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!