Hybrid motors

Hybrid Motors

Hybrid motors overcome some of the limitations of cost and storage of the AP motors, but introduce some drawbacks of their own. Because they contain no explosives, they do not require licensing to buy or use them, and they are much cheaper to run.

However, they are much more complex to light, are much larger and heavier for a given impulse, and are more expensive to buy in the first place.

Hybrids are technically any type of motor that uses more than one type of propellant, but for amateur rocketry this will inevitably refer to a solid fuel with a liquid oxidiser - usually plastic fuel and nitrous oxide.

Nitrous oxide (known variously as laughing gas, dentist gas or party gas) is not an oxidiser at room temperature, but disassociates into nitrogen and oxygen at 570 degrees Celsius. It is stored in pressurised bottles as a liquid, then the motor is filled immediately before flight. Once full, the trick is to raise the inside of the motor to this critical temperature, then start the flow of nitrous - chemistry and physics do the rest.

It is this complexity that makes hybrids so expensive to set up and more complex to fly, but since the plastic fuel grains and nitrous oxide are both quite cheap (one-third the cost of an equivalent AP reload, or less), they are economical to operate once all the ground support equipment, or GSE, has been purchased.

There are several ways to achieve this high temperature.

One common way is to introduce gaseous oxygen into the motor and use a high-voltage spark to start the fuel grain burning. The heat or thrust (depending on the motor) from this is used to start the flow of nitrous.

The need to have oxygen bottles and an extra solenoid valve and more plumbing raises the cost of the GSE even higher, but it is a reliable method.

The second way is to enhance the performance of a standard igniter by chipping off a small amount of AP from a solid motor fuel grain. This is popular in the USA, but has only recently been interpreted as legal in the UK. One drawback of this method is that pieces of AP can break off and block the nozzle, with catastrophic results for the motor.

A third way is to use a piece of plastic initiator cord (PIC) and a standard igniter or 'electric match', which is quite straightforward but not suited to all motors.

A fourth way is to use Pyrodex pellets, again lit by a standard igniter.

One additional complexity is that hybrid motors do not provide a separation charge, so a different arrangement needs to be made. Typically, an altimeter, timer or magnetic apogee detector would be used to fire a black powder or pyrodex charge to deploy a parachute.

demonstration hybrid motor 

Demonstration hybrid motor

Of course, it's not usually possible to stand close to a rocket motor when it's firing. With hybrid motors in particular, you need to be at least 30 metres away if the motor is in the open. However, that doesn't allow anyone to see how they actually work.

We have built a safe motor that has a see-through case and fuel grain that we can fire in the lab, and people can see exactly what's happening. This can be done because the motor operates at low pressure and under complete control.

We use this motor for demonstrations to students, and also to applicants at our open days. The motor is made from plexiglas (PMMA or polymethyl-methacrylate) and uses gaseous oxygen as the oxidiser. Strong bolts and careful calculations ensure the pressure stays where it is supposed to.