[Spaceships] Magsail Braking

[Trachmyr]

I am a bit unclear as to when a magsail/plasma sail can begin decelerating. Is it at the Heliopause? Or can a magsail use stellar winds to brake?

If it’s the former, it seems magsails are ineffective is your DV was more than a few hundred mps (multiple magsails would raise this, but not significantly enough for interstellar travel).

If it’s the latter, then what would be the lowest DV allowed to decelerate to, I’m guessing about 375mps.

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Thanks in advance,
Trachmyr

[Ulzgoroth]

Originally Posted by Trachmyr

I am a bit unclear as to when a magsail/plasma sail can begin decelerating. Is it at the Heliopause? Or can a magsail use stellar winds to brake?

If it’s the former, it seems magsails are ineffective is your DV was more than a few hundred mps (multiple magsails would raise this, but not significantly enough for interstellar travel).

If it’s the latter, then what would be the lowest DV allowed to decelerate to, I’m guessing about 375mps.

-----------------------------

Thanks in advance,
Trachmyr

A magsail should be able to decelerate (or potentially accelerate, depending on the local currents) in interstellar space, so long as the ISM is reasonably dense and ionized.

In theory you could decelerate to rest, but thrust is going to be limited by a factor proportional to, if I'm thinking right, the square of velocity relative to the medium.

[munin]

This was explained to me recently. A magsail has a current passing through its loop which creates a magnetic field around the magsail. When the magnetic field hits ionized particles in the interstellar medium (particles in the space beyond the heliopause, between the stars), it exerts a force on them, which exerts an opposite force on the spacecraft. Thus the spacecraft is slowed, as if it were a really big light object being slowed by movement through air. This can be done anywhere and doesn't require any additional energy once the current has been generated in the magsail.

Basically, you've got a "wind" of particles blowing out from the stars until they hit the heliopause, and "still air" between the stars. You can sail on the wind, but in the still air you can only brake.

Based on lwcamp's drag force math, a magsail could maintain its 0.001 G acceleration in ISM down to about 169 mps. Below that, its acceleration would decline:

A = MIN[0.001, 3.51e-8 * V^2] (in Gs and mps)

[Trachmyr]

Thank you very much!
This will greatly help my seedship and probe desgins.

One additional question while on this topic, would the speed of a star's solar wind affect acceleration performance of sails?

For example, the solar wind of Epsilon Eridani is noted as being 30x as energetic as sol's. Would that increase acceleration to 0.03g, 0.0055g, something else or not at all?

[teviet]

Originally Posted by Ulzgoroth

A magsail should be able to decelerate (or potentially accelerate, depending on the local currents) in interstellar space, so long as the ISM is reasonably dense and ionized.

In theory you could decelerate to rest, but thrust is going to be limited by a factor proportional to, if I'm thinking right, the square of velocity relative to the medium.

Yes and yes.

If the magsail has a sufficiently versatile reeling mechanism, it can basically maintain a constant deceleration force F=constant, trading off between a tight intense field at high velocities down to a wide sparse field at low velocities.

Once the cable is fully unreeled, the magsail will have a deceleration proportional to current velocity squared: F=D*A*v^2 where F is the braking force, D is the density of the ISM, v is the current speed, and A is the effective interaction cross-section area of the magsail.

TeV

[Joseph Paul]

Sol is in the middle of the Local Fluff (~on the order of a few light years across) which is in the middle of the Local Bubble (~300LY across). The Local Fluff has twice the Interstellar Medium (.1 atoms/cc) that the Local Bubble has (.05 atoms/cc) but the Local Bubble has something like 10% of the ISM that the rest of the Orion Arm exhibits (.5 atoms/cc). Differences in ISM density may affect propulsion systems that use the ISM and there may be a need to make adjustments when in the Orion Arm/Local Bubble/Local Fluff locales. Of course if your setting is a 'galaxy far, far away' you can set the ISM density to your own satisfaction.

Any thoughts?

EDIT

Ah Teviet! Thanks for a formula that illustrates what I just wrote. So the effectiveness of the braking is cut to 10% of what it is in a standard ISM when used in the Local Bubble. In the Local Fluff it would be at 20% of the standard ISM. What units is the ISM expressed in?

Ignore the above. I assumed that ISM would be set at a whole integer. So my question is again how is the ISM expressed?

[Fred Brackin]

Originally Posted by Trachmyr

Thank you very much!
This will greatly help my seedship and probe desgins.

One additional question while on this topic, would the speed of a star's solar wind affect acceleration performance of sails?

For example, the solar wind of Epsilon Eridani is noted as being 30x as energetic as sol's. Would that increase acceleration to 0.03g, 0.0055g, something else or not at all?

It's more like total energy than just speed. Speed and density of the solar wind would probably cover it.

This is for inside the heliopause at reduced speeds of course. At max speed in interstellar space it's almost totally about transferring your vehicle's kinetic energy to the local medium. All you need is a medium to interact with.

At any rate it's always about transfer of energy. Density of the ISM will limit your transfer rate but total energy of the local solar wind will determine what you can get out of that.

[teviet]

Originally Posted by Trachmyr

One additional question while on this topic, would the speed of a star's solar wind affect acceleration performance of sails?

For example, the solar wind of Epsilon Eridani is noted as being 30x as energetic as sol's. Would that increase acceleration to 0.03g, 0.0055g, something else or not at all?

Hmm, looked up my old notes on magsails. Assuming that a magsail consists of an arbitrarily long length of arbitrarily fine wire, then the optimum performance comes from balancing the magnetic pressure of the sail with the wind pressure of the plasma. You end up with a thrust that scales as the plasma wind speed times the square root of the plasma density.

At some point (depending on sail construction), though, you can't unreel any more sail. At that point the thrust scales as the plasma density times the square of the plasma wind speed. But let's neglect this complication and stick with the former case.

I assume you don't want the full formula for magsail thrust (expressed in terms of current and mass densities of the sail material). So first let's define a normalized proton wind pressure W=nv^2, where n is the plasma density in atoms/cc and v is the wind speed in km/s (or you can give n in 1/m^3 and v in m/s and get the same number). The "average" Solar wind in the vicinity of Earth has n=4 atoms/cc and v=500km/s, giving W=1 million.

Let's assume a nominal thrust of Fe in the vicinity of Earth. Then the thrust in another environment will be F=Fe*sqrt(W/1e6) where W is the normalized proton wind pressure in the other environment.

Originally Posted by Joseph Paul

Sol is in the middle of the Local Fluff (~on the order of a few light years across) which is in the middle of the Local Bubble (~300LY across). The Local Fluff has twice the Interstellar Medium (.1 atoms/cc) that the Local Bubble has (.05 atoms/cc) but the Local Bubble has something like 10% of the ISM that the rest of the Orion Arm exhibits (.5 atoms/cc). Differences in ISM density may affect propulsion systems that use the ISM and there may be a need to make adjustments when in the Orion Arm/Local Bubble/Local Fluff locales. Of course if your setting is a 'galaxy far, far away' you can set the ISM density to your own satisfaction.

Any thoughts?

So contrary to what I wrote earlier (different assumptions!), the braking thrust will scale as square root of density. So the local bubble is only 3 times worse at braking than the average ISM, and only 1.5 times worse than the local fluff. But remember the speed factor: at 0.1c, even the local bubble has a normalized proton wind pressure of 45 million (45 times greater than the Solar wind near Earth), giving a braking thrust 7 times more than the nominal magsail thrust near Earth.

TeV

[Joseph Paul]

Very interesting scenarios develop out of that. Thanks Teviet.