Comments on: MHD Aerobraking and Thermal Protection Part III: Aerobraking and Aerocapture http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/ Random Musings from the Warped Minds of Jonathan Goff, Ken Murphy, John Hare, and Kirk Sorensen Sat, 05 May 2012 20:53:30 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: Jonathan Goff http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7969 Jonathan Goff Sat, 20 Mar 2010 05:12:27 +0000 http://selenianboondocks.com/?p=1431#comment-7969 Randy, That sounds like it might be plausible, though the devil is probably in the details. The problem is that at the point where the MHD effects start weakening, you're still going ~3.5km/s. If it's a ground-based laser, I don't know how long you could provide that ionization before the rocket has passed too far beyond you. I guess it also depends on the deceleration rate and such. It's not totally implausible, and depending on the energy requirements might not even be that unreasonable. But I'm not sure if it actually would end up making financial sense even if it could be made to work technically. ~Jon Randy,
That sounds like it might be plausible, though the devil is probably in the details. The problem is that at the point where the MHD effects start weakening, you’re still going ~3.5km/s. If it’s a ground-based laser, I don’t know how long you could provide that ionization before the rocket has passed too far beyond you. I guess it also depends on the deceleration rate and such. It’s not totally implausible, and depending on the energy requirements might not even be that unreasonable. But I’m not sure if it actually would end up making financial sense even if it could be made to work technically.

~Jon

]]> By: Randy Campbell http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7967 Randy Campbell Fri, 19 Mar 2010 19:16:29 +0000 http://selenianboondocks.com/?p=1431#comment-7967 Jon: Quik question: I recall a bit of talk on both an aerobraking thread somewhere and a beamed propulsion thread that just came together in my head and I'm wondering if you can "check" me to see if I'm talking out of my keester here :o) The crossover point was where someone pointed out that the method used to "clear" the air for ground based telescopes with a laser deals with ionizing an air tunnel for the scope. (Which in the suggested case could be used to reduce laser propulsion losses for orbital manuevering) Then it was mentioned in the aerobraking thread that using a set of lasers in a similar manner could allow a way to 'even' out the atmosphere in the path of an aerobraking vehicle. Crossover thought: In addition to seeding for better ionization of the plasma could a set of laser pulses increase the ionization in the path of an aerobraking vehicle? RAndy Jon:

Quik question: I recall a bit of talk on both an aerobraking thread somewhere and a beamed propulsion thread that just came together in my head and I’m wondering if you can “check” me to see if I’m talking out of my keester here :o )

The crossover point was where someone pointed out that the method used to “clear” the air for ground based telescopes with a laser deals with ionizing an air tunnel for the scope. (Which in the suggested case could be used to reduce laser propulsion losses for orbital manuevering) Then it was mentioned in the aerobraking thread that using a set of lasers in a similar manner could allow a way to ‘even’ out the atmosphere in the path of an aerobraking vehicle.

Crossover thought: In addition to seeding for better ionization of the plasma could a set of laser pulses increase the ionization in the path of an aerobraking vehicle?

RAndy

]]> By: Chris (Robotbeat) http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7620 Chris (Robotbeat) Wed, 24 Feb 2010 06:24:48 +0000 http://selenianboondocks.com/?p=1431#comment-7620 I'm working on a model in Scilab (a good MATLAB clone) for a magnetic radiation shield. I'm in the quite early stages (and am going to use the magnetostatic approximation for now, which should work for orbital velocities...), but it should allow one to model drag occurring from deflecting ions at altitudes of about 100km (where the mean-free path is bigger than the size of the magnet). If anyone wants the code, I can email it to them... just private message me on NASASpaceflight.com (or, if you're a selenianboondocks blogger, I think you can directly email me). BTW, if you want to get a feel for the magnetic fields produced by different electromagnet configurations, check out this site (really one of the best basic physics sites on the internet) : http://www.falstad.com/mathphysics.html And for 3d magnetostatics: http://www.falstad.com/vector3dm/ I’m working on a model in Scilab (a good MATLAB clone) for a magnetic radiation shield. I’m in the quite early stages (and am going to use the magnetostatic approximation for now, which should work for orbital velocities…), but it should allow one to model drag occurring from deflecting ions at altitudes of about 100km (where the mean-free path is bigger than the size of the magnet). If anyone wants the code, I can email it to them… just private message me on NASASpaceflight.com (or, if you’re a selenianboondocks blogger, I think you can directly email me).

