ALITEC Propellant

[RobertH3]

https://aerospaceamerica.aiaa.org/year-in-review/improving-efficiency-and-fuel-ignition/
https://www.purdue.edu/newsroom/rel...-million-contract-to-extend-firing-range.html
Cheers / Robert

 

[Syclone]

Interesting. By the pictures on their website, it looks like they were doing test firings on the Aerotech test stand.

https://www.adranos.com

 

[prfesser]

From their web page and an Aerotech post, it would appear that they are using an aluminum-lithium alloy both to produce higher specific impulse and to scavenge the chlorine from AP. I do wonder how effective that scavenging is.

Best -- Terry

 

[aerostadt]

Some more information is here. This article talks about reducing hydrogen chloride, which is toxic and corrosive. The article claims performance increases. Both aluminum and lithium are bound to create solid particles, which cannot contribute to thermodynamic gas expansion.
https://aerospaceamerica.aiaa.org/year-in-review/improving-efficiency-and-fuel-ignition/

 

[AeroTech]

Yes, AeroTech/RCS partnered with Adranos in the development testing of Alitec. AeroTech/RCS has an exclusive license to use Alitec in hobby rocket motors. I personally assisted them in formulating a version of Alitec that was castable for their flight demonstrations.

Interesting. By the pictures on their website, it looks like they were doing test firings on the Aerotech test stand.

https://www.adranos.com

 

[Johnly]

Did the lithium component decrease the D-ISP or was there sufficient performance increase to offset the low density of the lithium component?

 

[AeroTech]

I don’t remember the exact numbers but they conducted flight tests using an RMS-98/7680 motor and the Alitec propellant outperformed an otherwise identical propellant with aluminum only.

Did the lithium component decrease the D-ISP or was there sufficient performance increase to offset the low density of the lithium component?

 

[AeroTech]

Additional information of note is that I originally formulated a version of Alitec propellant using our now-patented no-plasticizer binder system. Unfortunately the lithium-aluminum alloy reacted with the tin cure catalyst (not isocyanate) and caused foaming. We decided to revert to a standard MDI/R-45 binder with no catalyst and it worked fine.

Interesting. By the pictures on their website, it looks like they were doing test firings on the Aerotech test stand.

https://www.adranos.com

 

[Nytrunner]

Is this one of those propellants that is only worthwhile in larger motors where the metals can burn properly in chamber?

 

[Walter Longburn]

AeroTech/RCS has an exclusive license to use Alitec in hobby rocket motors.

So are you planning to start making and selling motors with this propellant to us hobbyists? Does this propellant have higher ISP than Propellant X?

 

[aerostadt]

It would be interesting to run the formulation through a thermochemical simulation like NASA-Lewis and see the theoretical results.

 

[prfesser]

It would be interesting to run the formulation through a thermochemical simulation like NASA-Lewis and see the theoretical results.

I just ran a quick n dirty PROPEP 3.0 on a hypothetical 10% Li, 10% Al, 64% AP, 14% R45, 2% E744. Isp is 250-259. Virtually the same result was obtained by removing the lithium and adding another 10% aluminum.

Density of the lithium-containing mixture was quite low, 0.0498 lb/in^3. With all-aluminum it's 0.063.

Since Aerotech has indicated significantly-higher performance from ALITEC, a working hypothesis is that the Li in the alloy aids in combustion of aluminum inside the chamber rather than outside. That is consistent with the flame shown on the Adranos web page; high-aluminum propellants usually produce a fairly bright white flame, from aluminum particles that exit the motor before combustion is complete.

The chemistry that reduces HCl in the exhaust is going to be interesting. The chlorine *has* to go somewhere (rocket flame is hot, but not hot enough to transmute an element ). PROPEP shows a great reduction---three orders of magnitude!---in HCl in the exhaust. It appears that much of chlorine in the exhaust would be in the form of gaseous Li2Cl2.

Best -- Terry The Generator Of SWAGs. (scientific wild-@$$ guesses)

 

[aerostadt]

Maybe the amount of free hydrogen in the exhaust might be going up, lowering the average molecular weight in the exhaust increasing gas exit velocity.

 

[AeroTech]

It's very likely that combustion efficiency would be improved over straight Al, especially in motors less than 54mm diameter.

