# Tacking a solar sail to increase duration of acceleration

Because the utility of a solar sail diminishes the further it travels away from the sun (max. distance ~50 AU), the longer it can stay in closer proximity to the sun the longer it can accelerate.

Is it theoretically possible to angle a solar sail in order to create a heliocentric orbit so that the time duration of acceleration can be extended via a sustained angular acceleration? Or, will the photonic radial pressure only push the sail outward away from the sun no matter the attitude of the sail?

Tacking proper involves sailing into the wind at an angle. This is not possible in space. You can't tack like a boat, because a boat pushes against the water with its keel, giving an overall velocity into the wind. There is no medium in space which the spacecraft can push against, so tacking like a boat cannot be done.

"so that the time duration of acceleration can be extended..."

No. It's not possible to store up energy like that. I assume you are imagining the sail remaining in the same orbit, getting faster and faster before suddenly departing at high speed. Sadly, without a medium to push against it doesn't work that way. The orbit shape/size, and the orbital speed are absolutely linked, you cannot change one without getting a change in the other. So the solar sail gives the vehicle an acceleration, and this acceleration increases the size of the orbit. The gross energy imparted will always be outwards, since the wind is always blowing outwards.

It's possible to deflect this outward force, and to "dive bomb" to get closer to the sun, but this slings you out high on the other side, so its only the shape of the orbit which is being changed, and made more eccentric. To get into a totally lower orbit requires bleeding off speed, and the sail can't do that, since its always being accelerated away from the sun.

In your question, you are proposing that this acceleration outwards can be used to hold the craft at a constant distance, so as to get more of this acceleration, to hold even longer, just does not work.

• I appreciate your answer and correcting me on my misuse of the work 'tacking', I was meaning something along the lines of angular deflection. I changed the wording of my question to reflect this. Many thanks! May 6, 2019 at 22:06
• @solarisra it's possible to use the sail for angular deflection, and to approach the source of the wind by rotating the orbit periapsis. However, this also increases the apoapsis. It's not possible to get into a general lower orbit, this requires slowing down in the prograde direction. You should not accept the other answer, it is incorrect. May 7, 2019 at 6:26
• So, in order to maintain a lower solar orbit, the spacecraft must not exceed the escape velocity for that orbital radius. Anything faster than the orbital escape velocity will fling the craft away from the sun unless acted upon by a stronger, opposing force. With the orbital escape velocity decreasing as distance from the sun increases, an opposing force will have to get exponentially stronger as the spacecraft accelerates so as to maintain a stable orbit. Since such a powerful opposing force would not exist, a stable orbit at increasing acceleration is impossible. Is this correct? May 7, 2019 at 18:51
• I imagine then, that at a certain distance dependant upon the size of the sail, more acceleration can be generated when moving linearly from the sun (with the sail perpendicular to the sun) than a spiraling orbit (with a canted sail) between the same intersecting points of the spiral and line. May 7, 2019 at 19:24

A solar sail can meet the sunlight at any angle, and that's essentially the only way to operate it because the light pressure is too small to blow you arbitrarily far away. And by choosing your deflection angle you could go to higher or lower orbits or change your orbital plane--you could go anywhere in the solar system, eventually.

The coolest method I've seen for attitude control is in the Ikaros solar sail of the Japanese Aerospace Agency. They used LCDs at the fringes of the sail to control the reflectivity and thereby apply torques.

https://www.space.com/25800-ikaros-solar-sail.html

• That is not tacking. And he didn't ask about attitude control, you could remove that irrelevant part. May 4, 2019 at 20:22
• That is tacking as the OP defined it. But you are correct that it is not tacking as the dictionary would define it, and I have adjusted my answer to reflect that. As for attitude control, that is how it is done, and I don't understand why the subject should be avoided. It seems relevant to understanding the process, plus it's a very clever technique, so I left it in.
– Greg
May 4, 2019 at 23:37
• This answer should not be accepted. It's not possible to do as this answer suggests. Moving against the solar wind (ie lowering the orbit) is not possible. May 7, 2019 at 6:17
• Also. incorrect that a sail can meet the solar wind at any angle. It can only meet the wind within a 180 degree angle, meaning the resulting force is always away from the source of the wind. May 7, 2019 at 6:28
• While it might be possible to go anywhere in the solar system with a sail, it is not possible to get to arbitrary orbits, especially lower ones. May 7, 2019 at 6:31