Author Topic: NASA boosts nuclear thermal propulsion with BWXT contract  (Read 6839 times)

Online Joffan

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http://www.world-nuclear-news.org/ON-NASA-boosts-nuclear-thermal-propulsion-with-BWXT-contract-04081701.html

some quotes
Quote
The National Aeronautics and Space Administration (NASA) has awarded BWXT Nuclear Energy a $18.8 million contract to initiate conceptual designs for a nuclear thermal propulsion reactor in support of a possible future manned mission to Mars. BWXT Nuclear Energy is a subsidiary of nuclear components, fuel and services provider BWX Technologies, which is based in Lynchburg, Virginia.
The reactor, based on low-enriched uranium (LEU) fuel, would be part of a nuclear thermal propulsion (NTP) rocket engine designed to propel a spacecraft from Earth orbit to Mars and back. According to NASA, an NTP system can cut the voyage time to Mars from six months to four and "safely deliver human explorers" by reducing their exposure to radiation. That also could reduce the vehicle mass, enabling deep space missions "to haul more payload".
:
Part of NASA's Game Changing Development (GCD) Program, the NTP project "could indeed significantly change space travel", NASA said, largely due to its ability to accelerate a large amount of propellant out of the back of a rocket at very high speeds, resulting in a highly efficient, high-thrust engine. In comparison, a nuclear thermal rocket has double the propulsion efficiency of the Space Shuttle main engine, "one of the hardest-working standard chemical engines of the past 40 years". That capability makes NTP "ideal for delivering large, automated payloads to distant worlds", it said.

http://www.spacedaily.com/reports/NASA_taps_BWXT_for_spacecraft_reactor_design_999.html
Quote
"BWXT is extremely pleased to be working with NASA on this exciting nuclear space program in support of the Mars mission," Rex D. Geveden, president and chief executive officer of parent company BWX Technologies, said in a press release. "We are uniquely qualified to design, develop and manufacture the reactor and fuel for a nuclear-powered spacecraft."
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Offline Elmar Moelzer

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #1 on: 08/06/2017 02:04 AM »
This is good news! Nuclear thermal engines are definitely something worth looking into.

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #2 on: 08/06/2017 09:30 AM »
I think KiloPower offers a more near term deployment, mostly for surface power and as a source to drive ion thrusters of various types.

That said NTR is about the only approach that can deliver any significant improvement on Isp at thrust levels that can shorten human missions with a TRL that's anywhere close to deployment.

The shift to LEU is a big change though as historically these systems have assumed the availability of about 97% enriched U235, which with modern proliferation and safety concerns is a complete non starter.  :(
"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 clongton

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #3 on: 08/06/2017 07:42 PM »
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.
Chuck - DIRECT co-founder
I started my career on the Saturn-V F-1A engine

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #4 on: 08/07/2017 07:33 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 might agree with nuclear power but the single use NERVA architecture seemed epically wasteful to me.  :(

IIRC wasn't NTR in the DRM 5.0 mission a $13Bn line item? this is about 722x smaller.   :(

The KiloPower team managed to raise $63m to prove that you could run a Stirling generator off of a nuclear reactor and their reactor test (the first of a space rated design in the US since 1965) is costed around $200m. That unit operates at about 1Kw, with stretch potential up to 10Kw and will be complete except for the radiators, which are deemed too dependent on the final application. 

Incidentally radiator design is one of those areas where I think a lot of improvement is possible without hugely exotic TRL0 designs. I'd love to see more "tailored" emission surfaces that maximize heat radiation at the radiator operating temperature while minimizing heat absorption.

But NTR's are in the MW class for even small ones (the one studied here, an upgrade to the SNRE studied under NERVA
http://www.neofuel.com/Schnitzler-Borowski-2009_NTR_25klbs_3.5TtW_AIAA-2009-5239-234.pdf
is 550MW(th) for 25 000lb of thrust. 

I think it's fair to say that for this project to get beyond the Powerpoint stage it's going to take a lot of commitment from NASA 
"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 #5 on: 08/07/2017 10:03 AM »
Update.

I found this on the NASA website for NTP

https://gameon.nasa.gov/gcd/files/2016/05/FS_NTP_160525.pdf

But there is something a bit odd about the description.  :(

An earlier report on the subject said that Graphite Composite fuels had much more maturity than cermets (20 reactors built Vs no cermet unit ever tested, not a trivial difference in this field).

