7 ms·

Happy to see this getting some attention. It's seemed like one of the only viable propulsion methods for interstellar purposes, though nuclear is unfashionable

Happy to see this getting some attention. It's seemed like one of the only viable propulsion methods for interstellar purposes, though nuclear is unfashionable at the moment. I remember seeing a chart showing that it was possible to reach other stars using a technique like this. Unfortunately the problem is slowing down. You could get there, but you'd be going too fast to stop. And since the more fuel you carry, the harder it is to reach that speed, it ends up becoming completely impractical to carry enough fuel to both get there and slow down. Maybe it'd be viable to stop by smashing into a planet, but it's hard for anything to survive that kind of impact force.

sien 10y ago
This was considered by project Orion. Basically you turn the vehicle around and use the propulsion to slow you down. It would use a heap of fuel to do this but it was possible. Edit, also, great find of this by the poster.
- api 10y ago
  But since you already used a bunch of your pulse propellant during acceleration, this is partly compensated for by the vehicle weighing less.
  - wiredearp 10y ago
    In "Death’s End", that’s a science fiction book by Cixin Liu, they use a variant of nuclear propulsion where the nukes are pre-distributed throughout the Solar System instead of being carried by the ship itself. This could free up even more mass and make room for an even bigger magnetic sail to be used for deceleration. Later in the same book, and much later in the fictional timeline, they build a circumsolar particle accelerator, which is easier than you would think [1], and this could be used to produce massive amounts of antimatter in space with a lot more propulsion power than conventional nukes. So I remain optimistic. It’s by the way a remarkable series of books that will freak out a lot of hackers, I recommend sticking with the series to the end. [1] https://books.google.dk/books?id=A_1oCAAAQBAJ&pg=PA451&lpg=PA451#v=onepage https://books.google.dk/books?id=A_1oCAAAQBAJ&pg=PA451&lpg=P...
- sillysaurus3 10y ago
  In order to have fuel to do that, you'd need to accelerate it to begin with. It ends up being an exponential increase in the quantity of fuel required, far more than could be carried. The chart from the paper was pretty convincing. I'll see if I can dig it up. Making the explosions more efficient can only get you so far. I'd prefer to be optimistic about it, but the math was showing that even with an ideal ability to convert mass into energy, it would require too much fuel, to put it mildly.
  - sien 10y ago
    According to the wikipedia page on Project Orion: Later studies indicate that the top cruise velocity that can theoretically be achieved are a few percent of the speed of light (0.08-0.1c).[17] An atomic (fission) Orion can achieve perhaps 9%-11% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the max. speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.[18] https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Theoretical_applications https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
    - sillysaurus3 10y ago
      The referenced paper [18] is informative: http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_superconducting_coils_for_near-term_interstellar_transportation_LenardAndrews.pdf http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_s... === 2.3. Deceleration: the canonical issue While accelerating to a sizable fraction of the speed of light is a daunting task, deceleration is a yet more challenging task. The reason is that no realistic scenario to provide the gravy-train approach to either momentum transfer or pellet energy transfer at distances of 4+ light years seems within known physics. We are therefore challenged with either sending sufficient numbers of pellet-sheath elements so that the spacecraft can decelerate with on-board energy, or to come up with some other innovative concept. 2.3.1. Fueling at departure point One option is to provide all necessary propellant at the departure location. If the specific power and specific impulse is sufficiently high, this is a potential approach. However, for our MMO concept, it is not particularly feasible. For example, our specific power is certainly high enough, but our specific impulse is dictated by the pellet-sheath combination, which limits the Isp to 128,955 s. While this may seem like a prodigious specific impulse, the ratio of spacecraft speed .1 c to g * Isp is 23. This means that the deceleration propellant must be e^23 times the spacecraft mass, which is an unworkable solution. 