Author Topic: Fusion with space related aspects thread  (Read 1112952 times)

Offline Lampyridae

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Re: Fusion with space related aspects thread
« Reply #2480 on: 12/14/2009 02:01 am »
Stars dump heat radiatively. Is there no way to duplicate that process for a deep space vehicle?
WTH ?  :o

A star does this by being made entirely of hot plasma, having enough gravity to hold together in that state, and generally not caring what gets cooked.

I KNOW how stars do it. I was asking if there had been any thinking among the physicists out there of any way to duplicate, at least in principle, what the star does for an end result. It was a think-outside-the-box question.
Yes, the issue comes in energy-in vs energy-out.  The Farnsworth Fusor does a fair impersonation of a star, it just consumed more than it could ever generate.

I'm familiar with inertial electrostatic confinement. It's an attempt to create a fusion reactor. That's not what I was getting at. I was asking about ways to dump waste heat. Any fusion reactor we build is going to have a tremendous waste heat problem. The question is how do we dump it without melting the spacecraft? I was looking for innovative ideas other than huge honking radiators. Maybe we can create a plasma confinement radiator that can be as large as we need it that just switches on whenever the reactor is running. That's just a wild ass thought I thought I'd throw out there. Does everything we build actually have to be solid matter?

Here's a wild idea. How about using the plasma as lasing medium and ejecting waste heat as a laser? I've seen similar ideas based on laser cooling.

Offline 93143

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Re: Fusion with space related aspects thread
« Reply #2481 on: 12/14/2009 02:37 am »
So any inter-planetary polywell fusion vehicle is going to use some amount of reaction mass, probably LH2. There's your cooling mechanism right there.

...no.

Assume you can get the coolant up to 3000 K (maybe a bit optimistic - okay, a lot optimistic, but whatever).

Starting from 20 K, that's 48.3 MJ/kg, if my calculations are correct.  Multiply by five for the total energy imparted to the propellant from a reactor with an 80% direct-conversion efficiency (so the previous figure is 20% of the total output).  Assume 100% jet power efficiency.  Then back-calculate a specific impulse.

2241 seconds.

Do you really want to limit your interplanetary spacecraft to that?

Here's a wild idea. How about using the plasma as lasing medium and ejecting waste heat as a laser? I've seen similar ideas based on laser cooling.

That doesn't sound particularly respectful of the Second Law.  The waste heat isn't in the plasma; it's generated by electrons hitting the magrid and ions slamming into collector plates with imperfectly matched voltages.  It's genuine high-entropy waste heat in solid material.  How do you profitably run a laser on it?

Now, I'm not real familiar with laser cooling, so you may be onto something.  But IIRC it has more to do with supercooled masses like Bose-Einstein condensates than with high-temperature systems like this...

Offline kkattula

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Re: Fusion with space related aspects thread
« Reply #2482 on: 12/14/2009 05:26 am »
So any inter-planetary polywell fusion vehicle is going to use some amount of reaction mass, probably LH2. There's your cooling mechanism right there.

...no.

Assume you can get the coolant up to 3000 K (maybe a bit optimistic - okay, a lot optimistic, but whatever).

Starting from 20 K, that's 48.3 MJ/kg, if my calculations are correct.  Multiply by five for the total energy imparted to the propellant from a reactor with an 80% direct-conversion efficiency (so the previous figure is 20% of the total output).  Assume 100% jet power efficiency.  Then back-calculate a specific impulse.

2241 seconds.

Do you really want to limit your interplanetary spacecraft to that?


Yes I do actually. That would do very nicely. Maybe even a bit high. Can we reduce it to 2000?

That would allow single stage from Earth to Mars and back, without refueling.

With top-up refueling in LEO, one way could take less than a month.
« Last Edit: 12/14/2009 05:30 am by kkattula »

Offline 93143

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Re: Fusion with space related aspects thread
« Reply #2483 on: 12/14/2009 07:09 pm »
1) What about manned missions to the asteroid belt?  How about Callisto, or Titan?

2) The reactor is both very heavy (even with just shadow shielding) and VERY powerful.  6 GW jet power at 2000 seconds is 612 kN.  For a 1000 mT spacecraft, that's 0.062 gees, or about what the shuttle's OMS can pull.

Oh, and it would use about 80,000 mT of propellant in a month...

3) Most importantly, that number is dependent on getting the hydrogen coolant up to 3000 K, which is a dubious prospect to say the least.  For lower coolant temperatures, the maximum ARC Isp will drop.

I think using some sort of radiator system in space is unavoidable.  If the temperature is high enough, the mass penalty shouldn't be too great, and it completely decouples the propellant mass flow rate from the power output.  Bussard envisioned drives with Isp values up to 70,000 seconds in this mode, for large expeditions to the outer planets.  For the very outer planets, or for interstellar probes, you'd use diluted fusion product propulsion, and the cooling requirements would actually go down (though not to zero)...

