Author Topic: NASA boosts nuclear thermal propulsion with BWXT contract  (Read 6939 times)

Offline Jim

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #20 on: 08/10/2017 05:19 PM »

....
IOW in principal going LEU saves a lot of money
...... a cost claim without argument.


That one is intuitively obviously and doesn't need an argument.

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #21 on: 08/10/2017 08:35 PM »
That doesn't make any sense. Please elaborate and/or correct your comment.
There is thermal efficiency (% of heat converted to electricity) and system efficiency (Kw/Kg of mass)
Thermodynamics says the bigger the temperature difference the more efficient the heat engine. With the background temperature of the universe being around 3K that makes a temperature difference of (maybe) 2800K.

But that means a temp difference between the radiator (which in space works purely by radiation) of maybe a few Kelvin. Given that the ISS radiator is around 50W/M^2 at around 303-340K that means the hypothetical  radiator would be enormous and the system efficiency (Kw/Kg) would be very low.

For any NEP system there's a graph of radiator temp Vs overall system weight for a given output.

Improving space radiator efficiency for NEP is a really good idea (actually improving space radiator efficiency in general at various temperatures would be a good idea)

Quote from: Propylox
You're talking about HEU systems, which this is not, and contradicted your desire for efficiency by proposing a low temperature generator and associated systems.
There are actually 3 systems being talked about here. The NASA legacy HEU NTP (that's a rocket) the Russian fast NEP (electric power reactor running an ion thruster) and the NASA BWXT programme which is LEU NTP.

Since you brought the Russian system up you should keep track of what's being talked about.
NTP's have short run times. Their pressure vessel and nozzle typically needs regenerative cooling (which is what drives the turbopump). While running some of that flow can drive a generator. During shut down there would be no flow through the reactor, so how does the power get from the core to the wall?
Quote from: Propylox
Another contradiction, and a cost claim without argument.
Based on your previous posts, I'd request the old JS19 reply  ;)
There are 2 different costs here. Raw material and system costs.

KiloPower uses HEU. It is cheap because the US has a surplus of HEU. It is being tested in areas that already have high security because they deal with nuclear weapons.  It is part of the payload. It is switched off when it's fitted to the rocket.

Once you get to NTP for propulsion you have to do a lot of ground testing at NASA facilities. It's not just a % of enrichment it's a step change in the security and planning costs involved in each stage of the design and mfg process. It's like the cost differences between using Hydrogen Peroxide/Kerosene (coveralls, gloves, goggles, water shower) and  NTO//UDMH (full rubber suite with self contained air supply).
Quote from: Propylox
-- Previous quote --
Re1) Why have you proposed tungsten will be part of the core and not part of the rocket - ie; the throat and upper nozzle? Isotopic requirements on neutron absorption/reflection would be much more applicable to the rocket architecture or core's casing than the core's matrix.
While it's possible, the process is so expensive that you'd only use it if absolutely necessary. The key issue is LEU makes a design more sensitive to neutron absorption by the structure of the core. That's why you'd want to strip the more absorbent isotopes out of the core raw material.
Quote from: Propylox
Re2) Agreed, but a LEU fast reactor still doesn't produce the heat desired for NTP without reflecting (W?), or otherwise encouraging, neutrons back into the matrix to accelerate fission and temperature. This is why I asked about creating higher temps or use of tungsten around the throat - the LEU doesn't cut it otherwise. And there's still the issue of keeping propellant in contact long enough to extract temperature - conflicting with keeping it moving and building velocity. I don't see how that's solvable without a working fluid. Thoughts?
Well that's sort of the point of a design contract. To see if an NTP engine with LEU is possible.

You do realize that the "working fluid" you're talking about is the propellant in an NTP system, right?

There are multiple materials that can be used as neutron reflectors. AFAIK Isotope enriched or depleted W has never been one of them. Why bother with something when higher TRL materials already exist?

