Re: Z-Pinch Renaissance
Posted: Mon Oct 19, 2020 5:21 pm
What is the destination?
a discussion forum for Polywell fusion
https://talk-polywell.org/bb/
Saturn maybe?
F=MAGiorgio wrote: ↑Sun Oct 18, 2020 4:43 pmIf we assume for easy of calculations that the ship mass at destination still to be 30000 Tons and keep the engine thrust to 300 KN than we have with rough calculations:
Ship Mass: 30.000 Tons
Fuel Mass: 240.000 Tons
Acceleration phase (first half):
Best time to destination: ~300 days
Average time to destination: ~400/450 days (depends on planetary relative position)
Deceleration phase:
Best time to destination: ~170 days
Average time to destination: ~200/220 days (depends on planetary relative position)
Total time:
Best time to destination: ~470 days
Travel time calculated as acceleration till half way and deceleration for other half of the trip.
What kind of trajectory are you assuming? What I was trying to say is that the more DeltaV you can generate the less like a semi-circle type Hohmann transfer orbit trajectory and the more like a flat ellipse. The orbital mechanics get a lot "flatter" the faster you can go Getting closer to a straight line almost; that would cut down on trip distance and time.Skipjack wrote: ↑Mon Oct 19, 2020 8:16 pmOk, why such a huge ship?
30,000 tonnes is quite big, IMHO, especially when using only one engine (engine mass is not really that relevant).
At the current distance to Saturn I am roughly estimating about maybe 300 days total trip time for a 660 take off mass ship (30 tonne dry and 630 tonne fuel).
Joe's (great) online DV calculator gives me a total DV of ~152,000 m/s assuming the 5,000 Isp.
Total burn time would be around 85 hours or so, if I did not miscalculate anywhere. Thing is that you can spend a lot more of that time accelerating than you need to spend decelerating since the ship will be much lighter then and you can decelerate at much higher g forces.
And acceleration will increase over time too. So the burn time might actually be quite a bit shorter... Hmmm
I wished Joe's otherwise excellent DV calculator would include the total burn time. Would also be nice to have an option to include deceleration to 0 in the calculation to see what the total burn time is.
Wrong formula and also you didn't include the fuel mass in the calculations.
I estimated the mass according the ZAP engine estimate you made few posts before:
With only 30 tons of dry mass you can't fit a reactor able to give you 300 KN thrust at 5,000 ISP. Than all the assumptions and calculations I made are not anymore meaningful.
You can quickly calculate Fuel flow rate as = Thrust / (ISP x 9.8 ) , result is fuel flow in kg/s, Thrust is in Newton.
I deliberately didn't use the fundamental equation of rocketry because you said the ship mass was:
Based on the vague info that I have, I estimated the mass of the ZAP engine at ~15 tonnes. It is really just two 10 meter long hollow and thin tungsten tubes. The heaviest part would be the power supply and switches. I don't understand how you would estimate it based on a submarine nuclear reactor. Those are completely different technology (fission and not fusion).
viewtopic.php?f=10&t=3957&p=130405&hili ... +5#p130405The SpaceX BFR (Big Falcon Rocket or Big frick Rocket) has a planned payload of 150,000 kg (330,000 lb) when flying reusable or 250,000 kg (550,000 lb) when flying expendable, making it a super heavy-lift launch vehicle.
I think we will have to agree before if we are doing a "theoretical analysis" based on yet to come technologies or implementing a practical application based on the "state of the art" technology we have available today.
Capacitor bank.
I don't think that this is some sort of hard physics limit. It is just all that they need. AFAIK you can make the reactor longer and/or increase the input current to increase the fusion power and Q, but why do more than what is needed? I think Uri chose the Q of 2 to have just enough fusion power for the system.
I am not sure where you get 60% efficiency from. D+He3 releases almost all of it's energy in charged particles. Those can be directly converted into electricity.Giorgio wrote: ↑Wed Oct 21, 2020 12:34 amAll of this is anyhow subject to an efficiency factor. The biggest loss should be in the direct conversion process that (to the best of my knowledge) can reach a 60% efficiency. But even if you should reach a 80 or 90% conversion, the limiting factor is still that starting Q=2.
5,000 is pretty much top of the line of any propulsion system we have ever had and this ship would have a comparably enormous amount of thrust.Giorgio wrote: ↑Wed Oct 21, 2020 12:34 amThird, an ISP of 5000 is not that great in terms of fuel efficiency.
Unless we can reach an ISP of one or 2 order of magnitude bigger, the fuel will "always" be the predominant factor of the ship, and we can't escape this point due to the very definition of ISP.
The reactor is by it's nature pulsed. It just can do a lot of pulses per second. I am not sure how that makes a separate power generator necessary. The P(in) vs P(out) should be the same either way.Giorgio wrote: ↑Wed Oct 21, 2020 12:34 amFourth, there is still the huge and yet unanswered issue of the thermalization of the Alphas that could prevent continuous operation and force to use a slower pulsed operation moving most of the electrical load needs to a secondary generator as default design.
I think those are already included in the design of the ZAP, but you still need to charge them from a source before the reactor first shot and also in any event when there is not enough energy charge to allow for another shot.
