The most important stage of building a rocket engine is testing. First flow testing the injectors, then the fun can begin. Its time to do an injector plate hot test, this is basically removing the nozzle and doing an “open” burn to see that everything is working as intended, most importantly that the chamber can handle the extreme temperatures produced by these reactions. The temperatures in Epoch 1 were expected to top 5000 degrees, that’s more than hot enough to melt the majority of structural metals. This is why we went with a large piece of aluminum for our chamber, it is capable of radiating the heat away without melting, to a point of course.
Now it gets scary, putting the nozzle on. All we can do is stand back and hope our beautiful and very expensive rocket preforms as it is designed to, and not turn into a glorified pipe bomb. Sitting behind a tree stump and few rocks as a makeshift blast shield, double checking gauges, checking that injectors are all free of clogs, I have to say it was a very tense moment. Would our work be rewarded or would the rocket gods laugh in our faces? A lot to think about when prepping to fire an amateur build Bipropellent rocket engine.
As the countdown goes it only gets more stressful, at 8 the pit in my stomach turns into a dense knot, at 5 I can barely stand to watch. At 2 the oxygen flow starts, its already deafening as over 100 psi oxygen gas surges through the injector manifold. T minus zero, the fuel valve is opened and there is no turning back. Diesel flows through at 10 gallons per hour, and explodes into a fireball, beautiful and scary. We throttle up the oxygen flow, 150 psi, now its roaring. 200 psi, what in the world are we doing, the flame is now retreating into the cone expected from a rocket. 220 psi, the engine is roaring at nearly full throttle, the most rewarding sound to date, so much louder than expected. So much more satisfying than I could have ever imagined.
We may never know how much thrust the test produced, or what class our engine falls into, or the specific impulse of Epoch 1. However, what we can say is that it was a rousing success, and that it only gets better from here, bring on Epoch 2.
Without a well designed nozzle the rocket engine is doomed to fail. It needs to be able to take the heat and pressure of the combustion while still performing the necessary tasks of compressing the flow on the converging side and expanding the flow to hypersonic velocities on the diverging sides. With this in mind we chose graphite as the material for the nozzle, although we did go on to using stainless steel eventually.
Naturally the most important part of a biprop engine is the fuel system, as unlike other engine types the two fuels must actually be pumped into the combustion chamber where they mix and react. The fuels we chose for Epoch 1 were diesel and gaseous oxygen. Diesel is a great fuel because it is very dense, and it burns very hot with a relatively simple combustion reaction. Gaseous oxygen was chosen because it is fairly safe and very easily available, the biggest reason was our lack of experience with biprops. We felt that jumping right into Nitrous Oxide or Liquid Oxygen would end in a very large explosion.
Given the choices we made with fuels, the fuel system was left with very few options. The lack of a liquid state oxidizing agent immediately ruled out the standard impinging streams design. Therefore we decided on using a commercially available misting nozzle to disperse the diesel fuel into the chamber.
The oxygen was a little bit tougher. The oxygen injector needs to help disperse the fuel and be able to mix with it very quickly so all of the combustion stays within the given combustion chamber length. We decided on 16 holes drilled in 2 concentric circles and fed by a manifold. The manifold being fed by a custom oxygen "regulator" from our large welding tank.
In the center is the fuel misting nozzle, while the small holes around it are the oxygen injectors
Here we have the plumbing to feed the injector plate. The tube going into the center is the fuel line, this would lead to a compression fitting to connect to a long flexible fuel hose from our fuel tank, in which we store under pressure to remove the need for a fuel pump. The outer pipe feeds the oxygen manifold, this also connects to a flexible high pressure hose to our oxygen tank.
