# Propellantless interstellar travel using electric fields that is better than laser driven sails up to 5% of light speed

Laser-driven sails are limited to about 6.67 Newtons per gigawatt. The Three Gorges Dam has a capacity of about 22.5 GW. If this was transmitted with 100% efficiency to a light sail it would provide thrust equivalent to the force required to lift a 15 kg mass on Earth.

A new propulsion system wants to use electric fields to push against the charged particles of the interstellar medium.

Recently Robert Zubrin published his independent work on a ‘dipole drive’ concept which bears a striking resemblance to this concept (called SWIMMER) which they describe.

The Magsail and electric sail concepts are based on the fact that we can interact with significant mass in the ISM (or the heliosphere) using relatively low mass structures consisting of charged or current carrying wires. How then can we interact with the medium to accelerate rather than decelerate?

Above – A schematic representation of a paddy-wheel SWIMMER in operation. The top frames show the paddy wheel with sail A positively charged. The inset shows sail A in its frame of reference pushing against the positive ions in the ISM which appear to be streaming towards it. Bottom frame shows a later time in the rotation cycle during which neither sail is moving with negative velocity, and the electric potential difference is removed with both sails neutrally charged.

## Rotating version with cycling of the positive and negative charges

Two electric sails are mounted opposite each other at the ends of two long tethers which are electrically connected, and which we can apply a potential difference across. The tethers are mounted to a reaction wheel in the center, and the whole system is set spinning with the spin axis perpendicular to the direction of travel (which we define as the positive direction). As one of the electric sails (sail ‘A’) approaches the portion of the cycle when its velocity is negative with respect to the ISM, we apply a potential difference, charging sail A positively and sail B negatively. At this point in the cycle in the frame of sail A, ions in the ISM are streaming towards it and pushing it in the desired direction of travel. We note that simultaneously the negatively charged sail B will be reflecting electrons in the positive direction causing some drag. Fortunately, as ions out-mass electrons by at least a factor of mp/me=1836, the electrons contribute negligible drag, and in general we will ignore them here and throughout the analysis.

## Two layer sail with positive and negative charges but pulsing power

Another configuration described is a large pair of wire grids with opposite charges to push on the ambient ISM, is very similar to that recently described by Robert Zubrin as the dipole drive. In the case of the dipole drive, however, the electric field is apparently static rather than pulsed, the wire grids are separated by some distance, and they push on the charged particles as they pass between the plates. At first look, this seems like a reasonable and simpler approach. Introductory E&M tells us that two oppositely charged infinite plates produce a strong electric field between them and no electric field outside, so if we can simply push the heavy ions between the plates in the correct direction this static electric field should give us thrust. Unfortunately, the approximation of infinite plates leads us astray here. In fact, a finite system of parallel plates will produce an electric field outside the plates pushing in the opposite direction. Although these fields will be weaker than the field between the plates, they will also extend over a larger region, canceling out the thrust gained from particles between the plates. Indeed, any system of static charges over a finite area must leave the electric potential zero at infinity.

## Space probe rendezvous at α Cen

A relatively early stage SWIMMER mission might have the goal of transporting a modest space probe, mpay = 1000 kg to α Cen A and then decelerating to allow gravitational capture for a permanent orbital space telescope. We will assume a modest electrical power delivered to the SWIMMER of 10 MW. The pusher plate will be made up of several long tethers. In practice these tethers will consist of very fine braided filaments to prevent failure due to micrometeoroid and interstellar dust collision, as described for the electric sail (Janhunen 2004), but we will consider them to be single wires with an effective diameter of 30 µm. This is equivalent in material to eight filaments with diameters of about 10 µm. We will also include strategically weak breakpoints in our tethers which can be activated by simply increasing the spin rate such that the centripetal force exceeds the breakpoint capacity. As we reach higher velocities then, we may leave behind mass from the pusher plate. Given the pulsed nature of the SWIMMER electric field, the wire tethers should be made out of superconducting materials.

The total mass of the SWIMMER ship is comprised of mpay=1000 kg, mpower=2500 kg (given by our 10 MW electric power supply and its assumed specific power), and mpusher. At the moment it is unclear how much mass to devote to mpusher, however we will show that a mass of 7400 kg is useful. The mass for the tethers could be mined in situ from asteroids. This mass provides for a total summed tether length of 4.1×109 meters.

While this is seemingly a very long tether, it does not in any way represent the spatial scale of the SWIMMER as the pusher plate will be made up of several thousand tethers, possibly splitting off from each other at greater radial distances. The summed length is merely a useful value for determining the total cross-sectional area in plasmas of different temperatures and densities.

After 1.5 years the SWIMMER enters the ISM at 100 AU with a velocity of 4.0×105 meters per second.

Upon entering interstellar space, the SWIMMER begins normal mode operations. Simultaneously the ion density drops and the cross sectional area of our tethers increases by a factor of λD(ISM)/λD(helio) = 2.3. At this distance from the sun our tethers will be superconducting, and we can begin applying our 10 MW of power.

