Arunn's Notebook

Nano-aluminium and Rocket Science

Arunn Narasimhan

Let us talk rocket science. And explore the curious case of the conjecture of using nanometer sized aluminium particles to save rockets from exploding.

A particle of metallic aluminium can be of any desirable size. In the beginning of this post let us have it in microns. These aluminium particles are used in rocket propellants (fuel) for two reasons.

The first one, which is apparent, is that since they are metals, while they burn along with the propellants at over 4,100 K and releases a large amount of energy that significantly expands the gases within the combustion chamber. The expanding combustion gases within a fixed volume results in pressure increase leading to higher exit velocity for the exhaust gases that escape through the nozzle. This results in increased specific impulse of the motor.

The second one is less apparent and a bit more involved.

While burning, these aluminum particles turn into alumina droplets inside the combustion chamber of the rockets. These particles are carried along with the burnt gases that provide the thrust for the rocket - and come out from the rocket as smoke that we see.

Meanwhile, the burnt propellants in gaseous from that are ejecting out of the combustion chamber cause acoustic oscillations, which affect the entire rocket in many ways. For instance, the rocket motor by default has a natural frequency of vibration and if these acoustic oscillations match their frequency, the resulting resonance could cause irrevocable damage. Solid rockets suffer from unsteady acoustical waves that may resonate within the combustion chamber and cause pressure spikes [1]. Further, these acoustic oscillations may lead to the propellant getting burnt very strongly and quickly (faster rate of combustion) ending up at times in the explosion of the rocket nozzle (flanges will fall apart due to excess internal thrust). These acoustic oscillations also could mess with the electronic controls of the rocket leading to their malfunction and disrupt the guidance and control of the rocket.

The second reason for using aluminium particles with rocket propellants is that the resulting alumina particles mix in the gaseous propellant exhaust. They interfere with the traveling acoustic waves mentioned earlier and dampen them, resulting in a steadier, uniform, burning of the propellant.

Figure 1: Cutaway drawing of a solid rocket motor (courtesy [1])

But there are enough caveats in this. For instance, these alumina particles render the exhaust as two phase flow (gaseous flow with solid alumina particles). This two phase flow is locally “dragged” by the solid alumina particles. The resulting locally lagging flows (from the main flow of the exhaust) lead to reduced thrust (momentum loss) for the rocket motor.

Further, when the aluminium particles burn, they don’t do it in tandem with the propellants. Once the propellant begin to burn, the aluminium particles begin to melt a while later upon coming into contact with the flame released by the propellants and they form a sort of a big flame ball fusing many such aluminium particles together. The resulting oxidized aluminum bulk forms alumina droplets of a certain size, which while traveling with the exhaust, can reduce the acoustic oscillations.

But not always.

Just like a flautist uses a shorter flute to play a Jagadhanandhakaraka in Naattai raga at the start of the concert and a longer flute at the end, while playing, say, Jagadhodharana in Kaapi raga, larger the rocket motor, shorter their natural frequency. Hence the size of alumina particles to dampen the acoustic oscillations should be suitably tailored for a particular size rocket motor.

So the question one is left with is what should be the size of the original aluminium particles that are to be added to the propellants, so that after burning they would result in alumina droplets of suitable size that are able to dampen the acoustic oscillations of a particular rocket motor.

To answer this question, one should first answer the question of how pure the initial micron-sized aluminium particle should be.

These aluminum particles are very reactive with atmospheric oxygen, and a thin passivation layer of alumina (Al2 O3) quickly forms on any exposed aluminium surface. This layer envelops the micron size aluminium particle as a hard spherical shell of nanometer thickness (~ 5nm) and protects the metal from further oxidation. This alumina in the annular shell can exist in three forms, the a, b and g types - the former two in crystalline form while the latter g in amorphous form. Alumina in all of these forms is harder than aluminium and has a melting point a few times larger (~ 2300 C) than the aluminium metal (660 C) that resides inside the core.

