Can pyroxene and other silicon compounds be used as a theoretical spacecraft fuel?

Question

I have been reading this patent, which led me to this paper.

The first uses polysilane, which basically replaces the $\text{C}$ in a $\text{-R-C-R-}$ group, making an $\text{-R-Si-R-}$ group. I also read that carbon and silicon are quite similar in chemistry. The second paper goes into detail. We find that silanes burn with $\text{N}_2$ and produce solid silicon nitride.

My question :

1. How do you get thrust, if you are producing solid silicon nitride? Or is the trust coming only from the $\text{H}_2\text{O}$?

2. Please consider a common silicon compound: pyroxene : $\text{XY(Si,Al)}_2\text{O}_6$. It has silicon and aluminum in it, as well as oxygen. Can you burn similar common minerals as well to produce thrust?

3. Can we use solar energy to basically vaporize the silicon solids? How much delta-v can we realistically generate out of that?

Thank you.

Answer 1

When we talk about the combustion of elements in pure oxygen, per overall reactant mass, silicon is in the fifth place when comparing the highest oxidation states. Seen in descending order (Be>Li>B>Al>Si). Beryllium is very toxic and expensive and has potential application only in space, Lithium is a very reactive element with low density. Boron is expensive and does not burn completely so it is difficult to use its potential. Aluminum already has applications in rocketry especially in solid propellants.

Silicon offers a little less energy during combustion than aluminium, but it is four-valent and can bind more hydrogen than aluminum. Organo-silicone compounds (where polysilicon belongs) have a higher mass fraction of hydrogen than organo-aluminum compounds.

In addition, silicon belongs to the group of elements which can burn in nitrogen. This property also have boron and lithium. This is an additional benefit when oxidizers such as HNO₃, N₂O, N₂O₄ are used and in scramjet applications where fuel is burnt in atmosphere. In addition, there is an option to burn silicon in ammonia (NH₃) and hydrazine (N₂H₄). Although the energy of combustion is lower per unit of mass, hydrogen makes bulk of reaction gases, and it has the lowest possible molar mass. This is important for achieving high ISP.

Therefore, it is not surprising that there is interest in silicon and its compounds in patents. However, silicon has not been widely used. The big disadvantage is that pyrophoric gases such as silane (SiH₄), disilane (Si₂H₆) are used for the production of pure silicon and its compounds. This raises the level of safety necessary in production, which in turn means a higher price. At the end, this price increase is not justified by the benefits compared to aluminum and that is why aluminium is used much often.

As far of question regarding silicon nitride - yes propulsion is achieved buy gaseous combustion products and vapors - H₂, H₂O, N₂, NH₃, CH₄, etc. depending on composition of polysilane and oxidizer used. Silicon nitride heats these gases by releasing energy to surrounding. Silicon nitride particles have negative impact and, in general, reduce ISP. As particles grow larger, their negative impact on ISP increases.

Pyroxenes cannot be used as fuels or oxidizers because they are actually mixtures of highest oxidation states of Mg, Ca, Fe, Si, Al. For example, one pyroxene compound - diposide MgCaSi₂O₆ can be written as MgO ∙ CaO ∙ 2SiO₂. This means that elements are completely oxidized and there is no more energy to be used. At the other hand oxygen is strongly bonded to elements and it cannot be used to burn other materials. Exceptions are Be, Li, B because of their high combustion energies, but this mixtures belong to thermite mixtures not to propellants because of little to no gaseous products.

In theory, if you vaporize solid silicon compounds, including silicon nitride, you will get a small amount of propulsion impulse. This would be a kind of thermal rocket propulsion. But this is highly inefficient because solids have high vaporization energies (energy required to turn solids into vapors) and have large average molar masses. Much more efficient way is to use this solar energy to heat medium (metal or ceramic) and pass light gases, such as hydrogen and helium, through holes made in this medium.

Answer 2

Silicon atomic weight is 28, vs 12 for Carbon. The energy of the silane Si-H bond is 318, vs 412 for C-H (and 432 for H-H).

So probably not for rockets, maybe yes for a "green fuel" that leaves sand rather than CO2 as a waste product. (fine silicates do have issues as lung irritants).

Polyvalent metals, such as finely ground aluminum, do find use in solid fuels, as their oxidation is highly energetic. Their waste products are metal oxides.