One of the initial project goals is to purify Tungsten (the matrix material for the cermet designs) to 90% purity affordably, then to look at a reactor built with LEU at a regular engine test site IE Stennis).

I'm baffled by this.  ???

Going from HEU to LEU will make the core larger. I can only presume they think a thermal spectrum reactor (all that graphite makes it a thermal reactor, epithermal at most) will be simply too large to launch (IE from 97% U235 to <20%) and the only way to cope with the reduction is to go with a fast reactor, hence the cermet approach.

But TBH I did not realize there was any problem with W purity to begin with, unlike say the issue of all commercial Mo (low capture cross section) having enough Hf (high capture cross section) in it to affect its use in nuclear applications without serious processing.  :(

Clearly shifting to LEU changes the preferred options by a very long way.



 
"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.

Online Joffan

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #6 on: 08/08/2017 12:14 AM »
Update.

I found this on the NASA website for NTP

https://gameon.nasa.gov/gcd/files/2016/05/FS_NTP_160525.pdf

The video referenced by that document - not really adding technical information but quite nicely done:



One of the initial project goals is to purify Tungsten (the matrix material for the cermet designs) to 90% purity affordably, then to look at a reactor built with LEU at a regular engine test site IE Stennis).

I'm baffled by this.

Hmm, I wonder if the clue is in the phrase "isotopically pure tungsten" - do they just want one isotope? That does sound difficult and expensive.
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Offline yg1968

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #7 on: 08/08/2017 01:52 AM »
I don't know if this helps but they expand a little bit on this here:

Quote from: page 2
Initial project goals are to demonstrate the ability to purify tungsten to a minimum of 90 percent purity and determine the production costs at that purity level; to determine the technical and programmatic feasibility (pre-phase A level) of an NTP engine in the thrust range of interest for a human Mars mission; and to determine the program cost of an LEU NTP system and the confidence level of each major cost element.

Offline Propylox

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #8 on: 08/08/2017 03:34 AM »
I think it's fair to say that for this project to get beyond the Powerpoint stage it's going to take a lot of commitment from NASA
This seems like NASA dipping a toe in case they suddenly need to try and play catch-up to Russia. In 2016 Rosatom received experimental fuel for their NEP design, which began development around '09-'10, and are expected to unveil the prototype next year.
It's a 4MWt ~ 1MWe high temperature gas-cooled fast reactor for 100-150kw nominal ion propulsion.

January 2014 informative article http://osnetdaily.com/2014/01/russia-advances-development-of-nuclear-powered-spacecraft/
March 2016 short article https://sputniknews.com/business/201603211036691748-russia-rosatom-fuel/
« Last Edit: 08/08/2017 03:36 AM by Propylox »

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #9 on: 08/08/2017 11:36 AM »
This seems like NASA dipping a toe in case they suddenly need to try and play catch-up to Russia. In 2016 Rosatom received experimental fuel for their NEP design, which began development around '09-'10, and are expected to unveil the prototype next year.
It's a 4MWt ~ 1MWe high temperature gas-cooled fast reactor for 100-150kw nominal ion propulsion.

Not really. Nuclear Electric <> Nuclear Thermal.  :(

The Russian NE is very big reactor by space nuclear standards (the biggest the US fielded was 30Kw(t) 500W(e) in 1965). It would mate very well with some of the NASA Ion thruster development projects.

Nuclear thermal is in the 100s of MW of thermal power. Even the Small Nuclear Engine ( basically a nuclear RL10) was around 350MW(t).

The upside of NTP for the US is they've had a substantial programme in it already, so there's a fair knowledgebase (although not perfect. It was shut down in some haste  :( ) to draw on and it converts heat directly into thrust with no intermediate conversion. That's important because radiators in space are true radiators (no convection  :( ). The more efficient your generator the lower its waste output temperature and the bigger the radiator you need.
For any system there will a  "break even" mass where making the generator 1% more efficient increases the radiator mass too much to be worth it.

I don't know if this helps but they expand a little bit on this here:

Quote from: page 2
Initial project goals are to demonstrate the ability to purify tungsten to a minimum of 90 percent purity and determine the production costs at that purity level; to determine the technical and programmatic feasibility (pre-phase A level) of an NTP engine in the thrust range of interest for a human Mars mission; and to determine the program cost of an LEU NTP system and the confidence level of each major cost element.
I'd already seen this in the presentation.
I took it to mean that normally tungsten has impurities (not a formally made alloy) that knock down its suitability as a reactor material, which is tough, but not as bad as requiring isotopic purity

Hmm, I wonder if the clue is in the phrase "isotopically pure tungsten" - do they just want one isotope? That does sound difficult and expensive.
That's the usual meaning and AFAIK you're right. For isotope sep you're talking exactly the sort of systems that do Uranium enrichment, which are specialized, complex and very expensive.   :(

I had sort of hoped they meant that Tungsten is normally found with other metals in it's chemical group in the same way that Molybdenum is found with Hafnium. Mo has a very low neutron capture cross section and very good high temperature strength, as does Hafnium, so most people don't bother to separate them.

