[Spaceships] Query: Activating Mag-Sails

[DaltonS]

A magsail requires one Power Point to activate, but no power to maintain once powered.

Spaceships 1, page 25

How long does it need one Power Point to activate? Could a small power system and an energy bank work? How long does it take to "deploy"? If it has to be "deployed" to work, can it be "undeployed" and stowed inside the hull (after being "deactivated" of course)? I searched for the word "retract" in Spaceships 1 and I only found it referring to landing gear and exposed radiators. Are any other "exposed systems" retractable? Or is this a special feature that costs money?

Dalton “who hates exposing his ignorance” Spence

[PTTG]

I'd say it takes about an hour, maybe more if it's large or crude, less if it's small or advanced.

It's not a physical object -- it's a charged plasma field.

[DaltonS]

Originally Posted by PTTG

I'd say it takes about an hour, maybe more if it's large or crude, less if it's small or advanced.

It's not a physical object -- it's a charged plasma field.

A huge superconducting loop mounted in front of the spacecraft that interacts with charged particles from a star’s solar wind to generate thrust.

Spaceships, page 25

That's the physical structure of it. Whether it can be retracted or "deactivated" I don't know. I'll go with the 1 PPh energy estimate though; it makes sense. (A small "Energy Bank: Battery" should be enough and could be recharged by 2 small Fission Reactors in 1.5 hours.)

Dalton “now only if my other questions are answered” Spence

[Fred Brackin]

Originally Posted by PTTG

I'd say it takes about an hour, maybe more if it's large or crude, less if it's small or advanced.

It's not a physical object -- it's a charged plasma field.

I'm afraid you've confused "plasma sails" with "mag sails". As Dalton points out the mag sail is a big loop of superconducting cable.

The solar wind bouncing off the superconducting magnetic field of that cable transfers momentum to the cable and whatever is attached to it. This is similar to what happens at the periphery of the plasma sail but the two things are different ways of skinning the same cat.

[DaltonS]

Originally Posted by Fred Brackin

I'm afraid you've confused "plasma sails" with "mag sails". As Dalton points out the mag sail is a big loop of superconducting cable.

The solar wind bouncing off the superconducting magnetic field of that cable transfers momentum to the cable and whatever is attached to it. This is similar to what happens at the periphery of the plasma sail but the two things are different ways of skinning the same cat.

Do you think it's retractable? Should it be discharged first? How would that be done?

Dalton “always interested in informed opinions” Spence

[ericbsmith]

Originally Posted by PTTG

I'd say it takes about an hour, maybe more if it's large or crude, less if it's small or advanced.

Lacking anything else to go on, an hour sounds like a good timeframe. I mean, theoretically it might take days to build up the necessary charge, but like all things would depend on the technology assumptions of the gameworld.

Originally Posted by DaltonS

Do you think it's retractable?

Possibly. It's a superconductor loop, so if that can be built with wire that wire may be coilable. OTOH, if it's made from something that's not flexible, like the ceramics used in modern superconductors, the loop may not be flexible; though such loops may be foldable. So it's very much materials dependent which is entirely dependent on your worldbuilding assumptions.

Originally Posted by DaltonS

Should it be discharged first? How would that be done?

It would have to be. As a magnetic superconductor the magnetic field would tend to push it into a circular shape. Trying to "reel it in" while it's charged would result in the magnetic field pushing back against you. Also, if you could manage to reel it in the coil would warm to the ships temperature, which could be above it's superconductor temperature... and would result in a boom. Big badda boom. (above the superconducting temp the charge generates resistance, which generates heat, which warms the loop more, which results in more resistance, which generates more heat, which quickly turns into a catastrophic failure of the loop). If it's a solid loop, breaking the loop (to fold it) would result in a catastrophic failure once the loop is broken.

Discharging it would just be draining the electromagnetic field from it, which could be done as a direct energy drain or by using opposing electromagnetic fields, which would result in an electric charge being transferred to the opposing field generator. Either way you would wind up with energy which you could either use for something useful (charging batteries, firing weapons, running systems) or could be used to generate heat and dumped into an external radiator.

[DaltonS]

Thank you Eric, this is exactly what I needed. By giving me a valid reason why it might not be retractable you saved me modifying my Mars Trans-Orbital Taxi since it might be damaged when entering Mars atmosphere while it remains a good option for interplanetary craft that stay in space. Does the one hour at 1 PP activation time sound good to you?

Dalton “who loves reasons to mix up tech” Spence

[thrash]

Sean Barrett's 1992 (!) Roleplayer article, "Stellar Windjammers: The Magnetic Space Sail Drive," has the following:

Magsails are stored on huge reels attached to the outside of the payload's hull. To deploy, the reels are simply freed to turn, and current applied to the wire. The magnetic field created will unfurl it automatically. The time required to deploy the magsail is the time necessary for thepower plant to fully charge the wire, or one minute per mile of radius, whichever is greater. To retract a magsail takes one minute per mile of radius. Magsails take no hull volume even when furled, and can enter the atmosphere of a planet that has a usable magnetosphere, at very low speed.

Radius (mile) = 200 * sqrt(thrust in tons)
Energy (MW-hrs) = 25.7 * sqrt((thrust in tons)^3)

Note that since Mars lacks a magnetosphere, any magsail (or plasma sail, for that matter) maneuvering would be in relation to the solar wind, not the planet as such.

[DaltonS]

Originally Posted by thrash

Sean Barrett's 1992 (!) Roleplayer article, "Stellar Windjammers: The Magnetic Space Sail Drive," has the following:

Radius (mile) = sqrt(thrust in tons)
Energy (MW-hrs) = 25.7 * sqrt((thrust in tons)^3)

Note that since Mars lacks a magnetosphere, any magsail (or plasma sail, for that matter) maneuvering would be in relation to the solar wind, not the planet as such.

Wow! Talk about a blast from the past! I found this article online if anyone wants to read the whole thing. (The radius should be 200*sqrt(thrust in tons) miles according to the article.) Now to translate this into Spaceships terms. To keep things simple, let's assume the charging time is the deployment/retraction rate of one minute per mile of radius. Thus the energy required would be 1 PPh per mile of radius / 60. Thrust in tons would be 0.001 * LWT of ship making the radius=sqrt(40*LWT) miles. (The units might not exactly match but I don't want to get into the MW per PP debate.)

Dalton “Looks like I may have to retract my previous post” Spence

[DaltonS]

Just in case anyone is interested, here's a quick table of magsail radiuses and energy requirements.

| Size Radius  Energy|Size Radius  Energy|Size Radius  Energy|Size Radius  Energy|
| (SM) (miles) (PPh) |(SM) (miles) (PPh) |(SM) (miles) (PPh) |(SM) (miles) (PPh) |
+--------------------+-------------------+-------------------+-------------------+
|  +3   10.95   0.18 | +5.5 48.99   0.82 |  +8   200.0  3.33 | +10.5 894.4 14.91 |
| +3.5  15.49   0.26 |  +6  63.25   1.05 | +8.5  282.8  4.71 |  +11  1095  18.26 |
|  +4   20.00   0.33 | +6.5 89.44   1.49 |  +9   346.4  5.77 | +11.5 1549  25.82 |
| +4.5  28.28   0.47 |  +7  109.5   1.83 | +9.5  489.9  8.16 |  +12  2000  33.33 |
|  +5   34.64   0.58 | +7.5 154.9   2.58 |  +10  632.5 10.54 |                   |

* A spacecraft cannot be given a space sail if its SM exceeds its TL.

Dalton “I hope this is helpful.” Spence