Author Topic: Fluorine  (Read 134685 times)

Offline ijuin

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Re: Fluorine
« Reply #30 on: October 23, 2010, 07:57:15 PM »
What are the obstacles to building an engine that can use both? I'm thinking of a low-thrust H2 mode with maximized ISP for sustained burns, while being able to switch to an onboard He tank for a thrust boost (e.g. for ejecting from LEO or inserting into planetary orbit at the destination). Would this be more effective than just dumping extra H2 propellant through the reactor of a NTR optimized for H2?

And speaking of using multiple propellants, what obstacles exist with respect to a dual mode H2/H20 NTR engine? For example, let's say that you are landing on Callisto or Ganymede, and you use H2 propellant to get there from Earth (due to superior ISP), but you use local H2O to refill your tanks for the return journey (instead of using your reactor to separate out the H2, which might take more time than you can afford to wait around).

Offline Satanic Mechanic

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Re: Fluorine
« Reply #31 on: October 24, 2010, 12:15:18 AM »
Now we are getting into a subject I love, the NERVA.  I have read many papers about this project and may I suggest the white paper written by Lt.Col. Timothy Lawrence from the Air Force Academy titled "Nuclear Thermal Rocket Propulsion Systems".
As for fuels, my choice would be ammonia (NH3) mainly because of the low boiloff of the fuel.  If you have to send up multiple tankers to fuel the NERVA and it will take time (months) to do the job, I would pick ammonia.  The drawback for ammonia is it has a higher molecular mass than hydrogen, so it will have a slower Isp than hydrogen.
To answer Ijuin's question about a solid core NERVA with the ability to take on different fuels, the only problem I see is the ratings of the pumps and storage tanks to take on multiple fuels.  It is all in the engineering.  You might need different pumps for the molecular hydrogen vs. the light water since the water has a higher molecular mass.
Another thing that comes to mind is the size of channels in which the propellant flows across the fuel rods.  I don't know... maybe use one size for the channels and control the heat transfer via the control rods.  Use a heavier fuel, so you have to pull out the control rods a little bit more to generate more heat.
I can see an engine that can use multiple propellants but those propellants would have similar characteristics like ammonia and methane.


SM

Offline Bob B.

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Re: Fluorine
« Reply #32 on: October 24, 2010, 05:53:17 PM »
I investigated ammonia a while back and concluded that it was one of the best options after hydrogen.  It will produce a specific impulse comparable to a LOX-LH2 chemical engine.

Regarding the possibility of a dual fuel engine, I've come up with some numbers to ponder.  I performed a simple design of a hydrogen engine and then recalculated the numbers with alternate fuels to see how they compare.  The design parameters used included a 2,000 megawatt reactor, an operating temperature of 2,500 K, and a 40:1 nozzle expansion ratio.  For the first set of calculations, i.e. the hydrogen engine, I used a combustion chamber pressure of 40 atmospheres.  Below is what I came up with:

Propellant:  Liquid Hydrogen
Propellant flow rate:  50.5 kg/s
Reactor power:  2,000 MW
Combustion chamber temperature:  2,500 K
Combustion chamber pressure:  40.0 atm
Gas molecular weight:  2.012
Specific heat ratio:  1.30
Nozzle throat diameter:  27.63 cm
Nozzle exit diameter:  174.8 cm
Expansion ratio:  40:1
Thrust (vacuum):  434.8 kN
Specific impulse (vacuum):  878 s

Propellant:  Liquid Helium
Propellant flow rate:  154.7 kg/s
Reactor power:  2,000 MW
Combustion chamber temperature:  2,500 K
Combustion chamber pressure:  79.9 atm
Gas molecular weight:  4.003
Specific heat ratio:  1.67
Nozzle throat diameter:  27.63 cm
Nozzle exit diameter:  174.8 cm
Expansion ratio:  40:1
Thrust (vacuum):  778.5 kN
Specific impulse (vacuum):  513 s

