Extracting Power from Nuclear in Space

[PB666]

50 minutes ago, wumpus said:

So it looks like the choices for dealing with post-burn heat are:

* Open-cycle cooling: send propellent through the system, possibly into a smaller bell to act efficiently at 6-10% propellant mass, possibly switching to a non-propellant with either higher thermal mass/mass ratio or would suck heat better during a phase change (or two if using a solid).  Ideally this can be ISRU ice, but still it involves mass.
* Closed cycle cooling: This is highly complex and has most of the issues PB666 just described.  Expect *lots* of dry mass needed to do this.
* Dumping the fuel: obviously you need all the fuel mass each burn, but expect it not to be much more massive than the cooling mass needed in open-cycle designs.  Just don't count on any ISRU uranium (unlike the ice example).

The major advantage here for dumping fuel is simplicity (it probably requires less total mass than closed cycle cooling as well).  If you have a meltdown (and I'd expect it to happen when leaving Earth or possibly leaving for Earth with failing ISRU material) you aren't going  to be doing a capture burn and can expect to be lost in space in some sort of transfer orbit oscillating between Earth orbit and some other orbit.  Trying to move the fuel into a graveyard orbit might be avoided simply to avoid failure: for some reason people will object if you try to avoid a .001% chance of Earth being hit by radioactive slag and fail vs. simply assume that "it is all right" and ignore it.

Forget about NTR and go with another type of system.

[DerekL1963]

13 hours ago, DDE said:

A quick glance through the apex of reliability, Wikipedia, suggests the events too place over the timeframe of tens of minutes. From what I heard full nuclear plant shutdown/start-up requires up to a day.

Yes, the events that lead up to the final disaster took place over tens of minutes.  The actual accident occurred within a few tens of seconds of the control rods starting to move.  The power surge that lead to the explosion only spanned a few seconds between the rods being jammed in place and the first explosion.   In the same way, the events that lead up to Challenger's loss (exposure of the o-rings to subzero temperatures) started almost twelve hours before launch.  The actual damage took around a minute after ignition to start fully manifesting (venting gasses from the SRB impinging on the ET).  Six seconds later the ET begins to fail and leak... Six seconds after that, structural failure (the RH SRB tearing loose) begin...  and then the actual breakup spans four seconds.

But nobody would claim the Challenger accident took twelve hours, would they?  Misunderstanding the difference between the "events leading up to the accident" and "the actual accident" had lead you to a mistaken impression over the speed of the reactor's response to control inputs.

And yes, a full start-up takes a significant period of time...  But the majority of that time is spent running down a checklist and performing tests to verify the reactor is ready to start and all systems are operational.  During the actual startup, they shim the rods out of the core very slowly to ensure the measured conditions match the calculated conditions - and they heat the reactor very slowly both to avoid thermal shock and to avoid sudden changes in reactivity and response.  (Basically staying well the heck away from any condition that might lead to prompt critical.  Prompt critical means a Very Bad Day is in the offing.)  Again, these bear no relation to normal operations and have lead you to a mistaken impression of actual throttle response time.

Not to mention, unless reactors responded quickly, we'd never be able to scram them...

[shynung]

7 hours ago, PB666 said:

Forget about NTR and go with another type of system.

NTR, both fission and fusion types, are more energy efficient than nuclear electric propulsion systems, because their energy transfer efficiency is much better. Long story short, NTR converts heat->velocity, while NEP converts heat->electricity->velocity; that extra conversion step brings with it a huge energy-efficiency hit. The only thing holding NTR mass-efficiency (ISP) low is that their cores often can't get too hot, putting a limit on chamber temperature, so to speak. This is why NTR is often envisioned using LH2 propellant, because their low atomic mass means for the same input thermal energy, they carry more momentum, thus higher ISP.