Unfortunately Hf has a huge thermal neutron capture cross section (it's used in reactor control rods).

Fortunately there are chemical separation processes that work (but multiply the cost of "reactor grade" Mo  :( )

I can only presume that some of the natural Tungsten isotopes have very poor properties for a nuclear reactor and stripping them out makes a really big improvement.  If so that's going to be tough.  :(
"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 #10 on: 08/08/2017 03:29 PM »
Digging into the NASA progress report on the project they state they are looking for 90% pure Tungsten 184 isotope. Dynetics (who I associated with robotics, then the SLS boosters)  is the company doing the work. they need to deliver kilograms of Tungsten to test the fabrication process but so far they only managed about 50g+ of 50% pure W, but they think they know what the problem is.  :(
"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 #11 on: 08/09/2017 06:03 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?

Online KelvinZero

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #12 on: 08/09/2017 06:34 AM »
I think NTR basically always uses hydrogen. The point is that smaller particles move faster for the same temperature, so assuming you run you engine as hot as you can without melting it, you get better ISP if you use the smallest molecule for your propellant.

Lunar water does contain hydrogen though, so, yay.

Offline Asteroza

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #13 on: 08/09/2017 08:49 AM »
That's the usual meaning and AFAIK you're right. For isotope sep you're talking exactly the sort of systems that do Uranium enrichment, which are specialized, complex and very expensive.   :(

Would SILEX/laser enrichment schemes from uranium (re)processing be applicable here? I understood that the laser wavelength is output isotope specific so you can't use the same designs directly, but the basic principles should still apply, right?

Though there is the whole reprocessing taboo from the Carter era that made SILEX development run for so long. Trying to push SILEX tech for non-uranium use now may raise some technology dissemination concerns again.

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #14 on: 08/09/2017 05:00 PM »
Would SILEX/laser enrichment schemes from uranium (re)processing be applicable here? I understood that the laser wavelength is output isotope specific so you can't use the same designs directly, but the basic principles should still apply, right?
I know nothing of "The technology that must not be named,"  :) but I'd guess that broadly speaking you'd be right.

I might (semi randomly) observe that a lot more materials have been tested for laser action since the 1970's and I could imagine (but not know) that either a new laser type (AFAIK high power laser diodes in the visible range were science fiction then, today people are building actual weapon systems out of them), new laser material or new way way of using an existing material (EG harmonic generation) could produce a laser intense enough to be useful at just about the right wavelength to do the job in a way that systems of that era could not.

[EDIT One thing that did strike was the (no doubt superficial) resemblance between the descriptions of the various laser enrichment systems and atomic clocks designs, in particular using various combinations of light sources, RF and/or magnetic fields to get separation of atoms in different atomic states.
Obviously the clocks seek to minimize the number of isotopes to begin with so a "clock like" system would be the worlds least accurate atomic clock.  :) .
The materials used in clocks are also chosen for how easily the can be vaporized, which would not be the case for Tungsten, which is just about the hardest element to vaporize there is.  :(  TBH I'm still a bit vague why isotoically pure Tungsten is needed. Tungsten has multiple isotopes. I guess some of them have just too high a capture cross section before the fission neutrons get to the preferred operating energy of the design.  :( ]

TBH I quite liked the proposal that one of the National Laboratories had of dissolving it in molten Bismuth and centrifuging the mix so the stuff separated into layers. I presume you then inserted a hollow needle down to the right layer and extracted the relevant isotope after it had spun down, but before the layers mixed together. Sadly it didn't go anywhere as I guess it was just too tough to engineer. I think the physics didn't quite work.  :( .