BTW, if you want to get a feel for the magnetic fields produced by different electromagnet configurations, check out this site (really one of the best basic physics sites on the internet) :
http://www.falstad.com/mathphysics.html
And for 3d magnetostatics:
http://www.falstad.com/vector3dm/

]]> By: Nels Anderson http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7616 Nels Anderson Wed, 24 Feb 2010 05:42:33 +0000 http://selenianboondocks.com/?p=1431#comment-7616 Jon, do Otsu et al. or the others give indications as to how large the conductivities? That's the biggest single thing missing from the toy model above; some parts it's implicitly assumed that the conductivity is small enough that the B-field penetrates the plasma to a depth of L or so. Another question we can ask is, how much energy does it take to pump up the B-field in the first place. I don't know how flux pumps work (@Ruediger Klaehn?), but it's easy to estimate the total energy that must be put into the magnetic field. The energy density of the field is B^2/mu0. If it's scale size is L, then the total energy, U, is on the order of B^2*L^3/mu0. For B = 1 T and L = 3 m, U ~ 2e7 J ~ 6 kW h, before losses. That doesn't sound like a lot to me, but I don't know how much power a Centaur would likely need for other purposes anyway. Since B ~ mu0*I/L, it follows that U ~ mu0*I^2/L, which pushes us toward larger L if power is a constraint. Jon, do Otsu et al. or the others give indications as to how large the conductivities? That’s the biggest single thing missing from the toy model above; some parts it’s implicitly assumed that the conductivity is small enough that the B-field penetrates the plasma to a depth of L or so.

Another question we can ask is, how much energy does it take to pump up the B-field in the first place. I don’t know how flux pumps work (@Ruediger Klaehn?), but it’s easy to estimate the total energy that must be put into the magnetic field. The energy density of the field is B^2/mu0. If it’s scale size is L, then the total energy, U, is on the order of B^2*L^3/mu0. For B = 1 T and L = 3 m, U ~ 2e7 J ~ 6 kW h, before losses.

That doesn’t sound like a lot to me, but I don’t know how much power a Centaur would likely need for other purposes anyway. Since B ~ mu0*I/L, it follows that U ~ mu0*I^2/L, which pushes us toward larger L if power is a constraint.