Of course, if mass efficiency (ISP) is all that matters, and energy conversion loss doesn't get into the equation, then by all means, go with NEP systems. The acceleration afforded would be pathetic, sure, and all those radiators would eat into the payload mass, but if propellant is scarce, it's the best. Also, the ship now has a power reactor by default, which can be used to power a laser or do other things; a NTR-propelled ship would need a separate reactor, if their NTR isn't a bimodal-type.

TL;DR - In NTR, nearly all of the energy the reactor outputs goes into accelerating propellant, while NEP throws away much of the reactor power through the radiators. On the other hand, NEP often gets better ISP than NTR because electrostatic/electromagnetic does a better job of chucking propellant as fast as possible than a de Laval nozzle, so that's that.

[DDE]

2 hours ago, shynung said:

TL;DR - In NTR, nearly all of the energy the reactor outputs goes into accelerating propellant, while NEP throws away much of the reactor power through the radiators. On the other hand, NEP often gets better ISP than NTR because electrostatic/electromagnetic does a better job of chucking propellant as fast as possible than a de Laval nozzle, so that's that.

On top of that: SCNTR chamber temperatures are lower than many chemical motors. It has to win at ISP primarily by not having oxidizer... although the massive increase in propellant density found in my LANTR designs has been an interestig effect. Hell, I can get more ISP from a craft of the same size - which, given how huge LH2 tanks can get, is not insignificant.

[shynung]

16 minutes ago, DDE said:

On top of that: SCNTR chamber temperatures are lower than many chemical motors. It has to win at ISP primarily by not having oxidizer...

Actually, SCNTR gets a better ISP by ejecting lower-molecular-mass exhaust than their chemical counterparts. For the same chamber conditions (temperature, pressure), a gas of low molecular mass will achieve higher velocity post- expansion through a de Laval nozzle compared to that of a high molecular mass.

So, in theory, you can boost the SCNTR ISP even further by using atomic hydrogen (single-H) as opposed to molecular hydrogen (H2), provided that you can convince the hydrogen atoms to not recombine before they leave the nozzle.

[DDE]

51 minutes ago, shynung said:

Actually, SCNTR gets a better ISP by ejecting lower-molecular-mass exhaust than their chemical counterparts.

...and, as a rule, oxidizers are heavier than NTR propellants. Which is what I meant.

52 minutes ago, shynung said:

So, in theory, you can boost the SCNTR ISP even further by using atomic hydrogen (single-H) as opposed to molecular hydrogen (H2), provided that you can convince the hydrogen atoms to not recombine before they leave the nozzle.

In pure theory. IIRC as a chemical monopropellant mono-H has ISP in the 2500 sec region.

[shynung]

49 minutes ago, DDE said:

...and, as a rule, oxidizers are heavier than NTR propellants. Which is what I meant.

NTRs pretty much gobble any propellant without too much trouble. Favorite propellants of Children of a Dead Earth warships, besides hydrogen, are methane, decane, water, ammonia, and hydrogen deuteride (H-D). The last one have better ISP than regular hydrogen (H2), for reasons which escape me.

[DDE]

4 minutes ago, shynung said:

NTRs pretty much gobble any propellant without too much trouble. Favorite propellants of Children of a Dead Earth warships, besides hydrogen, are methane, decane, water, ammonia, and hydrogen deuteride (H-D). The last one have better ISP than regular hydrogen (H2), for reasons which escape me.

You’re preaching to the converted.

Even though there are nutters who experiment with mercury propellants in an attempt to get obscene mass ratios.

[wumpus]

10 hours ago, shynung said:

TL;DR - In NTR, nearly all of the energy the reactor outputs goes into accelerating propellant, while NEP throws away much of the reactor power through the radiators. On the other hand, NEP often gets better ISP than NTR because electrostatic/electromagnetic does a better job of chucking propellant as fast as possible than a de Laval nozzle, so that's that.

Power is typically only a concern as an engineering problem in expelling waste heat.  The energy from the nucleus might as well be unlimited (solar power scoffs, but don't expect to travel past Mars with solar).

