OK, Time to start documenting.

Buzzards exciting adventure into Hybrids.

cant wait to see it go up dude should be very cool

Alllroight then, lets get started.

Why get involved in Hybrids when composite motors have come down in price so much.

Simple, flexibility.

What makes a Hybrid Motor go:

Hybrid rockets are no different to any other type of combustible model rocket. A fuel is converted via an oxidiser into a large amount of gas that is expelled through a nozzle to produce thrust.

In Blackpowder Motors, Charcoal reacts with an Oxidiser (Potassium Nitrate)  to produce volumes of gas that is then squeezed out of a nozzle to produce thrust. The sulfur is present to reduce the ignition temperature and is just consumed as a fuel in the normal course of events.

In Composite Motors, rubber and Aluminium reacts with an oxidiser (Ammonium Perchlorate and sometimes other stuff to change the colour of the jet and plume colour) to produce volumes of gas.

In Hybrid motors, PVC or some other type of fuel (HTPB, PBAN, Italian Sausage etc) reacts with Nitrous Oxide to produce volumes of gas.

The theory is basically the same for all the different type of motors. Ignite some stuff with an oxidiser, squeeze the result through a nozzle and you should get a rocket motor.

Why doesn't everyone get into Hybrids?

Cost is a major factor.

Hybrids require specialised ground support equipment and techniques.

You need:

• A Hybrid motor
  
• A container for the Nitrous Oxide
  
• Some way of getting your container filled with Nitreous Oxide.
  
• A rocket that will accept a hybrid motor.
  
• A way of connecting the Nitreous Oxide Container to the Motor so that it can be filled prior to launch.
  
• A way of controlling the release of Nitreous Oxide into the motor.
  
• A way of venting the gas in the motor if you decide that you don't want to fire the rocket (Things go wrong)
  
• A way of firing your primed Nitreous oxide filled rocket
  
• A way of making sure your rocket wafts slowly down to earth (instead of augering a hole into the ground and frightening the cows).
  
• Other things that will become patently obvious during the course of this thread.
  

So what is the cost factor advantage over composite motors?

Hybrids allow you to select the impulse of the motor you wish to launch simply by changing the injector size, casing length or fuel type for a particular launch.

What does this mean?

I have decided to use Contrail Hardware. Right or wrong I don't really care. You simple need to make a decision to fly Contrail, Rattworks, Aerotech or anything else that turns up. The principles are basically the same.

The Hardware I have purchased is 38 mm Contrail with a 28 and 36 inch casing. I will soon be adding a 48 Inch casing to inventory as well but there is actually no rush as at present as I am not certified to fly any of these motors.

I have also purchased 3 lots of reloads (Packs of three). Two PVC and one HP with 1/4 inch Injectors.

To launch an I-333 I will need:

• 36 Inch motor casing
  
• 12 Inch PVC Grain
  
• 1/4 Inch Injector
  
• 460cc Nitreous Oxide
  

Total cost:

• $15 USD Fuel Grain
  
• $15 AUD Nitreous Oxide
  
• $5 AUD Sundries
  
• $40 AUD Total
  

To launch a H-248 I will need:

• 28 Inch motor casing
  
• 12 Inch PVC Grain
  
• 1/4 Inch Injector
  
• 275cc Nitreous Oxide
  

Total cost:

• $15 USD Fuel Grain
  
• $7 AUD Nitreous Oxide
  
• $5 AUD Sundries
  
• $30 AUD Total
  

The important thing to notice is that the the amount (and cost) of nitreous oxide is the only variable.

If I had a 48 Inch casing then the cost to launch a J318 is:

• 48 Inch motor casing
  
• 12 Inch PVC Grain
  
• 1/4 Inch Injector
  
• 735cc Nitreous Oxide
  

Total cost:

• $15 USD Fuel Grain
  
• $22 AUD Nitreous Oxide
  
• $5 AUD Sundries
  
• $45 AUD Total
  

Quoted from Buzzard

Buzzards exciting adventure into Hybrids.

Go Buzzard some great information above keep it coming.

Quoted from star destroyer

Go Buzzard some great information above keep it coming.

