Extracting Power from Nuclear in Space

[PB666]

6 minutes ago, Nuke said:

iter is why fusion is always 20 years away. thats just how long it takes to build a massive tokamak. so every time a physicist says ' we need a bigger machine' add 20 years to the time table. its also why it will never yeild a fusion reactor that is commercially viable even if demo makes breakeven. it will also never yeild a spaceworthy reactor because tokamaks are massive machines. 'its good science'  but thats all it will be. 

the real breakthroughs are going to come out of the smaller machines. they lend themselves to rapid iteration, many are compatible with direct conversion schemes, and are all small and light enough for spacecraft use. the same makes them viable for commercial application. those are what you want to pay attention to. my money is on polywell and they are giving 3 years instead of 20. 

 

There is no impetus for them to finish as long as 50 countries are willing to dump billions into it. The way to do it is to set it up like a grant, you reach this hall mark by this date or we fund the other guy.

 

[Nuke]

4 minutes ago, PB666 said:

There is no impetus for them to finish as long as 50 countries are willing to dump billions into it. The way to do it is to set it up like a grant, you reach this hall mark by this date or we fund the other guy.

 

its a cushy job. why give that up to work in a makeshift laboratory in an abandoned strip mall and routinely have to dumpster dive for parts. lm might be the winner on the small reactor front because of the funding they have available to them, but i really wish they would publish stuff. of course if they do it it will likely be classified for 20 years.

Edited December 10, 2017 by Nuke

[PB666]

15 hours ago, Mayer said:

About what levels of power supply are we talking about? A fusion reactor is by all means a power generator.

Fusion power doesn't exist? The sun begs to differ

 

A fusion reactor by itself does not have the power to start its reaction. Fusion power exists but there are costs to harnassing it, the cost of harnassing it is often not considered, For example it may cost 90% of a reactors output to initiate the next reaction.

15 hours ago, Mayer said:

That is fiction.., that is utopian.., that doesn't mean anything. The same buzzwords with which rockets and airplanes were disregarded in the past. You need some solid arguments for the infeasibility of the concept.

I for one trust the British Interplanetary Society(if their name is supposed to be funny, remember what a similar sounding Society for Space Travel gave us) and NASA who had a similar idea with Project Longshot.

First off at the turn of the last century we did not have the base of phycisicist we have now, no did anyone, in fact the whole world dump a considerable amount of GDP into a rocket to go to the moon. All-things-being held equal it like comparing airplane concepts, BUT all things are not equal, the scientific base is much better now than then. I can point out a few other flaws with Daedelus, the fuel for the reaction comes in pellets, you would not store them in spherical tanks, the would be dispensed from a hopper (like a grain silo). Project Longshot evolved, the BIS basically has not. The tank structure is nothing more than targets for granule strikes. 

Once upon a time people believed they could make gold from lead and mercury. That has never happened. Once upon a time people believed that comet dust would kill them. Myth is a wonderful thing as long as your realize that its value is in the story, not in its factuality.

 

15 hours ago, Mayer said:

Every form of nuclear fusion is hard to initiate because you have to overcome the Coulomb barrier. That's why we have thermonuclear bombs and large plasma containers. Small fusion pellets would be a big step forward but the technology is still in its infancy.

Yes, but we are not talking about Nuclear bombs are we, we are talking about controlled reactions that require a continuous power input to be sustained, the drive essentially assumes that this power comes from their drive (handwaving). The power input per cycle could be a high percentage of the power generated. You cannot wave this problem away, no amount of trust in the British Interplanetary society solves the problem. I detest hand-waving arguments, they deserve to be picked to pieces.

15 hours ago, Mayer said:

If nozzles are so wasteful, why are we stll using rocket thrusters? The principle behind utilizing a chemical explosion or a nuclear equivalent is the same.

Because its not a chemical rocket, when you talk about ISP in the -3 to -1 c magnitude(10) you are talking about an extremely erosive and destructive particle velocities. In essence you are creating cosmic rays, something to be carefully disposed of in the opposite direction in which your are moving.