The missing parameter is time/thrust.  If you have a crew, you can't take too long in the Van Allan belts nor extend your journey far past what a standard Hohmann transfer would take.  This removes nearly all ion systems from consideration and leaves maybe VASIMR for electric propulsion.  NTR has no such issues (the thrust is less than chemical rockets, but it will get the job done).  You are left with the choice of either NTR or producing a sufficiently massive amount of power (nuclear, solar, or plain old chemical*) to use electric thrust to get beyond the Van Allan belts, and then off to your destination (which will still need tremendous amounts of power, but without the hard limits of the Van Allan belts).  Remember, VASIMR trades efficiency for thrust so this burn will likely be hardest on the power source.

If you have an interstellar probe, your best bet is to use as many planetary slingshots as possible until you have the highest speed possible in the right direction (obviously slingshots are limited after hitting escape velocity, they become flyby only).  Once you've hit escape velocity, expect an TNG powered ion burn lasting decades.  Power (and thus thrust) only serves to add dry mass, so should be minimized and traded for time.  Sending a message back home becomes a tricky proposition: your TNG will have to maintain enough power (Americanium?) to at least wake up the probe and then presumably firing up a new TNG (presumably getting the mass up to criticality and producing *some* power, simple radiation is unlikely to be enough) so you will have enough power to send data back to Earth.

* I'm assuming a fuel cell is out of the question.  Typically these engines are lousy at energy efficiency, but energy is nowhere near as difficult to supply in space as mass.  But if not,  certainly a fuel cell would be on the table.

[PB666]

4 hours ago, DDE said:

You’re preaching to the converted.

Even though there are nutters who experiment with mercury propellants in an attempt to get obscene mass ratios.

Mercury is much easier to store, but has a lower ISP. Since most NTRs require certain sustained flow characteristics one does not want to employ something like chromium or iron which might gum up the works.

For chemical rockets reductants can be anything. For example Fe⁰ (Cast iron) is a reductant, so is sodium metal. Can you imagine the piping system required to turbopump cast iron. lol.

Basic energy. https://labs.chem.ucsb.edu/zakarian/armen/11---bonddissociationenergy.pdf and find the bond energy for

Take the energy and divide by the weight.

H-H is 436 kJ per mole. O=O is 498 kJ per mole. O-H is 460 kJ/mole. 872 + 498 (it takes 1370 kJ/rxn-mole to take them apart)   -----> (1840 Kj/rxn-mole recieved to put them back together)  = 470kJ per mole. .004 + .032 kg/mole. This means 470kJ/0.036 kg  13MJ per kg. A joule is a Newton of force accelerated over a length. Suppose a rocket nozzle is 3 meters in length then on can accelerated at 4.3 Ma. T = 0.00118. Ve =5079 ISP(sec) maximum of 517 sec. However this is not what you get from the reaction. Approximately 5-10% of the energy is bled off to run turbo pumps, generators, in H2/02 rockets energy is bled to evaporate the cryoliquids. Then there is heat within the nozzle that is lost and finally there is differential heat in the rocket gas (since the flow from the nozzle is not perfectly laminar).

C-H is  4 @ 431 kJ per mole. 2 @ O=O 498.7 -----> C=0 2 @ 749 + O-H 4@460. 3338 - 2724.1 = 613.9 kJ/rxn-mole.  (.016 + 0.064 = 0.080 rxn-mole per kilogram) = 7673 Mj/kg. Once again 3 meter nozzle. a = 2.557Ma  T = 0.00153. Ve =  3916 ISP is  399. This means that no methane powered rocket can ever produce more ISP than 399 sec. Take note of the fact that CH4 produce more energy per reaction mole than H2, but it adds an additional 0.012 kg/mole of reductant weight and an additional .032 kg/mole of oxidant. . . .its the light weight of the H-H bond that makes all the difference.

