In my experience with hybrid rockets, the igniter is always triggered prior to or concurrent with the oxidizer flow into the chamber (which already has fuel in it, hopefully lit). This is to prevent igniting a chamber full of both propellants instantaneously - i.e., hard-starting the engine.

Similarly, the RS-25 (SSME) uses a fuel-lead start (confirmed in a comment by OrganicMarble, and also by this document (which I got from from theradicalmoderate's answer here)).

However, I also know that at least a couple of engines currently in use have a LOX-lead ignition sequence, specifically in order to prevent a hard start. I can't find a public source on this at the moment; any help there would be appreciated. I also welcome the possibility that I'm misinformed. The explanation I've generally heard is that LOX-lead prevents a hard start or 'detonation' of the propellants.

Sutton (9th edition) notes in Chapter 8.6, "it is difficult to exactly synchronize the fuel and oxidizer feed systems so that propellants reach the chamber simultaneously from all injection holes or spray elements. Frequently, more reliable ignitions are assured when one of the propellants is intentionally made to reach the chamber first. For example, for fuel-rich starting mixtures the fuel is admitted first." He also mentions this AIAA paper (98-3204) but it's paywalled and I'm not that frustrated yet. Chapter 11.4 of Sutton also replicates the sequencing summary of the RS-25 startup linked here.

I've had a hard time articulating the question so I'll pose it in multiple parts, and hope that's ok since it should just be a singular answer. What is the mechanism by which too much of an ox or fuel lead would cause hard start or 'detonation'? Aside from the 'fuel lead for fuel-rich' method Sutton mentions, how is it decided what the sequence for an engine will be? Am I misinformed about LOX-lead in modern engines, or is Sutton's above rule not universal?

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    $\begingroup$ I can confirm that SSMEs led with H2. That's why they had the "sparklers" or ROFIs at the pad. space.stackexchange.com/a/35991/6944 $\endgroup$ Feb 17, 2020 at 1:14
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    $\begingroup$ @OrganicMarble Cool thanks. I edited slightly to remove the uncertainty there. $\endgroup$ Feb 17, 2020 at 1:22
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    $\begingroup$ I've been thinking about this and I can't think of any way to answer it that you haven't covered in the question! Except for examples of lox-lead engines. $\endgroup$ Feb 17, 2020 at 18:57
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    $\begingroup$ The only one I'm really familiar with is SSME and that was fuel-lead because they were super concerned about ever having a high mixture ratio. $\endgroup$ Feb 17, 2020 at 20:36
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    $\begingroup$ are they lox-lead to avoid a hard start, or are they single-propellant-lead to avoid a hard start, with lox as the choice? $\endgroup$
    – user20636
    Feb 20, 2020 at 12:42

1 Answer 1


I think this has to be evaluated for each propellant combination and situation individually. I was involved in a horizontal ground test program at AeroAstro in the mid 90's for a LOX/RP-1 (kerosene) engine that was designed to be LOX lead to start. After a very short burn was computer aborted due to an accidental trigger of the "high thrust" alarm, the next burn ended up hard starting the engine because there was some unexpected fuel pooled in the chamber from the abort. It turns out that the pooled Kerosene, when chilled to cryogenic temperatures, can become explosive in the presence of LOX. This was something most of had not heard of except for some of the real old guys who had worked with RP-1 for a long time. There's an old NASA paper describing some detonation testing that had been done for RP-1/LOX here: NASA Detonation Research Thus when the engine was restarted, the pool cooled when the LOX from the next start hit it (which according to our gray beard can cause RP-1 to form a gel with LOX) and then our igniter lit and the gelled mix detonated. But we were using LOX to regeneratively cool the faceplate of the injector so we wanted the pre-chill that the LOX lead gives, so we kept this timing and changed our inspection criteria between runs. BTW, our "High Thrust" alarm was triggered because we initially set this very conservatively at just 20% over MEOP. We ended up having some small combustion instabilities which weren't completely eliminated with the first revision resonator cavities we drilled, so we saw ~ 30% pressure spikes above MEOP. The system was designed to take 3x MEOP for this early test, so we had room to move our trigger to 50% and that's what we did for the second run.

For a NTO/Hydrazine engine we recently tested at Stellar Exploration, we elected to run with an NTO lead because spilled hydrazine is harder to clean up/decontaminate than NTO, which readily evaporates and is dispersed.

At a third organization, we were testing a LOX/methane engine and had elected to use LOX lead for the ignition to avoid building up potentially explosive methane gas in the test cell. That engine still found a way to blow up, it just wasn't due to methane buildup in the test cell.

  • $\begingroup$ These are great examples. $\endgroup$ Feb 17, 2020 at 23:01
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    $\begingroup$ These are good. Thank you. I'm going to hold off accepting because I'm hoping for more info on the "what is the mechanism" part of the question. You might actually be able to address that if you can tell me where that "high thrust" alarm came from. I'm coming around to accepting that there's not a short list of first-principle, intuitive rules to answer this. $\endgroup$ Feb 18, 2020 at 1:40

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