2.3.2. Mag Sail deceleration In 2003 both Andrews and Lenard postulated using a large superconducting ring to intercept charge particles in interstellar space to slow the spacecraft down from high speeds. Additionally, the solar wind emanating from a star system provides an additional source of charged particles that can interact with the magnetic field. Deceleration can actually begin a sizable distance from the target star system. The following two charts indicate deceleration, velocity and time as a function of distance from the target star system. In this case the first phase of the deceleration starts at 21600 AU with a twoturn superconducting carbon nano tube reinforced loop. This loop captures the charged interstellar medium and deflects it to decelerate the spacecraft. This initial hoop size is 500 km in radius and carries 1,000,000 A of current. The spacecraft decelerates from .1 c to 6300 km/s by the time the spacecraft reaches 5000 AU. This will be quite a light show, so if there are any intelligent life forms with an observing system, they should be able to see the arrival.
narag 10y ago
I remember seeing a chart showing that it was possible to reach other stars using a technique like this. Unfortunately the problem is slowing down. So this is like being concerned how you will spend all that money, once you make it :-) There's a lot of discussion on interestellar travel, but "we" have almost frozen solar system travel. Yes, there are very nice probes out there doing important work, but manned vehicles are limited to keep ISS's lights on. The "impossible" tech needed to reach the stars will most probably be discovered or refined while conquering the solar system. Big sailing ships were developed along four centuries of sailing. The analogy may not be perfect, but I think it's eye opening.
- triplesec 10y ago
  I agree with you in innovation terms, but there's a big economic difference: sailing used free energy and therefore could be developed over the long term by many actors. There are huge energy cost barriers to solar system travel which require bigger bets on economic return to be viable
andygates 10y ago
> nuclear is unfashionable at the moment It's more than that - this isn't just hippies, it's every major power on the planet. To fly Orion you have to put hundreds of nuclear explosives in orbit and pinky-swear that you're not going to use them to obliterate your enemies. Nobody is going to accept that. Orion's a fun paper rocket but will never be more. Engineers and they "yay nukes!" crowd will drag it out every couple of months for a repost, though.
- jessriedel 10y ago
  > To fly Orion you have to put hundreds of nuclear explosives in orbit and pinky-swear that you're not going to use them to obliterate your enemies. To reiterate berntb's point: Every nation who has the ability to do this already has ICBMs that can deliver nukes anywhere on the planet in 20 minutes. So what difference does this make?
  - DennisP 10y ago
    True, but we still have a treaty that prohibits bombs in orbit, because they'd reduce the warning time enough to threaten deterrence. Maybe one day we can launch Orion from Mars.
    - maxerickson 10y ago
      Drive it to Earth, have lots of bombs left. From a strategic point of view, what's the difference between building on Mars and building it in orbit? Obviously there is a difference because the hostile faction has to seize control of it and move it to Earth, but once they do that, the calculation becomes the same. Do you commit to obliterating them the second they start moving it to Earth?
      - masklinn 10y ago
        Best case scenario, Mars is 55 million kilometres from earth. Even using a ridiculous engine going full blast the whole way (in which case your payload will probably disintegrate when it hits the atmosphere) you won't get the transfer time under a month. If you want to actually deliver your payload in a useful manner you'll need twice that at the very least. 2+ months is a long lead time, and the engine you'll need will make the craft impossible to miss. What's the difference to building it in orbit? Building it in earth orbit you're dropping it from under 500km and your assembly makes 10~20 orbits a day, you can deliver it quickly at pretty much any time. The former allows countermeasure, the latter not really.
deleted 10y ago
[deleted]

