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The concept of hybrid rocketry is very interesting, and seems like it has many advantages over its solid and liquid forebears. But having only a superficial idea of the challenges preventing it from mainstream use (namely combustion instability and adverse grain regression rates) and not wanting to fall for a case of "what you see is all there is", I must ask what else there is that is preventing hybrid rocketry from being used for more large scale applications, instead of just suborbital joyrides and small scale amateur endeavours.

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    $\begingroup$ I suspect the answer has more to do with institutional culture than technical challenge. $\endgroup$ – Jerard Puckett Mar 12 '14 at 16:20
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    $\begingroup$ You might find this High Performance Hybrid Upper Stage Motor (PDF) document interesting, it's a good read with lots of information relevant to your question in it. $\endgroup$ – TildalWave Mar 13 '14 at 16:06
  • $\begingroup$ Thanks TidalWave, after reading that it seems like the combustion instabilities have been solved, and paraffin based fuels are now the holy grail of hybrid fuels due to their high regression rates. But I still see organizations continuing to use HTPB even with its complex multi-port requirements. I know that the article was written by some of the SPG founders, but are paraffin based fuels not actually as great as they're said to be, or are they just difficult to manufacture? $\endgroup$ – InquisitiveInquirer Mar 25 '14 at 7:34
  • $\begingroup$ One of the challenges remains the oxidizer to fuel flow rate as the oxidizer path through the fuel grain expands. I've asked a question about the possibility of using additive manufacturing techniques and the use of 3D grain profiles to improve O/F flow rate and more consistent burn rate as the solid core fuel is consumed, hopefully it'll produce some interesting insight into that. $\endgroup$ – TildalWave Mar 31 '14 at 14:51
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    $\begingroup$ You have to remember that since you can't regulate the fuel flow, any throttlability comes at cost of heavily fuel-rich, non-optimal combustion. Hybrids have a definite advantage over SRBs that they can be shut down. But they are bad at throttling down partially. $\endgroup$ – SF. May 29 '18 at 8:22
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According to this recent article by one of the Space Propulsion Group partners, paraffin-based fuels give hybrid rockets more oomph, because the fuel that is exposed to combustion melts, atomizes, and becomes entrained in the flowing oxidizer [inset]. This enlarges the surface area over which the fuel can vaporize and react.

Wax Fuel Delivers More

More fuel in the mix, faster burn, higher thrust. This does seem to address the problem with slow grain regression seen with other hybrid fuels. Since this makes possible single channel combustion, the problematic multi-channel design can be avoided (thus more fuel in the cylinder & no chunks of fuel flying loose during burn). Working with NASA Ames, they've hot fired hybrids with LOX and nitrous oxide. Recently they have been testing a 56-cm motor capable of 100,000 newtons of thrust.

However, they are still subject to low-frequency combustion instabilities, along with high-frequency instabilities common to all kinds of rockets. It really flickers like a candle, as you can see in this video:

The SPG website is here.

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  • $\begingroup$ It's visually flickering, but does that also correspond to fluctuating thrust? $\endgroup$ – Vedant Chandra Jul 13 '15 at 10:23
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Just a quick response here, but from doing some hybrid engine research as part of a student organization I think there is also the issue of layers beneath the actual boundary layer of wax melting and causing some instabilities with the grain. I believe additives such as carbon black help this issue by lowering radiation being transmitted through to lower layers of the wax, but I'd imagine on larger rockets it also presents itself with just the sheer size of the fuel grain. I'm not entirely sure on this, however, and am still a student.

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IMO it's just pretty new. Liquid fuel rockets have the advantage of the wealth of knowledge generated during the space race. Liquid fueled rocket engines are a pretty much solved problem with respect to science. The difficult/expensive part is engineering and manufacturing. If you already know liquid fueled engines then hybrids don't bring much to the table. However, the real advantage to hybrids is their accessibility to amateurs and startups.

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    $\begingroup$ Sources to back up your claims of accessibility would make this answer better, for instance potential startups/small groups who have successfully integrated the hybrid designs into a project. $\endgroup$ – Magic Octopus Urn Jul 18 '18 at 14:24

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