Will do

So, looking at the above, If I have the right hardware available and only one PVC fuel grain, 1/4 inch injector and a tank of nitreous, I can choose to fly anything between a G-300 to a J-345 simply by using a different casing length and a different quantity of Nitreous. (Contrails have 16,20,28,36 and 48 inch casings for their 38mm motors).

Pretty flexible in other words.

So, what does the motor look like. Here is a picture of the motor hardware with a reload to give you an some idea.



From Left to right:

• Vent Tubing
  
• Top Vent/End Closure and Snap Ring
  
• Resistor Ignitors and O-Rings
  
• Engine Casing (28 Inch)
  
• Fill Tube (Long black thing)
  
• Bulkhead and injector (1/4 inch Prestoloc fitting)
  
• Fuel Grain (PVC 12 Inch)
  
• Graphite nozzle with snap ring and seal disk
  
• Thrust ring
  

When you buy a pack of reloads from Contrail it includes fill tubes, O-Rings, ignitors, injectors and vent tube.

I got mine from http://www.contrailrockets.com/38store.htm. I originally ordered it from Pratt hobbies but cancelled the order after several months as it had not been filled. Dissapointing. Contrail supplied it within two weeks via USPS but messed up the order and only supplied one pack of reloads instead of three. (Reloads come in packs of three so easy to see where the problem came from, next time I will order 2 or 4 packs of reloads to avoid confusion) Contrail have since completed the order at no extra cost and are fairly easy people to deal with.

They offer their 38mm Motors in different sets:

Complete Motors:
    (1) Motor Casing (Buyer Selects Length)
    (1) Bulkhead
    (1) Injector
    (1) Nozzle (User Selected Speed)
    (1) Top Vent Assembly
    (2) Snap Rings
    (1) Thrust Ring

This is enough hardware to fire a Hybrid rocket. You can purchase additional motor casings and nozzles if you require to give you more options.

Motor Systems:
    (1) Motor Casing (Buyer Selects Length)
    (1) Bulkhead
    (1) Injector
    (3) Nozzles (Slow, Medium and Fast/X-Fast)
    (1) Top Vent Assembly
    (2) Snap Rings
    (1) Thrust Ring

This the same as the complete motor but has a full set of nozzles. I didn't get this one as I could not see a full use for the nozzles supplied. In hindsight probably a better way to go as at the moment I only have one nozzle (graphite) and if I drop and break it I'm out of the game. I will correct that before I go flying, promise.

Complete Motor System
    (5) Motor Casings (16, 20, 28, 36 and 48 Inch Casings Included)
    (1) Bulkhead
    (1) Injector
    (3) Nozzles (Slow, Medium and Fast/X-Fast)
    (1) Top Vent Assembly
    (2) Snap Rings
    (1) Thrust Ring

This one has the full set of motor casings and nozzles. I couldn't justify the 16 and 20 inch casings so price difference didn't come under buying separately.

I only ordered a 36 Inch 38mm Motor Hardware set with an additional 28 Inch casing.

So for $450 AUD delivered, I got the engine hardware and 9 reloads.

Reloads come in the mail???????

Hybrids have another big advantage over composite motors.

Until put together and charged with gas, they are completely inert. No hazardous material problems.

(Not 100% true as to ignite the motor you need a Pyrodex slug or a piece of AP propellant. A G-64 motor supplies about 5 starters.)

Even the Nitreous Oxide isn't a big problem to carry in your car. (as long as you don't hook it up to the fuel system in some states or leave the valve open)

Speaking of Nitreous Oxide, lets look at how I am going to cart it around.

Originally I was going to use a BOC D size bottle and get the gas from BOC. The deal I was offered was pretty good. No bottle rental and gas for $22- a kg. Excellent, but I do some work for BOC and was basically offered a staff deal which the normal person is not going to get. There are also some problems with the deal. Allow about a week to get your cylinder exchanged and you have to use a BOC bottle without modification. BOC D bottles are not ideally suited to Hybrid motors. The biggest issue is that they have a flow restrictor installed to prevent them being used in car applications.

