15 ms·

Summary: burn lots of fuel to go to periphery of solar system, dive back toward the sun and skim it very closely to maximize velocity and use a bunch of fuel ne

by xt00 7y ago

Summary: burn lots of fuel to go to periphery of solar system, dive back toward the sun and skim it very closely to maximize velocity and use a bunch of fuel next to the sun at max V (oberth maneuver) https://en.m.wikipedia.org/wiki/Oberth_effect https://en.m.wikipedia.org/wiki/Oberth_effect and then go hurtling out of solar system.. main challenge is solar shielding.. the deeper into the gravity well you can go the faster you can go due to the deltaV performed next to the sun..

deleted 7y ago
[deleted]
pdonis 7y ago
It looks like the initial outward leg would only be to Jupiter; the idea appears to be to use a gravity assist around Jupiter to get more velocity diving towards the Sun. It doesn't make sense to me to use very much fuel trying to get farther out from the Earth before diving in towards the Sun; the additional speed just from the increased distance of fall is negligible, and you could take the same fuel and burn it during the Oberth maneuver at the Sun to get more bang for the buck out of it. It might be worth it for the additional speed from a Jupiter gravity assist if things were aligned just right.
- thatcherc 7y ago
  The reason for a detour to Jupiter is that it's actually fairly hard to get closer to the Sun. Somewhat surprisingly (if you haven't seen it before), it takes quite a bit of energy to drop from Earth's orbit around the Sun to ie Mercury's. In this case, it looks like it would take less energy to fly out to Jupiter and get the gravity assist (which is very substantial because of the planet's mass), and fall back toward the Sun than it would be to drop straight into a low solar orbit from Earth. This type of trajectory was actually considered for the Parker Solar Probe [0], but was eventually rejected because it would take too long.* If you're going on an interstellar voyage, the time to get out to Jupiter might not impact the timeline as much. [0] - https://wikipedia.org/wiki/Parker_Solar_Probe https://wikipedia.org/wiki/Parker_Solar_Probe * notably, the new trajectory still includes gravity assists off Earth and Venus, underscoring the amount of delta-V needed to lower a solar orbit
  - xt00 7y ago
    Yea interesting! Parker solar probe will hit a whopping 0.064% of the speed of light.. and the orbit is quite interesting as you said — needs to slow down to cancel out the speed of earths orbital velocity. So ya know, for a reference point, the 1% of the speed of light that the Trisolaran fleet is flying at is just a mere 6.7 billion miles per hour.. so 0.5 million ain’t too bad...
    - thatcherc 7y ago
      Always good to have the Trisolarans to put things in perspective! Just gotta watch out for those pesky droplets...
      - topkeks 7y ago
        And that fucking sophon block...
  - pdonis 7y ago
    > it takes quite a bit of energy to drop from Earth's orbit around the Sun to ie Mercury's More precisely, you have to lose a significant amount of energy relative to the Sun, and it takes significant delta-v for a spaceship starting at Earth (to cancel out at least part of Earth's orbital speed about the Sun) to do that. Yes, that's a valid point.
the_pwner224 7y ago
With some more detail on the mechanics: Here's a screenshot of an orbit in Kerbal Space Program, an excellent NASA-simulator game (you have a spaceport and make + fly rockets and do science): https://wiki.kerbalspaceprogram.com/wiki/File:KerbalEccentricity.jpg https://wiki.kerbalspaceprogram.com/wiki/File:KerbalEccentri... On the blue orbit, you can see Ap and Pe. Ap is the apogee; the place in the orbit where the spacecraft is closest to the earth. Pe is the perigee, the place where the spacecraft is farthest from the earth. Since the blue orbit is mostly circular, they are at about the same altitude. If you look at the large white orbit, the apogee would be the leftmost point, close to the apogee of the blue orbit. The perigee would be all the way at the right of the image. When your spacecraft is at position X, thrusting your engines will not meaningfully change your current position - the acceleration they generate is negligible to the speed at which you are moving. However, the maneuver will change your speed, which affects your orbit. At the apogee, if you thrust in the direction you are moving, your perigee will increase - the orbit will become even more eccentric. Example: at the apogee you are moving at 100 miles per second. Thrusting the engines will not change your current position much; your current position is determined by your 100 miles per second speed and where you came from. But the maneuver will change your speed to 105 miles per second, which will cause your perigee to become much farther away from the earth. However your apogee will stay in the exact same position. Why? Because in orbit you are basically flying sideways while gravity pulls you down. In a circular orbit this balances out into you flying around in a circle. But if you go faster, you're going more sideways while gravity pulls the same amount. So your orbit becomes more elongated. You can thrust in the direction of movement at any point in orbit, and it will increase the altitude of the orbit on the opposite side of where you are at - if you thrust at apogee the perigee altitude increases, and in the white elongated orbit if you thrust between the apogee and perigee your orbit will become more circular and bigger. Similarly to make an eccentric orbit circular, you get to the apogee and then use the engines to slow down, which brings down the perigee. Since thrusting changes your position on the opposite side of the orbit, the most efficient way to increase your perigee is to thrust while at the apogee. The NASA thing is basically going into an elongated orbit between the Sun and Jupiter, coming back to the apogee near the Sun, and then thrusting. The difference in efficiency between thrusting at the apogee of an orbit and just thrusting in a straight line out into space is really huge. Though I'm not sure why the extra gravity of the Sun makes the maneuver more efficient. If you're interested in this you should definitely play KSP. It's a great game and gives you a great intuitive sense of how these things work.
p1mrx 7y ago
The Oberth effect is interesting. You want to get as low/fast as possible (converting your fuel's gravitational potential energy into kinetic energy), and then do a burn to harvest that energy, dropping the exhaust into the sun while the spacecraft zips away at high speed. It's basically the "tennis ball on a basketball" trick, with rockets.
mabbo 7y ago
No matter how it's explained to me, the Oberth effect still seems like magic, or at least some kind of physics violation. Somehow, we use the same delta-V to get more delta-V.
- greglindahl 7y ago
  You're adding less energy to the exhaust and more to the rocket. The Wikipedia article is a pretty good explanation.
rbanffy 7y ago
> main challenge is solar shielding.. If you carry a solar sail to add a little more push from the Sun's radiation you can spare a little material to build a shade on the other side of the sail.