We will also begin discarding mass from the pusher plate as it accelerates. The optimal rate to discard mass will change based on the specific details of any given mass distribution, power, and journey length.

A minimum 1 pc travel time of 263 years for our SWIMMER with χ = 0.13, and ψ = 0.53. The ship arrives with a velocity of 6.02×106 meters per second (0.02 c, 2% of lightspeed). Without allowing the pusher plate mass to be discarded en route, the journey would take slightly longer at 340 years. For comparison, an ideal light sail dominated by mpay = 1000 kg (IE ignoring the light sail mass and assuming perfect reflectivity) pushed with the same delivered power, would take 793 years to complete the same journey.

As the SWIMMER approaches α Cen A it begins destination braking. This would begin in nearby interstellar space at a distance of ∼12500 AU from α Cen A. By this point the SWIMMER has significantly reduced the mass of its pusher plate to 518 kg, with a corresponding interaction area of 1.25×1010 m2. After 23 years of braking in the ISM, the SWIMMER enters the α Cen A heliosphere at a distance of 100 AU and a velocity of 1.13×106 meters per second relative to α Cen A

Arxiv – Spacecraft With Interstellar Medium Momentum Exchange Reactions: The potential and limitations of propellantless interstellar travel

Researchers propose a new mode of transport which relies on electric-field moderated momentum exchange with the ionized particles in the interstellar medium. While the application of this mechanism faces significant challenges requiring industrial-scale exploitation of space, the technological roadblocks are minimal, and are perhaps more easily addressed than the issues presented by light sails or particle beam powered craft. This mode of space travel is particularly well suited to energy efficient space travel at velocities less than 5% of light speed, and compares exceptionally well to light sails on an energy expenditure basis. It therefore represents an extremely attractive mode of transport for slow (~multi-century long) voyages carrying heavy payloads to nearby stellar neighbors. This could be very useful in missions that would otherwise be too energy intensive to carry out, such as transporting bulk materials for a future colony around Alpha Cen A, or perhaps a generation ship.

## Ark ship

Due to their extremely favorable performance at lower power and velocities, SWIMMERs would make excellent transporters for large masses that can take long timescales. This could be used as the basis of a generation ship, or perhaps a transporter for bulk colony materials sent out ahead of time before a fast moving low mass people transporter arrived. For this example we will assume a payload mass, mpay=8×109 kg, equivalent to the Super Orion ship discussed by Dyson (2002). Since such a mission would likely only be attempted after significant technological advances, we will slightly improve our material properties by assuming a specific power in our power conversion systems of 10 kW kg−1 , and superconducting materials which are able to passively operate beyond 3 AU. We will take our delivered SWIMMER power to be 10000 GW, thus mpower=1×109 kg. We will use a pusher plate of mass mpusher=3.7×1010 kg, with a summed tether length of 2.0×10M16 m. As before, this pusher plate mass is based on our optimization of the travel time during the normal SWIMMER operation as a function of velocity, ψ, and χ.

By disposing of onboard reaction mass they circumvent the rocket equation, and by exchanging momentum
with ions in the ISM they improve by orders of magnitude over the energy efficiency of traditional
light sails. The key to this momentum exchange is the changing electric field which allows us to create inhomogeneities in the surrounding plasma, and then push on these inhomogeneities to create thrust. SWIMMERS perform exceptionally well at lower velocities, with their advantage over light sails diminishing quickly at velocity over 0.05 c. Furthermore, by relying on the ambient ISM as a momentum exchange medium, they are quite versatile, able to accelerate either away or towards a beamed energy
source, opening up myriad opportunities to serve as oneway transport, roundtrips, or even statites in stationary positions.

The examples discussed here only scratch the surface of the possible roles for SWIMMERs in our spacefaring future. Their characteristics make them ideal for any mission with large masses in which relatively low velocities (v less then 0.05 c) are acceptable. They are unlikely to be the sole mode of space transport due to their diminishing advantages at high velocities and their structural complexity which requires onboard power conversion systems with significant mass. Nonetheless, SWIMMERS will play an important role in future space exploration and augment other modes of transport. They might, for
instance, also be well suited to aiding the construction of a fast interstellar highway by transporting massive particle beam stations along with their fuel supply out to stationary positions between us and our target destinations. These particle stations could be used to swiftly carry light weight Magsails along the path, and simultaneously augment the power of future SWIMMERs by replacing the stationary ISM with a corridor of fast moving beamed particles.