15 hours ago, Mayer said:

The chance of colliding with a larger object is ridiculously low and you can't compare interstellar space with our solar system because the sun attracts a lot of junk.

You keep changing the argument to suit your opinion. We are not talking about a larger object, we are talking about the probability of crossing a grains of sand to grain of silt sized particle.

Let me convert the argument to facts at least an object person would find useful.

Size of a speck of silt or
medium sand 0.0003 meter, area 27E-12 cubic meters at 1000 kg per meter^2 = 2.7E-8 kg. a grain of silt is about 1/1000 the weight or 2.7E-13
Here are the impact energies at various speeds: silt, sand, gravel.

0.001c = 0.012, 12 J, 12000 J
0.01c = 1.2 J, 1200 J, 12000000 J
0.1c = 120 J, 120,000 J, 120,000,000 J
0.8c = 12000J, 12,000,000 J, 12,000,000,000 J

Here is the energy of a 50 calibre mounted. it can basically penetrate 1 cm of solid titanium.  Assuming a muzzle velocity of 2000 feet/sec (860 m/sec) and a grain size of .1174grams  <  43,400 J

So if we assume are vessel walls are on the same order as a titanium version of the ISS, then here is what would kill that craft at the given speeds (maximally, without including the melting for caused by a higher velocity object hitting over a much smaller area. No possible survivals are not included.

0.001c - would survive a silt sized impact, probably survive a sand sized impact, probably would not survive a gravel sized impact.
0.01c   - would probably survive a silt sized impace, might survive a sand size impact.
0.1c     - might survive a silt sized impact
0.8c     - improbably survive a silt sized impact.

Particle densities in space.
Of the gas in the ISM, by number 91% of atoms are hydrogen and 9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium,^[3] known as "metals" in astronomical parlance. By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements. Assuming hydrogen and helium will not solidify we can eliminate these. However we might want to know what the mass is, since along the  journey we will have to move this out of the way, the force exherted is the square of the velocity. of the 1.5% 66% is in the form of dust. I will take the value of 35E6 hydrogen masses per cubic meter. This corresponds 4.98 x 10E-23 kg of gas per meter. Its about what we expect 5E-25 kg of dust. So how much dust will we collide with in our trip to a nearby star. 10²¹x 5x10^-25, about 0.0005 kg of dust or larger particles. That is to say the weight of 18,518 grains of sand, 18,000,000 grains of silt.

In addition images of certain interstellar events have lead scientist to believe there are aromatice carbon compounds in space, these compounds are known to autoattract into ring-on-ring structures. Therefore we also have potent nuclei formers in interstellar space.
 

 

 

 

 

 

 

 

 

Edited December 10, 2017 by PB666

[MatterBeam]

@PB666, @Mayer:

Interstellar dust particles are deal killers when it comes to high transit velocities. No hull can withstand the energy released by a single impact, so withstanding years of impacts is out of the question...

but there are smart ways to deal with the problem.

A plasma shield is a type of shield where a plasma is held in front of the spaceship using magnetic fields. 

When the dust particle strikes ions in the plasma, it converts some of its energy into heat. This heat breaks up the particle, until you have a small, hot gas cloud travelling through the plasma shield. This gas strikes many more atoms, thereby converting most of its kinetic energy into random thermal motion. It reaches a high enough temperature to become a plasma... which becomes affected by the magnetic fields you are using to hold the shield in place. It can then be deflected out of the way or simply absorbed.

Another option is to shoot out massive, thin disks ahead of the spaceship. Dust particles will run into these disks and annihilate themselves well clear of the spaceship. 

[DerekL1963]

20 hours ago, Nuke said:

my money is on polywell and they are giving 3 years instead of 20.

Polywell has been three years away ever since the late 00's.

[Nuke]

1 hour ago, DerekL1963 said:

Polywell has been three years away ever since the late 00's.

its still an improvement.

[PB666]

10 hours ago, Nuke said:

its still an improvement.