For our first biprop we decided to go with a modular design, such that we would be able to take it apart after each test fire and clean the interior of the engine. The design is all aluminum extruded tubing from McMaster-Carr, with the exception of the nozzle which was originally graphite but later changed to 304 stainless steel, and the retaining rods which are 1/4-20 threaded 316 stainless steel rods. The combustion chamber is 1.5 inches inside diameter with .75 inches of wall for an outside diameter of 3 inches (so that it would fit in a standard 75 mm motor mount), this overly thick wall was to allow for a very simplistic external ignition method. The external ignition causes a minor flashback into the chamber spiking pressure very rapidly, thus the need for very thick combustion chamber walls. However, this does cause the engine to be very heavy and thus not able to fly in its current configuration.
What is a Bipropellant Rocket Engine?
A bipropellant engine, often referred to simply as a Biprop, is the most commonly used form of rocket engine for larger craft and also by far the most efficient form of rocket engine. Let’s start by defining the various kinds of chemical rocket engines.
First is the simplest and oldest form of rocket engines, the solid rocket. This is the type of engine encountered in virtually all hobby store engines. A solid engine is magnificently simple, comprised of no moving parts, the engine is made of a casing, usually an aluminum or steel tube, a bulkhead to contain the pressure, and a nozzle, usually steel or graphite. The casing is loaded with a solid fuel, which is generally Ammonium Perchlorate Composite Propellant, or APCP. APCP is made of Ammonium Perchlorate and some rubber or epoxy that is used as both a binder and a fuel. Once lit there is no turning back with a solid engine, and this leads to obvious dangers, and is the sole reason that we turned away from using high power solid engines.
The next form of rocket engine is the least common, and that is the hybrid. The name hybrid is very fitting as it takes the best elements of both solid and liquid engines. The engine is comprised of a solid fuel grain and either a liquid or gaseous oxidizing agent, usually gaseous oxygen or liquid oxygen. These are slightly more complicated, as containing the oxidizer requires external tanks that are capable of either dealing with the high pressure required for gaseous oxygen or nitrous oxide, or the extremely low temperatures of liquid oxygen. They also require valves to control the flow of oxidizer. The great part about hybrids is that they are totally throttleable and can even be shut off, which adds a large safety factor as the engine can be turned off if the burn starts to become unstable.
The final form of chemical rocket is the liquid bipropellant engine, this is what we are pursuing. In a biprop engine both the fuel and the oxidizer are kept completely separate as liquids. This means biprops are by far the most complicated, as multiple valves and pumps are usually required to move the propellants into the combustion chamber. Biprops are also the most efficient type of rocket by a fairly wide margin. They are also even safer than hybrids as they can be throttled by adjusting both the fuel or the oxidizer, and the chamber pressure is relatively low compared to solids or hybrids, leading to less chance of a catastrophic failure. We are using the standard combination of Diesel and Oxygen for our engine, Epoch 1.
Why Pursue the Bipropellant?
We chose to point our rocket adventures in the direction of the bipropellant rocket engine despite its complexity for two reasons, safety and efficiency. As mentioned above biprops offer a large safety margin due to lower chamber pressure, usually in the range of 1000 psi or so, at least for our tests. For reference solid motors are capable of spiking to over 10,000 psi in a very short period of time if something goes wrong, and this will end in failure of the engine. We also like the idea of the throttleability of the engine for both safety and usability, as it is far more practical to have an engine that we can control the output of, rather than a purely binary on/off switch.
The second, and probably main reason that we chose to go with the biprop is efficiency. Rockets and jets are both use a unit called specific impulse to measure the efficiency. Simply put, it’s the thrust per unit mass of propellant. Solid engines have a specific impulse that tops out around 200-250 seconds. Hybrid engines can push upwards of 300 seconds if very well designed. Biprops hold the record for highest specific impulse on a chemical rocket of over 500s. Although this was done in a vacuum with non-realistic propellants. The rs25 engine, otherwise known as the space shuttle main engine, has a specific impulse of 458s, which is worlds above any solid or hybrid engine, and this is why we chose to pursue the bipropellant engine.
Stay tuned for more info on Epoch 1!