Figure 2: Aluminum oxide particles photographed using a scanning electron microscope (enlarged 8000x) (courtesy [1])

If such an “impure” aluminium particle (of micrometer size) coated with nanometer sized outer shell of alumina is used in the propellant, while combustion, the aluminium will melt first, become a liquid with about 6 percent volume increase and try to come out by cracking the hard alumina shell. The successful liquid aluminium oozing out of the shell at random places will grow on the surface of the shell by further oxidation and form a bridge with a similarly behaving adjacent aluminium particle. This leads to a porous aluminium/alumina bed, which then burns in the flame (sintering) resulting from the combustion and forms the fireball that was mentioned earlier, resulting in the alumina droplets.

In the above scenario, the efficiency and size of the alumina droplet formation is determined by how much of the aluminum actually comes out of the original “shell” and was able to burn to form the alumina particles of a particular homogeneous size. For instance, if not enough aluminium coalesce in the first place to form the porous bed, the end result of alumina will be of one size - which may not be the desired one to control acoustic oscillations.

As we saw earlier, the alumina can exist in three forms of variable hardness. So the oozing of the liquid aluminium from the shell of alumina could be a random event, as where exactly on the surface the shell will crack depends on the composition of the alumina.

We can appreciate now the importance of the purity of the aluminium particles that are to be mixed with the propellants. As far as possible aluminum particles with an outer shell of alumina are to be avoided (although this is impossible on Earth) so that one doesn’t have to address the issue of random cracking and insufficient aluminum during combustion.

But, for instance, this was not known to rocket engineers when they started practicing these technologies and by 1967 NASA US (military?) lost a rocket because of insufficient damping of acoustic oscillations - a result of mismatch of alumina droplet size and rocket motor size, which in turn is a result of impure aluminium mixed in the propellant in the first place. A simple case of giving the contract for preparing the aluminium to two companies, neither of them being aware of the implications of the issue and preparing aluminium particles by solidification of liquid aluminum with two different cooling rates, resulting in particles with a shell of alumina of markedly different compositions (think of amorphous versus crystalline). The 1967 mishap was explained only by 1994 after understating the entire process of how aluminium and alumina particles dampen acoustic oscillations.

Now, where does nanometer sized aluminium come into the picture?

In the original micrometer sized particle, there will be around a million aluminum atoms. In the nanometer sized particles this number is reduced to about a few hundreds. Such an agglomeration results in asymmetric atomic level forces, preventing strong oxidation of the aluminium particle. In other words, in a nanometer sized aluminium particle, the alumina shell, a result of the inevitable oxidation, is absent rendered non-uniform and may be even absent in a few places on its surface (because of the non-uniform atomic force fields, becuase of only the agglomerate having only a few hundred atoms). This results in a chain of advantages.

For instance, there is direct immediate ignition of aluminum particles in the combustion chamber as there is no non-uniform protective alumina shell around it (so at “weak spots” on the surface of the aluminium particle where the alumina shell is minimal or absent, ignition chance is enhanced). So there is no sintering effect and no aluminium fireball formed as before. The aluminum particles burn and directly result in alumina droplets. Of course, the alumina size is still a variable but the factors that influence the formation of alumina are now minimized because of the use of nanometer sized aluminium particles. Further, the nanometer sized aluminum particles result in superior combustion purposes because of reduced melting point and direct ignition and complete combustion. The resulting alumina droplets even if are not in proper size - only micron sized alumina is useful for acoustic oscillation damping; not a nano-sized alumina droplet - it is still an advantage. One can inject separately micro-sized alumina particles directly into the combustion chamber to control acoustic oscillation.

We shall stop here exploring the case of the nano-aluminium saving rocket explosions. All this is in the research stage. But much of it involved years of patient experiments and hard work.

After all, it is not a detective story; just rocket science.

[1] http://www.aerospaceweb.org/question/propulsion/q0246.shtml

[I thank S. R. Chakravarthy, my colleague and collaborator from the Aerospace Department of IIT Madras for sharing this exciting story - one of his research interests - with me.]

© Arunn Narasimhan | Original version written ~ Feb 2007 | Last revision on Apr 01, 2012

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