to reach delta-v 160km/s (i.e. current multi-year missions to Jupiter/etc. would get in well under a year, and it will be with like 10 ton payloads). Don't see solar sails coming close to that - https://en.wikipedia.org/wiki/Solar_sail#Inner_planets https://en.wikipedia.org/wiki/Solar_sail#Inner_planets. And as i mentioned earlier - let say we got thrusters with 400km/s. The same rocket will get to 800km/s - 1500 years to the nearest star - in 20 years. 3 stages - 500 years to the nearest star. 1 ton final payload if starting with 1000 ton rocket like the one described above. Gathering reaction mass ram style - it needs big apparatus and needs to be efficient. Doesn't seem realistic with current tech, yet i'm sure will be on the table once the tech matures.
  - rcxdude 2y ago
    You don't need to slow down, the region where you can do the observations is basically a slowly expanding cone the further you get away from the sun. But it does sound like the current plan involves a 25 year journey before observations start.
- Cthulhu_ 2y ago
  They're hesitant to put nuclear reactors in a rocket though, in case it goes wrong.
  - trhway 2y ago
    that will pass. With Starship you can deliver reactor into orbit in a "safe box" such that if anything happens with the rocket during launch and acceleration the box with the reactor will fall without breaking apart. Such box can be made in self-stabilizing shape similar to Dragon capsule so that it will slow down in atmosphere. One can imagine 10 cm thick tungsten and steel walls, etc. for the "box" to not break on fall/reentry or in the rocket explosion. And you don't need it for the whole reactor, only for the nuclear fuel.
    - perihelions 2y ago
      You don't need anything at all. A reactor that has never been turned on is not radioactive. The only protection you need is to simply not start the reactor until it's on a planetary escape orbit, one that does not return.
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if to use existing nuclear reactor tech and already existing, as tested by NASA (and drives Starlink satellites), ionic drive - about 3500 ISP - that focal point would take about 10 years to reach. I hope that SpaceX flights to Mars will, after the probably first chemical ones, be done using ionic drive with solar as it is just faster, thus getting tech developed and with adding nuclear for beyond Mars - so in 10-20 years we'll have the stuff flying. (note that "small" reactors - 100MW - we have for submarines, and with MS, ORCL, GOOG, AMZN getting into nuclear we'll have such small reactors productized into normal commercial use which will simplify space use too as commercial use require higher reliability/etc. compare to military)

kibwen 2y ago
> that focal point would take about 10 years to reach Is this taking into account the time needed to slow down?
- trhway 2y ago
  It is the napkin scale, not precise mission calculation. Doing 2 stages you can get faster, doing higher voltage you can get faster, etc. Slow down would of course take time and delta-v, changing observation station would also take them, etc. What interesting is that increasing Isp 10x seems to be doable with the today's/near-future tech, and that would even allow 1000 year mission to the closest star using 3 stages (unfortunately even my napkin breaks though when trying to stretch to the 100 years mission to the star using the today's/near-future tech).
  - kibwen 2y ago
    Right, but assuming constant acceleration, there's an enormous difference between accelerating all the way to the target and only accelerating halfway to the target, and then decelerating the rest of the way.
    - trhway 2y ago
      >but assuming constant acceleration it isn't realistic assumption. Until you're talking pure solar, the amount of acceleration is limited by the reaction mass available. Actually to get there in 10 years with the Isp 3500 3 stages are necessary, or better the Isp should be increased 2x-4x - still seems doable - to get with like 2 stages with realistic [today] parameters of the reactors/etc.
      - oneshtein 2y ago
        We can use solar sail for initial acceleration, then we can attract surrounding ion/particles/dust using long wires with negative charge, to have more mass for further acceleration.
        trhway 2y ago
        i think solar sail is red herring kind of. Really slow and very big, and hardly working beyond Mars. Compare to nuclear powered ion thruster. Say we get reactor plus generation at 5KW/kg total - takes some engineering, yet nothing unrealistic for current tech (even 10KW/kg seems pretty reachable). Reactor is on a long pole with only small protection wall directly between reactor and payload. Say 5 ton reactor, 25MW. 100 ton whole rocket, 80 ton of it reaction mass. At current NASA 40km/s ion trusters we get delta-v 80km/s in 60 days. If we get thrusters with 80km/s - wikipedia mentions that current ones reach 50km/s, so don't see why we can't increase voltage and thus ejection speed further - then it would take 240 days to reach delta-v 160km/s (i.e. current multi-year missions to Jupiter/etc. would get in well under a year, and it will be with like 10 ton payloads). Don't see solar sails coming close to that - https://en.wikipedia.org/wiki/Solar_sail#Inner_planets https://en.wikipedia.org/wiki/Solar_sail#Inner_planets. And as i mentioned earlier - let say we got thrusters with 400km/s. The same rocket will get to 800km/s - 1500 years to the nearest star - in 20 years. 3 stages - 500 years to the nearest star. 1 ton final payload if starting with 1000 ton rocket like the one described above. Gathering reaction mass ram style - it needs big apparatus and needs to be efficient. Doesn't seem realistic with current tech, yet i'm sure will be on the table once the tech matures.
- rcxdude 2y ago
  You don't need to slow down, the region where you can do the observations is basically a slowly expanding cone the further you get away from the sun. But it does sound like the current plan involves a 25 year journey before observations start.
Cthulhu_ 2y ago
They're hesitant to put nuclear reactors in a rocket though, in case it goes wrong.
- trhway 2y ago
  that will pass. With Starship you can deliver reactor into orbit in a "safe box" such that if anything happens with the rocket during launch and acceleration the box with the reactor will fall without breaking apart. Such box can be made in self-stabilizing shape similar to Dragon capsule so that it will slow down in atmosphere. One can imagine 10 cm thick tungsten and steel walls, etc. for the "box" to not break on fall/reentry or in the rocket explosion. And you don't need it for the whole reactor, only for the nuclear fuel.
  - perihelions 2y ago
    You don't need anything at all. A reactor that has never been turned on is not radioactive. The only protection you need is to simply not start the reactor until it's on a planetary escape orbit, one that does not return.

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