Talk-Polywell.org

The three main engines on the space shuttle generate 23 times the power of the Hoover Dam. That's nearly 50 GW. Of course SSMEs are thermal, but that's the kind of power it takes to reach orbit, not to mention the solid boosters.

Can anyone estimate the volume and mass of a 50 GW BFR? Is it even possible, assuming the 100 MW - 1000 MW Polywell works of course.

I guess a lessor BFR could power a spacecraft with wings that flies to orbit but that wouldn't work on other planets or moons.

You're also forgetting how much of that energy is being wasted just lifting fuel/oxidizer.

Aero wrote: Can anyone estimate the volume and mass of a 50 GW BFR? Is it even possible, assuming the 100 MW - 1000 MW Polywell works of course.

Do go on. Amuse me. How is this thing going to take off if you have no fuel exhaust to push out of the back of it?

Do go on. Amuse me. How is this thing going to take off if you have no fuel exhaust to push out of the back of it?

Quite irrelevant to this discussion.

All he's asking is how big the reactor would be. Designing a VTO machine around it is another problem entirely, but quite manageable once his big unknown--size and weight of the reactor--are solved.

Since a fusion rocket would use less propellant that's easier to store, you have some margin there. You'd still end up with a big honking tank like the shuttle has, but there would be savings.

Come on guys, give me a little credit here. How big and heavy would be a 50 GW BFR?

The BFR is just the prime mover. The balance of the spacecraft include many other issues but breakthrough propulsion is not considered. No inertial drives, no Mach thrusters, probably not Vasimar. Just an engine and reaction mass that obeys the rocket equations. Whatever form it takes, one kg of BFR will gain energy to the extent of the potential and kinetic energy added. I know that Dr. Bussard had his own ideas about this but the balance of the spacecraft is not pertinent to the question, OK?

Of course I do agree that at this point in time, if I accept that the Polywell works I might as well accept that the Heim drive works, too. That simplifies the problem a whole bunch. But if you insist, I'll throw you a bone in return for your answer.

Wasn't the last consensus that you run into some byproduct issue (heat?) with reactors over a certain size? And that the optimal size was something in the hundreds of MW? As anal as chrismb's comment is, it probably is getting too far ahead of oneself to assume a 50GW polywell is a given. A cluster of smaller ones might also make more sense for redundancy.

Also consider MSimon and 93143's BOEs that you might as well go all out and build flying carrier vessels. In which case it seems to me that you'd want multiple engine modules. darn that's sexy.. Something the size of a carrier thrusting away with multiple engines, each rated in the dozens of GWs.

What I can say, using TallDave's observations regarding Polywell's exponential power versus cost, is that a 50GW Polywell reactor should cost a mere $4.9million.

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Assuming the 100MW BFR optimally designed and is ~2m radius, and the gain scales with the previously published r^5, 2*(50/.1)^(1/5) = ~7m radius. This of course TOTALLY ignores first wall effects and a bunch of other stuff. But it doesn't seem it can get better than that.

Also consider MSimon and 93143's BOEs that you might as well go all out and build flying carrier vessels. In which case it seems to me that you'd want multiple engine modules. darn that's sexy.. Something the size of a carrier thrusting away with multiple engines, each rated in the dozens of GWs.

That's kind of my point. One SSME is rated at about 16 GW. Sure, it only runs for a few minutes, but anything that replaces it is going to need to match those few minutes. The gravity well doesn't take time-out. So how big is it? I can guess that it is 15 meters in radius assuming a low Q but how massive is it, and I see that I need a high Q for more like a 13 meter radius. If I don't get the high Q, the BFR is going to melt into a very expensive puddle very quickly. (Of course it would break containment and shut down before it melted very much.)

I'm not being a jerk, really.

For that craft, to operate in any lunar or Martian environment would mean it required far less reaction mass.

Even if you feel like landing on Venus, Mars, or jumping in and out of gas giants... big gravity wells mean you have gas to use as delta-V.

You never need to push as hard with a reactor, because you are never lifting so much non-cargo for every lb of cargo.

KitemanSA wrote:Assuming the 100MW BFR optimally designed and is ~2m radius, and the gain scales with the previously published r^5, 2*(50/.1)^(1/5) = ~7m radius. This of course TOTALLY ignores first wall effects and a bunch of other stuff. But it doesn't seem it can get better than that.

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. ???

Heath_h49008 wrote:I'm not being a jerk, really.

For that craft, to operate in any lunar or Martian environment would mean it required far less reaction mass.

Even if you feel like landing on Venus, Mars, or jumping in and out of gas giants... big gravity wells mean you have gas to use as delta-V.

You never need to push as hard with a reactor, because you are never lifting so much non-cargo for every lb of cargo.

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.

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.

Aero wrote: 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.

Doesn't engine power required depend on the mass?

KitemanSA wrote:

Aero wrote: 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.