There are questions whether polywell is theoretically possible to stabilize, at least no-one seems to know how badly electrons which are supposed to maintain highest density at the center and the wells, are actually thermalized and wasting all the power generated in undesired hv.

[PB666]

This delayed response is in reply to something @MatterBeam stated a few weeks back and I have been working on the problem.

Let me preface this thread with the following, since I previously posted here I have been studying the issue of energy transfer with regard to space. If fusion was a thing today and we had a rocket to get it into space a reactor, and the reactor could operate for at least a few seconds stably. IOW it meets all the criteria for functional fusion energy in space . . .the vast majority of space missions would not choose this. If you are following the Isaac Arthur videos . . .I am sorry . . . . but fusion is the smaller of the problems. Fusion electric  is premised on the idea that you have to get  more out as you put into it. The problem with fusion in space is compounded by the fact that the ability to get power out is worse in space than it is on land. To begine with we may not get enough power out to reinitiate. The second problem is you have to store charge to get it started, requiring mass. The process itself is hampered by the fact that steam generation is the most efficient way to get power out, which on land is benefited by the fact we can tap into lakes (such as in fission power plants, cooling towers) you can bring water from miles away to cool if you had a very powerful plant, like a fusion power plant. In space there is only two ways to get rid of energy, either as kinetic energy of exhaust or as waste radiation. As the raw nuclear power output power increases, the mass (radiator) cost of waste increases faster. So at some-point one is almost always better off using solar since its power is cooled at the source. That limit point on power is probably in the fission reactor range, not fusion.

The issue of heat is very important, not because it just increases weight, but it points at the exact problem is that finding a solution to waste (i.e increasing efficiency) is a more serious problem than solving fusion power. If we had a way to efficiently utilize nuclear thermal heat, we don't need Fusion, fission would do for now. Fusion only appears to be a solution to space energy problems because we don't have it. If we had fusion we would suddenly realize how far we are from interstellar power. Of course, if you are on Europa or some other Jovian moon, fusion might be very attractive, given all the ice you have to melt, but its not in a mode in near or mid future to be plucking gasses off the sun. Its just not. Thus the Fusion powered tug I designed could never exist, it does not have enough radiator mass, probably off by a factor of 100 in constituitive vessel mass. (meaning that thus is more like  0.0001 instead of 0.01.

OK so here is the analysis

Spiralling out of orbit using Nuclear power is very wasteful. In this case we have 82114 Exhaust velocity engine ~330000 kg of payload and 38000 kg of fuel (dv/dt = 0.0105 factoring fuel loss as this ship spiralled out.
If one where to magically burn from 160,00 km to escape it would require a 117% more fuel to spiral out instead. So matter beam you are wrong, such setups would required many days, maybe a month to leave earths sphere of influence by kicking at the periapsis to the best of the ships ability.

The crux of the problem with spiraling is the the specific mechanical energy gained per unit time of burn decreases from 82.4  j/sec down to 22 j/sec with an average of about 61% efficiency, this inefficiency is compounded by the fact that the highest thrust  is when the fuel is the lowest corresponds to the period when SME gain is also the lowest.

This does not include the burn to Mars which from low earth orbit would only add a few hundred dV from high earth orbit would require the addition of almost 2000.

The tug was designed to go to Mars drop off a load and return to earth, spiralling into orbit and burning to mars it burnt all its fuel leaving none to stop at mars, thus one would have to carve out payload to add fuel, just to reach mars.

The secret to electric power spacecraft is getting weight down while simultaneously increasing power

 

[DDE]

@PB666 Fusion-electric has to me always seemed utterly asinine. It’s a case where chasing Isp is counterproductive.

[magnemoe]

On 10.12.2017 at 8:25 PM, PB666 said:

A fusion reactor by itself does not have the power to start its reaction. Fusion power exists but there are costs to harnassing it, the cost of harnassing it is often not considered, For example it may cost 90% of a reactors output to initiate the next reaction.