So, the next best option is to use your own bottle and get it filled at a performance car centre.

What type of bottle to use. There are three candidates for bottles. Standard N20 bottles from performance car centres (Medium cost), CO2 bottles from brewery supply shops (Low Cost) or air tanks from dive shops (high cost of buying new). These bottles all have the same if not better pressure capacity and all fall under the same ratings. The problem is that when you take your CO2 or dive tank to the filling station you might get a cold stare. Some shops I talked to flatly refuse to fill anything other than a standard NOS bottle.

So looking at CO2 bottles. These are available through Ebay for around $180 delivered for a 1.9 kg bottle. (filled with CO2 that you need to dispose of sensibly). The local brew supply sells the same bottle for $205 if you want one today. They also have a 2.5 kg bottle for $275-. The local performance shop wanted $495 for a 10 lb NOS bottle (4kg).

For my use I felt the 1.9 kg bottle was too small (not for the immediate future but certainly for 12 months time) and I found a NOS bottle from here http://www.vpw.com.au/product.asp?CatID=788&PrdGrpID=2548&ProdID=68835 for $340 AUD delivered. For my money I felt that for the extra $70 for a bottle that no-one is going to have a problem filling and the extra 1.5kg capacity was a no-brainer.

Here it is:



The local drag shop will fill it for $14 a pound and the one in Rockingham for $12 a pound. (In Melbourne there is a home fill up company that comes to you and charges $5 a lb ($80 delivery fee mind you))

The drag shops will also simply top up bottles (for the cost of whatever you need) whereas BOC give no refund on unused quantities.

So buzzard, one thing that has always confused me is how people always refer to the PVC Fuel Grain. Does the PVC compound act as an oxidiser and actually gets consumed in the burn of the motor (so you have to replace the fuel grain each time yes?)

Quoted from Dogzilla

So buzzard, one thing that has always confused me is how people always refer to the PVC Fuel Grain. Does the PVC compound act as an oxidiser and actually gets consumed in the burn of the motor (so you have to replace the fuel grain each time yes?)

You might have to replace the fuel grain each time.

Lets look at how you fire this thing with a bit of explanation as to what happens at firing.

You park your NO2 bottle and ground support equipment next to the launch pad. Fairly close actually as you don't want a long fill line running between the support equipment and the motor.

The ground support equipment is just an electrically operated solenoid valve which allows it to be opened remotely. (Two solenoids in a good setup as the other allows you to vent the NO2 in case of a misfire)

The motor is assembled as follows:

• The injector (Prestoloc fitting) is inserted into the bulkhead.
  
• The fill line is attached to the Injector
  
• A preheater grain (Pyrodex pellot or slug of AP) is attached to the fill line right next to the injector.
  
• An Ignitor is secured to the outside of the preheater grain.
  
• The fill line is wrapped in Aluminium Insulation tape from the preheater slug to out past the nozzle.
  
• The fill line and igniter line is passed through the Fuel grain and the bulkhead made to rest on top of the fuel grain.
  
• The above assembly is slid into the motor casing with lots of grease.
  
• The nozzle and seal disk is inserted into the motor casing
  
• A Snap Ring is inserted into the inside groove in the motor casing to hold the nozzle in place. (You don't need to worry about everything being squashed together properly as filling the nitrous tank will take care of that. Thats why Contrail motors are referred to as a floating bulkhead type)
  
• A thrust ring is installed to the outer groove on the motor casing.
  
• The vent assembly is inserted into the other end of the motor
  
• a Snap ring is inserted into the top inner groove to retain the Vent Assembly.
  

The motor is now assembled and looks like a big metal tube with a plastic fill line and an ignitor lead hanging out of the nozzle.

To install it into the rocket:

• Vent tube is fed into the vent aperture and down the motor mount out the end of the rocket.
  
• The vent tube is attached to the vent assembly on the motor.
  
• The motor is installed into the rocket pushing the vent tube forward as it is inserted.
  
• The motor retention system is fitted
  
• The vent line is trimmed flush with the side of the airframe.
  

The rocket is now ready to go.

