The Germans have developed a gel-fuel rocket motor, and are getting ready to flight test (paid subscription required).

I’ve not worked with the technology, but I did work with someone who did work with liquid propellant artillery, and I imagine that the tech is rather similar.

Basically, it allows for a monopropellant rocket, much like solid fuel rockets, with the ability to easily throttle the motor by varying the flow of the propellant into the combustion chamber.

Additionally, since it’s the consistency of toothpaste, or at least it was in the LP gun, it means that if the tank is punctured, you do not get leakage.

The Bayern–Chemie demonstrator uses a single gas generator designed to support a range of missions. It is used to drive a piston that forces the gelled propellant and, in a bi-propellant mode, oxidizer, into the combustion chamber. The gel passes through a valve to regulate its flow, and then through injectors that effectively turn it into a liquid as it enters the combustion chamber to optimize the burn. The motor will use a pyrotechnic ignition system.

An entirely non-explosive configuration has also been conceived. It would replace the solid propellant gas generator with an air pressure reservoir and the ignition would be triggered through a gas burner.

It’s more complex and bulkier than solid fuel motors, but should have interesting applications where a complex thrust profile is required.