Designing and Buying Rockets suitable for Hybrids

Generally speaking a Hybrid motor is going to be a lot longer than a conventional solid motor based on APCP.  Rattworks uses a UC valve fill system which can create very long motors.  The M900 is 6 feet long.  That’s a long motor.

With the Rattworks motors you can take 3 basic approaches to designing your rocket.

The first is a minimum diameter rocket.  With a motor like the M900 this will tend to lead to designs with a L/d ratio of close to 50/1.  This is generally considered to be the limit as it can lead to large buckling loads being imposed on the airframe.  If you want to understand this take a long thin rod and press on the top of it while the other end is on a desk.  You press up to a point then it buckles. When this type of force is applied to your rocket it happens much more dynamically.  The rocket may experience a side wind as it moves through the atmosphere.  This could start the bend and the load on the nose would finish the job off.

This is one of the reasons l/d ratios over 50/1 have to be considered carefully.  Also small diameter designs are difficult to successfully deploy a parachute from.  There are ways to achieve this but you really need to test what you are going to do AND do what you test.

A high l/d ratio generally gives rockets with multiple calibres of stability.  This is a good thing isn’t it.  Nope is isn’t.  If your rocket is too stable then you can get big wind cocking problems easily.

The next at the opposite end is a design based around a narrow core.  You have the motor in the centre and arrange your deployment gear and chutes around it in a larger diameter design concentricly. This will result in a low l/d ratio.  The problem you now face is that the stability of a design is specified in calibres of stability with 1 being the minimum difference between your cp and cg.  the cg tends to go back on a Hybrid during flight as the Nitrous gets used.  This depends on where the cg of your motor was in relation to the cp of your motor airframe assembly to start with.  But the short of it is you need to know what is going to happen to your motor cg as it uses oxidiser.  Also your parachute and other bits are going to move backwards under thrust.  With a low l/d ratio this has the potential to result in an unstable flight.

The third is a hybrid rocket for Hybrids.  This is sort of an egg lofting design.  A large diameter tube on top of a slender motor mount tube for the motor.

I have gone for type 1 and 3 for my current fleet and Ron has gone for type 2 flying Hypertek

Whichever way you decide to go Ron can advise you and I’ve flown the odd Hybrid too

Norman Mcgeoch

TRA 12957 L3 ( all on hybrids)