Braking Against a Stellar Wind

by Paul Gilster on March 19, 2012

This morning I want to pick up on the ‘problem of arrival’ theme I began writing about on Friday, and we’ll look at interstellar deceleration issues a good bit this week. But I can’t let Monday start without reference to the Icarus results from Gran Sasso that finds neutrinos traveling at precisely the speed of light. All of this adds credence to the growing belief that the earlier Opera experiment was compromised by equipment problems. The news is all over the place (you might begin with this BBC account) and while we’ll keep an eye on it, I don’t plan to spend much time this week on neutrinos. We still have much to get done on the subject of slowing down.

Magsails and Local Resources

When you begin to unlock the deceleration issue, the options quickly multiply, and you find yourself looking into areas that weren’t remotely the subject of your earlier research. As we saw on Friday, the concept of magnetic sails grew organically out of Robert Bussard’s idea of an interstellar ramjet. Bussard didn’t want to slow down — he wanted to go very fast indeed. Read the comments on that post and you’ll find Al Jackson’s entertaining reminiscences of a dinner with Bussard (Tau Zero author Poul Anderson was present too), and a reminder that the scientist always claimed to have come upon the ramjet idea because of an encounter with Mexican food. The usual story has it that it was a burrito which, bitten down upon, suddenly opened for Bussard the splendors of matter being forced into a cylinder at high speeds.

Or maybe he was eating huevos rancheros — the story seems to have varied a bit over the years. Whatever the case, the idea of scooping up interstellar hydrogen and fusing it turned into a 1960 paper for Acta Astronautica and, along the way, into a critique by Robert Zubrin and Dana Andrews that showed just how much drag an electromagnetic scoop could generate. Andrews was working for Boeing at the time, and had grown interested in using Bussard concepts right here in the Solar System, thinking that a big enough scoop could gather hydrogen for use in an ion engine that could be powered up by an onboard nuclear reactor. A self-fueling ion drive might not be adaptable for interstellar missions, but for interplanetary work it seemed worth a look.

But the numbers were intractable. The magnetic scoop Andrews hoped to deploy created more drag than the ion engines produced thrust. The two researchers quickly found that the scoop’s best function was as a magnetic sail, and their work on the idea appeared in the literature in the early 1990s. In his 1999 book Entering Space, Zubrin recalls that the time was right for the magsail given that Paul Chu (University of Houston) had just invented the first high-temperature superconductors, which a magsail could theoretically use to create the magnetic field that would allow it to ride on the solar wind. Practical high-temperature superconducting wire born out of this work might one day allow magsails to achieve higher thrust-to-weight ratios than solar sails.

Magsails have clear propulsion implications, but Zubrin states the obvious about their most effective uses:

…the most interesting and important thing about the magsail is not what it can do to speed up a spacecraft — what’s important is its capability for slowing one down. The magsail is the ideal interstellar mission brake! No matter how fast a spaceship is going, all it has to do to stop is deploy and turn on a magsail, and the drag generated against the interstellar plasma will do the rest. Just as in the case of a parachute deployed by a drag racer, the faster the ship is going, the more ‘wind’ is felt, and the better it works.

Which takes us to the idea of using in-situ resources to tackle the deceleration problem. If your goal is to launch a starship that can decelerate in the destination system to explore it, the magsail lets you do the job without carrying the deceleration fuel aboard the vehicle. Play around with the numbers long enough and you’ll see what a huge boost this would be, for otherwise you’re carrying all the fuel needed to slow down a starship (moving, perhaps, at .10 c!), and that means you’ve got to get all of that fuel up to cruise in the first place. The idea of creating drag against the interstellar medium and a destination stellar wind thus has a powerful appeal.