sien 10y ago

This was considered by project Orion. Basically you turn the vehicle around and use the propulsion to slow you down. It would use a heap of fuel to do this but it was possible. Edit, also, great find of this by the poster.

api 10y ago
But since you already used a bunch of your pulse propellant during acceleration, this is partly compensated for by the vehicle weighing less.
- wiredearp 10y ago
  In "Death’s End", that’s a science fiction book by Cixin Liu, they use a variant of nuclear propulsion where the nukes are pre-distributed throughout the Solar System instead of being carried by the ship itself. This could free up even more mass and make room for an even bigger magnetic sail to be used for deceleration. Later in the same book, and much later in the fictional timeline, they build a circumsolar particle accelerator, which is easier than you would think [1], and this could be used to produce massive amounts of antimatter in space with a lot more propulsion power than conventional nukes. So I remain optimistic. It’s by the way a remarkable series of books that will freak out a lot of hackers, I recommend sticking with the series to the end. [1] https://books.google.dk/books?id=A_1oCAAAQBAJ&pg=PA451&lpg=PA451#v=onepage https://books.google.dk/books?id=A_1oCAAAQBAJ&pg=PA451&lpg=P...
sillysaurus3 10y ago
In order to have fuel to do that, you'd need to accelerate it to begin with. It ends up being an exponential increase in the quantity of fuel required, far more than could be carried. The chart from the paper was pretty convincing. I'll see if I can dig it up. Making the explosions more efficient can only get you so far. I'd prefer to be optimistic about it, but the math was showing that even with an ideal ability to convert mass into energy, it would require too much fuel, to put it mildly.
- sien 10y ago
  According to the wikipedia page on Project Orion: Later studies indicate that the top cruise velocity that can theoretically be achieved are a few percent of the speed of light (0.08-0.1c).[17] An atomic (fission) Orion can achieve perhaps 9%-11% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the max. speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.[18] https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Theoretical_applications https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
  - sillysaurus3 10y ago
    The referenced paper [18] is informative: http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_superconducting_coils_for_near-term_interstellar_transportation_LenardAndrews.pdf http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_s... === 2.3. Deceleration: the canonical issue While accelerating to a sizable fraction of the speed of light is a daunting task, deceleration is a yet more challenging task. The reason is that no realistic scenario to provide the gravy-train approach to either momentum transfer or pellet energy transfer at distances of 4+ light years seems within known physics. We are therefore challenged with either sending sufficient numbers of pellet-sheath elements so that the spacecraft can decelerate with on-board energy, or to come up with some other innovative concept. 2.3.1. Fueling at departure point One option is to provide all necessary propellant at the departure location. If the specific power and specific impulse is sufficiently high, this is a potential approach. However, for our MMO concept, it is not particularly feasible. For example, our specific power is certainly high enough, but our specific impulse is dictated by the pellet-sheath combination, which limits the Isp to 128,955 s. While this may seem like a prodigious specific impulse, the ratio of spacecraft speed .1 c to g * Isp is 23. This means that the deceleration propellant must be e^23 times the spacecraft mass, which is an unworkable solution. 2.3.2. Mag Sail deceleration In 2003 both Andrews and Lenard postulated using a large superconducting ring to intercept charge particles in interstellar space to slow the spacecraft down from high speeds. Additionally, the solar wind emanating from a star system provides an additional source of charged particles that can interact with the magnetic field. Deceleration can actually begin a sizable distance from the target star system. The following two charts indicate deceleration, velocity and time as a function of distance from the target star system. In this case the first phase of the deceleration starts at 21600 AU with a twoturn superconducting carbon nano tube reinforced loop. This loop captures the charged interstellar medium and deflects it to decelerate the spacecraft. This initial hoop size is 500 km in radius and carries 1,000,000 A of current. The spacecraft decelerates from .1 c to 6300 km/s by the time the spacecraft reaches 5000 AU. This will be quite a light show, so if there are any intelligent life forms with an observing system, they should be able to see the arrival.

narag 10y ago

I remember seeing a chart showing that it was possible to reach other stars using a technique like this. Unfortunately the problem is slowing down. So this is like being concerned how you will spend all that money, once you make it :-) There's a lot of discussion on interestellar travel, but "we" have almost frozen solar system travel. Yes, there are very nice probes out there doing important work, but manned vehicles are limited to keep ISS's lights on. The "impossible" tech needed to reach the stars will most probably be discovered or refined while conquering the solar system. Big sailing ships were developed along four centuries of sailing. The analogy may not be perfect, but I think it's eye opening.

triplesec 10y ago
I agree with you in innovation terms, but there's a big economic difference: sailing used free energy and therefore could be developed over the long term by many actors. There are huge energy cost barriers to solar system travel which require bigger bets on economic return to be viable

andygates 10y ago

> nuclear is unfashionable at the moment It's more than that - this isn't just hippies, it's every major power on the planet. To fly Orion you have to put hundreds of nuclear explosives in orbit and pinky-swear that you're not going to use them to obliterate your enemies. Nobody is going to accept that. Orion's a fun paper rocket but will never be more. Engineers and they "yay nukes!" crowd will drag it out every couple of months for a repost, though.

jessriedel 10y ago
> To fly Orion you have to put hundreds of nuclear explosives in orbit and pinky-swear that you're not going to use them to obliterate your enemies. To reiterate berntb's point: Every nation who has the ability to do this already has ICBMs that can deliver nukes anywhere on the planet in 20 minutes. So what difference does this make?
- DennisP 10y ago
  True, but we still have a treaty that prohibits bombs in orbit, because they'd reduce the warning time enough to threaten deterrence. Maybe one day we can launch Orion from Mars.
  - maxerickson 10y ago
    Drive it to Earth, have lots of bombs left. From a strategic point of view, what's the difference between building on Mars and building it in orbit? Obviously there is a difference because the hostile faction has to seize control of it and move it to Earth, but once they do that, the calculation becomes the same. Do you commit to obliterating them the second they start moving it to Earth?
    - masklinn 10y ago
      Best case scenario, Mars is 55 million kilometres from earth. Even using a ridiculous engine going full blast the whole way (in which case your payload will probably disintegrate when it hits the atmosphere) you won't get the transfer time under a month. If you want to actually deliver your payload in a useful manner you'll need twice that at the very least. 2+ months is a long lead time, and the engine you'll need will make the craft impossible to miss. What's the difference to building it in orbit? Building it in earth orbit you're dropping it from under 500km and your assembly makes 10~20 orbits a day, you can deliver it quickly at pretty much any time. The former allows countermeasure, the latter not really.

deleted 10y ago

[deleted]