Benzene or coal are very poor choices for fuel because they both have resonance energy stabilization due to 4n+2 pi orbital arrangement. Also because the C-H C-C ratios are lower. kerosene is some where between Methane and Benzene in terms of performance. Note that gasoline has alot of benzene in it, for this reason it gets lower mileage than diesel fuel. You don't want to use gasoline as rocket fuel, but you could use paint thinner or hexane, pentane, pentanes, butanes, propanes, ethane, and methane. . . .from lower to higher ISPs

Hydrogen MAY someday be the choice chemical reductant for long range space craft, but there are many issues that need to be solved.

1st. The weight of fuel tanks need to come down.
2nd. While reducing the weight of tanks, the leakage rate also needs to come down.
3rd. Durable gas refrigeration needs to evolve and the weight needs also to come down.
4th. The weight of secondary containment needs to come down.

This may also involve light movable solar shields that protect the cryogenic fuels in flight from latent-heat evaporation.

 

[PB666]

2 hours ago, wumpus said:

Power is typically only a concern as an engineering problem in expelling waste heat.  The energy from the nucleus might as well be unlimited (solar power scoffs, but don't expect to travel past Mars with solar).

The missing parameter is time/thrust.  If you have a crew, you can't take too long in the Van Allan belts nor extend your journey far past what a standard Hohmann transfer would take.  This removes nearly all ion systems from consideration and leaves maybe VASIMR for electric propulsion.  NTR has no such issues (the thrust is less than chemical rockets, but it will get the job done).  You are left with the choice of either NTR or producing a sufficiently massive amount of power (nuclear, solar, or plain old chemical*) to use electric thrust to get beyond the Van Allan belts, and then off to your destination (which will still need tremendous amounts of power, but without the hard limits of the Van Allan belts).  Remember, VASIMR trades efficiency for thrust so this burn will likely be hardest on the power source.

If you have an interstellar probe, your best bet is to use as many planetary slingshots as possible until you have the highest speed possible in the right direction (obviously slingshots are limited after hitting escape velocity, they become flyby only).  Once you've hit escape velocity, expect an TNG powered ion burn lasting decades.  Power (and thus thrust) only serves to add dry mass, so should be minimized and traded for time.  Sending a message back home becomes a tricky proposition: your TNG will have to maintain enough power (Americanium?) to at least wake up the probe and then presumably firing up a new TNG (presumably getting the mass up to criticality and producing *some* power, simple radiation is unlikely to be enough) so you will have enough power to send data back to Earth.

* I'm assuming a fuel cell is out of the question.  Typically these engines are lousy at energy efficiency, but energy is nowhere near as difficult to supply in space as mass.  But if not,  certainly a fuel cell would be on the table.

Don't take what media hype says as truth. The HiPEP system is rated at 24kw weight 10kg and can produce up to 8900 ISP. VASIMR can produce up to 5102 ISP, it weighs tons, and can use 200 kw.

If you simply take 8 24kw (.38 m2 x 8 = 3.04 m2 =  Circle of radius 0.983 meters) you have better performance than VASIMR at 192 kw. at at least 90% of the efficiency of VASIMR but at 1/100th of the weight. With the weight saved by using HiPEP over VASIMR you can add the weight of theoretical 1000+ m2 of solar panels at 300 w/ meter = >300000 KW. . . . .easily making up any efficiencies you have lost. VASIMR looks like a space ship engine, but its little more than space junk.

All electric propulsion are hard and expensive. Even if the Cannae drive for instance is 100 times higher than a photon drive, its power requirement is 3E6 KW per N. You don't use fuel but you have to carry 100 times the weight of fuel you would use in solar panels.

I am not knocking ION drives or other handwaving electric propulsion, they have their place. . .(Station keeping and tugging things about) . .but they are not fairy dust or 39 or 89 days to Mars under any known circumstance.

