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Hybrid rocket engine usually used rubber or plastic as fuel but why not sugar? Just like many amateur solid rocket motors used sugar. What are the advantages of using rubber in hybrid engine over sugar?

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2 Answers 2

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A supplement to TrySCE2AUX:

  1. Is it even possible to use sugar in a hybrid rocket? According to Clarks' Ignition, the engineering of workable hybrid rocket engines is very complicated and tricky to get right (much worse than solid fuel or liquid fuel). Getting the solid fuel (called a "grain") to erode and burn properly tended to dominate fuel engineering, and the book describes major efforts as focusing on more plasticky or rubbery materials rather than sugar which also has the issues of being brittle, meltable, sticky, and hygroscopic. Lately one hears more about waxy materials. So it is an open question whether sugar would even be workable.

  2. Are carbohydrates a better fuel than other options? Sugar can be approximated as CH2O, meaning that the hydrogen is "pre-burned" and therefore cannot generate energy when burned with oxygen the way that a hydrocarbon such as rubber (closer to CH2) will be able to.

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    $\begingroup$ Those are interesting insights. We've seen how hard it is with Spaceship two. I thought about mentioning that the solubility of sugar will probably play no role in this because the contact time is too short. But did not think about the hygroscopic nature uf sugar... imagine a rocket is on the pad and liquified sugar is dripping out of the nozzles after a few days of delays (or bees starting to feed on the propellant). $\endgroup$
    – TrySCE2AUX
    Jul 18, 2022 at 9:50
  • $\begingroup$ The cost would be an interesting factor... would sugar or the other stuff be cheaper in those quantities? $\endgroup$
    – TrySCE2AUX
    Jul 19, 2022 at 6:31
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    $\begingroup$ I think that other propellants are cheap enough, and the simple materials cost of fuel is a small enough factor, that it probably barely matters. $\endgroup$
    – ikrase
    Jul 19, 2022 at 6:49
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Short answer: Rubber gives you better performance

Longer answer:

We first have to find out, why some propellants are used in different situations. There are different parameters that matter. Just from the top of my head:

  • performance (Specific impulse)
  • use case (i.e. can it be switched off or throttled)
  • price (propellant and rocket motor)
  • availability
  • handling
  • storeability
  • igniteability

Now we have to ask, where sugar stands in there:

Information for hybrid (liquid / solid rocket motors) is not easy to find. I'm therefore taking the performance figures of solid/solid propellants as a guide and assume that the comparison will hold up (since energy density and rectivity of the components is more or less the same).

In the "Performance" section of the wikipedia article about Rocket Candy:

Sugar based rocket propellants have an average Isp(specific impulse) of between 115 and 130 seconds. Compare that to the average Isp of an APCP (Ammonium perchlorate composite propellant), which is 180 to 260 seconds. Sorbitol and KNO3 based propellants with a typical 35:65 ratio are capable of an Isp of between 110 and 125 seconds. However, sorbitol and KNO3 rockets with additives have been recorded as having specific impulses of up to 128 seconds.[4]

Now, the answer is already in this text but for good measure let's see what NASA has to say about specific impulse of different propellants. They write in "Astronautics and its Applications" in Chapter 6, Propellants.

Propellant Family Propellant combinations Isp Range (sec)
Monopropellants ( liquid ):
Low-energy monopropellants 160 to 190
Hydrazine
Ethylene oxide
Hydrogen peroxide High-energy monopropellants:
Nitromethane 190 to 230
Bipropellants (liquid):
Low-energy bipropellants 200 to 230
Perchloryl fluoride-Available fuel
Analine-Acid
JP-4-Acid
Hydrogen peroxide-JP-4 Medium-energy bipropellants 230 to 260
Hydrazine-Acid
Ammonia-Nitrogen tetroxide High-energy bipropellants 250 to 270
Liquid oxygen-JP-4
Liquid oxygen-Alcohol
Hydrazine-Chlorine trifluoride Very high-energy bipropellants 270 to 330
Liquid oxygen and fluorine-JP-4
Liquid oxygen and ozone-JP-4
Liquid oxygen-Hydrazine Super high-energy bipropellants 300 to 385
Fluorine-Hydrogen
Fluorine-Ammonia
Ozone-Hydrogen
Fluorine-Diborane Oxidizer-binder combinations ( solid ):
Potassium perchlorate:
Thiokol or asphalt 170 to 210
Ammonium perchlorate:
Thiokol 170 to 210
Rubber 170 to 210
Polyurethane 210 to 250
Nitropolymer 210 to 250
Ammonium nitrate:
Polyester 170 to 210
Rubber 170 to 210
Nitropolymer 210 to 250
Double base 170 to 250
Boron metal components and oxidant 200 to 250
Lithium metal components and oxidant 200 to 250
Aluminum metal components and oxidant 200 to 250
Magnesium metal components and oxidant 200 to 250
Perfluoro-type propellants 250 and above

And what we see here is, that the specific impulse of all of those propellants is significantly higher than sugar candy.

So sugar is readily available and relatively easy to handle. That's important for amateur rocketry but not so much in professional settings.

So there would have to be other huge benefits of sugar over other propellants to make it interesting to use. And they just seem to be not there.

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