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Have there been rocket engines or even test engines that have used diesel as a fuel? If so, what oxidizer had they used and what ISP did they achieve? If not, is it possible to say what ISP is at least theoretically possible?

Also, if available, what ISP was achieved using hydrogen peroxide as the oxidizer, and what fuel mixer was likely used?

Kozuch
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R. Hall
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    I slightly reworded your question because ISP is very dependent on the details of an engine design. It's not possible to say what is the ISP of a fuel, we can only say what ISP actually achieved or theoretically possible. – uhoh Sep 02 '20 at 02:20
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    @uhoh cheers makes more sense now. thanks for the tip about fuel ISP as well – R. Hall Sep 02 '20 at 02:26
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    Related: why use kerosene? – Russell Borogove Sep 02 '20 at 03:28
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    they are going to be basically identical to RP-1. – ikrase Sep 02 '20 at 04:48
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    Comparing diesel to RP-1 is a bit like comparing a grass-weave loincloth to an EVA suit, or asking why we haven't considered using leather, horn, or bronze instead of kevlar to shield the ISS. Diesel is just a blind dart-throw away from crude muck out of the ground - RP-1 is an insanely engineered and refined product that is technological leaps and bounds beyond the sophistication of plain diesel. We don't use stealth bombers to drop heavy rocks on people either, and nobody has considered it because we already have technology that is vastly superior to heavy rocks. Same with rocket fuels. – J... Sep 02 '20 at 12:08
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    I immediately flashed back to Salvage-1 which used a cement mixer as the crew capsule. https://nostalgiacentral.com/wp-content/uploads/2018/11/salvage1-002.jpg – Organic Marble Sep 02 '20 at 12:39
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    @J... RP-1 is carefully formulated to avoid forming solid deposits at high temperatures and to have well defined low temperature behavior, but it's fundamentally just a more controlled mix of heavy hydrocarbons, and is going to be almost identical to RP-1 in performance. Something like the SpaceX Kestrel (pressure fed, ablatively cooled combustion chamber and nozzle, no turbopumps or cooling channels to clog) could probably run without modification on diesel. – Christopher James Huff Sep 02 '20 at 13:54
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    @ChristopherJamesHuff It could maybe run, sure, but it would make a terrible mess and the engine would certainly need to be completely overhauled before using it again. It would also incur a much higher risk of failure, disaster, etc, for no reason whatsoever. You could probably rig a diesel engine to run on whale oil too - but that doesn't mean that anyone has seriously considered doing it, or that it's even remotely a good idea. – J... Sep 02 '20 at 14:14
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    @J... "ablatively cooled combustion chamber" and "using it again" don't often go together. As for "seriously considered diesel fuels", they were originally conceived to run on coal powder, and modern diesels have been run on a slurry of coal powder in water. – Christopher James Huff Sep 02 '20 at 14:32
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    @ChristopherJamesHuff SpaceX is all about efficiency and cutting costs. As a rhetorical question - if they could run diesel in the Kestrel, at 1/20th the cost of RP-1, why don't they? As for coal slurry, I'd argue that's also a highly engineered product - much moreso than diesel. The particle size must be controlled to nanometer scales and the ash content also must be very carefully controlled. It's not a technological step backwards to a less-refined, less-controlled product. – J... Sep 02 '20 at 14:35
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    @J... even for the Falcon 9, fuel costs are negligible, and we're talking about only replacing the fraction of the Falcon 1's fuel that's in the upper stage. One of SpaceX's biggest ambitions is to get Starship operation costs low enough that fuel cost matters. Also, at the time they were flying the Kestrel, they were still trying to get payloads into orbit, and in the end they only did so twice (only once with a paying payload) before moving on to the Falcon 9. They had bigger fish to fry. – Christopher James Huff Sep 02 '20 at 15:23
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    @ChristopherJamesHuff The point of all of these rhetorical questions is to point out the numerous ways in which considering diesel is a bad idea - you save very little and you take on an array of problems that the more highly refined and engineered RP-1 has already solved. It's a pointless step backwards to even consider, so people don't. – J... Sep 02 '20 at 15:59
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    How about Jet A, then? JP-8? More expensive than diesel but not by a lot. Also, running diesels on SVO/WVO is commonplace. – Harper - Reinstate Monica Sep 02 '20 at 17:18
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    @Harper-ReinstateMonica But why Jet A? Why go backwards? It's still a less-controlled soup of hydrocarbons that adds nothing but problems, risk, and contaminants. Why would you try to solve the engineering problem of burning crude oil in a diesel engine when you already have diesel? Why would you put JetA in a rocket when you already have RP-1? It just doesn't make sense. What do you hope to achieve? – J... Sep 03 '20 at 10:53
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    @J... cost, and consolidation down to fewer fuel types... – Harper - Reinstate Monica Sep 03 '20 at 14:21
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    @Harper-ReinstateMonica But fuel cost is not really an issue. So why go solving a problem that isn't a problem? Especially when that solution introduces risk and requires a massive engineering effort to now re-solve previously solved problems. Seems a lot like reinventing the square wheel to me. – J... Sep 03 '20 at 14:27
  • @Harper-ReinstateMonica Early kerosene rockets often were meant to use jet fuels. RP-1 was made as an improved alternative. These were generally military weapons and there was significant logistics pressure to make them share fuel with jet aircraft, though it was eventually settled for using a special fuel that could be burned by jet engines. – ikrase Dec 26 '23 at 01:44

