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update:

This answer links to Larry Russell Kellogg which links to NASA Tests Environmentally Friendly Rocket Fuel where images of Test firing of new Paraffin-Based Fuel at NASA Ames Research Center 1/15/2003 can be seen.

enter image description here

For more on paraffin-based work see https://www.nasa.gov/centers/ames/news/releases/2003/03images/paraffin/paraffin.html

I've just read the NASA news item From Pedicures to the Peregrine Rocket, Paraffin Wax Proves Its Worth.

The project started when researchers at Stanford discovered that paraffin fuel burns three times faster than conventional fuels, and therefore can provide more thrust and higher performance than existing hybrid rockets. They approached Ames for assistance in measuring the burn rate at larger scales and to contribute to the maturation of the technology.

I think the "three times faster" means that a given volume (or mass) of the solid wax propellant in a given shape could be consumed in one third the time of a conventional solid propellant for hybrid engines, and so that factor of three speaks directly to thrust-to-weight ratio of the engine.

I'm not sure what conventional is, but I'm guessing it's one of the synthetic "rubbers" like HTPB.

Why would wax burn roughly three times faster than other fuels? And why the sudden interest now — is there some specific technical challenge that has been potentially solved allowing for greater thrust/weight, or is there a need for a low temperature robust hybrid rocket on Mars (see below and in link) and the factor of three is a secondary benefit?

The paraffin-based fuel also works under challenging environmental conditions, like the very low temperatures found on the surface of Mars. The Mars Ascent Vehicle, currently under development by NASA’s Jet Propulsion Laboratory, aims to use the paraffin technology to return a sample from the surface of the planet to an orbiting spacecraft.

CAUTION: lower your volume before playing...

An nice summary of hybrid fuel combinations was linked in this insightful answer, and some of the links in Does the NK-33 engine require subcooled kerosene so cold that it turns to wax? helps one appreciate that one can think of wax as being more like solid kerosene than it is like rubber.

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  • $\begingroup$ So- it's kind of like a solid rocket in that the surface area of the wax exposed increases thrust output while decreasing longevity? Or did I misunderstand what a hybrid wax rocket was? $\endgroup$
    – Magic Octopus Urn
    Jan 24, 2019 at 3:02
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    $\begingroup$ @MagicOctopusUrn With a low melting point, as well as a low boiling point (in the 350 to 500 C range depending on composition) I think that combustion in a paraffin (or even sugar) hybrid engine operates very differently than a solid. Remember that solids have the oxidizer incorporated within the matrix itself, so burn rate is determined by a different set of phenomenon. $\endgroup$
    – uhoh
    Jan 24, 2019 at 3:13
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    $\begingroup$ @MagicOctopusUrn have a good look at all the sources linked in answers and comments at What properties define a good solid propellant for a hybrid engine? (e.g. why not use wood?) $\endgroup$
    – uhoh
    Jan 24, 2019 at 3:15
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    $\begingroup$ Hobby rockets have been using parafin powder in e.g. epoxy (with N2O) for over a decade now (personally saw many of them fly). The idea is that the expansion of the parafin while melting causes the matrix to fracture on the surface resulting in an increased surface area. Moreover particles and molten wax are probably hurled into the oxydizer stream by this. I'm not aware of any hobbyist actually being able to verify or even publish about this but the bigger ablation rates are well known within the community (by looking at the wax/epoxy afterwards you can see the difference). $\endgroup$
    – Christoph
    Aug 8, 2019 at 7:05
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    $\begingroup$ @MagicOctopusUrn Hybrids feed a liquid into a solid. They are a lot simpler than a fully liquid fueled engine. $\endgroup$
    – Loren Pechtel
    Oct 3, 2022 at 2:32

1 Answer 1

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Wax/Paraffin has gained interest for the last ~5 years as a fuel of choice for Mars Sample Return because the unique requirements of this mission mean being able to withstand more thermal cycle extremes than most most missions, making it really tough for solids to be a solution. The small vehicle size and layout restrictions (carried by a rover) also make it hard for an all liquid design. Hybrids have some greater freedom in regard to form factor and have relatively good mass fractions in this range. Wax also has challenges in terms of cracking and separating from the liner under thermal cycling but they've done a lot of work in overcoming this. Personally I favor HTPB (rubber) as a fuel in this application due to better ability to withstand the temperature ranges without cracking, but I think the paraffin researchers got funding for this development earlier so they appeared to be further down the design road at the time when NASA needed to chose a technology path.

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    $\begingroup$ Thanks for your answer! I'm now wondering if sufficient electrical power were available the wax could be re-melted and solidified pre-flight to deal with cracking and separation from the liner. I suppose that it could be quite impractical; the wax would probably just flow out the bottom of the rocket. $\endgroup$
    – uhoh
    Dec 12, 2019 at 22:36
  • $\begingroup$ They've discussed this, but to keep the wax in the right annulus configuration one would have to keep the engine spinning to force it against the liner while it was melted and cooled. They have looked into adding electrical heaters to minimize the low temperature exposure and cut out a lot of the temp cycle range. $\endgroup$
    – Terrance Yee
    Dec 13, 2019 at 4:43
  • $\begingroup$ Wouldn't it be a lot easier to keep the wax warm by releasing a trickle of oxidizer? Chemical energy has a great energy density. You'd just need to put a thin foil with a small pinhole in the exhaust nozzle. $\endgroup$
    – MSalters
    Jun 19, 2020 at 11:53
  • $\begingroup$ Solar energy is still "free" so for the long duration the rocket may have to sit on the surface (nearly 2 years), this is a better long term trade to use solar powered electric heaters. $\endgroup$
    – Terrance Yee
    Jul 7, 2020 at 17:57

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