Rise of the Superconductor

When Bussard studied how his ramjet could operate in a region of interstellar space where the density of hydrogen was roughly 1 hydrogen atom per cubic centimeter, he saw that he would need a collecting area of 10,000 square kilometers. This is so vast that even if it were made of 0.1-centimeter mylar, a physical scoop would weigh something on the order of 250,000 tons. But a much smaller collector generating a magnetic field seems practical given the advances in superconducting alluded to above, with a loop of superconducting wire deployed from the spacecraft, the current applied to it cycling continuously to generate the magnetic field. Here’s how Zubrin and Andrews described it in a paper based on their presentation at the 1990 Vision-21 symposium at NASA’s Lewis Research Center (now Glenn Research Center):

The magnetic sail, or Magsail, is a device which can be used to accelerate or decelerate a spacecraft by using a magnetic field to accelerate/deflect the plasma naturally found in the solar wind and interstellar medium. Its principle of operation is as follows: A loop of superconducting cable hundreds of kilometers in diameter is stored on a drum attached to a payload spacecraft. When the time comes for operation the cable is played out into space and a current is initiated in the loop. This current once initiated, will be maintained indefinitely in the superconductor without further power. The magnetic field created by the current will impart a hoop stress to the loop aiding the deployment and eventually forcing it to a rigid circular shape.

Image: A space probe surrounded by a magnetic sail. Early work on these concepts has taken place at the University of Washington under Robert Winglee, with reports available at NASA’s Institute for Advanced Concepts site. Credit: NASA/University of Washington.

Thus the hybrid concept Andrews and Zubrin came up with in the Vision-21 work, extending ideas they had first presented in a 1988 paper: Use laser beaming technology to push a sail to interstellar cruise speeds, then deploy a magsail upon arrival to reduce deceleration time. The authors looked at the numbers and worked out 0.8 years for acceleration, 17.4 years of coasting at almost half the speed of light, and 18.8 years for deceleration. This gets you about 10 light years out in around 37 years, a mind-bending pace that uses a huge sail and some generous assumptions about laser power that we’ll look at tomorrow. For there are other ways to use lasers, even for deceleration, and other ways, too, to exploit the local interstellar medium.

Zubrin and Andrews’ paper from Vision-21 is “Use of Magnetic Sails for Advanced Exploration Missions,” in the proceedings of Vision-21: Space Travel for the Next Millennium” (NASA Conference Publication 10059. The citation for their 1988 work is given in yesterday’s Centauri Dreams post.

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{ 12 comments }

Paul D. March 19, 2012 at 11:48

It may be possible to combine magsails and laser propulsion in another way. Use the magnetic field to store a plasma of partially ionized ions, for example singly ionized alkali earth elements, that have a strong absorption line. Illuminate the plasma with laser light at resonance, where the scattering cross section is extremely high. The laser would have to operate at variable frequency to compensate for doppler shift.

Such a sail should be able to tolerate extremely high laser intensity, since it comes pre-vaporized. If the laser is properly tuned, it could even remove thermal energy from the plasma by preferentially scattering off ions with velocities (in the frame of the vehicle) directed toward the laser.

Interstellar Bill March 19, 2012 at 13:44

Won’t there be a back-emf on the loop
that requires power for deceleration?
If this were so if would make things worse
because of the inefficiency of generating electricity.

Adam March 19, 2012 at 15:44

Interstellar Bill, the braking isn’t via a magnetic motor effect at all. Instead the ISM plasma is being deflected by the magnetic field and it’s that change in momentum which is then transferred to the sail. There’s no electrical currents or EMFs involved in the plasma flow itself as such. The spatial density of the charges is low enough that their electrical interactions can be ignored, and there’s no net charge in the ISM. There is some charge separation in the magnetopause that forms in the initial shock front, which only enhances the coupling between mag-sail and ISM.

Interstellar Bill March 19, 2012 at 19:00

Then it would be nice if the destination star had a strong stellar wind, with a dense plasma for the loop to grab. High-mass stars look good.

As for superconducting loops, for sure unshielded, do we know their vulnerability to the considerable radiation hazards of interstellar flight?
For example, their superconductivity may get poisoned by radiation-generated defects, their mass eroded by collision-induced spallation.

The longer and faster the trip the worse are such problems. Perhaps the loop could be continuously repaired by crawling nanobots, or perhaps it could be kept in a shielded compartment to be deployed at:

ShowTime,
the Big SlowDown,
the Mandatory Halt,
the Blessed Trip-Arrival.
I can see a considerable potential here for the Galactic Religion to have tremendous ceremonies of euphoric triumph and elated congratulation to greet starship arrivals.