Propellant:  Liquid Ammonia
Propellant flow rate:  158.2 kg/s
Reactor power:  2,000 MW
Combustion chamber temperature:  2,500 K
Combustion chamber pressure:  61.0 atm
Gas molecular weight:  8.505
Specific heat ratio:  1.30
Nozzle throat diameter:  27.63 cm
Nozzle exit diameter:  174.8 cm
Expansion ratio:  40:1
Thrust (vacuum):  664.0 kN
Specific impulse (vacuum):  428 s

Propellant:  Liquid Water
Propellant flow rate:  245.5 kg/s
Reactor power:  2,000 MW
Combustion chamber temperature:  2,500 K
Combustion chamber pressure:  67.5 atm
Gas molecular weight:  17.884
Specific heat ratio:  1.18
Nozzle throat diameter:  27.63 cm
Nozzle exit diameter:  174.8 cm
Expansion ratio:  40:1
Thrust (vacuum):  779.8 kN
Specific impulse (vacuum):  324 s

Helium, ammonia, and water all produce higher thrust because the propellant flow rates are much higher than hydrogen, however hydrogen has by far the best specific impulse.  The high flow rates of  helium, ammonia, and water are needed to keep the engine from over heating.  The flow rate is the mass of propellant that 2,000 MW can raise to 2,500 K in one second.  The combustion chamber pressure is that necessary to force the require gas flow through the nozzle throat (which is of fixed dimension).
« Last Edit: October 25, 2010, 08:11:26 AM by Bob B. »

Offline Bob B.

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Re: Fluorine
« Reply #33 on: October 24, 2010, 11:12:39 PM »
I was just looking for other compounds that might make a good NTR propellant and I think I may have found something - pentaborane (B5H9).  The numbers seem to indicate that it will decompose into hydrogen and boron, and if so, will yield a good specific impulse.  Another advantage is that it is liquid over a good temperature range (-46.8 to 60.1 C).  It's density is not quite as good as ammonia, but it's still not too bad (0.618 g/ml).  However it is toxic, and I have no idea if it's suitable to be used as a coolant.  For comparison, here are the numbers when used with the engine from my previous post:

Propellant:  Pentaborane
Propellant flow rate:  154.75 kg/s
Reactor power:  2,000 MW
Combustion chamber temperature:  2,500 K
Combustion chamber pressure:  70.9 atm
Exhaust molecular weight:  6.655
Specific heat ratio:  1.13
Nozzle throat diameter:  27.63 cm
Nozzle exit diameter:  174.8 cm
Expansion ratio:  40:1
Thrust (vacuum):  845.3 kN
Specific impulse (vacuum):  557 s

A significant improvement with this propellant can be attained with a bigger expansion ratio.  If I make the expansion ratio 80:1, the specific impulse improves to 577 seconds.  That's still not nearly as good as hydrogen, but it's a big improvement over chemical propulsion.  Furthermore, pentraborane's density impulse is better than anything else I've tried - it gets about 5.5 times more thrust per liter than hydrogen.  It also yields the most thrust per megawatt of reactor power of anything tried so far.
« Last Edit: October 25, 2010, 08:47:10 AM by Bob B. »

Offline ijuin

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Re: Fluorine
« Reply #34 on: October 25, 2010, 02:52:38 AM »
I asked about water as a propellant mainly because of the fact that it would be the most readily available propellant for In-Situ Resource Utilization at most destinations (Venus excluded). Certainly you could use your reactor's energy to crack it into oxygen and hydrogen, but getting a tankful of H2 in this way may take longer than you can afford to stick around (e.g. a manned Europa landing, where radiation levels dictate that EVA times be kept as short as possible). For an SF example, I am thinking of how the Chinese ship Tsien was refilling its fuel tanks with Europan water when it tragically encountered the native fauna.

Offline Bob B.

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Re: Fluorine
« Reply #35 on: October 25, 2010, 08:40:03 AM »
I asked about water as a propellant mainly because of the fact that it would be the most readily available propellant for In-Situ Resource Utilization at most destinations (Venus excluded).

It's a big advantage when your propellant is free and easily available.  Although water's performance in a NTR is not nearly as good as some other propellants, it looks like it will yield a specific impulse as good as any common chemical propellant aside from LOX-LH2.  Water certainly seems like a viable propellant.

