Big Polywell size.

[KitemanSA]

Yes, your math is correct. Do you know where the 100 MW at 2 meter radius came from? If that is a scale-up from WB-6, then WB-6 should have generated 2.4 Watts. ??? or maybe 240 Watts ??? or maybe 1.4 Watts scaled r^7. ???

I believe I first heard this in Dr. B's Google talk. 1.5m for D-D, 2m for pB11, IIRC.

Well, when I run the scaling backwards, 7 meter down to 15 cm, I get more power than WB-6 delivered. Maybe there is justification for that, I just don't know what it is.

WB6 is not comparable/scalable from WB100. For one thing, the B field of WB100 will be MUCH stronger than simple scaling (superconductive vice copper, so MUCH lower losses, etc). Also, this is GAIN scaling which doesn't scale to no-gain systems. The r^5 gain scaling is "all else being equal", and it just isn't tween WB6 and WB100.

[Aero]

Doesn't engine power required depend on the mass?

Well, yes. But there are plenty of ways to add mass. If that's to much power, just add cargo. But if the BFR is to massive then it will replace cargo. I understand that net power scales as r^5, is it fair to say that mass scales as r^2 (Magrid) or r^3 (chamber)? If so, then is there a mass estimate for any radius?
Thinking a little, I'd guess that the Magrid mass scales a bit faster than r^2, as the B field increases as r which I'd bet will make the coils a little thicker as radius increases. But I was hoping for a little more confidence than I get by making my own guesses.

[Heath_h49008]

I appreciate your point. But don't overlook the simple fact that the craft must escape Earth at least one time. Escaping with the use of an air breathing propulsion system seems impractical unless the craft is going to stick around Earth. Of course collecting atmospheric gasses and using them strictly as reaction mass can work.

The idea of a SSTO/shuttle system seems to imply working close to earth, or another planet of origin. Interplanetary craft wouldn't need all the mass required for heat shielding, aerodynamic concerns, or air-breathing engines. I just don't see a shuttle being forced to deal with more than one large body.

I would love to be so overpowered we could take the heat shields away and enter atmo at non-orbital speeds. You don't need a heavy fuselage to protect you if you enter at 1500kts instead of just under 25,000.

I'm wondering if your super-sized powerplant might be able to accomplish that for a better mass value than shielding. What do you think?

As for landing on Venus, I don't think so. The 800 degree C surface temperature makes it good as a crematorium but little else.

I always wondered how we could possibly thin Venus' atmosphere out a bit, or heat up it's core to create an EM field. It would seem we could just add reactants to cause unwanted substances to precipitate out, build scrubbers, etc for the air. But, how would you heat up the solid core of Venus or Mars to give it some resistance to solar products?

(I know, it's a subject for a different thread.)

[Betruger]

Isn't that plasma window tech impervious to heat damage - wouldn't that, plus a cooling scheme with the abundence of power from something like a working polywell, be enough to make a venus lander/return feasible? The power needed for the plasma window would still be huge, though. Something like kilowatts per inch IIRC.

[Aero]

I'm wondering if your super-sized powerplant might be able to accomplish that for a better mass value than shielding. What do you think?

I think it depends on the mass of the powerplant. But I doubt that shielding will mass out as much as the reaction mass needed for the de-orbit burn. Remember, orbital velocity is 7.2 km/sec, so you're looking for at least 4 km/sec. delta V. That will take almost half as much reaction mass as is used to get into orbit in the first place. Of course my guess might be way off, and design compromises could result in significant de-orbit burn allowing significant reduction in heat shielding.

Again, it depends on the mass of the powerplant.

[Aero]

I guess I’ll take a stab at the mass of a BFR.
We did some work last year during which we concluded, IIRC, that the Magrid shadowed about 20 % of the chamber walls when viewed from the center of the Polywell. That was for a 2-meter radius, but I will assume it remains constant at 20 % for any radius. So the area of the surface of a sphere at the distance of the Magrid (7 meters) from the center is 615.75 square meters and the 20 % in shadow is 123.15 square meters. That is for the whole sphere which includes 6 coils, of 7 meter radius. The total length of all 6 coils, 6* 2 pi*r is 263.89 meters. That means the width of the coils is 0.46667 meters. That is near enough the diameter of the cross-section. Coil cross sectional area then is 0.1710 square meters. The coil volume is length times cross-sectional area, or 45.137 cubic meters, the volume of the Magrid.

I don’t know what is inside that coil, but it is wire, coolant or structure. I will estimate coolant the same as water, one metric ton per cubic meter, and structure and wire the same as copper wire at 8.92 metric tons per cubic meter, then if the coil volume is half filled with coolant, the balance with wire and structure, the average density of the Magrid is 5 metric tons per cubic meter and the Magrid mass is 225.685 metric tons for a 50 GW machine. Of course the balance of plant brings it up and I don’t have a good guess for that. I’ll just double my guess and add some to say it masses out at 500 tons. (Mulltiply by 2.222 Using the same approach I guessed at the mass for other radii, here is what I came up with.