Offline kkattula

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Re: Fusion with space related aspects thread
« Reply #2484 on: 12/15/2009 02:27 am »
I suggest you read this paper: http://www.askmar.com/Fusion_files/QED%20Direct%20Electric%20Fusion%20Propulsion.pdf if you haven't already.  ( [F] is engine thrust to weight ratio. )

Quote
Fusion electric QED engines typically have 2 < [F] < 6 and 1500 < Isp < 5500 sec

Quote
Two approaches can be followed for QED engine configurations. First is an all regeneratively cooled [ARC] system, in which that fraction of the fusion power not delivered to the e-beam and deposited into the propellant gas is taken up by the in-flowing propellant stream before entering the thrust chamber. Second is a system that utilizes a separate waste heat radiator ... for controlled-space-radiation [CSR] cooling to handle some specified fraction of the regenerative cooling requirement.

Offline 93143

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Re: Fusion with space related aspects thread
« Reply #2485 on: 12/15/2009 10:16 pm »
I've probably read that paper at some point.  (It's a fairly old one; 1993, I believe.)  However, I had not remembered that Isp formula (Isp=1500/sqrt(fcM), where fc is the waste heat power fraction), which is considerably more optimistic than my own calculations.

If his formula is accurate, I suspect it is because of dissociation, which becomes significant for hydrogen above 1500 K and can soak up a lot of heat.  I was using the Cp curve for undissociated H2.

Still, that's quite a discrepancy - with undissociated coolant I get a constant of about 800, rather than 1500, even ignoring the energy loss resulting from dissociation during the REB heating stage.

Using his formula, we get a maximum Isp of 3341 seconds for an 80% efficient reactor (keep in mind this assumes an 1800°C heat dump).  Not bad at all, but for anything past the moon (and maybe even for a lunar shuttle) it's still better to be able to go higher.  Remember, these reactors are putting out GW, not MW; even at 3341 seconds an engine with 6 GW of jet power would use hydrogen at the rate of 11.2 kg/s and develop over 82,000 lbs of thrust...

Regarding your quotes, I don't see how either of them is relevant to the argument, and the description of ARC and CSR modes is one I've already given.  The T/W ratios are undoubtedly calculated without taking radiation shielding into account; a T/W ratio of 6 at 1500 seconds implies a 6 GW reactor weighing just 14 mT, even with 100% jet power efficiency (VASIMR gets 60-65%).

In fact, the calculations for reactor system mass are over-optimistic, because he assumed an IXL reactor, which he already knew wouldn't work.  A magnetic-electrostatic EXL reactor (ie: a Polywell) is significantly heavier.

Offline Cinder

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Re: Fusion with space related aspects thread
« Reply #2486 on: 12/17/2009 08:19 pm »
Kind of a stupid question, but how exactly do you estimate power by the radius^7 rule of thumb?  I don't get anything like some magnitude of 100 (100 MW as a 3m diameter polywell is roughly supposed to put out) when I use standard radius 1.5m.  I do get ~112 if I raise to the seventh power a spherical volume of 1.5m radius.
I got r^7 relation from the Google video.  Is that out-dated?
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Offline 93143

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Re: Fusion with space related aspects thread
« Reply #2487 on: 12/18/2009 12:37 am »
It's a scaling factor, not an absolute value.

All it means is that keeping fuel and drive voltage constant, and with magnetic field strength scaling linearly with radius due to engineering considerations (the assumption that results in that scaling; the physical scaling is (B^4)(R^3), where B is the magnetic field), a Polywell will produce 128 times as much power if you scale it up by a factor of 2 in linear dimensions.

This scaling is not really in dispute.  The trouble with extrapolating from existing units is (a) we don't have much data due to the Navy data embargo, and what we do have is low-quality, and (b) we don't know how the losses scale.

WB-6 had about a 0.15 m radius and put out something on the order of a milliwatt of fusion power.  Of course, it wasn't operating at the optimum voltage, and it was in one-off pulse mode with a very primitive fuel feed mechanism...  Losses are predicted to scale as r^2, but there are a lot of things that could go wrong multiplying the size of the test units by a factor of 10 (and their magnetic fields by a factor of 100; superconductors are helpful for this)...

Incidentally, multiply 1 mW by (100^4)(10^3) and you get 100 MW...

