Saturday, July 3, 2010

When you hear the term ‘Hybrid Motor’ you probably think of a low(er) emissions automobile that runs at least partially on some alternative form of fuel, anything other than oil that is. Although that is pretty much what it means to the automotive industry, in the Aerospace field a ‘Hybrid Motor’ is something else entirely.

A Hybrid Rocket Motor is in fact a rocket that gets its propulsion power by producing a chemical reaction using two separate components, one is solid, and the other is Fluid. Typically, a solid inert propellant is used along with a liquid oxidizer. The reaction requires an ignition source to be lit in the presence of both in order to get the party started. What does that actually mean? It’s simple.

Let’s start with the solid component. The solid fuel is cast into what’s called a grain, in a cylindrical shaped casing, with a hole down the middle, called a port. Think of it like a big roll of paper towel. The paper part is the solid fuel, and the hollow interior is the port, which is where the oxidizer is going to flow through. There are several materials that are commonly used to make the solid portion of the fuel: Hydroxyl Terminated Polybutadiene (HTPB), Polyethylene, Paraffin and various additives. Each fuel composition will give rise to different burn characteristics, and different motor performance. The port can be of various shapes, it could be circular, pie shaped, wagon wheeled, and you can have multiple ports instead of just one. Changing these parameters will affect the way the oxidizer flows through the grain, and hence will change the characteristics of the reaction as well.

The grain is then placed in what’s called a combustion chamber. This is where the reaction is going to take place. It’s important that the combustion chamber be well designed, and built. It’s going to have to withstand a whole lot of pressure, once that reaction gets going. The goal is to get the pressure in the chamber, produced by the chemical reaction of the decomposition of the solid grain in the presence of the ignited oxidizer, to be expelled out the back nozzle, thus producing thrust and propulsion in the proper direction. If the combustion chamber were to fail, you wouldn’t get the thrust you’re after.

Next, you have the oxidizer, typically Nitrous Oxide (N2O) or Liquid Oxygen (LOX), which is routed to flow through the port or ports of the solid fuel grain. The oxidizer resides in its own tank, under pressure and temperature conditions that are selected for performance. Changes in the state of the oxidizer will of course change the burn characteristics as well.

Below is a picture of what the set up looks like:

The enclosure the green fuel grain would be placed in is the Combustion Chamber. Often you would find more plumbing between the Combustion Chamber and Oxidizer tank.

Hybrid motors have a few major advantages compared to other types of rocket fuel. First and foremost, you can turn them off. Because the reaction in a hybrid motor relies on the presence of the Oxidizer to keep going, you can literally shut down the motor, terminate the reaction, by simply cutting off the flow of oxidizer. This comes in handy if anything were to go wrong. This also allows for throttling using a simple valve mechanism to control the amount of oxidizer flowing into the grain, which is great for stuff like landing, docking, and maneuvering.

Hybrids, as their name suggests, actually are far more environmentally friendly than other types of rockets. The materials used in hybrids can be selected such that the reaction will not produce hydrochloric acid, aluminum oxide or any other undesirable exhaust constituents. Big plus on being more green!

Another nice thing about Hybrids is that the materials used to make them are cheap. Really cheap if compared to solid and liquid fuels.

Next, the materials used to make Hybrids are easy to work with seeing as they are inert, and are nontoxic – Also a big plus, especially if you happen to be the one mixing said fuel. And, seeing as they will not spontaneously burst into flames or explode no matter how hard you hit them or how much heat you subject them to, they are an ideal candidate for easy and cheap transport and storage. Yup, they are safe. I’m a fan of safe. Especially since I see myself as a future Moon dweller… Perhaps even ruler.. but we can discuss that in a later post.

Ok, this was a very top level basic overview, I can provide more detail, if you so desire.

Here are some cool videos to check out if you have a few minutes.

CCB 98mm Hot Fire Test – This is a standard sub-scale test, during those burns data is collected regarding new fuel formulations, new port design, new valves, mechanical assembly, anything that’s changed, there is value in testing on small scale prior to the full size motor tests.


Lunar Lander – This was a fun project, basically it’s a grouping of 4 motors surrounding one larger oxidizer tank. The goal here was proof of concept for landing small payloads on the Moon. The guide wires and stops at the top are there since there were no fins or steering, the test was done to show the hybrid throttling capabilities. That’s me in the back with the blue sweatshirt. If I look far less enthusiastic than the rest of the gang, it’s cause I’d had about an hour sleep in the 3 days leading up to this test, I was quite busy setting that whole thing up.


Btw, the dude that high five's me at the end, that's controls guy. You might have heard of him in my posts about 'My First Job Interview' part 1 and part 2. He's the one who pulled out the notepad, and asked about equations.

HUS Hot Fire Test – This was a pretty successful full scale hybrid upper stage test. That was a good day! Oh yea, you can’t see me there, but that was me pressing the fire button!