Imagine being finally slowed down and seeing a Sun again,
after all that time HURTLING THE COLD DANGEROUS DARK.

Now that’s a Salvation Experience.

Eniac March 19, 2012 at 21:36

The incredibly low density of the interstellar medium and the very short distances (compared to interstellar distances) that stellar wind will be available are formidable problems here. Another one is that the same wind you want to brake against also tends to blow away the field you’d like to brake with. There is going to be a maximum current density in that wire, and with that comes a maximum extent of magnetosphere around it, which will diminish with increasing speed.

I have heard very little quantitative arguments, mostly hand waving. Dana and Zubrin, JBIS 1990, have an interesting quantitative treatment (path-2.narod.ru/design/base_e/msit.pdf), and they claim an acceleration in the solar wind of 0.01 m/s^2, and interstellar breaking of a factor of e in five years. Not great, but perhaps just enough to be useful? Neither of these take payload into account, I think.

The “sweeping away” of magnetic field lines, which is well known from the magnetospheres of the planets, has not been considered as far as I can tell. It should make a huge dent in the already weak performance, I would think. Has anyone looked into this?

Christopher Phoenix March 19, 2012 at 21:48

Many scientists have picked up the “brain bug” that says that starships must carry all the fuel and propellent they use to speed up and slow down- but the whole concept of magnetic sails shows that this utterly untrue. I am confident that magnetic sails will be a liberating technology, both in our solar system and beyond. Magnetic sails can be used to shield orbiting space colonies from cosmic rays, gather hydrogen from the solar wind, propel spacecraft, and brake interstellar craft without expelling propellent- that is a lot of uses for one technology!!

Michael March 20, 2012 at 1:56

Could it be possible to use a laser of the most useful wavelength in a cone shape configuration ‘the base edge’ to move gas ahead of the craft into the magsails path and ionise it at the same time, a sort of long range gatherer. It would collect a much larger amount of interstellar and alien solar wind gases.

jkittle March 21, 2012 at 0:18

There are a few big problem with making a big sail.
First the energy is a function of the SIZE of the field as well as its strength. thus a big sale takes a lot of energy to construct (inflate? energize?) . Second the forces on the wire loop are not trivial, with the loop tending to collapse as the field builds. a really big loop is hard to keep open since it need to be kilometers in diameter.
I thought of spinning the loop to supply an inertial ( “centrifugal”) force to keep it open.

Eniac March 21, 2012 at 18:31

@jkittle: I believe the loop would tend to expand from the field it generates, thus be stabilized. It is even possible that its tensile strength puts a more stringent limit on the field strength per mass than the current density. Spinning the loop is a good idea, but will not be necessary.

Christopher Phoenix: Indeed, it would be great if magnetic sails could do all these wonderful things, but unfortunately when you do the numbers with realistic assumptions they come up short on most counts. Sometimes a “brain bug” is simply a realization that fantasy won’t move your ship.

jkittle March 21, 2012 at 23:24

actually the lopp will tend to crunch closed, this is one of the limitations on practical construction of high magnetic field coils used in NMR ( nuclear magnetic Resonance) or in particle accelerators like CERN. the force on the wires become extreme

Tom March 22, 2012 at 20:09

When I was a teenager, the Bussard Ramscoop had a lot of appeal for serious Interstellar man rated travel. The sufficiency of Alpha-Hydrogen, the architecture for advance fusion powered propulsion…. about several dozen ‘sobering’ assessments that put the idea on the shelf until science ‘catches up’ or supercedes this design?
But what seems to be contradictory in speculating about advanced/exotic deep space flight is that half the conditions are ‘listed’ as improbable or infeasible to accelerate a spaceship… and the same list will erode or ‘fry’ a spaceship!
I understand the principle of diminishing return, but there should come a point where a ‘technology’ field demonstration can answer these concerns.
I’m tired of the ‘about 300 years to never’ comments in solving these problems… its almost like arguing with Creationists about ‘great’ certainty’ without ‘confirming’ evidence.

Eniac March 23, 2012 at 15:33

@jkittle: Actually, the force is outwards. Read the section “Lorentz Forces” here: http://en.wikipedia.org/wiki/Electromagnet.

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