'Hypermag' propellant concept
I've been looking at highly-metallised fuel grain concepts for hybrids and I think I've come up with something unexpected. I call it "Hypermag", short for "hybrid peroxide magnesium".
The oxidiser is 85% HTP, and the fuel is Elektron magnesium-aluminium alloy (I've assumed 90% Mg 10% Al w/w), mixed in various ratios with a hydrocarbon binder (for calculations I've used paraffin, as that's what CEA supports). At high metal loadings, it may be preferable to introduce molten hydrocarbon into a metal matrix rather than trying to slurry metal powder into the hydrocarbon. Isp peaks at 70% metal in the fuel and density impulse peaks at 80%. Even a pure-Elektron fuel grain (assuming you can get bulk metal, rather than powder, to burn smoothly) has an Isd about 10% higher than neat paraffin. There are some potential operational advantages to these highly metallised fuel grains, such as the mechanical strength of Elektron, and its thermal conductivity (which should improve regression rates and may also make chamber heat available for regenerative cooling, driving thermal decomposition of peroxide to power pumps if that's your jam), but the main benefit is in some interesting properties of the propellant chemistry.
When run rich, the Mg strips the oxygen out of water (the magnesium-steam reaction), producing a significant amount of H₂ in the exhaust (about 70% of all the hydrogen in the propellants ends up in this form). It's also reducing enough that the carbon in the paraffin goes to CO, rather than CO₂ (the reverse water-gas shift reaction). These light diatomic molecules make for a γ that's surprisingly good for something that's half ceramic smoke by mass. When run lean, these equilibria shift to favour H₂O and CO₂, and the increase in energy release is almost exactly balanced by the decrease in nozzle efficiency. This is especially noticeable with the 70% metal fuel, whose Isp peaks at 1.2 O/F but is less than a percent lower at 3.0. (Tc is 3100K in both cases.) This almost complete insensitivity to mixture ratio is handy for a hybrid (where O/F is often poorly controlled), and may even improve stability (e.g. by eliminating mixture-mediated Pogo).
Massive caveat: this is all based on CEA simulation results. Two-phase flow losses from all the condensed MgO could be higher than predicted, and whether the metal particles actually burn to completion or just coat themselves in an oxide layer can only be settled by experiment. But it certainly feels worth investigating! Has it been looked at before?
Performance calculations (O/Fs chosen for max Isp, except the extra 70% row) with Pc=30bar, Pe=0.75bar, shifting iac:
| Paraffin% | Elektron% | O/F | ρ (g/cc) | Isp (s) | Isd (s·g/cc) |
|---|---|---|---|---|---|
| 100 | 0 | 8.25 | 1.321 | 270.22 | 356.87 |
| 50 | 50 | 4.4 | 1.358 | 278.67 | 378.46 |
| 40 | 60 | 3.0 | 1.370 | 280.65 | 384.37 |
| 30 | 70 | 1.2 | 1.393 | 283.06 | 394.29 |
| 30 | 70 | 3.0 | 1.396 | 280.92 | 392.20 |
| 20 | 80 | 1.8 | 1.434 | 278.37 | 399.22 |
| 10 | 90 | 2.4 | 1.462 | 270.62 | 395.68 |
| 0 | 100 | 2.6 | 1.492 | 262.16 | 391.17 |
Exhaust composition (70% Elektron, 1.2 O/F, by mass): 45% MgO, 26% CO, 14% H₂O, 8% MgAl₂O₄, 4% H₂, 3% CO₂. (Notice that there's no Al₂O₃ in the exhaust; it all goes to spinel.)
The other interesting thing to note is that this is very specifically a propellant for the dilute peroxide. If you sim the same thing with pure H₂O₂ oxidiser, the Isp gain over paraffin is negligible (peak is a little over 1% at 50% metal). Performance of the 100% Elektron fuel is almost identical with both oxidisers (with pure HTP: O/F=3.0, Isp=264.02, Isd=391.37). Conversely, if you use 50% strength peroxide, where hydrocarbon fuels basically give up (Isp=199, O/F=14), hypermag keeps on chugging (258s for 70% metal, 257s for 100%). And that 70% metal is running at O/F=0.9 where (to within a rounding error) all of the hydrogen in the propellants ends up as H₂ (more of it winds up as CH₄ than H₂O, and there's even a bit of C₂H₂ formed. That is one seriously reducing flame). The limiting case of this is the magnesium-water rocket, which in theory is possible, but I couldn't get CEA to converge using Elektron fuel or liquid water. With pure Mg and 375K steam it gets Isp=257.61, Isd=323.03 at 1.1 O/F.