Tungsten's specific gravity is about the same as pure Uranium. The T/W of NTP systems is bad to begin with. There are easier materials to work unless you absolutely need the maximum temperature Tungsten can give you. [EDIT TBH If isotopic enrichment techniques are on the table I'd go with separating Molybdenum from Hafnium. Mo with 5%Re is quite ductile and weldable. It's thermal conductivity is good and it's about 1/2 the density of W. But W is what the previous programme focused on so that's where the knowledge base is.   :( ]

From your questions you don't know as much about this subject as you seem to think you do.  :(
« Last Edit: 08/10/2017 11:17 PM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline ZachF

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #22 on: 08/16/2017 07:50 PM »
You could probably make an NTR on LEU if you use heavy hydrogen (deuterium) as the remass. maybe even heavy Methane.

You'd lose a some Isp from the heavier element, but fuel would be denser.

CANDU reactors can run on natural uranium because of the use of heavy water as moderator.

Offline Asteroza

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #23 on: 08/16/2017 11:03 PM »
One might be able to make the argument that a NTER might fit this case better, since a NTER's turboinductor will need lots of tungsten anyways and the turboinductor overcomes some of the fuel matrix heat limitations. Though that's like arguing a LEU NTER is equivalent to a HEU NTR, which may or may not be the case, all things considered.

Offline Robotbeat

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #24 on: 08/16/2017 11:10 PM »
You could probably make an NTR on LEU if you use heavy hydrogen (deuterium) as the remass. maybe even heavy Methane.

You'd lose a some Isp from the heavier element, but fuel would be denser.

CANDU reactors can run on natural uranium because of the use of heavy water as moderator.
You can also make a LEU NTR using Hydrogen/protium. That's what this thread is about.
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Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #25 on: 08/17/2017 11:21 PM »
You could probably make an NTR on LEU if you use heavy hydrogen (deuterium) as the remass. maybe even heavy Methane.

You'd lose a some Isp from the heavier element, but fuel would be denser.

CANDU reactors can run on natural uranium because of the use of heavy water as moderator.
It's true that CAnadian Natural Deuterium reactors can do this but that does not translate to NTR systems for several reasons.

Deuterium is a very small proportion of all hydrogen, so a major extraction process would be needed to generate it, otherwise the proportion of would not be high enough to sustain the reaction (regular H2 has too high a capture cross section). CANDU's neat feature is the way it separates the tasks of coolant and moderator, keeping the expensive Deuterium Oxide in one place. 
Deuterium is very expensive due to the extraction process, so making it in bulk is not going to be cheap, unless you want to pay for a whole new mfg plant to do it.
The kicker.  LEU = thermal spectrum reactor and HEU = Fast spectrum

In fact most of the NTRs built by the US have been HEU thermal spectrum reactors. They have been compact because their U235 density has been 24-33x higher than civilian PWR's.

Weapons grade Uranium is no longer an option for NTR but you still want it to be small, so the plan is to make it small by running at the top end of LEU and stripping out all the moderator. It is now a fast spectrum reactor (and in principal a breeder as well with this spectrum).

However AFAIK no one's actually done this before which is what this design exercise is all about.  :(
[EDIT Yes it should be possible but sometimes things are counter intuitive, which is when things get tricky, or exciting, depending on your PoV ]
« Last Edit: 08/18/2017 08:44 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline ZachF

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #26 on: 08/19/2017 07:45 PM »
haha tricky is exciting  ;)

Fast spectrum with LEU is going to have a pretty large fissile start-up charge. I'd bet this is going to a pretty heavy rocket.

It's too bad we don't have a better infrastructure for the production of U233.

Offline bradjensen3

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #27 on: 08/19/2017 08:12 PM »
Combine nuclear thermal propulsion with water from the Moon as reaction mass, and you could tool around the solar system at a high rate of speed. Get water from the Moon, drop to Earth orbit and pick up passengers, and get to Mars orbit in a month maybe?


seems obvious to me, but there is a whole thread where I get beaten to a pulp suggesting it.

Offline bradjensen3

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #28 on: 08/19/2017 09:01 PM »
I'm confused by this article. It states among other things that the NTP engines have higher thrust than chemical engines.

I'm being told over and over again on another thread that the Thrust to Weight Ratio (TWR) of nuclear thermal engines is too anemic to get a rocket with an NTR mainstage off the ground.

I see from historical figures that the NERVA engine was 6800 pounds, the later version was less than aton.