Quote from: Asteroza
Though there is the whole reprocessing taboo from the Carter era that made SILEX development run for so long. Trying to push SILEX tech for non-uranium use now may raise some technology dissemination concerns again.
Just another thing about the US that seems completely crazy to outsiders  :(. All those reactors (the most of any country on the planet?). All that spent fuel. All that potential for recycled fuel. But instead it's left to sit in a storage, waiting for a long term solution that's been coming "real soon now" since the original "Star Wars" had it's cinema release.  :(

What I don't get is why no one has tried to put together a "reprocessing plant in a shipping container" that can (slowly) chew through the pile of fuel rods and make new ones without them ever leaving a site. Obviously the economics are very tricky, and you'd want to prepare as much of the new fuel rods as possible centrally (and of course engineering a system capable of surviving exposure to intense radiation and highly toxic, volatile, abrasive and chemically aggressive chemicals) , but I think it could be possible.

However that's completely OT for this thread.   :(
« Last Edit: 08/09/2017 06:28 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.

Online Joffan

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #15 on: 08/09/2017 08:08 PM »
Urenco actually offers isotope separation on elements other than uranium:

https://media.urenco.com/corp-website/74/stableisotopes_2.pdf (p14)

Quote
Naturally occurring Tungsten has five stable isotopes...

All of these isotopes can be enriched or depleted by URENCO to any required concentration. Using our centrifuge technology, concentrations can be enriched to exceed 99.9% or depleted below 1%.
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Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #16 on: 08/09/2017 08:28 PM »
Urenco actually offers isotope separation on elements other than uranium:

https://media.urenco.com/corp-website/74/stableisotopes_2.pdf (p14)

Quote
Naturally occurring Tungsten has five stable isotopes...

All of these isotopes can be enriched or depleted by URENCO to any required concentration. Using our centrifuge technology, concentrations can be enriched to exceed 99.9% or depleted below 1%.
And they are already doing "low activation" Tungsten.  :)

Given that URENCO has a US operation (although their non fuel operation seems to operate out of their Netherlands site) the simple answer would be for Dynetics to call them saying they are on a USG contract and order up a Kg or two.

Presumably for some reason they are not doing it that way (too expensive for a govt contract?) and it's proving tougher than expected to do in house.  :(
"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 Propylox

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #17 on: 08/10/2017 02:18 AM »
4MWt ~ 1MWe high temperature gas-cooled fast reactor for 100-150kw nominal ion propulsion.
(snips) The Russian NE is very big reactor by space nuclear standards ...
Nuclear thermal is in the 100s of MW of thermal power ... and it converts heat directly into thrust with no intermediate conversion. That's important because radiators in space are true radiators. The more efficient your generator the lower its waste output temperature and the bigger the radiator you need.
For any system there will a  "break even" mass where making the generator 1% more efficient increases the radiator mass too much to be worth it.
Well aware, as I'm sure you are that the higher the temperature - the faster the heat transfer through radiance and convection. Rosatom's fast reactor design's high temperature therefor needs smaller radiators to create the working fluid's temperature differential for power production. NASA's NTP would traditionally use high temperatures to excite the propellant as quickly as possible - but they call it a "reactor" instead of a "core" and set a relatively low bar of ~900s isp, but which seems high for the LEU they've also proposed. That's curious.
http://www.world-nuclear-news.org/ON-NASA-boosts-nuclear-thermal-propulsion-with-BWXT-contract-04081701.html
Quote
... The reactor, based on low-enriched uranium (LEU) fuel, would be part of a nuclear thermal propulsion (NTP) rocket engine ... In comparison, a nuclear thermal rocket has double the propulsion efficiency of the Space Shuttle main engine ...
Whatever NASA is envisioning, it'll need to produce electricity (turbine, stirling, thermoelectric) and for reliability/efficiency purposes will probably use a working fluid. No surprises there.
With LEU they'll need to increase nuclear activity (temperature) right before the throat after the propellant spends considerable time circulating around the core, building temperature. This choice of LEU adds incredible complexity to the design while decreasing its performance - so why do it?

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #18 on: 08/10/2017 06:48 AM »
Well aware, as I'm sure you are that the higher the temperature - the faster the heat transfer through radiance and convection.
Again space radiators are true radiators. They are not surrounded by a gas layer as "radiators" are on Earth, which means convection, which is a very efficient way to move heat, doesn't exist.
Quote from: Propylox
Rosatom's fast reactor design's high temperature therefor needs smaller radiators to create the working fluid's temperature differential for power production.
True, but you sacrifice efficiency because you can only use a limited amount of that high temperature if you want the waste heat temp to be high. Conversion efficiency Vs radiator weight impacts overall system efficiency.

