6 ms·

One gee over the course of months is insane. Even if you converted the entire "fuel" reaction mass directly into momentum, you're still carrying tens or hundred

One gee over the course of months is insane. Even if you converted the entire "fuel" reaction mass directly into momentum, you're still carrying tens or hundreds of thousands of tons of fuel for any remotely habitable spacecraft. And no engine has perfect mass conversion. Matter-Antimatter has been simulated to be maybe sixty percent efficient, fusion is around 17%, nuclear pulse propulsion a surprising 7%, fission nuclear gas rockets might max out at 12% but those numbers are dodgy as we don't have sharp numbers on gas core reactors. As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations. I imagine a sort of neo-Polynesian culture, with travellers never quite knowing how the place will look if they ever come back that way.

Teever 3y ago
What if you didn't carry the fuel? Use stationary lasers from your starting point to power your journey half way until you lasers from your destination to slow your craft down.
- pc86 3y ago
  Of all the dangers associated with spaceflight it's a pretty interesting concept to add "lasers at the origin shut off for whatever reason" and "lasers at the destination never turn on / are misaligned / turn on too soon / turn on too late" to the list. Not sure I'd want to be in a spacecraft careening through the cosmos with no realistic way to slow it down.
  - jandrese 3y ago
    There is also the question of how you get the lasers to your destination in the first place.
    - Teever 3y ago
      Use lasers for the first half of the journey to send a rocket that can decelerate and build the laser station at the destination.
    - MilStdJunkie 3y ago
      Send a mirror ahead, bounce the beam back toward the ship to push it backwards. Loses a lot of efficiency, though. Laser propulsion, a lot of the thrust is coming from ablation of the surface.
      - eru 3y ago
        How do you 'send a mirror ahead'? Do you want it to stop, or just keep flying away indefinitely?
        MilStdJunkie 3y ago
        Yep, throwaway mirror, it goes off to wherever once it's done bouncing light back at your ship. Again, much less efficient, so when you get where you want to go, you'd want to put in a laser station there too.
- NotYourLawyer 3y ago
  The farther you get, the dimmer the laser gets. The faster you get, the less energy each photon has.
  - slow_typist 3y ago
    Note that the 2nd statement only applies because of the Doppler effect - energy of the photon is a function of its frequency.
- cypherpunks01 3y ago
  I think the perceived output from stationary lasers on Earth would appear to decrease as your speed increases, since you're accelerating away from the laser photon packets. Because of that behavior, I'm not sure it's a feasible way to get up to relativistic speed? I think the lasers would also need ~10,000 years of fuel, as opposed to ~10 years of fuel if you could somehow figure out how to carry it. But the energy to sustain in either case is probably measured in Dyson spheres, or some fraction of all energy in the universe pretty quickly too.
  - PeterisP 3y ago
    Time dilation also has an effect, since you're receiving 10000 Earth-years of laser energy over 10 ship-years.
- MilStdJunkie 3y ago
  That's the drive of the eponymous generation ship in Kim Stanley Robinson's Aurora. It sends a mirror ahead of itself, then the Sol-Mercury lasers bounce off the mirror, back on the ship to slow it down. I could see the light spacecraft with their matter-antimatter engines laying down "lightways", like railroads in the old west. Once a lightway is established, then it's much less of a big deal to get from system to system. Apart from the tens of thousands of years in subjective time, of course.
ryandrake 3y ago
> As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations. I guess this all depends on how you define simultaneity. Say I got on a relativistic rocket traveling near c and colonized Planet X in 10 years ship time and say 20,000 years Earth time. I put up my TV antenna as soon as I land, and I'd still be receiving Earth transmissions from approximately 10 years after I left, no? So even though civilization is long gone from Earth's point of view, from my point of view, I can keep up with the latest news as if I were still living there. So from my point of view I did not outlive anything. If I were to go back, yes, everything would be gone.
- cypherpunks01 3y ago
  While traveling near c, I believe you'd be receiving Earth transmissions from approximately the same time you left, Earth clocks would appear to slow down to a crawl. But if you land on another planet and change reference frame to be more "stationary" again relative to Earth, I think a massive amount of Earth time would suddenly appear to have elapsed very quickly during the process of slowing/landing because of the huge shift in simultaneity.
  - ryandrake 3y ago
    Pretty sure only those 10 ship years worth of transmissions would suddenly appear. You're still 20K light years away so the rest of those years' transmissions are still in flight.
    - cypherpunks01 3y ago
      Hmm I suppose you're right. I guess you'd see very few transmissions during the journey due to time dilation, and then those 10 years would arrive relatively quickly when slowing/stopping? I was thinking of this being the first half of the twin paradox, but perhaps for the apparent "time gap" in the spacetime diagram to appear, it's necessary for the traveling twin to turn and head back towards Earth quickly to shift simultaneity plane back in the other direction.