If the engine isn't super heavy and the thrust is higher,  why is the TWR many times lower?

And if the TWR is enough to get the rocket off the ground and control it, won't the higher ISP win out in the end?

I am understanding isp to basically be like  'fuel efficiency' where thrust is more like horsepower or torque of a mechanical engine. It's how much force you can apply how fast.

Am I thinking of this correctly?

Such that you could have a rocket engine design that doesn't have much 'oomph' but gets really great gas mileage, like an XEON engine.

Why can't a main stage NTR engine be engineered for higher thrust, like one expansion chamber where the heat is applied, and say three or four nozzles or six or eight where the reaction mass expands?  Couldn't you design the expansion chamber to handle higher heat if you didn't have to worry so much about cooling the nozzles?

Or the suggestion made by someone else of an engine that ingests air from the atmosphere and accelerates it along with the reaction mass, with an effect sort of like a turbofan in a high bypass jet engine?

The way I would be thinking about this is how would I design an NTR to launch mass into space, if it were the only tool I had to do the job?




Offline gongora

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #29 on: 08/19/2017 09:57 PM »
I'm confused by this article. It states among other things that the NTP engines have higher thrust than chemical engines.

They don't explicitly say which chemical engines they are using for the comparison.  The nuclear engine they are studying will be for in-space propulsion.  They won't be comparing it to booster engines, they'll be comparing it to other engines they might use on a spacecraft.  Something like the surplus OMS engines that will be used on Orion only has a few percent of the thrust of a Merlin 1D, which is a fairly small engine for a booster.

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #30 on: 08/20/2017 12:18 AM »
haha tricky is exciting  ;)

Fast spectrum with LEU is going to have a pretty large fissile start-up charge. I'd bet this is going to a pretty heavy rocket.

It's too bad we don't have a better infrastructure for the production of U233.
True.
I guess it's a case of swings and roundabouts.  :(

What you lose on having LEU you gain by having no moderator mass inside the reactor and (roughly) the volume you lose from eliminating the Graphite moderator you can pack with the 50% U238. Keep in mind this is not inert. It will breed to Pu239, going from "fissile" to "fissionable" which is good if you want to run the reactor for more than one burn.

Obviously the devils in the details. If the core does get bigger how big will you accept? Could you skip Uranium Oxide (which lowers density and absorbs neutrons) and coat straight U pellets with W, given Wis about as dense as U (at these densities Lead is actually a lightweight material  :). Not something you hear often). 
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline hkultala

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #31 on: 08/20/2017 05:34 AM »
what do "HEU" and "LEU" mean here?

Online Hauerg

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #32 on: 08/20/2017 05:47 AM »
Humanity will never do anything truly substantial in space until it uses NTR of some type.
I have always been an avid supporter of NTR technology so this is really good news.

Really?
As long as you are not able to simply refuel at destination it will not open the Solar System.
And expendable architectures will not get us anywhere in the long run.

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #33 on: 08/20/2017 09:52 AM »
what do "HEU" and "LEU" mean here?
Because Google might be a bit slow here let me help you.

Low Enrichment Uranium <20% U235
Highly Enriched  Uranium  > 20% 235

« Last Edit: 08/20/2017 01:11 PM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #34 on: 08/20/2017 09:55 AM »
Humanity will never do anything truly substantial in space until it uses NTR of some type.
I have always been an avid supporter of NTR technology so this is really good news.

Really?
As long as you are not able to simply refuel at destination it will not open the Solar System.
And expendable architectures will not get us anywhere in the long run.
Careful now. This is a little more subtle than it looks.

In an NTR propellant is reaction mass. The core could (should?) be capable of multiple burns.

So the question is (with current designs) is there a source of LH2 at the destination?  Could be (in principle) Ammonia, Water or Methane (CO2 ?) as well.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline blasphemer

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #35 on: 08/20/2017 10:17 AM »
Humanity will never do anything truly substantial in space until it uses NTR of some type.
I have always been an avid supporter of NTR technology so this is really good news.

