# Have there been rocket engines designed for diesel? What ISP theoretically possible?

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?

• 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 2 at 2:20
• @uhoh cheers makes more sense now. thanks for the tip about fuel ISP as well – Reuben Farley-Hall Sep 2 at 2:26
• Related: why use kerosene? – Russell Borogove Sep 2 at 3:28
• 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 2 at 12:08
• @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 2 at 13:54

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 this problem, widely used in modern rocket engines, but at the other extreme, diesel is more prone to coking, 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.

• 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 2 at 14:11
• If that were the case, would you not expect the mass-specific impulse figure to be significantly lower? – Russell Borogove Sep 2 at 16:12
• 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 2 at 17:21
• Wow, I think I like the idea of beryllium rocket fuel only slightly more than launching nuclear fuel on rockets – llama Sep 2 at 19:09
• @RyanC yes, but at least those are much nicer after combustion, instead of spreading finely divided beryllia powder all of the place – llama Sep 3 at 22:45