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Great breakdown of these propellant combinations!
To answer your first question regarding small-sat launch vehicles:
Solid propellants like APCP definitely hold a massive advantage when it comes to ground operations, mechanical complexity, and launch readiness. For dedicated small-sat launchers, omitting complex turbo-pumps, plumbing, and cryogenic conditioning significantly reduces the initial R&D capital expenditure. However, the fixed thrust curve and inability to reignite make precise orbit insertion (especially for multi-payload rideshares requiring different orbital planes) much harder without a liquid secondary kick-stage.
For small launchers, I believe hybrid systems or simple pressure-fed liquid engines might bridge the gap better than pure solids in the long run.
Regarding small-scale turbopumps, the biggest thermodynamic issue is usually the Reynolds number effect and tip clearance losses. When scaling down turbomachinery:
Efficiency drops rapidly: Viscous losses inside tiny impellers become proportionally much higher.
Extreme Rotational Speeds: To achieve the required pressure ratios at small diameters, the pumps have to spin at extremely high RPMs (often exceeding 60,000–100,000 RPM), which severely degrades bearing life and complicates dynamic seal design.
It will be really interesting to see how additive manufacturing (3D printing with Inconel or copper alloys) continues to solve these geometric challenges for micro-turbopumps.
Curious to hear if anyone here has simulated small-scale pressure-fed vs. electric-pump-fed cycles for sub-500kg payload launchers!
