# Does Lightsail-2 take significant advantage of the Oberth effect?

The cubesat Lightsail-2 has been deployed and has communicated with ground stations. See The Planetary Society's LightSail 2 Healthy as Mission Team Continues Spacecraft Checkouts

In the near future, the large solar sail will be deployed, and then it will be used in orbit-raising maneuvers.

For traditional, rocket-based propulsive apoapsis-raising maneuvers, due to the Oberth Effect impulsive boosts at periapsis where velocity is highest are the most efficient use of a given amount of impulse.

The animation shown in the video shows the solar sail being used to intercept sunlight (from the left, hat-tip to @Jack's comment) during the half of the elliptical orbit closest from the Earth.

Question: Does the Oberth effect apply to this kind of use of solar sails in the same general qualitative way as it does to impulsive maneuvers; being most efficient during the parts of the orbit closer to periapsis?

cued at 02:36 (warning, really loud music!)

• To me it looks like the 'burns' do occur at closest approach (sunlight from the left, perigee at the bottom)? Also, I'm not certain, but I believe the Oberth effect wouldn't apply here since it's to do with the mechanical energy available from the onboard propellant
– Jack
Jul 5 '19 at 13:00
• @Jack Oh, holy granola you are right! I'm going to adjust the wording of the question right now, thank you for bringing that to my attention!
– uhoh
Jul 5 '19 at 15:12

Their solar sailing algorithm is described in the Planetary Society blog:

Solar sailing: The spacecraft is attempting to raise its orbit using the solar sail. To do this, it must make two 90-degree turns each orbit. When flying towards the Sun, the sail orients itself edge-on, effectively turning off the thrust. When flying away from the Sun, the sail turns broadside to the Sun's rays and gets a slight push.

So they're not thrusting at lowest altitude, they're thrusting for the entire half of the orbit where they're flying away from the Sun.

The Oberth effect should still apply as its formulae don't depend on engine type (no mention of a change in mass due to thrust for example). But the Oberth effect won't be as critical as for chemical engines: instead of being a factor in deciding when to burn, it's just a factor in how much delta-V your solar sail will generate.

Lightsail-2 starts out in a circular orbit. The location of the periapsis and apoapsis will be the result of engaging the solar sail for the entire half-orbit moving away from the Sun, ie an inevitable effect and not something they chose deliberately.

• In the video, isn't the orbit oriented such that "the entire half of the orbit where they're flying away from the Sun." is nearly the same thing as "the half of the elliptical orbit closest from the Earth"? I'm thinking that this is done on purpose in order to take advantage of the Oberth effect.
– uhoh
Jul 7 '19 at 15:02
• So my question is about the orbital mechanics. Whether or not they say they are using the Orberth effect or not, I'm asking if Lightsail-2's orbit does benefit from it or not, and that's best answered with some math and a look at the orbit itself.
– uhoh
Jul 7 '19 at 15:18