From what I've heard, hard-starts are a very common, particularly in less refined amateur rocket engines, and the only way to avoid them is to either start the engine on an extremely low flow rate or use a particularly effective igniter, possibly similar to the ones used on the space shuttle, resembling a sort of blow-torch.

My question is: would it be feasible to lower the O/F ratio so much as to stop the fuels detonating, and is there a limit to how far the O/F ratio can be lowered to where there will be no ignition to speak of?

Additionally, would this prevent a hard start or simply prolong/lessen its severity?

  • $\begingroup$ "to start the engine on an extremely low flow rate" would be a waste of fuel. For lift off you need more thrust than the weight of the rocket. $\endgroup$ – Uwe Nov 10 '20 at 10:18
  • 2
    $\begingroup$ I think he's saying that you then ramp it up. $\endgroup$ – ikrase Nov 10 '20 at 20:40
  • $\begingroup$ The SSME started with a low MR - the "lead hydrogen" is why they had the "sparklers"- so it's certainly possible. More for reason of temperatures than hard start though AFAIK. $\endgroup$ – Organic Marble Nov 25 '20 at 3:13
  • $\begingroup$ Related: Why did the Agena leak oxidizer when starting its engine? $\endgroup$ – Russell Borogove Nov 25 '20 at 3:51

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