3 ms·

> that focal point would take about 10 years to reach Is this taking into account the time needed to slow down?

by kibwen 2y ago

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.
        lazide 2y ago
        The big issue with this - how would we ever get anything useful back? A 1 ton payload is probably not enough to even stick a sufficiently powerful laser on to see at all over those distances.
        trhway 2y ago
        1. by that time we can definitely use Sun as a receiver's lense 2. 1 ton is starting from 1000 ton rocket. The Saturn V and Startship are on the scale of 5000 ton and assembled on Earth. That interstellar rocket will be assembled in space anyway, so not being subject to any meaningful gravitational nor accelerational stresses, we can easily build a 100000 or even 500000 ton rocket - basically just the reactors and tanks of acceleration mass - and thus get 100-500 ton payload. If we get any [semi]hybernation going (may be combined with 3d printing or CRISPR-like repair of organs, whatever we get in 20-30 years) or more probably some brain [partial] upload integrated with AI into some capable cyborg, may be even some people or those merged human/AI cyborgs would be able to go. And by collecting some additional reaction mass ram-style over that distance and time (as long as we have enough reactor power to use part of the collected mass to avoid slow down resulting from the collection) we'd probably be able to slow down some small probes to land and orbit various objects in the target star system.
  - ndheebebe 2y ago
    Silly question. Can you do a "drive by". In other words not slow down. How much time you need to "take the photo". I am using terms like Randall in Thing Explainer here!! Maybe it has further missions in deep space after that. Or look in other directions and use other stars.
    - kibwen 2y ago
      I've actually just finished watching the video linked elsewhere in this thread and a drive-by is exactly what they propose, using multiple telescopes launched on staggered schedules in order to make repeated observations and gradually refine the image.
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.

> that focal point would take about 10 years to reach Is this taking into account the time needed to slow down?

by kibwen 2y ago

> that focal point would take about 10 years to reach Is this taking into account the time needed to slow down?

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.
      - lazide 2y ago
        The big issue with this - how would we ever get anything useful back? A 1 ton payload is probably not enough to even stick a sufficiently powerful laser on to see at all over those distances.
        trhway 2y ago
        1. by that time we can definitely use Sun as a receiver's lense 2. 1 ton is starting from 1000 ton rocket. The Saturn V and Startship are on the scale of 5000 ton and assembled on Earth. That interstellar rocket will be assembled in space anyway, so not being subject to any meaningful gravitational nor accelerational stresses, we can easily build a 100000 or even 500000 ton rocket - basically just the reactors and tanks of acceleration mass - and thus get 100-500 ton payload. If we get any [semi]hybernation going (may be combined with 3d printing or CRISPR-like repair of organs, whatever we get in 20-30 years) or more probably some brain [partial] upload integrated with AI into some capable cyborg, may be even some people or those merged human/AI cyborgs would be able to go. And by collecting some additional reaction mass ram-style over that distance and time (as long as we have enough reactor power to use part of the collected mass to avoid slow down resulting from the collection) we'd probably be able to slow down some small probes to land and orbit various objects in the target star system.
- ndheebebe 2y ago
  Silly question. Can you do a "drive by". In other words not slow down. How much time you need to "take the photo". I am using terms like Randall in Thing Explainer here!! Maybe it has further missions in deep space after that. Or look in other directions and use other stars.
  - kibwen 2y ago
    I've actually just finished watching the video linked elsewhere in this thread and a drive-by is exactly what they propose, using multiple telescopes launched on staggered schedules in order to make repeated observations and gradually refine the image.

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.