First off at the turn of the last century we did not have the base of phycisicist we have now, no did anyone, in fact the whole world dump a considerable amount of GDP into a rocket to go to the moon. All-things-being held equal it like comparing airplane concepts, BUT all things are not equal, the scientific base is much better now than then. I can point out a few other flaws with Daedelus, the fuel for the reaction comes in pellets, you would not store them in spherical tanks, the would be dispensed from a hopper (like a grain silo). Project Longshot evolved, the BIS basically has not. The tank structure is nothing more than targets for granule strikes. 

Once upon a time people believed they could make gold from lead and mercury. That has never happened. Once upon a time people believed that comet dust would kill them. Myth is a wonderful thing as long as your realize that its value is in the story, not in its factuality.

 

Yes, but we are not talking about Nuclear bombs are we, we are talking about controlled reactions that require a continuous power input to be sustained, the drive essentially assumes that this power comes from their drive (handwaving). The power input per cycle could be a high percentage of the power generated. You cannot wave this problem away, no amount of trust in the British Interplanetary society solves the problem. I detest hand-waving arguments, they deserve to be picked to pieces.

Because its not a chemical rocket, when you talk about ISP in the -3 to -1 c magnitude(10) you are talking about an extremely erosive and destructive particle velocities. In essence you are creating cosmic rays, something to be carefully disposed of in the opposite direction in which your are moving.

You keep changing the argument to suit your opinion. We are not talking about a larger object, we are talking about the probability of crossing a grains of sand to grain of silt sized particle.

Let me convert the argument to facts at least an object person would find useful.

Size of a speck of silt or
medium sand 0.0003 meter, area 27E-12 cubic meters at 1000 kg per meter^2 = 2.7E-8 kg. a grain of silt is about 1/1000 the weight or 2.7E-13
Here are the impact energies at various speeds: silt, sand, gravel.

0.001c = 0.012, 12 J, 12000 J
0.01c = 1.2 J, 1200 J, 12000000 J
0.1c = 120 J, 120,000 J, 120,000,000 J
0.8c = 12000J, 12,000,000 J, 12,000,000,000 J

Here is the energy of a 50 calibre mounted. it can basically penetrate 1 cm of solid titanium.  Assuming a muzzle velocity of 2000 feet/sec (860 m/sec) and a grain size of .1174grams  <  43,400 J

So if we assume are vessel walls are on the same order as a titanium version of the ISS, then here is what would kill that craft at the given speeds (maximally, without including the melting for caused by a higher velocity object hitting over a much smaller area. No possible survivals are not included.

0.001c - would survive a silt sized impact, probably survive a sand sized impact, probably would not survive a gravel sized impact.
0.01c   - would probably survive a silt sized impace, might survive a sand size impact.
0.1c     - might survive a silt sized impact
0.8c     - improbably survive a silt sized impact.

Particle densities in space.
Of the gas in the ISM, by number 91% of atoms are hydrogen and 9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium,^[3] known as "metals" in astronomical parlance. By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements. Assuming hydrogen and helium will not solidify we can eliminate these. However we might want to know what the mass is, since along the  journey we will have to move this out of the way, the force exherted is the square of the velocity. of the 1.5% 66% is in the form of dust. I will take the value of 35E6 hydrogen masses per cubic meter. This corresponds 4.98 x 10E-23 kg of gas per meter. Its about what we expect 5E-25 kg of dust. So how much dust will we collide with in our trip to a nearby star. 10²¹x 5x10^-25, about 0.0005 kg of dust or larger particles. That is to say the weight of 18,518 grains of sand, 18,000,000 grains of silt.

In addition images of certain interstellar events have lead scientist to believe there are aromatice carbon compounds in space, these compounds are known to autoattract into ring-on-ring structures. Therefore we also have potent nuclei formers in interstellar space.

You would want an APU like an standard fission reactor You will probably need an capacitor bank anyway to trigger next cycle. 
90% power is realistic for an engine, however it does not have to be idiotic amounts of power, no known designes has insane power demands on startup outside of laser compression and even here its simply restrict engine size, for an tokamak its the reactors size. 