Quote from: Propylox
NASA's NTP would traditionally use high temperatures to excite the propellant as quickly as possible - but they call it a "reactor" instead of a "core" and set a relatively low bar of ~900s isp, but which seems high for the LEU they've also proposed. That's curious.
http://www.world-nuclear-news.org/ON-NASA-boosts-nuclear-thermal-propulsion-with-BWXT-contract-04081701.html
Quote
... The reactor, based on low-enriched uranium (LEU) fuel, would be part of a nuclear thermal propulsion (NTP) rocket engine ... In comparison, a nuclear thermal rocket has double the propulsion efficiency of the Space Shuttle main engine ...
Whatever NASA is envisioning, it'll need to produce electricity (turbine, stirling, thermoelectric) and for reliability/efficiency purposes will probably use a working fluid. No surprises there.
With LEU they'll need to increase nuclear activity (temperature) right before the throat after the propellant spends considerable time circulating around the core, building temperature. This choice of LEU adds incredible complexity to the design while decreasing its performance - so why do it?
Not really. Historically reactor flow through they NASA cores has been pretty simple. In one end, out the other. I'd expect a longer core. Basically it will be a question of heat release per unit length of the core. Look at an Earth based AGR. It's gas temperature is around 500c but the clad fuel was much hotter, and the UO2 pellets higher still (because UO2 is a very poor heat conductor) operating at the 10s of bar level.

The temperature of the reactor vessel is like that of a rockets thrust chamber so it's regeneratively cooled. Piping some of that flow through a generator would be no problem. The issue is with the reactor shut down can you get enough heat of the core without a propellant flow between the core and the walls to extract the heat? If not you're looking at sticking some kind of plug in the throat to trap recirculating gas, or keep a (hopefully small) constant stream of propellant running through it for a very long period.  :(
Personally I like heat pipes. They can move a lot of heat and they can be made one way and switchable. IE turned off while the reactor is running but switched on to extract decay heat for electrical power.

As to why NASA are doing this. AFAIK HEU (or "weapons" grade) is quite cheap, hence it's interest by the Kilopower team.

However those systems can be delivered to the launch site as black boxes.

An engine needs a lot of testing at various NASA sites which are not geared up to the level of security and hazmat containment that has not existed at those sites for decades.  IOW in principal going LEU saves a lot of money
"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 Propylox

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #19 on: 08/10/2017 04:38 PM »
Rosatom's fast reactor design's high temperature therefor needs smaller radiators to create the working fluid's temperature differential for power production.
True, but you sacrifice efficiency because you can only use a limited amount of that high temperature if you want the waste heat temp to be high. Conversion efficiency Vs radiator weight impacts overall system efficiency.
That doesn't make any sense. Please elaborate and/or correct your comment.

(snips) Historically reactor flow through they NASA cores has been pretty simple. In one end, out the other. I'd expect a longer core. ... The temperature of the reactor vessel is like that of a rockets thrust chamber so it's regeneratively cooled.
Piping some of that flow through a generator would be no problem. The issue is with the reactor shut down can you get enough heat of the core without a propellant flow between the core and the walls to extract the heat?
Personally I like heat pipes. They can move a lot of heat and they can be made one way and switchable. IE turned off while the reactor is running but switched on to extract decay heat for electrical power.
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.

AFAIK HEU (or "weapons" grade) is quite cheap, hence it's interest by the Kilopower team.
IOW in principal going LEU saves a lot of money
Another contradiction, and a cost claim without argument.
Based on your previous posts, I'd request the old JS19 reply  ;)

-- Previous quote --
1) An earlier report on the subject said that Graphite Composite fuels had much more maturity than cermets (20 reactors built Vs no cermet unit ever tested, not a trivial difference in this field). One of the initial project goals is to purify Tungsten (the matrix material for the cermet designs) to 90% purity affordably, then to look at a reactor built with LEU at a regular engine test site IE Stennis).
.. and ..
But TBH I did not realize there was any problem with W purity to begin with, unlike say the issue of all commercial Mo (low capture cross section) having enough Hf (high capture cross section) in it to affect its use in nuclear applications without serious processing.

2) Going from HEU to LEU will make the core larger. I can only presume they think a thermal spectrum reactor (all that graphite makes it a thermal reactor, epithermal at most) will be simply too large to launch (IE from 97% U235 to <20%) and the only way to cope with the reduction is to go with a fast reactor, hence the cermet approach. 
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.

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?
« Last Edit: 08/10/2017 04:44 PM by Propylox »

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 »
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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?




Online 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?

Offline 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 »

Offline TrevorMonty

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.
Deliver return fuel with SEP, doesn't matter if it takes a year or two to get there.