### 60 thoughts on “Propellantless interstellar travel using electric fields that is better than laser driven sails up to 5% of light speed”

1. The key point about the Zubrin concept is that, because the positive ions are so much more massive than the negative ions, they end up spending much more time between the grids. The same force on electrons and protons barely budges the protons, but rapidly expels electrons from the inter-grid space. The result is that the plasma between the grids isn’t neutral, it is highly positive. And so there’s a net force generated. I have some concern about the real world behavior of the sail; It’s possible that it would produce an external space charge that would neutralize the grid potential. But it would certainly at least start out generating thrust. The concept requires a much more detailed simulation of the way the surrounding plasma interacts with the sail; I think it’s too complicated and contingent to resolve the question by thought experiments and cocktail napkins. I don’t see how the rotating version has any hope of producing thrust outside the case where you’re practically at rest with respect to the surrounding plasma. It just can’t rotate fast enough.

2. SWIMMER positive plates appear to repel about equal numbers of positive ions forward and backward, for no net gain from electric forces. Same for the negative plates. At best, it’d give a velocity forward about equal to the velocity of the “paddle” beating against the ISM – and that’s not going to be very fast at all. Zubrin’s dipole drive can be evaluated by imagining seeing it from so far away that it looks like a point of net neutral charge in a sea of other particles with net neutral charge – the net electric forces on it are zero.

3. The key point about the Zubrin concept is that because the positive ions are so much more massive than the negative ions they end up spending much more time between the grids. The same force on electrons and protons barely budges the protons but rapidly expels electrons from the inter-grid space.The result is that the plasma between the grids isn’t neutral it is highly positive. And so there’s a net force generated.I have some concern about the real world behavior of the sail; It’s possible that it would produce an external space charge that would neutralize the grid potential. But it would certainly at least start out generating thrust.The concept requires a much more detailed simulation of the way the surrounding plasma interacts with the sail; I think it’s too complicated and contingent to resolve the question by thought experiments and cocktail napkins.I don’t see how the rotating version has any hope of producing thrust outside the case where you’re practically at rest with respect to the surrounding plasma. It just can’t rotate fast enough.

4. SWIMMER positive plates appear to repel about equal numbers of positive ions forward and backward for no net gain from electric forces. Same for the negative plates. At best it’d give a velocity forward about equal to the velocity of the paddle”” beating against the ISM – and that’s not going to be very fast at all. Zubrin’s dipole drive can be evaluated by imagining seeing it from so far away that it looks like a point of net neutral charge in a sea of other particles with net neutral charge – the net electric forces on it are zero.”””

5. I love the idea of interstellar spacecraft with paddle wheel drives. Add in the fact that they’ll need huge flat heat radiators. And that the minimal mass construction will be a minimum of truss compression members held together with tension member membranes and cords, and you end up with spacecraft that look like 1850s attempts at aeroplanes. If we power them with the General Fusion style steam hammer reactors, then we may as well just go with it, and make the rest of it out of leather and polished brass. With a strict tweed and frock coat dress code for the entire project. HOWEVER, this seems to be rubbish. If you want to push stuff around using electric fields, you don’t move the multi-km-in-size electrodes. This isn’t the 1850s regardless of the dress code. You have multiple electrodes and you switch between the electrically. You know, like we actually do for real systems intended to accelerate subatomic particles. Like a linear accelerator. So your interstellar star craft has a 3-D mesh of fine (superconducting naturally) wires. And by switching the currents around you have a moving electric field that drives in ripples from the front to the back, accelerating +ve particles (the massive ones) to the rear and so pushing yourself forwards. So your final craft doesn’t look like an 1850s steam powered paddle wheel driven aircraft. It looks more like the giant dandelion seed that featured in the Carl Sagan version of Cosmos. We can still have the tweed frock coats though.

6. I’m pretty convinced that the ‘out of gap’ work will exactly cancel out the ‘in gap’ work done. Zubrin seems to have taken the idea that there’d be no external field near a ‘dipole with infinite plates’ and jumped to the idea that the small outside fields of a wide but finite dipole can be ignored – when in fact they have to be integrated over distances much larger than the distance between plates, and additionally will have edge effects. But sure, a good simulation that doesn’t make false assumptions would be great – I certainly wouldn’t expect Zubrin to give up on the idea without such as proof.

7. I love the idea of interstellar spacecraft with paddle wheel drives.Add in the fact that they’ll need huge flat heat radiators. And that the minimal mass construction will be a minimum of truss compression members held together with tension member membranes and cords and you end up with spacecraft that look like 1850s attempts at aeroplanes.If we power them with the General Fusion style steam hammer reactors then we may as well just go with it and make the rest of it out of leather and polished brass. With a strict tweed and frock coat dress code for the entire project.HOWEVER this seems to be rubbish.If you want to push stuff around using electric fields you don’t move the multi-km-in-size electrodes. This isn’t the 1850s regardless of the dress code. You have multiple electrodes and you switch between the electrically. You know like we actually do for real systems intended to accelerate subatomic particles. Like a linear accelerator.So your interstellar star craft has a 3-D mesh of fine (superconducting naturally) wires. And by switching the currents around you have a moving electric field that drives in ripples from the front to the back accelerating +ve particles (the massive ones) to the rear and so pushing yourself forwards. So your final craft doesn’t look like an 1850s steam powered paddle wheel driven aircraft. It looks more like the giant dandelion seed that featured in the Carl Sagan version of Cosmos.We can still have the tweed frock coats though.