]]> By: Nels Anderson http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7608 Nels Anderson Tue, 23 Feb 2010 08:09:13 +0000 http://selenianboondocks.com/?p=1431#comment-7608 I find it easier to think in terms of magnets interacting with each other, because I've played with magnets. In that spirit, what's going on here is that the currents in the magnet tend to induce in the plasma a mirror image of themselves, and that mirror image repels the magnet. The greater the plasma's conductivity, the higher the fidelity of the mirror image. If you've ever seen a high-temperature superconductor floating above a magnet, you've seen the case in which the conductivity is effectively infinite. Let's build a toy model to play with: imagine a circular loop of radius a in which a current I flows. In SI units the B-field along the loop's axis of symmetry at a distance x from its center is 0.5*mu0*I/a * (1 + (x/a)^2)^-1.5 . This drops off as x^-3 for x >> a. Just among friends, let's pretend the field is constant at strength B = mu0*I/L over a distance L and essentially zero beyond, L = 2*a being the size of the magnet. One thing this suggests is that in the quest to reduce heating rates by pushing the shock front out, it may be tough to push it much further away than L . If so, then this obviously pushes one to increase L in the trade between I and L. Plugging B into the equation for Qmhd, we get sigma*mu0^2*I^2 / (rho*V*L) . Taking into account that the effective area of the magnetic field goes as L^2, the MHD force scales as I^2*L . This starts to make large I look more attractive than large L. To generate a field of strength B, we need a current of B*L/mu0 . For B = 1 T and L = 3 m (a Centaur's diameter), the current is about 2 MA. The magnetic moment is just the area of the loop times the current, roughly L^2 * I = 2e7 A m^2. If the strength of the earth's own B-field is, say, 0.3 gauss = 3e-5 T, then the torque is (2e7 A m^2)*(3e-5 T) = 600 N m. That's appreciable and something to be dealt with, but it's not overwhelming. Maybe the drag, whether electrodynamic or aerodynamic, could be made asymmetric so as to generate an equal but opposite torque. I find it easier to think in terms of magnets interacting with each other, because I’ve played with magnets. In that spirit, what’s going on here is that the currents in the magnet tend to induce in the plasma a mirror image of themselves, and that mirror image repels the magnet. The greater the plasma’s conductivity, the higher the fidelity of the mirror image. If you’ve ever seen a high-temperature superconductor floating above a magnet, you’ve seen the case in which the conductivity is effectively infinite.

Let’s build a toy model to play with: imagine a circular loop of radius a in which a current I flows. In SI units the B-field along the loop’s axis of symmetry at a distance x from its center is 0.5*mu0*I/a * (1 + (x/a)^2)^-1.5 . This drops off as x^-3 for x >> a. Just among friends, let’s pretend the field is constant at strength B = mu0*I/L over a distance L and essentially zero beyond, L = 2*a being the size of the magnet.

One thing this suggests is that in the quest to reduce heating rates by pushing the shock front out, it may be tough to push it much further away than L . If so, then this obviously pushes one to increase L in the trade between I and L.

Plugging B into the equation for Qmhd, we get sigma*mu0^2*I^2 / (rho*V*L) . Taking into account that the effective area of the magnetic field goes as L^2, the MHD force scales as I^2*L . This starts to make large I look more attractive than large L.

To generate a field of strength B, we need a current of B*L/mu0 . For B = 1 T and L = 3 m (a Centaur’s diameter), the current is about 2 MA. The magnetic moment is just the area of the loop times the current, roughly L^2 * I = 2e7 A m^2. If the strength of the earth’s own B-field is, say, 0.3 gauss = 3e-5 T, then the torque is (2e7 A m^2)*(3e-5 T) = 600 N m. That’s appreciable and something to be dealt with, but it’s not overwhelming. Maybe the drag, whether electrodynamic or aerodynamic, could be made asymmetric so as to generate an equal but opposite torque.

]]> By: Doug Jones http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7600 Doug Jones Tue, 23 Feb 2010 02:01:03 +0000 http://selenianboondocks.com/?p=1431#comment-7600 The only good way to counter the torque on such a large high field magnet might be... ANOTHER large high field magnet. Doubles the mass of the system, but makes the controls far simpler. I think Chris (robotbeat) has the right thread- rather than a small, high field magnet, you may want a big honkin' coil, perhaps even a deployable hoop with a few spokes. Maybe some of the Magsail papers would be useful. The only good way to counter the torque on such a large high field magnet might be… ANOTHER large high field magnet. Doubles the mass of the system, but makes the controls far simpler.

I think Chris (robotbeat) has the right thread- rather than a small, high field magnet, you may want a big honkin’ coil, perhaps even a deployable hoop with a few spokes. Maybe some of the Magsail papers would be useful.

]]> By: Jonathan Goff http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7599 Jonathan Goff Mon, 22 Feb 2010 22:42:37 +0000 http://selenianboondocks.com/?p=1431#comment-7599 Tom, Re: induced heating in the magnet. No, so long as the magnet stays superconducting any induced currents or magnetic forces shouldn't generate any heat. ~Jon Tom,
Re: induced heating in the magnet. No, so long as the magnet stays superconducting any induced currents or magnetic forces shouldn’t generate any heat.