[DDE]

2 hours ago, PB666 said:

For example Fe⁰ (Cast iron) is a reductant, so is sodium metal. Can you imagine the piping system required to turbopump cast iron. lol.

What about pumping nanoscale metal dust, specifically reclaimed Lunar regolith aluminium, in liquid oxygen? Heterogenous monopropellant, what could possibly go wrong!?

[PB666]

14 hours ago, shynung said:

NTR, both fission and fusion types, are more energy efficient than nuclear electric propulsion systems, because their energy transfer efficiency is much better. Long story short, NTR converts heat->velocity, while NEP converts heat->electricity->velocity; that extra conversion step brings with it a huge energy-efficiency hit. The only thing holding NTR mass-efficiency (ISP) low is that their cores often can't get too hot, putting a limit on chamber temperature, so to speak. This is why NTR is often envisioned using LH2 propellant, because their low atomic mass means for the same input thermal energy, they carry more momentum, thus higher ISP.

Of course, if mass efficiency (ISP) is all that matters, and energy conversion loss doesn't get into the equation, then by all means, go with NEP systems. The acceleration afforded would be pathetic, sure, and all those radiators would eat into the payload mass, but if propellant is scarce, it's the best. Also, the ship now has a power reactor by default, which can be used to power a laser or do other things; a NTR-propelled ship would need a separate reactor, if their NTR isn't a bimodal-type.

TL;DR - In NTR, nearly all of the energy the reactor outputs goes into accelerating propellant, while NEP throws away much of the reactor power through the radiators. On the other hand, NEP often gets better ISP than NTR because electrostatic/electromagnetic does a better job of chucking propellant as fast as possible than a de Laval nozzle, so that's that.

This argument is a clear case of "a vanablackened cat calling a kettle black". I am not against NTR, I am not an admirer of the current NTR because frankly they suck. Here is why they suck. Except for the fact that no current fissile NTR design is in use and Solar electric propulsion is one of the most popular forms of propulsion. Proof of the pudding is in the eating, and no-one want to eat NTR, it leaves a bad taste in the mouth. 

NERVA 825 ISP = 8093 m/s. If we assume the nozzle is say 6.7 meters. a = 7,700,000  j = 30,732,000 J/kg

177 MeV is the amount of fission energy in a of ²³⁵U. This is 0.0000000000283585 J/²³⁵U. One ²³⁵U weighs 3.902E-25 kgs. Thats 72,000,000,000,000 J/kg. Lets assume that an NTR rocket has at ION drive efficiency converts 1 kg of Uranium-235 into dead fission products. How much fuel would you need to carry to make the reaction efficient you would need to carry 2364 tons of hydrogen. The big STS orange tank carried 106 t of H2.

The peewee project had 36.8 kg of uranium. 9200 m/s Ve. Lets be generous and say 40,000,000 J/Kg. The burn length was 80 sec at 12.5 kg/sec = 10t of fuel.

Available power = 36.8 kg (assuming pure U²³⁵) * 72TJ/kg = 2.6PJ of Energy available. Of this 400 GJ was used for an efficiency of .015%. If we compare this with solar electric power, the SA power utilization is 0.2 to 0.3, the power efficiency of the drive is .7 to .8 the total Power inefficiency is 14%. In terms of power utilization SEP is almost 1000 times more efficient than NTR. Even if we granted the Uranium235 mass ratio was unenriched at .0072:1 the energy efficiency of Peewee would have been 2.08%. So don't give this BS about ION drives being power inefficient. The NTR rockets are the least power efficient system right now, and it does not really give a spectacular product for the numerous risks and shortcomings. If you could come up with a closed loop fission system (such as a fast breed reactor) at 10% power efficient  with a coupled ION drive system would produce a better and safer result than the NTR. At least the Soviets repeatedly placed such reactors in space, NTRs have no space track record at all. . . . .and I don't even like fission electric.

