Lithium Nitrate / Sorbitol

Possible Propellant?



Before describing the lithium experiment here is some history of how my lithium questions came about. Earlier work with Strontium nitrate Sr(NO3)2 to give a red flame to KNO3/sorbitol motors was not successful. A large part of the problem was that once there was sufficient Sr(NO3)2 to give flame color the burn rate was too inhibited to provide thrust. Sr(NO3)2 does not seem to be an adequately active oxidizer for the purposes of motor thrust with sugar fuels. Heavy ash was also noted in the burn products. Some literature searching suggested this ash was Strontium nitrite Sr(NO2)2. This means that only one of the three oxygen molecules in the nitrate ion (two of the six Os in one Sr(NO3)2 molecule) went to combustion with the sorbitol fuel.
The literature suggests that better utilization of the Sr(NO3)2 oxygen release to fuel combustion can be obtained with higher burn temperatures. However the use of powdered metals to attain this can be a safety problem with nitrate oxidizers.
Looking at the periodic table one can see the difference between Sr and potassium (K) is that K is in the most active metals group: Group 1. But Sr is in the group 2 column.
For active nitrates the group 1 metals and the ammonium ion are the options of choice. Looking at the group one column we see H, Li, Na, K, Rb, Cs, and Fr.
There is our old friend K; we know KNO3 works great with sugars. Sodium is one periodic level smaller than K and it works as an oxidixer in NaNO3. It is very hygroscopic and so is not used as often as KNO3. One would think that the strong yellow sodium flame emissions would give a NaNO3/sugar motor a yellow flame but this was not the case in the work by Jimmy Yawn.
An interesting point Jimmy makes is that due to NaNO3's lower molecular weight it could theoretically yield a more efficient propellant as it would leave more room for fuel in a grain. (More on that idea later in this long winded rambling.)
The Rb, Cs and Fr ionic compounds are either too rare or too toxic or both. The H ion in combination with NO3 is nitric acid. This has been used in pure form (such as red fuming) as an oxidizer in professional liquid fuel rockets. It is horribly caustic and very dangerous. Then there is Lithium (Li)
It is directly above Na in the periodic table. Lithium is known for it's red flame color! A possible replacement for the inadequate Sr? Li does not give as bright of red as Sr but it might make a nice night launch propellant. However since sodium didn't provide yellow I doubt that Lithium is going to show emissions of red in a sugar motor.
There is another reason to pursue LiNO3 and that has to do with the molecular weight question. Same concept as the NaNO3 discussion above.
KNO3 has a molecular weight of 101 gms/mole LiNO3 has a molecular weight of 69 gms/mole. So to provide the same number of molecules of the nitrate ion (per sorbitol molecule) requires less oxidizer weight. This then gives a higher fuel % load in the grain.
Here is my math to calculate optimum fuel weight%
The mole weight ratio of LiNO3 to KNO3 is 69/101 or 0.68
So if 35 grams of sorbitol requires 65 grams of KNO3 to provide optimum amount of oxygen from the nitrate ions then it follows that 35 grams of sorbitol will need 65 times 0.68 or 44grams of LiNO3. So the LiNO3/Sorbitol formula would be 44/35.
Converting this to the standardized 100 gram total formula weight yields a LiNO3/Sorbitol ratio of 56/44.
Therefore the Li propellant contains more fuel by weight than the K propellant. For the same weight grain there will be 125% more fuel. But volume is important too. Propep says the density of KNO3/So is 1.84 gm/cc and LiNO3/So is 1.86 gm/cc.
Since the densities are almost identical we should be able to pack about 125% more fuel in the same size grain. However preliminary tests of actual grains by my preparation methods gave results that LiNO3/Sorbitol's density was about 85% that of KNO3! Yikes !
If this density difference holds true then for the same volume grain the Mole weight ratio advantage of LiNO3 will be diminished.

This all assumes that the nitrate in LiNO3 gives up it's oxygen identically to KNO3. This is probably reasonable since as mentioned Li and K are both in the group one metals.
But there could still be some differences as well. For example it is known that KNO3 is ionic but LiNO3 is partly polar as well. LiNO3 will dissolve somewhat in Ethanol. This partial polar attraction of the Li ion to the NO3 ion could affect the burn characteristics.

Propep gives the following results:

ISP 114.9
ISP* 134.1

That extra % of fuel seems to bump up the ISP like we hoped.

Another way to approach the optimum ratio of LiNO3 to Sorbitol is to use Propep in Multiple run mode to find the ratio of these chemicals that gives the highest predicted ISP.
Doing a multiple Propep run for LiNO3/Sorbitol gives a maximum ISP of 137.2 when using a ratio of 61/39
This same multiple Propep run approach for KNO3/Sorbitol gives a maximum ISP of 115.5 when using a ration of 69/31
Since the standard 65/35 KN/S works so well in practice I will use 65/35 for the KN/S, and 55/45 for the Li/S
On to the experiment attempts.

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