Offline Satanic Mechanic

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Re: Fluorine
« Reply #36 on: October 25, 2010, 10:48:25 AM »
I was just looking for other compounds that might make a good NTR propellant and I think I may have found something - pentaborane (B5H9).  The numbers seem to indicate that it will decompose into hydrogen and boron, and if so, will yield a good specific impulse.  

Bob,
One thing that concerns me is the boron itself.  Boron is used as a neutron absorber, it is used in control rods.  The first thought in my mind was that the flow of the pentaborane propellant would shutdown the reaction by capturing the free neutrons.
I am sure there could be a way to use the propellant as the moderator/control rod but once the propellant was gone you would have to switch over to the control rods before the fuel assembly overheats and melts.

SM

Offline Bob B.

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Re: Fluorine
« Reply #37 on: October 25, 2010, 02:45:59 PM »
One thing that concerns me is the boron itself.  Boron is used as a neutron absorber, it is used in control rods.  The first thought in my mind was that the flow of the pentaborane propellant would shutdown the reaction by capturing the free neutrons.
I am sure there could be a way to use the propellant as the moderator/control rod but once the propellant was gone you would have to switch over to the control rods before the fuel assembly overheats and melts.

That's a point I hadn't considered.  Hopefully it's something that some brilliant nuclear engineers can figure out.

Clearly nothing is going to beat hydrogen as a reaction mass because of its low mass.  Unfortunately its -253 C boiling point and 0.071 g/ml density are big problems.  There will be significant boil-off and leakage issues and it takes huge insulated tanks to store it.  This discussion got me thinking about what's the "next best" propellant.  Helium has twice the density of hydrogen but it is even colder at -269 C.  And with a molecular weight of 4, there is a big loss in performance versus hydrogen.  Ammonia has better physical properties than either hydrogen or helium, but its performance isn't that much better than chemical propulsion.

For a propellant to get a serious look it has to provide advantages that we don't get with either liquid hydrogen or chemical propulsion.  Since we'll never beat the specific impulse of hydrogen, we must gain a decided advantage in density and storability to make the alternative worth considering.  And to be considered over chemical propulsion, the alternative should give us significantly better performance.

I started looking for compounds that contained a large amount of hydrogen bonded to a low molecular weight element.  I began working my way up the periodic table looking for viable candidates.  I found some lithium-hydrogen compounds but they are solids at normal temperatures.  The first molecule I found that looked like what I needed and was liquid over a good temperature range was pentaborane.

The good thing about pentaborane is that it decomposes, beginning at a temperature of 150 C, into hydrogen and elemental boron (the lightest molecular weight form of boron).  It also has a low specific heat ratio, which gives it a better specific impulse than helium even though the exhaust has a higher molecular weight.  It also has the potential for much improved performance at higher temperatures because of the dissociation of hydrogen.  For instance, if one could build a nuclear engine that could handle 6,000 K, it looks like pentaborane would exceed a specific impulse of 1,000 seconds.  Of course at that temperature hydrogen would yield a specific impulse exceeding 1,500 seconds.

There may still be something better that I haven't yet considered, but pentaborane appears to have good potential as a nuclear rocket propellant where storability and density issues are more important than maximum specific impulse.  Of course the neutron absorption issue could be a show stopper.

(EDIT) ...  I recalculated the ISP of pentraborane at 6,000 K and it looks like the best that can be expected is about 950 s.
« Last Edit: October 25, 2010, 08:35:29 PM by Bob B. »

Offline Satanic Mechanic

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Re: Fluorine
« Reply #38 on: October 25, 2010, 04:01:47 PM »
Bob,
Thanks for crunching numbers and looking into other propellants.  Maybe there is a chemical out there similar to pentaborane but is neutron transparent.  Like I said before, maybe we can use pentaborane, run the core super critical (k > 1) and let the propellant flow to make the core critical (k = 1).  You are right, leave it to the nuclear engineers to figure it out.
It may look like that hydrogen is the fuel to use, now if we can haul up the fuel in one launch, that would take care of the on-orbit storability losses.

SM

Offline Bob B.