8. I’m pretty convinced that the ‘out of gap’ work will exactly cancel out the ‘in gap’ work done. Zubrin seems to have taken the idea that there’d be no external field near a ‘dipole with infinite plates’ and jumped to the idea that the small outside fields of a wide but finite dipole can be ignored – when in fact they have to be integrated over distances much larger than the distance between plates and additionally will have edge effects.But sure a good simulation that doesn’t make false assumptions would be great – I certainly wouldn’t expect Zubrin to give up on the idea without such as proof.

9. If your star ship was small enough you could use a particle accelerator to fling it.

10. How about a stream of high velocity particles to power a star ship.

11. If your star ship was small enough you could use a particle accelerator to fling it.

12. How about a stream of high velocity particles to power a star ship.

13. A relatively early stage SWIMMER mission might have the goal of transporting … to α Cen A” No, a relatively early mission would be within the solar system. Only when a propulsion system has been proven for those relatively short distances will it be tried for interstellar missions. “4.1×109” Sure I can tell that is meant to be 4.1×10^9, but can’t large numbers be written that way or 4.1E9.

14. A relatively early stage SWIMMER mission might have the goal of transporting … to α Cen A””No”””” a relatively early mission would be within the solar system. Only when a propulsion system has been proven for those relatively short distances will it be tried for interstellar missions.””””4.1×109″”””Sure I can tell that is meant to be 4.1×10^9″””” but can’t large numbers be written that way or 4.1E9.”””””””

15. 300 plus years? What would the point be? By the time the probe gets to Alfa Centuri A new technology will have passed it centuries before it arrives.

16. 300 plus years? What would the point be? By the time the probe gets to Alfa Centuri A new technology will have passed it centuries before it arrives.

17. If you click through to their paper they address exactly that option (the high velocity particles) and say that the issue is that you can’t get the aiming right once you get to interstellar distances. For one thing the beam of particles will diverge, from the thermal energy of the particles if nothing else.

18. If you click through to their paper they address exactly that option (the high velocity particles) and say that the issue is that you can’t get the aiming right once you get to interstellar distances. For one thing the beam of particles will diverge from the thermal energy of the particles if nothing else.

19. If I recall correctly, there were issues with that approach explored in much earlier subject threads (since lost to the mists of time and software changes, much as with the library of Alexandria). 1. The density of stuff like gas, ions, microparticles etc is much higher inside the heliosphere than outside, so if your velocity gets too high before you are out into interstellar space you suffer serious erosion/impact damage. (Note: we ASSUME interstellar space is cleaner. TBD!) 2. Dumping enough energy onto a sail to get to a decent C fraction within a short distance means GW/square meter, and that will pose some interesting materials tech challenges. Nonetheless, with semi-proven tech as it currently stands I think the best option we have on the table right now is 1. Electrostatic sail (ie. the positively charged wires spread out from a probe. Maybe coat them with a beta emitter radioisotope to keep them charged up without needing an actual power source?) 2. Start off nice and close to the sun 3. Be able to predict when and where a huge solar flare is going to happen, and surf that wave of high speed particles out as far as you can.

20. Accelerate like crazy. And don’t be afraid to waste a lot of power. Light sailing wastes a lot of power.

21. If I recall correctly there were issues with that approach explored in much earlier subject threads (since lost to the mists of time and software changes much as with the library of Alexandria).1. The density of stuff like gas ions microparticles etc is much higher inside the heliosphere than outside so if your velocity gets too high before you are out into interstellar space you suffer serious erosion/impact damage. (Note: we ASSUME interstellar space is cleaner. TBD!)2. Dumping enough energy onto a sail to get to a decent C fraction within a short distance means GW/square meter and that will pose some interesting materials tech challenges.Nonetheless with semi-proven tech as it currently stands I think the best option we have on the table right now is1. Electrostatic sail (ie. the positively charged wires spread out from a probe. Maybe coat them with a beta emitter radioisotope to keep them charged up without needing an actual power source?)2. Start off nice and close to the sun3. Be able to predict when and where a huge solar flare is going to happen and surf that wave of high speed particles out as far as you can.

22. Accelerate like crazy. And don’t be afraid to waste a lot of power. Light sailing wastes a lot of power.

23. 4.1×10⁹?

24. 4.1×10⁹?