AnimalMuppet 3y ago
Can you explain why matter-antimatter only has 60% efficiency?
- JumpCrisscross 3y ago
  > Can you explain why matter-antimatter only has 60% efficiency? We don’t know how to use all the reaction products for thrust [1]. [1] https://en.m.wikipedia.org/wiki/Electron–positron_annihilation https://en.m.wikipedia.org/wiki/Electron–positron_annihilati...
- mikeyouse 3y ago
  > This is the primary reason for selecting the annihilation of a proton (p+) and antiproton (p–-); the products include neutral and charged pions (πo, π+, π-), and the charged pions can be trapped and directed by magnetic fields to produce thrust. However, the pions produced in the annihilation reaction do possess (rest) mass (about 22% of the initial protonantiproton annihilation pair rest mass for charged pions, 14% for the neutral pions), so not all of the protonantiproton mass is converted into energy. This results in an energy density of the proton-antiproton reaction of "only" 64% of the ideal limit, or 5.8x1016 J/kg. I love that someone wrote and published a serious paper where the end design involved first stage thrust of 550 million ft-lbs and the total ship weighs something like 17 million metric tons and if I'm reading page 26 correctly, the payload on the ship has to be 7,500 kilometers from the ignition source to survive. https://www.relativitycalculator.com/images/relativistic_photon_rocket/ANTIMATTER_ROCKET_FOR_INTERSTELLAR_MISSIONS.pdf https://www.relativitycalculator.com/images/relativistic_pho...
- c12 3y ago
  While there may be a 100% anilation with matter-antimatter reactions the resulting thrust is not perfectly efficient due to energy consumed as the rest mass of charged and uncharged pions, energy consumed as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and energy consumed as neutrinos and gamma rays. I'm unsure if the resulting efficienty is 60% however, not all of the energy produced in the anilation is converted to "stuff" that is useful for thrust.
knodi123 3y ago
In the book series The Expanse, that was pretty much the only unrealistic technology that the humans had, which allowed them to colonize the solar system. Sure, there was some might-as-well-be-magic alien stuff later in the book, but by page 1, humans have colonized the solar system, purely with a hand-wavy engine that can generate sustained thrust without needing tons of reaction mass and without turning your exhaust port into an unimaginable hell cannon.
- neaden 3y ago
  Eh, there is a ton of unrealistic technology in book 1, the stealth ship for instance is impossible. By the authors own words they don't think of the books as being Hard Sci-Fi, harder then Star Trek sure.
  - mplewis9z 3y ago
    What makes you think the stealth ship is impossible? They explain the concept pretty well in the first book: extensive cooling systems to soak up waste heat, large liquid tanks to store that heat internally for a while, and radar-absorbing coatings. We literally have all of those pieces today, it’d just cost unfathomable amounts of money to build with our current technology. As a bonus, they even talk about the radar-absorbing coating not being perfect, and being able to detect the stealth ship as an object a few Kelvin above the background radiation when they pumped their radar into it.
    - fredthedeadhead 3y ago
      Project Rho gives several compelling reasons why there ain't no stealth in space https://www.projectrho.com/public_html/rocket/spacewardetect.php#nostealth https://www.projectrho.com/public_html/rocket/spacewardetect...
      - mplewis9z 3y ago
        I don't know when that was written, but it definitely feels like The Expanse authors read that exact page and built their stealth system to address those points - the stealth ships in the books aren't permanently stealthy (they have to radiate their heat from the internal heatsinks after some amount of time, seemingly on the order of months), and are immediately detected when they light up their engines (and hence coast until they reach their destination). The main characters also manage to find a stealth ship after coming across coordinates to it - it was parked and completely offline (and unmanned) in the orbit of a small asteroid, which would further obscure any signature it might give off.
EvanAnderson 3y ago
> I imagine a sort of neo-Polynesian culture This makes me think of the Quen Ho in Vernoe Vinge's "Zones of Thought" universe.
perardi 3y ago
Have you read Tao Zero? https://en.wikipedia.org/wiki/Tau_Zero https://en.wikipedia.org/wiki/Tau_Zero Classic hard sci-fi. A colony ship’s Bussard ramjet has a glitch, and it cannot stop accelerating—and basically forever, given time dilation.
agent281 3y ago
That's basically the plot of the Forever War. When you drop out of hyper drive (or whatever they called it) you never know if you are going to be battling a ship from your future or your past. The very society that you fight for is completely alien. It's incredibly alienating.
marmaduke 3y ago
What do you think about the gravity well "drive" in three body problem trilogy? I thought it was fairly clever, especially given the stories describing it beforehand (maelstrom iirc?)
ausp 3y ago
> As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations. Spin, a novel by Robert Charles Wilson, touches on this from a planetary perspective. Quite the interesting read, though I won't say more here for fear of spoilers.