0.8c is fantasy, 0.1 is pretty much max speed with an good fusion engine at least if you have to brake. Armor enough to stop an .50 BMG is pretty trivial for an huge ship. 
And you don't use an thick steel plate. you use an thin plate in front, some meter behind you have the armor, the energy released then hitting the thin plate will shatter the object making it easier for the armor to deal with. Gravel and you start getting an problem. That is unless you have an fuel tank in front, send this some kilometer ahead then cruising. 

[shynung]

2 hours ago, magnemoe said:

And you don't use an thick steel plate. you use an thin plate in front, some meter behind you have the armor, the energy released then hitting the thin plate will shatter the object making it easier for the armor to deal with.

Best to fill the space between the thin plate (Whipple shield, I presume) and the main armor with some sort of aerogel. It adds additional mechanical resistance to the rapidly-disintegrating projectile/debris, giving it a chance that it may never reach the main armor at all, stuck inside the aerogel. Also, it's a good idea to stick a layer of fibrous armor (like aramid/Kevlar) just behind the main armor, to catch any spalls.

Also a good idea is to angle the armor, to increase its effective thickness (a projectile impacting armor at an angle has to go through more armor compared to one impacting straight). Also, angling the armor gives the projectile a chance to bounce off instead of digging into the armor.

[magnemoe]

21 minutes ago, shynung said:

Best to fill the space between the thin plate (Whipple shield, I presume) and the main armor with some sort of aerogel. It adds additional mechanical resistance to the rapidly-disintegrating projectile/debris, giving it a chance that it may never reach the main armor at all, stuck inside the aerogel. Also, it's a good idea to stick a layer of fibrous armor (like aramid/Kevlar) just behind the main armor, to catch any spalls.

Also a good idea is to angle the armor, to increase its effective thickness (a projectile impacting armor at an angle has to go through more armor compared to one impacting straight). Also, angling the armor gives the projectile a chance to bounce off instead of digging into the armor.

Not sure if aerogel would help much, at relativistic speeds you will get an significant explosion hitting the front plate, having it thin you should not get too much energy transferred to it, you want most of the explosion to just hitting vacuum, you also want an good spacing so the fragments get time to spread out. 
Protecting the back of armor however is an good idea, 

[shynung]

1 minute ago, magnemoe said:

Not sure if aerogel would help much, at relativistic speeds you will get an significant explosion hitting the front plate, having it thin you should not get too much energy transferred to it, you want most of the explosion to just hitting vacuum, you also want an good spacing so the fragments get time to spread out.

The projectile is already mostly plasma after it goes through the frontmost plate. Slowing down small, high-velocity fragments are what aerogels are good for. Aerogels also have very low density, so putting on several meters thick of it would not give a significant mass penalty.

[magnemoe]

43 minutes ago, shynung said:

The projectile is already mostly plasma after it goes through the frontmost plate. Slowing down small, high-velocity fragments are what aerogels are good for. Aerogels also have very low density, so putting on several meters thick of it would not give a significant mass penalty.

Think energy not fragments, you was just hit by an artillery shell who went of passing the first plate. if anything the aerogel would increase damage to the thin plate as the plasma would turn part of the aerogel into plasma, you want space so the plasma can expand before hitting armor, slanting should be smart to direct that explosion outwards. 

[PB666]

I have to agree somewhat with the space argument for the sheer reason that if there is enough space, say 10s of meters the collision residue would have ample incidence angle x distance to avoid the second collision with the hull. The problem with objects traveling at near light speed is that you have other energies to also consider (for example fusion of nuclei) and the resulting atomized particles would best be to spread out.

However structurally, a thin plate being hit with an artillery shell energy rocket, you are going to have unexpected oscillations in the plate caused by the collision, so that the structure of the plate needs to be reinforced. So it might no be a bad idea to have two thin plates with an aerogel in between.