Code: Select all

Radius-meters      Mass - Metric tons      Power - Megawatts
     2	                       11.69613919                     100
     3		               39.47446976                     759
     4	                       93.56911351                   3,200
     5		              182.7521748                    9,766
     6	                      315.7957581                   24,300  
     7	                      501.4719677                   52,522

Does anyone know how these masses compare to fission. Of course I don't think you can match the 5, 6 or 7 meter BFRs with any fission machine. They don't make them that big. But if the BFR is going to take us into space, well, that mission requires big power.

[MSimon]

Doesn't engine power required depend on the mass?

Well, yes. But there are plenty of ways to add mass. If that's to much power, just add cargo. But if the BFR is to massive then it will replace cargo. I understand that net power scales as r^5, is it fair to say that mass scales as r^2 (Magrid) or r^3 (chamber)? If so, then is there a mass estimate for any radius?
Thinking a little, I'd guess that the Magrid mass scales a bit faster than r^2, as the B field increases as r which I'd bet will make the coils a little thicker as radius increases. But I was hoping for a little more confidence than I get by making my own guesses.

I was thinking of reducing the mass. First off - no LOX (except for minor amounts) second off LH2 for reaction mass.

I did some BOE a while back an IIRC 50 ton 6 GW engine. 600 tons of reaction mass. 100 tons of payload.

[MSimon]

The best way to get the power up is to increase the B field. Power scales at B^4. Coil mass probably B^1.5 for SCs.

Note: for SC MgB is probably the best. It may cut down your density number from 9 (Cu) to 5 or less.

Critical field strength = B*R = 3, with B in Teslas and R in meters. It is not hard and fast. It is determined by the gyroradius of 6 MeV alphas.

Of course you would like a B*R of 20 or more in the ideal case with 10 doable given current technology.

[Aero]

Waste heat is another issue to look at. I have scaled net power as r^5. Gross fusion power scales as r^7. How does one calculate the waste heat for a 100 MW net power reactor?

Is it legitimate to say that r^5 =100, so r^7 = 100 ^ (7/5) ? I hope not because that gives 630 MW total fusion, hence 530 MW of waste heat.

Is the waste heat of high quality, it general? High enough to drive a thermal system? I hope so, because r^7 - r^5 is going to be huge for any r much greater than 1.

For radius = 2 meters: 2^5 = 32, 2^7=128, so 2^7 - 2^5 = 96, so waste over net = 3.
For radius = 3 meters: 3^5 = 243, 3^7 = 2187, so 3^7 - 3^5 = 1944, so waste over net = 8

That looks to me like waste over net = r^2 - 1 .

[Heath_h49008]

I'm also curious how well we can shield the coils from the heat of the reactor.

Niobium-tin (Nb3Sn) has to be kept a bit on the chilly side... but it can produce 13.5T fields.

So, we crank up the B field, and increase the size a little less?

Can we simply encase the coils in a thin, mirrored, vacuum sleeve? Whatever hits the walls is ours, and if we limit what the coils see by conduction or radiation... You see what I'm thinking.

[MSimon]

I'm also curious how well we can shield the coils from the heat of the reactor.

Niobium-tin (Nb3Sn) has to be kept a bit on the chilly side... but it can produce 13.5T fields.

So, we crank up the B field, and increase the size a little less?

Can we simply encase the coils in a thin, mirrored, vacuum sleeve? Whatever hits the walls is ours, and if we limit what the coils see by conduction or radiation... You see what I'm thinking.

If the B field is high enough (3 T m) no charged particles (very few anyway) hit the casings. Actual results will be required to determine heat loads. It may be possible to do away with the water jacket and just have a LN2 jacket as the outer thermal shield.

[chrismb]

If the B field is high enough (3 T m) no charged particles (very few anyway) hit the casings.

Really!!!

So, er.... why aren't charged particles held back from hitting the casing in a tokamak????

[KitemanSA]

So, er.... why aren't charged particles held back from hitting the casing in a tokamak????

Because the ones with enough energy to perhaps begin to be able to reach the MaGrid are "lost" via the cusps to the chamber wall or direct conversion unit. Mostly. Except those that had been hitting the "nubs" which may have been mover or modified to reduce that path.

[Heath_h49008]

High energy photons from the reaction, and from Bremsstrahlung still have to go somewhere. And they can still heat up what they hit.

I might be wrong, but the B field at even 12T isn't going to be enough to make them invisible.

[KitemanSA]

High energy photons from the reaction, and from Bremsstrahlung still have to go somewhere. And they can still heat up what they hit.

True, but that wasn't Chris's point. His comment was re: "charged particles", not EM.

Yes, the Em will hit the MaGrid. Yes it will heat it up. That is what the TPS is for. At this point, the TPS is expected to be much smaller than was expected when we falsely believed that the alphas radiate straight out and would also impact the MaGrid. Whole different order of magnitude, I think.