25. As always, a delicious reply

26. LOL. Math, Mr. Stewart. Math. v = at d = ½at² e = ½mv² … = ½ma²t² f = ma … a = f/m So, just plug in the desired speeds, see what comes out! v = 0.02 c = 6,000,000 m/s. Choose an (a). Something unmanned, like a particle-accelerated probe at 1,000,000 G? No, that’d fall apart. How about ‘rifle bullet’ accelerations? v = at … t = √( 2d/a ) (from ½at²) v = √( 2da ) for a gun barrel: d = 0.7 m, (a) we don’t know yet, but v = 1,000 m/s for high-velocity “magnum” type rifle bullet. v² = 2da; a = v²÷2d = 1,000²÷1.4 = 700,000 m/s² OK, so now we have an (a). Let’s work the equations again: v = at… 6,000,000 = at = 700,000 t t = 8.4 seconds d = ½at² d = 0.5 × 700,000 × 8.4² d = 24,700,000 m … or 24,700 kilometers That is one HELL of a long linear accelerator. Fold into a spiral? OK, but what about the centripetal wall force? F = ω²⋅radius … or v²/radius (they’re equivalent) Let’s wind up the accelerator to be what, 10 km in diameter (space based)? We need 24,700 total km whether circular or straight. Circumference = πd = 31.4 km per revolution. 24,700 ÷ 31.4 ≈ 800 revolutions. F = V² / radius = 6,000,000² / (10 km dia × 1000 m/km ÷ 2) F = 7,200,000,000 m/s² centripetal. That’s what, 10,800 times higher than the actual forward acceleration. Clearly the thing needs to be bigger! Since the V²/radius equation is strictly linear, that’ easy: 10 km × 10,800 = 108,000 km diameter is needed. Well, that’s silly… the diameter is rather more than the length of runway … so might as well make it straight. Still, a 24 THOUSAND kilometer long linear accelerator in space is one hêll of a Space Force structure to put up. Even with mining asteroids. Its huge. Just thought you’d like the math. The ever reading, but now rarely posting GoatGuy

27. What’s the point?”” … well”” what if in 300 years”” we’ve NOT figured out how to get appreciably faster by some novel means? If the nay-sayers who wouldn’t launch it “”””today-ish”””” got their way”” 300 years hence it’s still just an idea on an obscure dusty notebook. But if it were launched reasonably soon then some 300 years hence it’d arrive and dutifully … be completely unable to communicate back to the Earthlings because their Civilization was taken over by AI’s deemed redundant and reduced to ash”” to Save the Earth’s species. LOLGoatGuy”””

28. Mmmm… the rotating version was obviously conjured to provide the centripetal force to keep the pole plates away from each other … without requiring a truss structure between. As I said: obviously. The effect on any far-field plasma interaction are at best hopeful but with our shared suspicions of neutral-at-a-distance likely self-negating. I think the idea of positive charges sit on the inside”” longer”” is perhaps tenable”” but more likely it all comes back to the simplest of energy and momentum equations:E = ½mv² and I = mv.Where m (where is my subscript?) of protons is 1836× m of electrons. There is in a net-neutral plasma equal quantities of (p) and (e). Since one of the lovely outcomes of early modern physics was the notion that a unit charge in a unit volt field is accelerated to a unit kinetic energy expressible as electron volts (eV) then in a field of 1 volt all the electrons and all the protons each pick up 1 eV of kinetic energy. E = ½mv² … v = √(2E/m)Therefore for constant (fixed) E for both particles”” we’re comparing different velocities:v = √( 1 ÷ 1 ) for an elecron (“”””v = 1″”””) … versus … v = √( 1 ÷ 1836 ) for a proton (“”””v = ¹/₄₃””””). But remembering that I = mv … then respectively””””I = 1 electron mass × 1 velocity unit … = 1 momentum quantum per electronI = 1836 proton mass × ¹/₄₃ velocity unit = 43 momentum quanta per proton.Well … there you are. Has nothing intrinsically to do with “”””spending more time in field””””. Just straight ol’ momentum and quantum normalization charge physics. The expelled protons will deliver 43× the reaction-mass momentum compared to the electrons”” for a NET of 42 aft. Throw in Planck’s constant mass of an electron ‘c’ and the price of rye bread in Moscow on a Friday night and you’ve got real physics”” lads! Real!!![b]Goat[/b]Guy”””””””

29. As always a delicious reply

30. LOL. Math Mr. Stewart. Math. v = atd = ½at²e = ½mv² … = ½ma²t²f = ma … a = f/mSo just plug in the desired speeds see what comes out! v = 0.02 c = 6000000 m/s. Choose an (a). Something unmanned like a particle-accelerated probe at 1000000 G? No that’d fall apart. How about ‘rifle bullet’ accelerations? v = at … t = √( 2d/a ) (from ½at²)v = √( 2da ) for a gun barrel: d = 0.7 m (a) we don’t know yet but v = 1000 m/s for high-velocity magnum”” type rifle bullet. v² = 2da; a = v²÷2d = 1″”000²÷1.4 = 700000 m/s²OK so now we have an (a). Let’s work the equations again:v = at…60000 = at = 700000 tt = 8.4 secondsd = ½at² d = 0.5 × 700000 × 8.4²d = 24700000 m … or 24700 kilometersThat is one HELL of a long linear accelerator. Fold into a spiral? OK but what about the centripetal wall force?F = ω²⋅radius … or v²/radius (they’re equivalent)Let’s wind up the accelerator to be what 10 km in diameter (space based)? We need 24700 total km whether circular or straight. Circumference = πd = 31.4 km per revolution. 24700 ÷ 31.4 ≈ 800 revolutions. F = V² / radius = 60000² / (10 km dia × 1000 m/km ÷ 2)F = 72000000 m/s² centripetal. That’s what10800 times higher than the actual forward acceleration. Clearly the thing needs to be bigger! Since the V²/radius equation is strictly linear that’ easy: 10 km × 10800 = 108000 km diameter is needed. Well that’s silly… the diameter is rather more than the length of runway … so might as well make it straight. Still a 24 THOUSAND kilometer long linear accelerator in space is one hêll of a Space Force structure to put up. Even with mining asteroids. Its huge. Just thought you’d like the math. The ever reading”” but now rarely posting GoatGuy”””””””