~Jon

]]> By: Tom D http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7598 Tom D Mon, 22 Feb 2010 22:40:58 +0000 http://selenianboondocks.com/?p=1431#comment-7598 Jon, From what you say it sounds like the energy dissipation associated with MHD-assisted aerobraking is taking place in a wider, broader shockwave around the vehicle. Is there any induced heating in the magnet itself? That could be a potential design challenge. Jon,
From what you say it sounds like the energy dissipation associated with MHD-assisted aerobraking is taking place in a wider, broader shockwave around the vehicle. Is there any induced heating in the magnet itself? That could be a potential design challenge.

]]> By: Jonathan Goff http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7589 Jonathan Goff Mon, 22 Feb 2010 14:16:48 +0000 http://selenianboondocks.com/?p=1431#comment-7589 Chris, The B^2 in the Qmhd equation is the field strength at the stagnation point. Yeah, dipoles drop off pretty quickly. As I said, modeling all of this is outside my area of expertise, so I'd love to get help analyzing this from people in the field. I wish there were more Americans working on this--it'd make it a lot easier to ask them questions if ITAR wasn't a concern. The magnetic torque issue is one I was wondering about though. That could be quite significant, huh? I wonder what impact it would have if you placed the magnet inside something that allowed you to rotate the vehicle with respect to it. It would be pretty easy to get big angles of attack, but I wonder what it would do to the trajectories of the particles and the resulting current loops. ~Jon Chris,
The B^2 in the Qmhd equation is the field strength at the stagnation point. Yeah, dipoles drop off pretty quickly. As I said, modeling all of this is outside my area of expertise, so I’d love to get help analyzing this from people in the field. I wish there were more Americans working on this–it’d make it a lot easier to ask them questions if ITAR wasn’t a concern.

The magnetic torque issue is one I was wondering about though. That could be quite significant, huh? I wonder what impact it would have if you placed the magnet inside something that allowed you to rotate the vehicle with respect to it. It would be pretty easy to get big angles of attack, but I wonder what it would do to the trajectories of the particles and the resulting current loops.

~Jon

]]> By: Chris (Robotbeat) http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/comment-page-1/#comment-7582 Chris (Robotbeat) Mon, 22 Feb 2010 04:50:16 +0000 http://selenianboondocks.com/?p=1431#comment-7582 Don't forget that a big magnet will be torqued by the earth's magnetic field. Also, the magnetic field strength in Teslas is proportional to the inverse _cubed_ distance to the magnetic dipole (a quite rough approximation, I know), which means that moving the shock layer far enough away from the magnet will make the magnetic field at that point far less. I think using magnetic field strength as a figure of merit is kind of weird, since it really depends quite strongly on how far you are away from the magnet. This is a rather interesting concept, though. I like the idea of using a lunar lander for cislunar travel, although a dedicated cislunar vehicle is probably better for integrating a magnetic aerobrake. Of course, no ferromagnetic implants for any astronauts! You have to be careful of any ferromagnetic bolts, etc, too. Don’t forget that a big magnet will be torqued by the earth’s magnetic field.

Also, the magnetic field strength in Teslas is proportional to the inverse _cubed_ distance to the magnetic dipole (a quite rough approximation, I know), which means that moving the shock layer far enough away from the magnet will make the magnetic field at that point far less. I think using magnetic field strength as a figure of merit is kind of weird, since it really depends quite strongly on how far you are away from the magnet.

This is a rather interesting concept, though. I like the idea of using a lunar lander for cislunar travel, although a dedicated cislunar vehicle is probably better for integrating a magnetic aerobrake.

Of course, no ferromagnetic implants for any astronauts! You have to be careful of any ferromagnetic bolts, etc, too.

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