by xt00 7y ago

deleted 7y ago

[deleted]

pdonis 7y ago

It looks like the initial outward leg would only be to Jupiter; the idea appears to be to use a gravity assist around Jupiter to get more velocity diving towards the Sun. It doesn't make sense to me to use very much fuel trying to get farther out from the Earth before diving in towards the Sun; the additional speed just from the increased distance of fall is negligible, and you could take the same fuel and burn it during the Oberth maneuver at the Sun to get more bang for the buck out of it. It might be worth it for the additional speed from a Jupiter gravity assist if things were aligned just right.

thatcherc 7y ago
The reason for a detour to Jupiter is that it's actually fairly hard to get closer to the Sun. Somewhat surprisingly (if you haven't seen it before), it takes quite a bit of energy to drop from Earth's orbit around the Sun to ie Mercury's. In this case, it looks like it would take less energy to fly out to Jupiter and get the gravity assist (which is very substantial because of the planet's mass), and fall back toward the Sun than it would be to drop straight into a low solar orbit from Earth. This type of trajectory was actually considered for the Parker Solar Probe [0], but was eventually rejected because it would take too long.* If you're going on an interstellar voyage, the time to get out to Jupiter might not impact the timeline as much. [0] - https://wikipedia.org/wiki/Parker_Solar_Probe https://wikipedia.org/wiki/Parker_Solar_Probe * notably, the new trajectory still includes gravity assists off Earth and Venus, underscoring the amount of delta-V needed to lower a solar orbit
- xt00 7y ago
  Yea interesting! Parker solar probe will hit a whopping 0.064% of the speed of light.. and the orbit is quite interesting as you said — needs to slow down to cancel out the speed of earths orbital velocity. So ya know, for a reference point, the 1% of the speed of light that the Trisolaran fleet is flying at is just a mere 6.7 billion miles per hour.. so 0.5 million ain’t too bad...
  - thatcherc 7y ago
    Always good to have the Trisolarans to put things in perspective! Just gotta watch out for those pesky droplets...
    - topkeks 7y ago
      And that fucking sophon block...
- pdonis 7y ago
  > it takes quite a bit of energy to drop from Earth's orbit around the Sun to ie Mercury's More precisely, you have to lose a significant amount of energy relative to the Sun, and it takes significant delta-v for a spaceship starting at Earth (to cancel out at least part of Earth's orbital speed about the Sun) to do that. Yes, that's a valid point.