31. Has nothing intrinsically to do with “spending more time in field”.” No, I think it does. And doesn’t. Frequently there are different ways of analyzing a physics problem which are facially different, but at a deeper level are just the same thing. This is one of those cases. Both ways of looking at it give the same answer.

32. What’s the point?” … well, what if in 300 years, we’ve NOT figured out how to get appreciably faster by some novel means? If the nay-sayers who wouldn’t launch it “today-ish” got their way, 300 years hence, it’s still just an idea on an obscure dusty notebook. But if it were launched reasonably soon, then some 300 years hence, it’d arrive, and dutifully … be completely unable to communicate back to the Earthlings because their Civilization was taken over by AI’s, deemed redundant, and reduced to ash, to Save the Earth’s species. LOL GoatGuy

33. Mmmm… the rotating version was obviously conjured to provide the centripetal force to keep the pole plates away from each other … without requiring a truss structure between. As I said: obviously. The effect on any far-field plasma interaction are at best hopeful, but with our shared suspicions of neutral-at-a-distance, likely self-negating. I think the idea of “positive charges sit on the inside, longer” is perhaps tenable, but more likely it all comes back to the simplest of energy and momentum equations: E = ½mv² and I = mv. Where m (where is my subscript?) of protons is 1,836× m of electrons. There is in a net-neutral plasma equal quantities of (p) and (e). Since one of the lovely outcomes of early modern physics was the notion that a unit charge in a unit volt field is accelerated to a unit kinetic energy expressible as electron volts (eV), then in a field of 1 volt, all the electrons and all the protons each pick up 1 eV of kinetic energy. E = ½mv² … v = √(2E/m) Therefore for constant (fixed) E for both particles, we’re comparing different velocities: v = √( 1 ÷ 1 ) for an elecron (“v = 1”) … versus … v = √( 1 ÷ 1836 ) for a proton (“v = ¹/₄₃”). But remembering that I = mv … then respectively, I = 1 electron mass × 1 velocity unit … = 1 momentum quantum per electron I = 1836 proton mass × ¹/₄₃ velocity unit = 43 momentum quanta per proton. Well … there you are. Has nothing intrinsically to do with “spending more time in field”. Just straight ol’ momentum and quantum normalization charge physics. The expelled protons will deliver 43× the reaction-mass momentum compared to the electrons, for a NET of 42, aft. Throw in Planck’s constant, mass of an electron, ‘c’, and the price of rye bread in Moscow on a Friday night, and you’ve got real physics, lads! Real!!! [b]Goat[/b]Guy

34. Has nothing intrinsically to do with “”spending more time in field””””.””””No”” I think it does. And doesn’t. Frequently there are different ways of analyzing a physics problem which are facially different”” but at a deeper level are just the same thing. This is one of those cases. Both ways of looking at it give the same answer.”””

35. Obviously you need particles that are large enough to have guidance systems. Actually, in principle laser Doppler cooling could lower the thermal energy of the beam to the point where it might even gravitationally converge.

36. I plus-1 your reply… about 15 seconds after i pressed the “POST” button, I realized that you were just as right looking at the answer from a time perspective, at the deeper level. In fact it is “righter” in a sense: the longer clearing time (43x longer) gives the 43x greater I=mv momentum transfer. Thanks!

37. Obviously you need particles that are large enough to have guidance systems.Actually in principle laser Doppler cooling could lower the thermal energy of the beam to the point where it might even gravitationally converge.

38. I plus-1 your reply… about 15 seconds after i pressed the POST”” button”” I realized that you were just as right looking at the answer from a time perspective”” at the deeper level. In fact it is “”””righter”””” in a sense: the longer clearing time (43x longer) gives the 43x greater I=mv momentum transfer. Thanks!”””