[DDE]

21 hours ago, magnemoe said:

You would want an APU like an standard fission reactor You will probably need an capacitor bank anyway to trigger next cycle. 
90% power is realistic for an engine, however it does not have to be idiotic amounts of power, no known designes has insane power demands on startup outside of laser compression and even here its simply restrict engine size, for an tokamak its the reactors size. 

I’m wondering if it’s possible to somehow harvest the gigawatt-level power burst from a TRIGA to have both “hotel power” and reactor start-up covered by the same power package.

[wumpus]

As far as hull armor goes, have you stopped to think what all that dry mass does to the rocket equation?  For .1c you could presumably have a dry/wet ratio near unity with an Isp of 30,000.  At this point you probably need to be using Argon (if delivered from space elevator) as a reaction mass or preferably something commonly available in space.  While I'm a big fan of Orion, I doubt that anyone could afford to supply enough H-bombs to get a near-unity wet/dry mass ratio, let alone get the Isp up to 30,000.

[MatterBeam]

@PB666:

Let me reiterate. 
It can be a fusion rocker or a fart rocket, what matters is specific power (kW/kg).

Spiralling trajectories allow for continuous acceleration. This paper suggests that the deltaV to escape a 7800m/s orbit using a 3 milligee acceleration is
7800*(1-0.754(3/1000)^0.25)=6423m/s.
The spaceship accelerates for 2.5 days to reach this velocity, and by then it reaches an altitude of 3103km.

Yes, this is wasteful compared to a chemical rocket - it would only need about 3500m/s to escape Earth. 

But, this is vastly more practical than whatever solution you had set up as a straw man argument. 

For a 100 ton payload and an Isp of 3000s, you need a thrust power of 43MW to achieve an acceleration of 3 milligees. 

[PB666]

1 hour ago, MatterBeam said:

@PB666:

Let me reiterate. 
It can be a fusion rocker or a fart rocket, what matters is specific power (kW/kg).

Spiralling trajectories allow for continuous acceleration. This paper suggests that the deltaV to escape a 7800m/s orbit using a 3 milligee acceleration is
7800*(1-0.754(3/1000)^0.25)=6423m/s.
The spaceship accelerates for 2.5 days to reach this velocity, and by then it reaches an altitude of 3103km.

Yes, this is wasteful compared to a chemical rocket - it would only need about 3500m/s to escape Earth. 

But, this is vastly more practical than whatever solution you had set up as a straw man argument. 

For a 100 ton payload and an Isp of 3000s, you need a thrust power of 43MW to achieve an acceleration of 3 milligees. 

6423 - 3211 = 3212 you think wasting half the fuel is acceptable!!!!!! ROFL. Don't apply for the captain-ship of a space tug.
The 3500 you quote is to escape Earth and intercept Mars, to just escape earth by bruning at at 160 km orbit only requires 3211.888. This is considering the effects of the sun and the hillsphere.

See other thread, a spiraling orbit is actually slower to break orbit than one that takes a rest for 90' per orbit (270 burn span versus 360' burn span).

Two things that you believe to be true are unncessary.
1st, that with an abundance of power (Such as in the fictitous fusion electric system . . .again weight is not established, you may well need to use the highest ISP) that you should not use high ISP.
Actually, if you want to take advantage of low ISP, the only portion of the system leaving where it would be advantageous is on the last passes, and in particular close to the periapsis, the lower the better. This is because if you have established a low ISP from previous kicks, you can now take advantage of an 10,000 m/s, If you cut your ISP by a factor of 3 but your moving 4 times faster, the energy you gain is still 25% higher than if you had spiralled out an using the highest ISP.
Using a lower than optimal ISP before the last pass is simply wasteful and wont save much time. During the last pass it will save both energy and time.

2nd.  For most of the burn out. . . 3/4ths of it at least your can save 900 dV with no loss of time by using highest ISP available. The reason for this is that burns at apogee are very wasteful. by burning at 3/4ths of the time you save a periapsis burn meaning that you increase the amount of energy put into the orbit versus at the wasteful apoapsis. Then paper you quote apparently did not try to optomize for burn span, must not be a very good paper, since in a few hour of work I did.