Teever 3y ago

What if you didn't carry the fuel? Use stationary lasers from your starting point to power your journey half way until you lasers from your destination to slow your craft down.

pc86 3y ago
Of all the dangers associated with spaceflight it's a pretty interesting concept to add "lasers at the origin shut off for whatever reason" and "lasers at the destination never turn on / are misaligned / turn on too soon / turn on too late" to the list. Not sure I'd want to be in a spacecraft careening through the cosmos with no realistic way to slow it down.
- jandrese 3y ago
  There is also the question of how you get the lasers to your destination in the first place.
  - Teever 3y ago
    Use lasers for the first half of the journey to send a rocket that can decelerate and build the laser station at the destination.
  - MilStdJunkie 3y ago
    Send a mirror ahead, bounce the beam back toward the ship to push it backwards. Loses a lot of efficiency, though. Laser propulsion, a lot of the thrust is coming from ablation of the surface.
    - eru 3y ago
      How do you 'send a mirror ahead'? Do you want it to stop, or just keep flying away indefinitely?
      - MilStdJunkie 3y ago
        Yep, throwaway mirror, it goes off to wherever once it's done bouncing light back at your ship. Again, much less efficient, so when you get where you want to go, you'd want to put in a laser station there too.
NotYourLawyer 3y ago
The farther you get, the dimmer the laser gets. The faster you get, the less energy each photon has.
- slow_typist 3y ago
  Note that the 2nd statement only applies because of the Doppler effect - energy of the photon is a function of its frequency.
cypherpunks01 3y ago
I think the perceived output from stationary lasers on Earth would appear to decrease as your speed increases, since you're accelerating away from the laser photon packets. Because of that behavior, I'm not sure it's a feasible way to get up to relativistic speed? I think the lasers would also need ~10,000 years of fuel, as opposed to ~10 years of fuel if you could somehow figure out how to carry it. But the energy to sustain in either case is probably measured in Dyson spheres, or some fraction of all energy in the universe pretty quickly too.
- PeterisP 3y ago
  Time dilation also has an effect, since you're receiving 10000 Earth-years of laser energy over 10 ship-years.
MilStdJunkie 3y ago
That's the drive of the eponymous generation ship in Kim Stanley Robinson's Aurora. It sends a mirror ahead of itself, then the Sol-Mercury lasers bounce off the mirror, back on the ship to slow it down. I could see the light spacecraft with their matter-antimatter engines laying down "lightways", like railroads in the old west. Once a lightway is established, then it's much less of a big deal to get from system to system. Apart from the tens of thousands of years in subjective time, of course.

ryandrake 3y ago

> As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations. I guess this all depends on how you define simultaneity. Say I got on a relativistic rocket traveling near c and colonized Planet X in 10 years ship time and say 20,000 years Earth time. I put up my TV antenna as soon as I land, and I'd still be receiving Earth transmissions from approximately 10 years after I left, no? So even though civilization is long gone from Earth's point of view, from my point of view, I can keep up with the latest news as if I were still living there. So from my point of view I did not outlive anything. If I were to go back, yes, everything would be gone.

cypherpunks01 3y ago
While traveling near c, I believe you'd be receiving Earth transmissions from approximately the same time you left, Earth clocks would appear to slow down to a crawl. But if you land on another planet and change reference frame to be more "stationary" again relative to Earth, I think a massive amount of Earth time would suddenly appear to have elapsed very quickly during the process of slowing/landing because of the huge shift in simultaneity.
- ryandrake 3y ago
  Pretty sure only those 10 ship years worth of transmissions would suddenly appear. You're still 20K light years away so the rest of those years' transmissions are still in flight.
  - cypherpunks01 3y ago
    Hmm I suppose you're right. I guess you'd see very few transmissions during the journey due to time dilation, and then those 10 years would arrive relatively quickly when slowing/stopping? I was thinking of this being the first half of the twin paradox, but perhaps for the apparent "time gap" in the spacetime diagram to appear, it's necessary for the traveling twin to turn and head back towards Earth quickly to shift simultaneity plane back in the other direction.

AnimalMuppet 3y ago

Can you explain why matter-antimatter only has 60% efficiency?