Is this one of those propellants that is only worthwhile in larger motors where the metals can burn properly in chamber?

 

[AeroTech]

Only if the cost of the alloy powder can be reduced significantly. It's currently at astronomical levels. I'm going to run the Isp program and see what it says. But it outperformed X in a flight test.

So are you planning to start making and selling motors with this propellant to us hobbyists? Does this propellant have higher ISP than Propellant X?

 

[AeroTech]

Yes.

Maybe the amount of free hydrogen in the exhaust might be going up, lowering the average molecular weight in the exhaust increasing gas exit velocity.

 

[AeroTech]

I think the alloy being used is something like 80% Al, 20% Li. Adranos showed that the alloy particles explode during combustion due to the difference in boiling points between the two metals, facilitating combustion efficiency as a result of the smaller metal droplets. Yes, the chlorine gets bound up as lithium chloride.

I just ran a quick n dirty PROPEP 3.0 on a hypothetical 10% Li, 10% Al, 64% AP, 14% R45, 2% E744. Isp is 250-259. Virtually the same result was obtained by removing the lithium and adding another 10% aluminum.

Density of the lithium-containing mixture was quite low, 0.0498 lb/in^3. With all-aluminum it's 0.063.

Since Aerotech has indicated significantly-higher performance from ALITEC, a working hypothesis is that the Li in the alloy aids in combustion of aluminum inside the chamber rather than outside. That is consistent with the flame shown on the Adranos web page; high-aluminum propellants usually produce a fairly bright white flame, from aluminum particles that exit the motor before combustion is complete.

The chemistry that reduces HCl in the exhaust is going to be interesting. The chlorine *has* to go somewhere (rocket flame is hot, but not hot enough to transmute an element ). PROPEP shows a great reduction---three orders of magnitude!---in HCl in the exhaust. It appears that much of chlorine in the exhaust would be in the form of gaseous Li2Cl2.

Best -- Terry The Generator Of SWAGs. (scientific wild-@$$ guesses)

 

[G_T]

An additional benefit may be reducing or eliminating the tendency for burning Al to form its own impermeable shield of Al2O3 protecting the melted aluminum droplet from further easy combustion. That may be another source of improvement, poisoning that protective shell, reducing the required residence time. Likely other alloying constituents could do the same thing. I know of at least one that I think would work, but it isn't nearly as good a fuel as Li.

Sorry, I haven't read the link yet.

Gerald

 

[AeroTech]

I ran the two propellants on the Air Force Chemical Equilibrium Specific Impulse Program. First, here are the thermodynamic properties of a standard 70% AP, 16% Al and 14% HTPB binder formulation:



And here are the properties of the same propellant but with an 80/20 aluminum & lithium mixture substituted for the straight aluminum:



There is a decent increase in Isp with the ALITEC propellant, but the density is reduced. Not sure if this is true when using the alloy itself, the program is calculating the density as if you were using discreet aluminum and lithium powders.

Now look at the exhaust products of interest, first for the standard propellant:



Then for the ALITEC propellant:



In the ALITEC propellant the H2 is slightly increased and the HCL is substantially reduced.

These are theoretical numbers of course. There may be more going on here, and I suspect the actual delivered Isp is higher with ALITEC because of increased combustion efficiency.

 

[dhbarr]

https://en.wikipedia.org/wiki/Aluminium–lithium_alloy#Crystal_structureEvery 1% by mass of lithium added to aluminium reduces the density of the resulting alloy by 3% up to the ~4% solubility limit. After that it seems you get different phases with weird immiscible crystal structures. Presumably the melting temperature is also lowered at some percentage, and probably more again with some Mg in the mix.

EDIT: Yup something like 1/20 at. Li 1/3at. Mg drops you a couple of hundred degrees F

 

[boatgeek]

An additional benefit may be reducing or eliminating the tendency for burning Al to form its own impermeable shield of Al2O3 protecting the melted aluminum droplet from further easy combustion. That may be another source of improvement, poisoning that protective shell, reducing the required residence time. Likely other alloying constituents could do the same thing. I know of at least one that I think would work, but it isn't nearly as good a fuel as Li.

Sorry, I haven't read the link yet.