The advantage of fission electric over NTR.
No need to carry liqH2. Xenon and argon pack quite nicely. Magnesium maybe a future fuel. You could provide a fuel that both blocks radiation and can be used in the ION drive.
Fission electric can be shut down if ²³⁵U is used as a fuel because its much easier with little weight to reach to approach prompt critical.
Its much easier to shield because you are not spewing products into the space your ship is flying through.
Solar power can both provide power for ship system and manuevering and low power operations through ION drives when reactor is shut down.
The ISPs are much, much better.


 

 

14 minutes ago, DDE said:

What about pumping nanoscale metal dust, specifically reclaimed Lunar regolith aluminium, in liquid oxygen? Heterogenous monopropellant, what could possibly go wrong!?

Remember that the inside skin of the nozzle both slows the gas and cools it down, eventually you will have accumulation.

[Nuke]

On 11/25/2017 at 9:32 AM, DDE said:

I think the tokomaks would (also) end up trying to recapture those pesky 80% of the emissions from the red-hot neutron radiation shielding.

with the bulk of your typical tokomak i dont really think its suitable for space applications. added to that you also have the same problem you have with fission, that you need a way to convert heat to electricity. now a direct converter on something like a polywell (which last i read was about 3 years away), where a good chunk of the energy output is converted to hvdc much more efficiency than with current space capable thermal->electric conversion technologies. this would greatly reduce waste heat and reduce radiator mass, and it does help that the polywell will be only 3 or 4 meters in diameter, and you can probibly dump the heavy vacuum chamber too if you never plan to take it into the atmosphere.

in reference to op: i wouldnt be so quick to write off fusion as vapor ware. this mostly comes from the fact that we keep dumping time and money into the trainwreck that is iter. you dont even need breakeven to make a fairly decent fusion rocket that would blow all the other options out of the water.

Edited November 30, 2017 by Nuke

[PB666]

26 minutes ago, Nuke said:

with the bulk of your typical tokomak i dont really think its suitable for space applications. added to that you also have the same problem you have with fission, that you need a way to convert heat to electricity. now a direct converter on something like a polywell (which last i read was about 3 years away), where a good chunk of the energy output is converted to hvdc much more efficiency than with current space capable thermal->electric conversion technologies. this would greatly reduce waste heat and reduce radiator mass, and it does help that the polywell will be only 3 or 4 meters in diameter, and you can probibly dump the heavy vacuum chamber too if you never plan to take it into the atmosphere.

in reference to op: i wouldnt be so quick to write off fusion as vapor ware. this mostly comes from the fact that we keep dumping time and money into the trainwreck that is iter. you dont even need breakeven to make a fairly decent fusion rocket that would blow all the other options out of the water.

https://www.nasa.gov/directorates/spacetech/niac/2012_phaseII_fellows_slough.html

https://www.nasa.gov/pdf/716077main_Slough_2011_PhI_Fusion_Rocket.pdf

 

[shynung]

@PB666 Again, if you go with nuclear electric, a good portion of the reactor's output goes out the radiators. In a NTR, almost 100% of that output goes into the propellant. NTRs have dismal ISP because they can't get hot enough - at least, the solid core rockets are. Open-cycle gas core rockets, along with nuclear salt water rockets, get pretty good ISP because they let their cores get hot enough to turn into plasma.

Also, perhaps you might want to take a look at fission fragment rockets. These are a class of NTRs that use their own reaction product as propellant. One Robert Werka figures that a first generation design can get 1.7%c exhaust velocity (~520 000 sec ISP). A refinement of this design involves plugging a LH2 injection system to boost thrust at the cost of specific impulse, creating a so-called afterburner system. ISP drops to 32 000 sec, but thrust increases massively (~4.5 kN, compared to vanilla FFRE's 43 N).

Edited November 30, 2017 by shynung

[DerekL1963]

2 hours ago, shynung said:

Open-cycle gas core rockets, along with nuclear salt water rockets, get pretty good ISP because they let their cores get hot enough to turn into plasma.