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #41 on: 08/22/2017 05:57 AM »
No, they said " a nuclear thermal rocket has double the propulsion efficiency of the Space Shuttle main engine"  which means ISP and not thrust
Which sounds about right, given the SSME is about Isp 450 and NTR is about 900secs.

OTOH An SSME thrust equivalent NTR would be an absolute monster, with a budget requirement to match.  :(
"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 Propylox

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #42 on: 09/13/2017 05:08 AM »
-snip- 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.
No one has built such an NTR, but LEU fast breeders have been around for a while - in Russia. They even produce experimental fuels for other countries as well as their own constantly-developing designs. Considering NTRs have long been politically impossible in the US, this program seems like cover for catching up with our nuclear power technologies and expertise base.

PS-
A previous comment correcting all the inaccuracies and consistent contradictions in JS19's post was erased and I was banned temporarily for being accurate and helpful. Rather than recorrect it now, you guys can stay ill-informed. Your system, your loss.
« Last Edit: 09/13/2017 05:13 AM by Propylox »

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #43 on: 09/13/2017 09:16 AM »
No one has built such an NTR,

If you mean that no nation has put an NTR into orbit that would be correct.

OTOH the US ran a very substantial NTR development programme from about 1958 to 1973 and fired a number of them into the atmosphere at a place called "Jackass Flats."

IIRC at least one of them was close to flight weight.

Are you unaware of this?

Quote from: Propylox

but LEU fast breeders have been around for a while - in Russia. They even produce experimental fuels for other countries as well as their own constantly-developing designs. Considering NTRs have long been politically impossible in the US, this program seems like cover for catching up with our nuclear power technologies and expertise base.

Nuclear power is especially challenging in the US (the "no reprocessing" rule. I can't really get my head around that one, for example  :( ).
 Nuclear power for rocket propulsion even more so.  :( As for "politically impossible," I guess NASA are optimists.  :)
Part of softening that attitude to NTR would be to make (to the layman) an NTR look less  like a nuclear bomb that's a hairs breadth away from going bang
(let me repeat, "to a layman." I'm quite well aware that there are plenty of safeguards built in, starting with the fuel and its cladding). IOW not having an engine that mandates "bomb grade" fuel.

To have a chance of running, and continuing to run, any such programme would need to support a number of goals. "Improving (or sustaining) US competitiveness" is fairly standard language when something like this is looking for an appropriation, even if it's only for a design study. I had not realized you are not from the US, (but then neither am I) and might not be aware of this.


OTOH the US is just as prone to "Not Invented Here" syndrome as anywhere else. Consider how long it has taken for them to consider doing an Oxidizer Rich Staged Combustion engine, for example.  :(

On a personal note welcome back to the site.
"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 #44 on: 09/19/2017 08:03 PM »

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.

You are absolutely correct! Thank you for pointing out my misunderstanding!





Offline Katana

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #45 on: 09/20/2017 02:04 AM »
-snip- 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.
No one has built such an NTR, but LEU fast breeders have been around for a while - in Russia. They even produce experimental fuels for other countries as well as their own constantly-developing designs. Considering NTRs have long been politically impossible in the US, this program seems like cover for catching up with our nuclear power technologies and expertise base.

PS-
A previous comment correcting all the inaccuracies and consistent contradictions in JS19's post was erased and I was banned temporarily for being accurate and helpful. Rather than recorrect it now, you guys can stay ill-informed. Your system, your loss.
Fast breeders are much more difficult on ground, let alone in space.

Offline john smith 19

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Re: NASA boosts nuclear thermal propulsion with BWXT contract
« Reply #46 on: 09/20/2017 06:43 PM »
Fast breeders are much more difficult on ground, let alone in space.
True.

I think it's important to separate the idea of a "fast" spectrum reactor and a "breeder" reactor with low breeding or high breeding.

Once you strip all that graphite out of the design I think it's quite difficult for any reactor to remain a thermal spectrum system. IRL most PWR's have been "breeding" fuel for decades, in the sense of turning U238 into Pu and some of that fissioning in situ, improving reactor burnup. More a tweak in the ConOps, rather than a full re-design for breeding.

But you're right few actual breeder reactors, designed to produce significantly more Pu than the U235 they've burnt, have been built.

Historically NTR have been HEU systems (98% U235 IIRC) so there's been no U238 to breed in the first place. Obviously this design changes that. We'll have to see how far it goes.
« Last Edit: 09/20/2017 06:44 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.

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