Gerald

One of the benefits along that line listed in the article was that the lithium alloy in the alloy tended to vaporize, blowing up the droplet of melted metal into smaller bits. So less poisoning the shell than blasting it away. That might also indicate that the lithium is crystallized independently of a true solution.

Take all of this with a large grain of salt. I'm an engineer (mostly structural), not a materials scientist and definitely not a propellant expert.

 

[0011001100]

That might also indicate that the lithium is crystallized independently of a true solution.

It must be because from what I found you can't dissolve that much lithium into the aluminum.

I wonder if any of the other alkali metals would work, like sodium or potassium. They would be denser and both of them have lower boiling points thus more likely to exist in a gaseous form though that would depend on the compounds formed.

 

[G_T]

Na and K may have advantages for dealing with the Cl in an environmentally friendly fashion. I haven't run the numbers to see if that is how the combustion would work out. I'm not likely to, because...

- if there is enough Na or K in there to result in an alloy that does not form a good enough Al2O3 shield at room temperature, it will be hypergolic with air at STP (Standard Temperature and Pressure). Quite the opposite of a material suitable for a solid rocket propellant. Ie, one should strive to meet the requirements for Stable Munitions, particularly for hobby motors. If one isn't familiar with the concept, look it up.

Note we are getting pretty far into this for not being in the Research forum.

Gerald

PS - There is another common alloying element which I think would be a better choice for various reasons, and be cheap and safe. But not on this forum.

 

[RobertH3]

I don't know a whole lot about the subject and don't plan on getting into research motors. Learned a bit from posting the article though!

 

[prfesser]

Of the alkali metals, lithium may be the only choice. Although sodium and potassium are much cheaper and more dense, they're also much more reactive. I would not want to test an Al-Na or Al-K alloy in any binder nor in close contact with an oxidizer. Rubidium is even worse (or better, depending on how you like chemical reactions). Cesium...well, at a hundred bucks a gram...I don't think so.

This demo of alkali metals reactivity was posted by the Royal Society of Chemistry so it's legitimate.


Best -- Terry

 

[DaHabes]

Chemistry was one of my favorite topics in high school because 1) you got to do experiments where you see the results in real time ( and sometimes things went BANG!) and 2) my brother-in-law was my teacher. The downside of the latter was that after that class until I graduated I prepared all of the chemicals for his classes after school hours. Actually an upside, though, as I got rides to and from school and didn't have to ride the damn school bus every day.

 

[aerostadt]

Even though lithium and aluminum form solid compounds. which do not contribute to thermodynamic gas expansion, some of the simplified calculations for propellant performance include an average molecular weight based on those compounds. Surprisingly, those calculations sometimes have better predictions than the more complicated two-phase flow CFD calculations. There are some equations in "One-Dimensional Two-Phase" that use the molecular weight of these solid compounds. Obviously, on this basis the light molecular weight of lithium is attractive.

 

[G_T]

Makes sense.

To add to the fun, those using high metals in small motors should be using unreactive aluminum in the computations (for aluminum fuel) at an appropriate percentage of the total aluminum, to compensate for lack of residence time (skipping all the details here). Then they'd see that using too much aluminum in a small motor can be less than ideal, compared to replacing that mass with, say, AP. How much aluminum you can get away with is very size dependent. Adding Li to the aluminum seems to be a viable approach to make the propellant work in smaller motors without sacrificing quite so much performance. For fixed size motor tubes, Density ISP is not a bad choice of metric for comparison, rather than straight ISP. Skipping burn rate related caveats affecting geometry choices, of course. So it is not quite that simple.

Actually, "not quite that simple" probably applies to any realm of rocket science more often than not.

Of course lots of "effects" motors deliberately use too much metal, to help achieve the desired visual result. But for those motors, the "go" part is not worth as much as the "show" part!

Gerald

 

[MClark]

We made propellants for ejection seat rocket/catapult motors. Density ISP was what we were after.
Lead nitrate as primary oxidizer was used in production motors but a green alternative was what we were seeking. Came up with a replacement with same D-ISP and same burn rate using AP and micron size Zr. Run PEP increasing Zr way past scary.
Works but has manufacturing issues

 

[G_T]

Yep, dust very flammable.