Presuming NSWR works as theorized, but I'm not aware of any actual formal analysis of their performance.  (Zubrin's paper is not formal analysis, it's the scientific equivalent of a bar napkin.)
 

2 hours ago, shynung said:

Also, perhaps you might want to take a look at fission fragment rockets. These are a class of NTRs that use their own reaction product as propellant. One Robert Werka figures that a first generation design can get 1.7%c exhaust velocity (~520 000 sec ISP).

Which sounds impressive if your goal is high ISP and high exhaust velocity.  But in terms of actual performance, the things that interest people designing actual spacecraft and missions... it's not all that useful.  It's thrust makes an ion engine look like a cluster of S-IC stages.

[shynung]

1 hour ago, DerekL1963 said:

Presuming NSWR works as theorized, but I'm not aware of any actual formal analysis of their performance.  (Zubrin's paper is not formal analysis, it's the scientific equivalent of a bar napkin.)

Well, yeah. NSWR is still sketchy as it is. Mostly because there's no one ballsy enough to test it out. Project Orion nuclear pulse rocket at least had small-scale test models using conventional explosives.

1 hour ago, DerekL1963 said:

Which sounds impressive if your goal is high ISP and high exhaust velocity.  But in terms of actual performance, the things that interest people designing actual spacecraft and missions... it's not all that useful.  It's thrust makes an ion engine look like a cluster of S-IC stages

Well, OP seems to be concerned primarily on ISP, and not much else. FFRE is at the higher end of ISP performance, so that's what I pointed out.

Of course, OP has found out about Slough's magneto-inertial fusion rocket independently. So that's that.

[wumpus]

9 hours ago, DerekL1963 said:

Presuming NSWR works as theorized, but I'm not aware of any actual formal analysis of their performance.  (Zubrin's paper is not formal analysis, it's the scientific equivalent of a bar napkin.)
Which sounds impressive if your goal is high ISP and high exhaust velocity.  But in terms of actual performance, the things that interest people designing actual spacecraft and missions... it's not all that useful.  It's thrust makes an ion engine look like a cluster of S-IC stages.

It all depends on what the design is for.  Five figure Isp numbers won't buy you anything significant within the solar system, but perhaps you want to get a probe elsewhere and report within the lifespan of whoever built it (unlikely).  The above also doesn't look like anything that will burn for a century, not even considering the issue of plutonium's half-life (you need real patience with epsilon-level thrust) [six digit Isp might be assumed to going further afield than conventional systems.  How do you shield it for going well past .1c?].

[PB666]

7 hours ago, shynung said:

Well, yeah. NSWR is still sketchy as it is. Mostly because there's no one ballsy enough to test it out. Project Orion nuclear pulse rocket at least had small-scale test models using conventional explosives.

Well, OP seems to be concerned primarily on ISP, and not much else. FFRE is at the higher end of ISP performance, so that's what I pointed out.

Of course, OP has found out about Slough's magneto-inertial fusion rocket independently. So that's that.

Yes, talking about ISPs 3000+.

 

11 hours ago, shynung said:

@PB666 Again, if you go with nuclear electric, a good portion of the reactor's output goes out the radiators. In a NTR, almost 100% of that output goes into the propellant. NTRs have dismal ISP because they can't get hot enough - at least, the solid core rockets are. Open-cycle gas core rockets, along with nuclear salt water rockets, get pretty good ISP because they let their cores get hot enough to turn into plasma.

 

But you are confusing two things in the Peewee 36 kg of uranium is brought on board, <0.72 kg of that is actually undergoes a contaminated decay, 35 kg either does not contribute at all to the heat or goes into producing latent heat or simply wasted. The reason ISP is low is because their is relatively poor control of the reaction in NTR. NTR is kind of like someone taking an 12 layer wedding cake, putting it in a giant blender and then handing it off as just as good as a 12 layer wedding cake. All the while everyone walks away from the table . . . .i mean this is what happened to NTR. In a fission reactor, especially fast breeder reactor, the fuel is efficiently depleted of fissile material. The issue is of course how to extract heat in space. Full scale fission reactors are two heavy and need extensive heat dissipation. Smaller scale reactors can use other fuels, like plutonium, which have a much lower critical mass.

