What exactly is the fuel plug on RATTWorks Tribrid?

I hope I'm not venturing too far into research/experimental territory with this question.

The background of the question is that I might someday want to fly a tribrid here in Germany or build something similar or retrofit an existing hybrid, and it would be next to impossible to import anything containing solid rocket fuel from the US.

I understand that the RATTWorks Tribrid https://www.rattworks.net/research_tribrid.html uses two U/C valves, a small one (1/8"?) for the alcohol and a large one (5/16", 3/8"?) for the nitrous. Before filling, the alcohol tank is plugged in the U/C valve using a "fuel plug", which will burn through shortly after the larger fill tube has burned through and the plastic fuel grain has been ignited.
What exactly is this fuel plug? Is it a solid fuel rod, or is it a nylon tube containing solid fuel at one end? What kind of fuel is it? Or is it actually just a plug made from plastic which will simply burn away due to the heat created by the burning nitrous and plastic fuel?

Thanks in advance
Oliver

Oliver

There is no rocket fuel in the rocket. The "solid propellant" is an ABS plastic tube, so the motor is totally non-hazmat for shipping purposes.

A tribrid is really a liquid fueled motor with a hybrid starter. The most difficult operating phase in a bi-propellant motor is the ignition. If there is any appreciable ignition delay, the combustion chamber is likely to overpressure and rupture. In the tribrid, the motor is ignited as a hybrid and then liquid fuel is injected into the combustion chamber. Since the motor has already ignited, there is no ignition delay of the alcohol making the liquid phase start a non-event.

https://www.rattworks.net/docs/rattworks_k350-curve.pdf shows the thrust curve. The thrust doubles when the alcohol ignites and the hybrid transitions to a liquid fueled motor at that point as the N2O-alcohol reaction kinetics is much faster than the N2O-ABS reaction kinetics and the ABS becomes the combustion chamber liner.

Any low molecular weight alcohol should work as the liquid propellant. As menthol is toxic, I would suggest that ethanol or isopropanol are more environmentally friendly. From as safety viewpoint, alcohol fires are readily put out by water unlike gasoline/kerosene fires which require foam so a major concern about a N2O/alkane fuel fire is eliminated.

Bob

Thanks, Bob, I understand that the Tribrid is, for the beginning of the burn, a "regular" hybrid with the ABS tube as fuel and the nitrous as oxidizer.

I was wondering more specifically about the plug that closes the U/C valve of the alcohol tank, see e. g. the thin tube in https://www.rattworks.net/images/tribrid_composite.jpg (the bottom left pictures) and the thin white rod/tube in https://www.rattworks.net/images/tribrid_valve.jpg

Oliver

Raketenolli said:

Thanks, Bob, I understand that the Tribrid is, for the beginning of the burn, a "regular" hybrid with the ABS tube as fuel and the nitrous as oxidizer.

I was wondering more specifically about the plug that closes the U/C valve of the alcohol tank, see e. g. the thin tube in https://www.rattworks.net/images/tribrid_composite.jpg (the bottom left pictures) and the thin white rod/tube in https://www.rattworks.net/images/tribrid_valve.jpg

Oliver

Click to expand...

It's a plastic tube that burns away. Hollow and open on the tank side, if I remember correctly.

CoyoteNumber2 said:

It's a plastic tube that burns away. Hollow and open on the tank side, if I remember correctly.

Click to expand...

That makes sense, thank you.

Oliver

The following link is helpful:

https://rattworks.net/tribrid_tutorial/index.html