the_pwner224 7y ago

With some more detail on the mechanics: Here's a screenshot of an orbit in Kerbal Space Program, an excellent NASA-simulator game (you have a spaceport and make + fly rockets and do science): https://wiki.kerbalspaceprogram.com/wiki/File:KerbalEccentricity.jpg https://wiki.kerbalspaceprogram.com/wiki/File:KerbalEccentri... On the blue orbit, you can see Ap and Pe. Ap is the apogee; the place in the orbit where the spacecraft is closest to the earth. Pe is the perigee, the place where the spacecraft is farthest from the earth. Since the blue orbit is mostly circular, they are at about the same altitude. If you look at the large white orbit, the apogee would be the leftmost point, close to the apogee of the blue orbit. The perigee would be all the way at the right of the image. When your spacecraft is at position X, thrusting your engines will not meaningfully change your current position - the acceleration they generate is negligible to the speed at which you are moving. However, the maneuver will change your speed, which affects your orbit. At the apogee, if you thrust in the direction you are moving, your perigee will increase - the orbit will become even more eccentric. Example: at the apogee you are moving at 100 miles per second. Thrusting the engines will not change your current position much; your current position is determined by your 100 miles per second speed and where you came from. But the maneuver will change your speed to 105 miles per second, which will cause your perigee to become much farther away from the earth. However your apogee will stay in the exact same position. Why? Because in orbit you are basically flying sideways while gravity pulls you down. In a circular orbit this balances out into you flying around in a circle. But if you go faster, you're going more sideways while gravity pulls the same amount. So your orbit becomes more elongated. You can thrust in the direction of movement at any point in orbit, and it will increase the altitude of the orbit on the opposite side of where you are at - if you thrust at apogee the perigee altitude increases, and in the white elongated orbit if you thrust between the apogee and perigee your orbit will become more circular and bigger. Similarly to make an eccentric orbit circular, you get to the apogee and then use the engines to slow down, which brings down the perigee. Since thrusting changes your position on the opposite side of the orbit, the most efficient way to increase your perigee is to thrust while at the apogee. The NASA thing is basically going into an elongated orbit between the Sun and Jupiter, coming back to the apogee near the Sun, and then thrusting. The difference in efficiency between thrusting at the apogee of an orbit and just thrusting in a straight line out into space is really huge. Though I'm not sure why the extra gravity of the Sun makes the maneuver more efficient. If you're interested in this you should definitely play KSP. It's a great game and gives you a great intuitive sense of how these things work.

p1mrx 7y ago

The Oberth effect is interesting. You want to get as low/fast as possible (converting your fuel's gravitational potential energy into kinetic energy), and then do a burn to harvest that energy, dropping the exhaust into the sun while the spacecraft zips away at high speed. It's basically the "tennis ball on a basketball" trick, with rockets.

mabbo 7y ago

No matter how it's explained to me, the Oberth effect still seems like magic, or at least some kind of physics violation. Somehow, we use the same delta-V to get more delta-V.

greglindahl 7y ago
You're adding less energy to the exhaust and more to the rocket. The Wikipedia article is a pretty good explanation.

rbanffy 7y ago

> main challenge is solar shielding.. If you carry a solar sail to add a little more push from the Sun's radiation you can spare a little material to build a shade on the other side of the sail.