If NTR could produce impressive ISPs and increase the moment of force associated with optimal fission reaction (not perfect but say 10 fold better), then you might be able to justify its use in space. 

But as it stands right now an ION drive with 2000 ISP accelerates slowly and you waste half the energy going to orbit if you don't kick, but its still better than NTR because it can muddle through LEO and once it is in interplanetary space its a champion. So the powers-that-be don't see the need for NTRs anymore. If the ISP of an NTR went to 9000, and if the rxn was efficient but did produce a small tail (say 1/4 of a kg) of radioactivity while escaping earths orbit . . . it could be justified (Chernobyl released 3.5t of fuel, but some of this fell immediately into the surrounding area).

This is where the political problem comes from. NTRs have the appearance of some Bubbas making a rocket in the fashion that Chrysler used to put big-block V8 (~6.5 liter) lead spewing engines in passenger coupes and then wondered why the company went out of business when oil hit 33 dollars a barrel and there are four door sedans running around getting 30 miles to the gallon. 

[Mayer]

I wouldn't rule out fusion power, the stellerator project in Germany is making progress. Its prospects could be immense.

"making a rocket in the fashion that Chrysler used to put big-block V8 (~6.5 liter) lead spewing engines in passenger coupes" These big motors had also advantages, i mean nowadays you get the extra power via turbocharger which wear out fast. Old cars were fairly reliable.

 

Edited December 1, 2017 by Mayer

[PB666]

24 minutes ago, Mayer said:

I wouldn't rule out fusion power, the stellerator project in Germany is making progress. Its prospects could be immense.

"making a rocket in the fashion that Chrysler used to put big-block V8 (~6.5 liter) lead spewing engines in passenger coupes" These big motors had also advantages, i mean nowadays you get the extra power via turbocharger which wear out fast. Old cars were fairly reliable.

 

As is, the weight is too high, assuming these are gigawatt ranges they might be useful in tugging stuff around, but getting them off the ground has problems at many levels.
Keeping the argument short and sweet, in the vacuum of space the scalar size of a heat source is a problem, getting those sizes down is the solution. Fusion has not demonstrated that it can be scaled to the small size.

 

[James Kerman]

2 hours ago, PB666 said:

Fusion has not demonstrated that it can be scaled to the small size.

I have been watching Lockheed Martin's progress since they announced work on a compact fusion reactor - they originally said they were building a 20t device that could fit on the back of a truck and power a city.

They are currently at the 4th generation (T4).
 

Quote

It was originally believed that the compact reactor would fit on a large truck. It looked like it might weigh 20 tons. After more engineering and scientific research, the new design requires about 2000 ton reactor that is 7 meters in diameter and 18 meters long.
https://www.nextbigfuture.com/2017/05/lockheed-compact-fusion-reactor-design-about-100-times-larger-than-first-plans.html

 

[PB666]

10 hours ago, James Kerman said:

I have been watching Lockheed Martin's progress since they announced work on a compact fusion reactor - they originally said they were building a 20t device that could fit on the back of a truck and power a city.

They are currently at the 4th generation (T4).
 

 

I put 2 kT into orbit, lol.

[wumpus]

On 12/1/2017 at 8:48 PM, James Kerman said:

I have been watching Lockheed Martin's progress since they announced work on a compact fusion reactor - they originally said they were building a 20t device that could fit on the back of a truck and power a city.

They are currently at the 4th generation (T4).

So instead of being "20 years from now" fusion will be require a steadily increasing size until it exceeds Earth?