3 ms·

> They have to wait a full year for the planets to realign This actually shouldn't be true for much longer. The only reason it's true now is that we don't have

by pdonis 5y ago

> They have to wait a full year for the planets to realign This actually shouldn't be true for much longer. The only reason it's true now is that we don't have spaceships that can produce constant thrust for the entire trip; our ships spend most of their time in free fall, which, as Robert Heinlein pointed out decades ago, is the equivalent of floating down the Mississippi on a raft. As he said, what we need are the equivalent of sailing ships: ships that can put out a constant thrust, even if it's small, for an entire trip. Even a thrust of 1/100 gee, which is well within our technical capability if we get serious about things like ion drives, would greatly reduce trip time to Mars (and would reduce trip times to more distant planets even more, since the advantage over free fall increases as the distance increases) and would open up a lot of options for travel that we don't currently have. Note that this doesn't change the fact that humans on Mars will have to be self-reliant; payload mass is still payload mass and a spaceship just won't be able to carry enough of it to Mars to supply life support for any length of time staying there. Sending unmanned missions in advance might help, but I don't think even that would be enough.

thaumasiotes 5y ago
> our ships spend most of their time in free fall, which, as Robert Heinlein pointed out decades ago, is the equivalent of floating down the Mississippi on a raft. As he said, what we need are the equivalent of sailing ships: ships that can put out a constant thrust, even if it's small, for an entire trip. Sails don't do that. In terms of propulsion, they're exactly the same as pole-less rafts. Ever heard of being becalmed?
- pdonis 5y ago
  > Sails don't do that. The propulsive force is not as controllable as it is for a low-thrust rocket, true. But Heinlein was not claiming an exact comparison. > they're exactly the same as pole-less rafts. No, they're not. Pole-less rafts don't have sails. > Ever heard of being becalmed? Ever hear of missing the point? Yes, sailing ships can be becalmed. But they can also not be becalmed. And sailing ships that were not becalmed created a worldwide system of trade. Pole-less rafts would not have been capable of doing that.
godelski 5y ago
You statement doesn't actually confer confidence that you understand space propulsion. No fret. This is something most people don't know about, but I guess a lot of people speak about it because it is cool. We already have ion drives. But butterfly farts won't get warheads to Mars faster. Your burn rate will increase dramatically depending on Mars/Earth alignment. This again adds to the cost that we've been talking about above and how historically oppressors have had a difficult time sustaining combat with subjugated people, and cost is directly proportional to distance. It's a matter of will mostly and how much they get from those subjugated people. But even with the best technology we can see ourselves building within the next decade or so we're talking 3 months on a good alignment (current is 9 months). For comparison, Columbus was on the sea for 36 days. People saying that "oh, it was just like ye old times" don't realize that we're still talking about substantially longer time frames and substantially higher (proportionally) costs of travel. And as for robotic warfare, I'll give the same answer to everyone that asks how guerillas fight tanks and jets. 1) you can't rule the dead, 2) international trade looks down upon countries that commit genocide, 3) genocide is typically pretty expensive.
- pdonis 5y ago
  > We already have ion drives Yes, indeed. > Your burn rate will increase dramatically depending on Mars/Earth alignment. Why? I am not talking about keeping trip time constant. I am just talking about significantly reducing trip time compared to free fall. That is easily done with a low thrust ion drive; even a hundredth of a gee makes a significant reduction in trip time, and the longer the distance, the more the reduction in time even holding the thrust constant (since the trip time now increases only logarithmically with the distance instead of linearly). And that makes trips more feasible even when the planets aren't in alignment. > even with the best technology we can see ourselves building within the next decade or so we're talking 3 months on a good alignment Now it's you who aren't conferring much confidence in your knowledge. Below are some numbers (travel times are one-way). I've even included numbers for 1/1000 gee to show that even those numbers are much better than the free-fall numbers. Closest approach: about 60 million km (6 x 10^10 m) Free-fall travel time: 180 days 1/100 gee travel time: 18 days 1/1000 gee travel time: 57 days Furthest away: about 400 million km (4 x 10^11 m) Free-fall travel time: 1200 days 1/100 gee travel time: 47 days 1/1000 gee travel time: 148 days The free-fall transit time for closest approach is taken from the Wikipedia article on free-return Earth-Mars trajectories; I have used the shortest time that appears to have a reasonable return trip (though the return trip transit time is longer). [1] Free-fall transit times for furthest away are taken by multiplying the closest approach time by 6.67, which is probably somewhat of an underestimate (it assumes straight linear dependence of trip time on distance, without confirming that there actually is a viable transfer orbit with that transit time). 1/100 gee and 1/1000 gee transit times are obtained by using the formula t = 2 sqrt(d / a), where d is the distance and a is the acceleration. This formula assumes constant acceleration to the halfway point, then turnaround and constant deceleration the rest of the way. (I also adjusted the units in the answers above to days instead of seconds, which is what the formula I just gave gives; I rounded up all fractions.) I have ignored orbital maneuvers and takeoff/landing at both ends for all travel times. https://en.wikipedia.org/wiki/Free-return_trajectory#Earth%E2%80%93Mars https://en.wikipedia.org/wiki/Free-return_trajectory#Earth%E... > Columbus was on the sea for 36 days Which was by no means the longest, or even a particularly long, voyage for the time period. Vasco da Gama was at sea for more than three months at a time during his voyage around Africa to the Far East and back. So was Magellan during his round the world expedition.