JumpCrisscross 3y ago
> Can you explain why matter-antimatter only has 60% efficiency? We don’t know how to use all the reaction products for thrust [1]. [1] https://en.m.wikipedia.org/wiki/Electron–positron_annihilation https://en.m.wikipedia.org/wiki/Electron–positron_annihilati...
mikeyouse 3y ago
> This is the primary reason for selecting the annihilation of a proton (p+) and antiproton (p–-); the products include neutral and charged pions (πo, π+, π-), and the charged pions can be trapped and directed by magnetic fields to produce thrust. However, the pions produced in the annihilation reaction do possess (rest) mass (about 22% of the initial protonantiproton annihilation pair rest mass for charged pions, 14% for the neutral pions), so not all of the protonantiproton mass is converted into energy. This results in an energy density of the proton-antiproton reaction of "only" 64% of the ideal limit, or 5.8x1016 J/kg. I love that someone wrote and published a serious paper where the end design involved first stage thrust of 550 million ft-lbs and the total ship weighs something like 17 million metric tons and if I'm reading page 26 correctly, the payload on the ship has to be 7,500 kilometers from the ignition source to survive. https://www.relativitycalculator.com/images/relativistic_photon_rocket/ANTIMATTER_ROCKET_FOR_INTERSTELLAR_MISSIONS.pdf https://www.relativitycalculator.com/images/relativistic_pho...
c12 3y ago
While there may be a 100% anilation with matter-antimatter reactions the resulting thrust is not perfectly efficient due to energy consumed as the rest mass of charged and uncharged pions, energy consumed as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and energy consumed as neutrinos and gamma rays. I'm unsure if the resulting efficienty is 60% however, not all of the energy produced in the anilation is converted to "stuff" that is useful for thrust.

knodi123 3y ago

In the book series The Expanse, that was pretty much the only unrealistic technology that the humans had, which allowed them to colonize the solar system. Sure, there was some might-as-well-be-magic alien stuff later in the book, but by page 1, humans have colonized the solar system, purely with a hand-wavy engine that can generate sustained thrust without needing tons of reaction mass and without turning your exhaust port into an unimaginable hell cannon.

neaden 3y ago
Eh, there is a ton of unrealistic technology in book 1, the stealth ship for instance is impossible. By the authors own words they don't think of the books as being Hard Sci-Fi, harder then Star Trek sure.
- mplewis9z 3y ago
  What makes you think the stealth ship is impossible? They explain the concept pretty well in the first book: extensive cooling systems to soak up waste heat, large liquid tanks to store that heat internally for a while, and radar-absorbing coatings. We literally have all of those pieces today, it’d just cost unfathomable amounts of money to build with our current technology. As a bonus, they even talk about the radar-absorbing coating not being perfect, and being able to detect the stealth ship as an object a few Kelvin above the background radiation when they pumped their radar into it.
  - fredthedeadhead 3y ago
    Project Rho gives several compelling reasons why there ain't no stealth in space https://www.projectrho.com/public_html/rocket/spacewardetect.php#nostealth https://www.projectrho.com/public_html/rocket/spacewardetect...
    - mplewis9z 3y ago
      I don't know when that was written, but it definitely feels like The Expanse authors read that exact page and built their stealth system to address those points - the stealth ships in the books aren't permanently stealthy (they have to radiate their heat from the internal heatsinks after some amount of time, seemingly on the order of months), and are immediately detected when they light up their engines (and hence coast until they reach their destination). The main characters also manage to find a stealth ship after coming across coordinates to it - it was parked and completely offline (and unmanned) in the orbit of a small asteroid, which would further obscure any signature it might give off.

EvanAnderson 3y ago

> I imagine a sort of neo-Polynesian culture This makes me think of the Quen Ho in Vernoe Vinge's "Zones of Thought" universe.

perardi 3y ago

Have you read Tao Zero? https://en.wikipedia.org/wiki/Tau_Zero https://en.wikipedia.org/wiki/Tau_Zero Classic hard sci-fi. A colony ship’s Bussard ramjet has a glitch, and it cannot stop accelerating—and basically forever, given time dilation.

agent281 3y ago

That's basically the plot of the Forever War. When you drop out of hyper drive (or whatever they called it) you never know if you are going to be battling a ship from your future or your past. The very society that you fight for is completely alien. It's incredibly alienating.

marmaduke 3y ago

What do you think about the gravity well "drive" in three body problem trilogy? I thought it was fairly clever, especially given the stories describing it beforehand (maelstrom iirc?)

ausp 3y ago

> As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations. Spin, a novel by Robert Charles Wilson, touches on this from a planetary perspective. Quite the interesting read, though I won't say more here for fear of spoilers.