1 Answers1

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Apparently, at least one OTRAG rocket test used diesel. OTRAG's intended fuel was kerosene with a nitric acid/$N_2O_4$ blend for oxidizer, so I would guess they used a similar oxidizer with diesel.

Most large rocket engines pass the fuel through tubes surrounding the combustion chamber for cooling; normal kerosene and other common hydrocarbon fuels tend to "coke" (polymerize) and block the cooling channels and/or partially vaporize, either of which creates hot spots, promoting more coking and/or vaporization causing a runaway thermal failure. RP-1 is a specification for narrow-cut kerosene that minimizes these problems, and is widely used in modern rocket engines (along with the similar Russian formulation RG-1). At the other extreme, diesel is more prone to coking than kerosene, making it unsuitable as rocket fuel.

Also, if available, what ISP was achieved using hydrogen peroxide as the oxidizer, and what fuel mixer was likely used?

The liquid propellants table from Wikipedia has a few entries with hydrogen peroxide, getting a few percent less specific impulse with the same fuels combusted with LOX. The highest-performing peroxide combination given there is with a hydrazine/beryllium mix, about 403 sec (3954 m/s exhaust velocity) vacuum specific impulse; I don't know if that was ever actually fired on a large scale.

Note that peroxide is denser than LOX, so you get some small additional benefit in smaller tankage, smaller structure, thus less drag, so the difference in total launcher system performance is smaller than the direct $I_{sp}$ comparison.

The only large rocket I know of that used peroxide was Black Arrow, which combusted kerosene with 85% peroxide/15% water. Hydrogen peroxide can be challenging to handle and store at higher concentrations, though its reputation is probably worse than it deserves.

Russell Borogove
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    Very high concentration hydrogen peroxide is denser than LOX, but as an oxidizer, one O2 molecule is equivalent to two H2O2 molecules. You're effectively carrying 9 kg of water for every 8 kg of oxygen, so you'll need more than double the mass in hydrogen peroxide, and the tanks needed are nearly twice the size. – Christopher James Huff Sep 02 '20 at 14:11
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    If that were the case, would you not expect the mass-specific impulse figure to be significantly lower? – Russell Borogove Sep 02 '20 at 16:12
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    Those numbers are higher than I'd expect, yes. A small part would be due to the decomposition heat of the HTP...that might be more significant at the low chamber pressure given. Part of it may be that you can run closer to stoichiometric with the water diluting your oxidizer (a larger oxidizer tank or smaller fuel tank). Note that the O:F ratio for LOX:N2H4 is 0.94, for H2O2:N2H4 it is 2.05...about 2.2 times as much oxidizer mass for the fuel mass. – Christopher James Huff Sep 02 '20 at 16:44
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    As a monoprop, peroxide can yield ~190 sec by itself, then you get the liberated oxygen and heat to combust your fuel on top of that. Don't think of it as carrying water, but as carrying high-pressure steam. – Russell Borogove Sep 02 '20 at 17:21
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    Wow, I think I like the idea of beryllium rocket fuel only slightly more than launching nuclear fuel on rockets – llama Sep 02 '20 at 19:09
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    @llama compared to other commonly used propellant components such as hydrazine and red fuming nitric acid, beryllium is only slightly hazardous. :) – Ryan C Sep 03 '20 at 22:29
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    @RyanC yes, but at least those are much nicer after combustion, instead of spreading finely divided beryllia powder all of the place – llama Sep 03 '20 at 22:45