by pdonis 5y ago

thaumasiotes 5y ago

> our ships spend most of their time in free fall, which, as Robert Heinlein pointed out decades ago, is the equivalent of floating down the Mississippi on a raft. As he said, what we need are the equivalent of sailing ships: ships that can put out a constant thrust, even if it's small, for an entire trip. Sails don't do that. In terms of propulsion, they're exactly the same as pole-less rafts. Ever heard of being becalmed?

pdonis 5y ago
> Sails don't do that. The propulsive force is not as controllable as it is for a low-thrust rocket, true. But Heinlein was not claiming an exact comparison. > they're exactly the same as pole-less rafts. No, they're not. Pole-less rafts don't have sails. > Ever heard of being becalmed? Ever hear of missing the point? Yes, sailing ships can be becalmed. But they can also not be becalmed. And sailing ships that were not becalmed created a worldwide system of trade. Pole-less rafts would not have been capable of doing that.

godelski 5y ago

You statement doesn't actually confer confidence that you understand space propulsion. No fret. This is something most people don't know about, but I guess a lot of people speak about it because it is cool. We already have ion drives. But butterfly farts won't get warheads to Mars faster. Your burn rate will increase dramatically depending on Mars/Earth alignment. This again adds to the cost that we've been talking about above and how historically oppressors have had a difficult time sustaining combat with subjugated people, and cost is directly proportional to distance. It's a matter of will mostly and how much they get from those subjugated people. But even with the best technology we can see ourselves building within the next decade or so we're talking 3 months on a good alignment (current is 9 months). For comparison, Columbus was on the sea for 36 days. People saying that "oh, it was just like ye old times" don't realize that we're still talking about substantially longer time frames and substantially higher (proportionally) costs of travel. And as for robotic warfare, I'll give the same answer to everyone that asks how guerillas fight tanks and jets. 1) you can't rule the dead, 2) international trade looks down upon countries that commit genocide, 3) genocide is typically pretty expensive.

pdonis 5y ago
> We already have ion drives Yes, indeed. > Your burn rate will increase dramatically depending on Mars/Earth alignment. Why? I am not talking about keeping trip time constant. I am just talking about significantly reducing trip time compared to free fall. That is easily done with a low thrust ion drive; even a hundredth of a gee makes a significant reduction in trip time, and the longer the distance, the more the reduction in time even holding the thrust constant (since the trip time now increases only logarithmically with the distance instead of linearly). And that makes trips more feasible even when the planets aren't in alignment. > even with the best technology we can see ourselves building within the next decade or so we're talking 3 months on a good alignment Now it's you who aren't conferring much confidence in your knowledge. Below are some numbers (travel times are one-way). I've even included numbers for 1/1000 gee to show that even those numbers are much better than the free-fall numbers. Closest approach: about 60 million km (6 x 10^10 m) Free-fall travel time: 180 days 1/100 gee travel time: 18 days 1/1000 gee travel time: 57 days Furthest away: about 400 million km (4 x 10^11 m) Free-fall travel time: 1200 days 1/100 gee travel time: 47 days 1/1000 gee travel time: 148 days The free-fall transit time for closest approach is taken from the Wikipedia article on free-return Earth-Mars trajectories; I have used the shortest time that appears to have a reasonable return trip (though the return trip transit time is longer). [1] Free-fall transit times for furthest away are taken by multiplying the closest approach time by 6.67, which is probably somewhat of an underestimate (it assumes straight linear dependence of trip time on distance, without confirming that there actually is a viable transfer orbit with that transit time). 1/100 gee and 1/1000 gee transit times are obtained by using the formula t = 2 sqrt(d / a), where d is the distance and a is the acceleration. This formula assumes constant acceleration to the halfway point, then turnaround and constant deceleration the rest of the way. (I also adjusted the units in the answers above to days instead of seconds, which is what the formula I just gave gives; I rounded up all fractions.) I have ignored orbital maneuvers and takeoff/landing at both ends for all travel times. https://en.wikipedia.org/wiki/Free-return_trajectory#Earth%E2%80%93Mars https://en.wikipedia.org/wiki/Free-return_trajectory#Earth%E... > Columbus was on the sea for 36 days Which was by no means the longest, or even a particularly long, voyage for the time period. Vasco da Gama was at sea for more than three months at a time during his voyage around Africa to the Far East and back. So was Magellan during his round the world expedition.