Offline Cinder

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Re: Fusion with space related aspects thread
« Reply #2488 on: 12/18/2009 07:03 am »
Is there some rough set of equations that ties all the parameters and variables together?  They don't have to be accurate, I'm just curious to see the full picture of all things involved. 
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Offline JohnFornaro

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Re: Fusion with space related aspects thread
« Reply #2489 on: 12/18/2009 02:05 pm »
"Heat your spaceship to a few million degrees..."

Is it hot in here, or is it just me?  After Chernobyl, one of my favorite jokes was the recipe for Chicken Keiv:

1.  Preheat city to 400 degreees....
Sometimes I just flat out don't get it.

Offline M Simon

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Re: Fusion with space related aspects thread
« Reply #2490 on: 12/19/2009 07:15 am »
Is there some rough set of equations that ties all the parameters and variables together?  They don't have to be accurate, I'm just curious to see the full picture of all things involved. 

Re: Polywell: Magnetic field to the 4th power. Radius to the third power. You have to assume a power at some magnetic field and reactor size. The data is sparse. As to rockets. It gets more complicated.


Offline space_man

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Re: Fusion with space related aspects thread
« Reply #2491 on: 12/21/2009 03:25 pm »
Can anybody please point me to an article related to fusion propulsion by means of plasma shockwave compression?

I have ran through some numbers and it seems a ramjet vehicle traveling at Mach 10 through the lower layer of the Jovian Hydrogen atmosphere would develop the sufficient pressure/temperature across the plasma shockwave to generate steady fusion and expel Helium. This is not like a Bussard ramjet since it travels through much thicker Hydrogen and is solely based on compressed sound waves in the plasma.

Thanks.

Offline mlorrey

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Re: Fusion with space related aspects thread
« Reply #2492 on: 12/21/2009 07:22 pm »
Can anybody please point me to an article related to fusion propulsion by means of plasma shockwave compression?

I have ran through some numbers and it seems a ramjet vehicle traveling at Mach 10 through the lower layer of the Jovian Hydrogen atmosphere would develop the sufficient pressure/temperature across the plasma shockwave to generate steady fusion and expel Helium. This is not like a Bussard ramjet since it travels through much thicker Hydrogen and is solely based on compressed sound waves in the plasma.

Thanks.

Run the temp numbers through for the materials that make up your ramjet. Won't be able to use EM field shielding at those pressures.
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Offline b ramsey

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Re: Fusion with space related aspects thread
« Reply #2493 on: 12/23/2009 01:35 am »
Tom Ligon and M Simon are going to be on The Space Show tonight Dec 22 at 10:00 eastern to talk about Polywell fusion, its archived later if you miss it. I believe the last time Tom Ligon was on the Spaceshow,  he was on with Dr. Bussard.

Offline marsavian

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Re: Fusion with space related aspects thread
« Reply #2494 on: 01/29/2010 03:49 am »
Laser fusion test results raise energy hopes

http://news.bbc.co.uk/1/hi/sci/tech/8485669.stm


"The bottom line is that we can extrapolate those data to the experiments we are planning this year the results show that we will be able to drive the capsule towards ignition," said Dr Glenzer.

Before those experiments can even begin, however, the target chamber must be prepared with shields that can block the copious neutrons that a fusion reaction would produce.

But Dr Glenzer is confident that with everything in place, ignition is on the horizon.

He added, quite simply, "It's going to happen this year."


background ...

http://news.bbc.co.uk/1/hi/sci/tech/7972865.stm
« Last Edit: 01/29/2010 04:00 am by marsavian »

Offline Cinder

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Re: Fusion with space related aspects thread
« Reply #2495 on: 03/17/2010 10:28 pm »
The EMC2 website has been updated.
http://www.emc2fusion.org/
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Offline mlorrey

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Re: Fusion with space related aspects thread
« Reply #2496 on: 03/17/2010 11:23 pm »
The EMC2 website has been updated.
http://www.emc2fusion.org/

100 MW demo fusion reactor, size comparison... oh yea, I'mma fit that inside my borg cube...
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Offline aero

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Re: Fusion with space related aspects thread
« Reply #2497 on: 03/17/2010 11:42 pm »
So that is what, 7.8 meters on a side? Assuming the man is 6 ft. tall.
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Offline mlorrey

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Re: Fusion with space related aspects thread
« Reply #2498 on: 03/17/2010 11:47 pm »
So that is what, 7.8 meters on a side? Assuming the man is 6 ft. tall.

Guess so. Whats nice tho is that once you've reached net power, a little more scaling upward produces a LOT more power output. Note also that its not a big solid mass like you'd see with a fission device, most of the interior is empty space. If simon is around maybe he can let us know how heavy this puppy is supposed to be.
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Offline Cinder

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Re: Fusion with space related aspects thread
« Reply #2499 on: 03/18/2010 12:01 am »
How does this demo package compare with a 100MW fission package?
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