I support nuclear propulsion research, but do you really think nuclear thermal will be a gamechanger? Merely doubling the specific impulse (while also making the engine heavier and more complex) does not sound like a big advance to me at all. It may not even be worth the added complexity. I do think future will belong to fusion drives or advanced nuclear drives like this:

https://en.wikipedia.org/wiki/Fission-fragment_rocket

https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

But in the medium term, for inner solar system, I dont really see chemical propulsion as insufficient or a limiting factor. Especially with propellant depots.
« Last Edit: 08/20/2017 10:19 AM by blasphemer »

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #36 on: 08/20/2017 10:48 AM »
Humanity will never do anything truly substantial in space until it uses NTR of some type.
I have always been an avid supporter of NTR technology so this is really good news.

I support nuclear propulsion research, but do you really think nuclear thermal will be a gamechanger? Merely doubling the specific impulse (while also making the engine heavier and more complex) does not sound like a big advance to me at all. It may not even be worth the added complexity. I do think future will belong to fusion drives or advanced nuclear drives like this:

https://en.wikipedia.org/wiki/Fission-fragment_rocket

https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

But in the medium term, for inner solar system, I dont really see chemical propulsion as insufficient or a limiting factor. Especially with propellant depots.
It's what NASA is prepared to fund.  :(

Yes it's a doubling of Isp, which looks good next to conventional rocket in Earth to Orbit, but then again air breathing systems can get trajectory averaged of 1300secs+. In space you've ion thrusters with 3000secs+.

NTR scores if you want
a)Short transit time (due to high thrust)
b)Do it on 1 launch.
c)Want a lot of payload in that launch.

The DIA did a study in the 70's of Mars in 70 days, with a Shuttle ET and set of SSME's. The problem is you need to lose about 21Km/sec of delta V at the end.  :(

But once you can split that process up (call it depots. Call it "distributed lift"), or don't want all of those features together,  things get rather more hazy.  :(

However BWXT is what NASA wants.

[EDIT. I recently (again) came across some work NASA had done on revisiting the old DUMBO idea of plates with radial slots in them to radically increase heat flow and improve T/W ratio.

This was in the context of a "tricarbide" design and used "washers" roughly 10cm OD, 2cm ID and 2mm thick, with spiral grooves running outside to inside. Multiple parallel stacks then feed a single nozzle for exhaust.

The design was 97% HEU, but I think it would be possible to transition it to LEU ]

« Last Edit: 08/21/2017 09:52 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline bradjensen3

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #37 on: 08/21/2017 11:06 PM »
I'm confused by this article. It states among other things that the NTP engines have higher thrust than chemical engines.

They don't explicitly say which chemical engines they are using for the comparison. 

I think they mention being twice as much thrust  as the Shuttle Main Engine.

Offline Jim

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #38 on: 08/22/2017 02:08 AM »
Combine nuclear thermal propulsion with water from the Moon as reaction mass, and you could tool around the solar system at a high rate of speed. Get water from the Moon, drop to Earth orbit and pick up passengers, and get to Mars orbit in a month maybe?


seems obvious to me, but there is a whole thread where I get beaten to a pulp suggesting it.

And rightly so. 
A.  You said earth's atmosphere and not orbit
b.  there is no sense in coming back to earth for passengers.  They can go with the vehicle from the moon
c.  There is not as much water as you think

Offline Jim

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #39 on: 08/22/2017 02:11 AM »
I'm confused by this article. It states among other things that the NTP engines have higher thrust than chemical engines.

They don't explicitly say which chemical engines they are using for the comparison. 

I think they mention being twice as much thrust  as the Shuttle Main Engine.

No, they said " a nuclear thermal rocket has double the propulsion efficiency of the Space Shuttle main engine"  which means ISP and not thrust

And no where in the article does it state "among other things that the NTP engines have higher thrust than chemical engines."  There is only one use of the word "thrust" and it has nothing to with comparisons.

The article states "Nuclear thermal power for spaceflight has a number of advantages over chemical-based designs, it said, primarily providing higher efficiency and greater power density resulting in lower propulsion system weight"

It is about efficiency and power density.  No mention of thrust.

That is two places where you have misquoted/misunderstood an article.  I suggest a little more careful reading.
« Last Edit: 08/22/2017 02:17 AM by Jim »

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