With no physical limits having a greater Q means immediately having an higher thrust and an higher ISP so it is an highly desirable situation.Skipjack wrote: ↑Wed Oct 21, 2020 8:19 amI don't think that this is some sort of hard physics limit. It is just all that they need. AFAIK you can make the reactor longer and/or increase the input current to increase the fusion power and Q, but why do more than what is needed? I think Uri chose the Q of 2 to have just enough fusion power for the system.
Energy needs anyhow to be extracted from the charged particles, be it with a "Venetian Blind" type of converter or an Inverted cyclotron converter (ICC from TriAplha design).Skipjack wrote: ↑Wed Oct 21, 2020 8:19 amI am not sure where you get 60% efficiency from. D+He3 releases almost all of it's energy in charged particles. Those can be directly converted into electricity.Giorgio wrote: ↑Wed Oct 21, 2020 12:34 amAll of this is anyhow subject to an efficiency factor. The biggest loss should be in the direct conversion process that (to the best of my knowledge) can reach a 60% efficiency. But even if you should reach a 80 or 90% conversion, the limiting factor is still that starting Q=2.
I totally agree that it would be inefficient, especially with those low ISP values is not practical. My feeling is that (like many scientists) he simply didn't stop to evaluate the whole picture but just focused on the engine design. This is a trend I see in the 99,9% of the publications I read. They focus on the research object but the integration is always demanded to another phase/paper/research grant that has to evaluate the technical or economical feasibility of the proposed solution.
I didn't express myself in a clear way.Skipjack wrote: ↑Wed Oct 21, 2020 8:19 amThe reactor is by it's nature pulsed. It just can do a lot of pulses per second. I am not sure how that makes a separate power generator necessary. The P(in) vs P(out) should be the same either way.Giorgio wrote: ↑Wed Oct 21, 2020 12:34 amFourth, there is still the huge and yet unanswered issue of the thermalization of the Alphas that could prevent continuous operation and force to use a slower pulsed operation moving most of the electrical load needs to a secondary generator as default design.
Skipjack wrote: ↑Wed Oct 21, 2020 8:19 am5,000 is pretty much top of the line of any propulsion system we have ever had and this ship would have a comparably enormous amount of thrust.Giorgio wrote: ↑Wed Oct 21, 2020 12:34 amThird, an ISP of 5000 is not that great in terms of fuel efficiency.
Unless we can reach an ISP of one or 2 order of magnitude bigger, the fuel will "always" be the predominant factor of the ship, and we can't escape this point due to the very definition of ISP.
Extrapolating from Project Rover, I would expect about 100 tonnes of thrust from an engine like this but at 6 times the Isp. Staying with an assumed 15 tonnes of engine mass, we would get a T/W ratio of 6.6 (about double that of Rover). Not too shabby and I actually think that it could be even better.
I strongly doubt. The 15/18 tons would probably not be enough even only for the capacitor bank and related equipment, let alone for all the other components of the engine. Than again to make a real evaluation we need more data and (possibly) experimental data.
I disagree with that.
Nope, the plasma is charged particles that are moving. Put a magnetic coil around the tube and induce a current.
Knowing Uri, this is unlikely.
I still don't see that as a thing to necessarily happen.Giorgio wrote: ↑Wed Oct 21, 2020 9:41 pmI didn't express myself in a clear way.
Thermalization of the Alpha would drastically drop the Q (even under 1).
This will create more dependence (need more time) from auxiliary electric generator to charge the system for the next shot, hence reducing the number of pulses available for second.
If you look at my numbers, you can see that the 18 tonnes would be additional reserves in case the 15 tonnes I put aside for the engine are not enough.Giorgio wrote: ↑Wed Oct 21, 2020 9:41 pmI strongly doubt. The 15/18 tons would probably not be enough even only for the capacitor bank and related equipment, let alone for all the other components of the engine. Than again to make a real evaluation we need more data and (possibly) experimental data.
We agree that we disagree, we will just need to wait until more info will become available.
Your point on energy extraction is valid only in absence of thermalization effect, but thermalization is a real issue that was recognized as a potential big concern from Clifton Lilly when he made his MS thesis under Uri Shumlack supervision (his thesis is the base of the ZAP engine design).Skipjack wrote: ↑Thu Oct 22, 2020 1:47 amNope, the plasma is charged particles that are moving. Put a magnetic coil around the tube and induce a current.
Venetian blinds are needed when the plasma motion is without a direction, which is not the case in this reactor (things move in a direction).I still don't see that as a thing to necessarily happen.
Not to interrupt you gentlemen's interesting technical discourse but what about a compromise? Say one of those small modular nuclear reactors of the type that Trump just approved funding for?
In the NuScale reactor, a core is kept cool by circulating normal fresh water, as happens in today’s operating nuclear plants on a much, much larger scale. Inside huge nuclear towers, most of the space is dedicated to cooling. The NuScale reactor uses gravity and buoyancy to naturally circulate the cooling water. The size difference is staggering: “About the size of two school buses stacked end to end, you could fit around 100 of them in the containment chamber of a large conventional reactor,” Wired reports. The reactor technology itself isn’t completely different than before, it’s just wildly more efficient and up to date. Each NuScale reactor rates 60 MWe, which sounds small because the reactor is small by design. Plants can install dozens at a time.