39. As a non scientist – Im presuming a sort of a very wide(multi-km) particle accelerator as a propulsion source? Seems like this and the Dipole drive need further research about the edge interactions of these concepts, Nevertheless, space amoeba as a propulsion source sounds intriguing 🙂

40. As a non scientist – Im presuming a sort of a very wide(multi-km) particle accelerator as a propulsion source?Seems like this and the Dipole drive need further research about the edge interactions of these concepts Nevertheless space amoeba as a propulsion source sounds intriguing 🙂

41. As a non scientist – Im presuming a sort of a very wide(multi-km) particle accelerator as a propulsion source?

Seems like this and the Dipole drive need further research about the edge interactions of these concepts,

Nevertheless, space amoeba as a propulsion source sounds intriguing 🙂

42. Obviously you need particles that are large enough to have guidance systems.

Actually, in principle laser Doppler cooling could lower the thermal energy of the beam to the point where it might even gravitationally converge.

43. I plus-1 your reply… about 15 seconds after i pressed the “POST” button, I realized that you were just as right looking at the answer from a time perspective, at the deeper level. In fact it is “righter” in a sense: the longer clearing time (43x longer) gives the 43x greater I=mv momentum transfer. Thanks!

44. “Has nothing intrinsically to do with “spending more time in field”.”

No, I think it does. And doesn’t. Frequently there are different ways of analyzing a physics problem which are facially different, but at a deeper level are just the same thing. This is one of those cases. Both ways of looking at it give the same answer.

45. “What’s the point?” … well, what if in 300 years, we’ve NOT figured out how to get appreciably faster by some novel means? If the nay-sayers who wouldn’t launch it “today-ish” got their way, 300 years hence, it’s still just an idea on an obscure dusty notebook. But if it were launched reasonably soon, then some 300 years hence, it’d arrive, and dutifully … be completely unable to communicate back to the Earthlings because their Civilization was taken over by AI’s, deemed redundant, and reduced to ash, to Save the Earth’s species.

LOL
GoatGuy

46. Mmmm… the rotating version was obviously conjured to provide the centripetal force to keep the pole plates away from each other … without requiring a truss structure between. As I said: obviously. The effect on any far-field plasma interaction are at best hopeful, but with our shared suspicions of neutral-at-a-distance, likely self-negating.

I think the idea of “positive charges sit on the inside, longer” is perhaps tenable, but more likely it all comes back to the simplest of energy and momentum equations:

E = ½mv² and I = mv.

Where m (where is my subscript?) of protons is 1,836× m of electrons. There is in a net-neutral plasma equal quantities of (p) and (e). Since one of the lovely outcomes of early modern physics was the notion that a unit charge in a unit volt field is accelerated to a unit kinetic energy expressible as electron volts (eV), then in a field of 1 volt, all the electrons and all the protons each pick up 1 eV of kinetic energy.

E = ½mv² … v = √(2E/m)

Therefore for constant (fixed) E for both particles, we’re comparing different velocities:

v = √( 1 ÷ 1 ) for an elecron (“v = 1”) … versus …
v = √( 1 ÷ 1836 ) for a proton (“v = ¹/₄₃”).

But remembering that I = mv … then respectively,

I = 1 electron mass × 1 velocity unit … = 1 momentum quantum per electron
I = 1836 proton mass × ¹/₄₃ velocity unit = 43 momentum quanta per proton.

Well … there you are. Has nothing intrinsically to do with “spending more time in field”. Just straight ol’ momentum and quantum normalization charge physics. The expelled protons will deliver 43× the reaction-mass momentum compared to the electrons, for a NET of 42, aft.

Throw in Planck’s constant, mass of an electron, ‘c’, and the price of rye bread in Moscow on a Friday night, and you’ve got real physics, lads! Real!!!

[b]Goat[/b]Guy

47. As always, a delicious reply

48. LOL. Math, Mr. Stewart. Math.

v = at
d = ½at²
e = ½mv² … = ½ma²t²
f = ma … a = f/m

So, just plug in the desired speeds, see what comes out! v = 0.02 c = 6,000,000 m/s. Choose an (a). Something unmanned, like a particle-accelerated probe at 1,000,000 G? No, that’d fall apart. How about ‘rifle bullet’ accelerations?

v = at … t = √( 2d/a ) (from ½at²)
v = √( 2da ) for a gun barrel:
d = 0.7 m, (a) we don’t know yet, but v = 1,000 m/s for high-velocity “magnum” type rifle bullet.
v² = 2da;
a = v²÷2d = 1,000²÷1.4 = 700,000 m/s²

OK, so now we have an (a). Let’s work the equations again:

v = at…
6,000,000 = at = 700,000 t
t = 8.4 seconds

d = ½at²
d = 0.5 × 700,000 × 8.4²
d = 24,700,000 m … or 24,700 kilometers

That is one HELL of a long linear accelerator. Fold into a spiral? OK, but what about the centripetal wall force?

Let’s wind up the accelerator to be what, 10 km in diameter (space based)? We need 24,700 total km whether circular or straight. Circumference = πd = 31.4 km per revolution. 24,700 ÷ 31.4 ≈ 800 revolutions.

F = V² / radius = 6,000,000² / (10 km dia × 1000 m/km ÷ 2)
F = 7,200,000,000 m/s² centripetal.

