Hybrid Rocket Engine

[MxPropulsion]

I've been designing and testing hybrid rocket engines for a while now, the following video is a GOX-Paraffin rocket engine running at 50 psi and 25 LPM. Estimated thrust is 6N (my test stand broke and I'm waiting on a new sensor). The convergent section of the nozzle is sub-optimal. The nozzle is made of machined graphite and is intentionally overexpanding the flow (hence the obvious flow separation). I've been having issues with combustion instability and I was wondering if anyone could give me any pointers.

View attachment Forum video.mp4

I will continue to test, next with a sea-level nozzle and will post any updates.

 

[Steve Shannon]

I've been designing and testing hybrid rocket engines for a while now, the following video is a GOX-Paraffin rocket engine running at 50 psi and 25 LPM. Estimated thrust is 6N (my test stand broke and I'm waiting on a new sensor). The convergent section of the nozzle is sub-optimal. The nozzle is made of machined graphite and is intentionally overexpanding the flow (hence the obvious flow separation). I've been having issues with combustion instability and I was wondering if anyone could give me any pointers.

View attachment 421707

I will continue to test, next with a sea-level nozzle and will post any updates.

I would reach out to Arif Karabeyoglu at Stanford.
https://wiki.stanfordssi.org/Arif_Karabeyoğlu_-_SPG,_Hybrid_Rockets

 

[AlphaHybrids]

A lot of the combustion instability looks to come from the paraffin melting and being ejected through the nozzle. Try a grain where you use 50% paraffin, 50% melted hot glue sticks (high melt)

Edward

 

[jderimig]

50 psi sound kinda low.

 

[rocket_troy]

50 psi sound kinda low.

Yeah, that's very low. It's theoretically high enough to achieve choked flow but it doesn't look like that's happening. Looking at the diameter of the CC, one does jump to the impression that GOX (as in oxidizer flux rate, not gaseous oxygen) would be so low that there's likely some dripping going on in the CC - that's assuming there's also a large accommodating core. Also, at such low chamber pressures, it's generally difficult to achieve stable combustion in rocket motors although using gaseous oxygen does help with that.

 

[MxPropulsion]

Yeah, that's very low. It's theoretically high enough to achieve choked flow but it doesn't look like that's happening. Looking at the diameter of the CC, one does jump to the impression that GOX (as in oxidizer flux rate, not gaseous oxygen) would be so low that there's likely some dripping going on in the CC - that's assuming there's also a large accommodating core. Also, at such low chamber pressures, it's generally difficult to achieve stable combustion in rocket motors although using gaseous oxygen does help with that.

Thank you! In regards to the chamber pressure although it's at 50psi I'm running 25LPM or GOX so I believe I choked flow is happenings, as evidence by how excruciatingly loud the motor is.

 

[jderimig]

Thank you! In regards to the chamber pressure although it's at 50psi I'm running 25LPM or GOX so I believe I choked flow is happenings, as evidence by how excruciatingly loud the motor is.

With a low chamber pressure small pressure fluctuations with cause fluctuations in your oxidizer flow. You are running 25LPM average, what is the standard deviation around that 25?

 

[SpaceEggs]

Low chamber pressure and using paraffin appear to be the issues here. Paraffin's okay for amateur hybrids, but waxes suffer from low grain integrity and low melting points. You can actually see all the unburned paraffin droplets being ejected and combusted in the exhaust. First though, I would focus on getting your chamber pressure up to where you have consistently choked flow.

Just looked at what forum I'm in - does this belong in the Research forum?