3 ms·

(another ksp'er here...) I think the problem is atmospheric drag, which increases when the sail is open. If you're looking to increase apogee, the most efficien

(another ksp'er here...) I think the problem is atmospheric drag, which increases when the sail is open. If you're looking to increase apogee, the most efficient place to do so is at the perigee, which also happens to be where the atmospheric drag is the worst. So setting the mission, you get to choose the orientation of the sun relative to your orbital ellipse. If the sun direction to the sun is perpendicular to the long axis, then you (presumably) want your sail wide at the perigee, where drag will also be greatest, but will also provide the most efficient raising of the apogee. OTOH, if the sun's direction is parallel to the long axis, you can try to avoid drag at the perigee, and then rotate into position to get more thrust during the journey from perigee to apogee. But this is a less efficient way to raise the orbit, and will deform (circularize?) the orbit. I might have to fire up KSP with 'cubesat' with an ion thruster to simulate this...

jessriedel 7y ago
Unlike Lightsail 1, Lightsail 2 operates at an altitude where the atmosphere is negligible.
- sdenton4 7y ago
  Hmm.... But the site says this: "In the past 4 days, the spacecraft has raised its orbital high point, or apogee, by about 2 kilometers. The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft." They also say this on the mission page: "LightSail 2's attitude control system does not have the precision to maintain a circular orbit and continuously fly the spacecraft higher. Therefore, as one side of LightSail 2’s orbit rises, the other side will dip lower, until atmospheric drag overcomes the forces of solar sailing, ending the primary mission. The spacecraft will remain in orbit roughly a year before entering the atmosphere and burning up."
  - jessriedel 7y ago
    Based on my previous reading, I was under the impression that the atmosphere would be negligible until the perigee dipped much lower than the initial orbit. (Obviously, if you keep lowering the perigee then eventually it becomes important.) That's consistent with your second quote, but your first quote suggests I was wrong and the atmospheric effects are detectable even now. Thanks for the correction.
- Rebelgecko 7y ago
  It's not quite negligible at 700km. The effects of the atmosphere are also proportional to the craft's B* value (atmospheric drag coefficient), which is partially based on the surface area/mass ratio. Id imagine that a sail's effectiveness probably increases as its area increases and mass decreases so it should have have a relatively high B* compared to similar objects. Looking at the published TLE's seems to back this up— Lightsail 2's B* is larger than any current cubesat's value (https://celestrak.com/NORAD/elements/cubesat.txt https://celestrak.com/NORAD/elements/cubesat.txt)
  - jessriedel 7y ago
    Very useful, thanks.
rtkwe 7y ago
I don't think it's atmospheric drag unless deployment took several days. On the graph the change in perigee doesn't start until several about 2-3 days after the sail was deployed. [0] https://planetary.s3.amazonaws.com/assets/images/charts-diagrams/2019/20190731_apogeegraph-02_f840.png https://planetary.s3.amazonaws.com/assets/images/charts-diag...

jessriedel 7y ago

Unlike Lightsail 1, Lightsail 2 operates at an altitude where the atmosphere is negligible.

sdenton4 7y ago
Hmm.... But the site says this: "In the past 4 days, the spacecraft has raised its orbital high point, or apogee, by about 2 kilometers. The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft." They also say this on the mission page: "LightSail 2's attitude control system does not have the precision to maintain a circular orbit and continuously fly the spacecraft higher. Therefore, as one side of LightSail 2’s orbit rises, the other side will dip lower, until atmospheric drag overcomes the forces of solar sailing, ending the primary mission. The spacecraft will remain in orbit roughly a year before entering the atmosphere and burning up."
- jessriedel 7y ago
  Based on my previous reading, I was under the impression that the atmosphere would be negligible until the perigee dipped much lower than the initial orbit. (Obviously, if you keep lowering the perigee then eventually it becomes important.) That's consistent with your second quote, but your first quote suggests I was wrong and the atmospheric effects are detectable even now. Thanks for the correction.
Rebelgecko 7y ago
It's not quite negligible at 700km. The effects of the atmosphere are also proportional to the craft's B* value (atmospheric drag coefficient), which is partially based on the surface area/mass ratio. Id imagine that a sail's effectiveness probably increases as its area increases and mass decreases so it should have have a relatively high B* compared to similar objects. Looking at the published TLE's seems to back this up— Lightsail 2's B* is larger than any current cubesat's value (https://celestrak.com/NORAD/elements/cubesat.txt https://celestrak.com/NORAD/elements/cubesat.txt)
- jessriedel 7y ago
  Very useful, thanks.

rtkwe 7y ago

I don't think it's atmospheric drag unless deployment took several days. On the graph the change in perigee doesn't start until several about 2-3 days after the sail was deployed. [0] https://planetary.s3.amazonaws.com/assets/images/charts-diagrams/2019/20190731_apogeegraph-02_f840.png https://planetary.s3.amazonaws.com/assets/images/charts-diag...