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Re: Fluorine
« Reply #39 on: October 25, 2010, 04:59:51 PM »
I don’t want to completely rule out ammonia either.  It gives the specific impulse of a LOX-LH2 chemical engine but without the storability and density problems.  If you don’t need a specific impulse any better than the best chemical propulsion, an ammonia nuclear rocket might be a good choice.  There won’t be the boil-off problems and the density impulse is more than double a LOX-LH2 system.  The drawback would be a heavy inert mass due to the reactor.

So I guess at this point our choices are:

System             Isp (s)    Advantages / Disadvantages

Hydrogen NTR       850-900    Excellent Isp, poor storability & density, heavy

Methane NTR        650-700    High Isp, fair storability & density, heavy

Pentaborane NTR    550-600    High Isp, good storability & density, heavy, neutron absorber?

Ammonia NTR        400-450    Medium Isp, good storability & density, heavy

LOX-LH2            400-450    Medium Isp, fair storability & density

Other Chemical     300-350    Low Isp, excellent storability & density, light

Solid              300        Low Isp, excellent storability & density, simple, little control

EDIT - Added Methane NTR
« Last Edit: October 26, 2010, 12:14:44 AM by Bob B. »

Offline Satanic Mechanic

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Re: Fluorine
« Reply #40 on: October 25, 2010, 05:41:09 PM »
Bob,
I dug up some of my documents and I found an old declassified Los Alamos paper called "A Metal Dumbo Rocket Reactor".  In one section it is about the use of hydrogen vs. ammonia.  The ammonia was decomposed through a preheater but later they talk about using liquid hydrogen since they now have the technology to store large quantities of it(printed in 1957).
Do you want a copy of it?  It also addresses some engineering concerns like propellant flow and reactor design.

SM

Offline Bob B.

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Re: Fluorine
« Reply #41 on: October 25, 2010, 08:12:19 PM »
Do you want a copy of it?

Yes, I think that would be an interesting read.  Can it be emailed?  I'll PM you my email address.

Offline ijuin

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Re: Fluorine
« Reply #42 on: October 25, 2010, 09:04:22 PM »
Out of curiosity, how would the performance of methane in an NTR compare to that of ammonia?

Offline Bob B.

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Re: Fluorine
« Reply #43 on: October 25, 2010, 09:49:18 PM »
Out of curiosity, how would the performance of methane in an NTR compare to that of ammonia?

It depends on the temperature.  At temperatures below about 3,500 K ammonia is better, and above methane is better.  This is because ammonia very easily decomposes into hydrogen and nitrogen, lowering the average molecular weight of the gas.  There is not a lot of methane decomposition until the temperature approaches 3,000 K, and over 4,000 K there is very little methane remaining.  Once dissociated, methane is the better propellant.  Up to about 3,000 K, the specific impulse of methane is about 90% of ammonia.

(edit)... I retract what I just wrote - my previous calculations (from a couple years ago) included a big problem.  I included in the exhaust products gaseous carbon rather than solid carbon.  With solid carbon in the calculations, it looks like methane will decompose much more readily.  This changes the outlook considerably and makes methane look like a very good propellant.  I'll post the data as soon as I complete the calculations.
« Last Edit: October 26, 2010, 12:18:18 PM by Bob B. »

Offline Bob B.

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Re: Fluorine
« Reply #44 on: October 26, 2010, 12:11:01 AM »
It looks like methane is the answer.  I'm upset I didn't included methane in our earlier discussion, but because of the mistake I made a couple years ago, I dismissed it because I thought it was worse than ammonia.  At first glance, methane looks like it should be a pretty good propellant, but I never checked or questioned my old results until now.  Specific impulse wise, methane is the second best propellant I've looked at.

Propellant:  Liquid Methane
Propellant flow rate:  115.54 kg/s
Reactor power:  2,000 MW
Combustion chamber temperature:  2,500 K
Combustion chamber pressure:  59.6 atm
Exhaust molecular weight:  5.396
Specific heat ratio:  1.09
Nozzle throat diameter:  27.63 cm
Nozzle exit diameter:  174.8 cm
Expansion ratio:  40:1
Thrust (vacuum):  731.5 kN
Specific impulse (vacuum):  646 s
« Last Edit: October 26, 2010, 08:51:14 AM by Bob B. »