Install on the pad, connect the ignitor and fill line, arm any electronics on the ground support equipment and rocket and retreat to the firing station.

So far, little difference in assembly time between a composite and hybrid rocket.

To fire the rocket:

• Warn everyone you are firing (same as any type)
  
• Trigger the remote solenoid to begin filling the motor
  
• Wait for a jet of gas from the vent line to indicate that the motor is full. (This is the extra bit for Hybrids over composites)
  
• Close the remote solenoid, count down and push the fire button.
  

What happens next. The ignitor fires and hopefully ignites the preheater grain. Its in an enclosed space so will ignite as per a normal composite motor. The preheater grain starts to ignite the PVC fuel grain and as it burns down will burn through the fill line allowing the Nitrous Oxide to flow into the combustion chamber. We want the fill line to rupture as close as possible to the injector. Aren't you glad you wrapped the rest of the fill line in Aluminium tape to stop it burning through at the other end of the motor!

At 500 Degrees C, Nitrous Oxide breaks down into pure oxygen and nitrogen gas. The Oxygen accelerates the burn of the fuel grain and pressure builds up in the combustion chamber and produces thrust through the nozzle sending the rocket into the air. The chamber pressure will often exceed the pressure in the oxidiser tank temporarily closing the flow of gas into the combustion chamber and shutting down the motor, as the combustion chamber pressure drops, gas flows once again and the motor re-ignites and the cycle repeats. This cycle is very rapid and produces the buzzing noise heard on most Hybrid motors.

Interestingly it is possible to alter the Nitrous flow during flight on some types of motors giving you throttle control if required.

The motor burns until the Nitrous Oxide is consumed, from there on in its just like a conventional rocket in terms of ballistics and recovery.

So PVC is the fuel and Nitrous Oxide is the oxidiser.

Upon recovery and disassembly of the motor.

• The trimmed vent line is useless (too short) so throw it away.
  
• The injector is badly burnt, replace it with a new one.
  
• The fuel grain will be partially consumed and more at the injector end than the nozzle end. It is possible to reverse the grain and use it again in many cases.
  
• The fill line should be lying at the launch pad but too badly damaged to be re-used. If not there, it may have been consumed as fuel within the motor.
  
• The nozzle should be OK to be re-used many times.
  

BTW, The PVC Fuel grain is nothing more than a bit of PVC Pipe. Nothing too scientific about it at all.

PK has supplied me with these cutaway diagrams of what it looks like inside a prepped motor.

This is the cross section of the bulkhead, injector, fill line, ignitor, fuel grain and preheater slug.



This is the cross section of the completed motor. This motor is slightly different from a contrail motor in that it has a siphon vent which vents through the piston, but the idea is very much the same.



Kudos to PK for his help in getting this thing set up by the way. The poor guy has suffered an endless torrent of dumb questions (often the same one over and over) and without whos help and input I would be miles behind.

yer talked to PK on the phone the other day he seems like i nice guy its good to share information across the clubs it helps us all  

Quoted Text

The chamber pressure will often exceed the pressure in the oxidiser tank temporarily closing the flow of gas into the combustion chamber and shutting down the motor, as the combustion chamber pressure drops, gas flows once again and the motor re-ignites and the cycle repeats. This cycle is very rapid and produces the buzzing noise heard on most Hybrid motors.

In large motors this is a low frequency buzz or rumble:

This spectrum is from an L class 450ish N motor. You can clearly see the resonance at 120Hz.


In smaller motors, you hear it as a high pitch "scream". Some fuels (polypropylene and wax) are more prone to inducing instability than others because of the way they burn.  Metalised fuel grains (commonly sold as SFX reloads) are the worst.

Here's a video of a really bad one

you can see and hear the motor backfire at the end of the burn as the tank pressure drops.

Most people think that this is the sound that a hybrid makes... Actually its the sound that a really badly designed hybrid that is trying really hard to cato makes.  If the pressure spikes in the combustion chamber get high enough to force hot burny stuff back into the tank, very bad things happen very quickly......
.

At best you end up with a thrust curve that looks like this:
.

And a very poorly performing motor..

PK