That’s what, 10,800 times higher than the actual forward acceleration. Clearly the thing needs to be bigger! Since the V²/radius equation is strictly linear, that’ easy: 10 km × 10,800 = 108,000 km diameter is needed. Well, that’s silly… the diameter is rather more than the length of runway … so might as well make it straight.

Still, a 24 THOUSAND kilometer long linear accelerator in space is one hêll of a Space Force structure to put up. Even with mining asteroids. Its huge.

Just thought you’d like the math.
The ever reading, but now rarely posting
GoatGuy

49. If I recall correctly, there were issues with that approach explored in much earlier subject threads (since lost to the mists of time and software changes, much as with the library of Alexandria).
1. The density of stuff like gas, ions, microparticles etc is much higher inside the heliosphere than outside, so if your velocity gets too high before you are out into interstellar space you suffer serious erosion/impact damage. (Note: we ASSUME interstellar space is cleaner. TBD!)
2. Dumping enough energy onto a sail to get to a decent C fraction within a short distance means GW/square meter, and that will pose some interesting materials tech challenges.

Nonetheless, with semi-proven tech as it currently stands I think the best option we have on the table right now is
1. Electrostatic sail (ie. the positively charged wires spread out from a probe. Maybe coat them with a beta emitter radioisotope to keep them charged up without needing an actual power source?)
2. Start off nice and close to the sun
3. Be able to predict when and where a huge solar flare is going to happen, and surf that wave of high speed particles out as far as you can.

50. Accelerate like crazy. And don’t be afraid to waste a lot of power. Light sailing wastes a lot of power.

51. If you click through to their paper they address exactly that option (the high velocity particles) and say that the issue is that you can’t get the aiming right once you get to interstellar distances. For one thing the beam of particles will diverge, from the thermal energy of the particles if nothing else.

52. 300 plus years? What would the point be? By the time the probe gets to Alfa Centuri A new technology will have passed it centuries before it arrives.

53. “A relatively early stage SWIMMER mission might have the goal of transporting … to α Cen A”
No, a relatively early mission would be within the solar system. Only when a propulsion system has been proven for those relatively short distances will it be tried for interstellar missions.

“4.1×109”
Sure I can tell that is meant to be 4.1×10^9, but can’t large numbers be written that way or 4.1E9.

54. If your star ship was small enough you could use a particle accelerator to fling it.

55. How about a stream of high velocity particles to power a star ship.

56. I love the idea of interstellar spacecraft with paddle wheel drives.
Add in the fact that they’ll need huge flat heat radiators. And that the minimal mass construction will be a minimum of truss compression members held together with tension member membranes and cords, and you end up with spacecraft that look like 1850s attempts at aeroplanes.

If we power them with the General Fusion style steam hammer reactors, then we may as well just go with it, and make the rest of it out of leather and polished brass. With a strict tweed and frock coat dress code for the entire project.

HOWEVER, this seems to be rubbish.
If you want to push stuff around using electric fields, you don’t move the multi-km-in-size electrodes. This isn’t the 1850s regardless of the dress code. You have multiple electrodes and you switch between the electrically. You know, like we actually do for real systems intended to accelerate subatomic particles. Like a linear accelerator.

So your interstellar star craft has a 3-D mesh of fine (superconducting naturally) wires. And by switching the currents around you have a moving electric field that drives in ripples from the front to the back, accelerating +ve particles (the massive ones) to the rear and so pushing yourself forwards.

So your final craft doesn’t look like an 1850s steam powered paddle wheel driven aircraft. It looks more like the giant dandelion seed that featured in the Carl Sagan version of Cosmos.

We can still have the tweed frock coats though.

57. I’m pretty convinced that the ‘out of gap’ work will exactly cancel out the ‘in gap’ work done. Zubrin seems to have taken the idea that there’d be no external field near a ‘dipole with infinite plates’ and jumped to the idea that the small outside fields of a wide but finite dipole can be ignored – when in fact they have to be integrated over distances much larger than the distance between plates, and additionally will have edge effects.

But sure, a good simulation that doesn’t make false assumptions would be great – I certainly wouldn’t expect Zubrin to give up on the idea without such as proof.

58. The key point about the Zubrin concept is that, because the positive ions are so much more massive than the negative ions, they end up spending much more time between the grids. The same force on electrons and protons barely budges the protons, but rapidly expels electrons from the inter-grid space.

The result is that the plasma between the grids isn’t neutral, it is highly positive. And so there’s a net force generated.

I have some concern about the real world behavior of the sail; It’s possible that it would produce an external space charge that would neutralize the grid potential. But it would certainly at least start out generating thrust.

The concept requires a much more detailed simulation of the way the surrounding plasma interacts with the sail; I think it’s too complicated and contingent to resolve the question by thought experiments and cocktail napkins.

I don’t see how the rotating version has any hope of producing thrust outside the case where you’re practically at rest with respect to the surrounding plasma. It just can’t rotate fast enough.