Rocket engines, where a solid fuel is combined with a liquid oxidizer or vise versa, is known as a hybrid rocket. As of 2014, all of the rockets launched by Copenhagen Suborbitals have been of this type. At CS we have developed a work horse hybrid engine known as the Hybrid Atmospheric Test Vehicle, or HATV. Our experience with this type of engine is so extensive that we are able to tailor just about any thrust time history that our mission may need, with hybrids.
We have produced motors with thrust levels ranging from 0,2 kN to over 100 kN using the nitrous oxide / polyurethane combination.
The HATV is a sturdy machine, build from iron, and it delivers what we ask for. For this reason, it was also chosen as the type of engine used in the development of jet vanes for active guidance and ultimately as the flying testbed for computer controlled rocket flight.
The most advanced rocket ever to be flown by Copenhagen Suborbitals to date, the active guided Sapphire, used a standard HATV as power plant.
The HATV rocket consist of two halves, the oxidizer tank and the combustion chamber. These are connected by a structure known as the intertank. The HATV is a valve less design, meaning that the flow of nitrous oxide and the timing of ignition sequence is done using a simple yet unique “plug start”.
This allows the hybrid HATV to be operated much like a solid propellant rocket. It can sit on the launch pad ready for action for days, loaded with its earth storable oxidizer. Some 20 milliseconds before lift off the pyrotechnic igniter is activated and the engine fires.
This video shows the rapid thrust build up in the 100 kN LES motor developed for the TDS capsule. The specific impulse is 215 sec. Both thrust and ISP values are compensated for the effect of the 30 degree angling of the four nozzles.
The HATV engine and its derived designs have been extremely useful workhorses for Copenhagen Suborbitals. However, the very low liquid density of nitrous oxide, and the low loading factor of the solid fuel. As a result, while the HATVs are incredibly reliable, their mass ratio is very poor. Only 40 % of the liftoff mass is propellant.
This is the reason that we never attempted to use nitrous oxide for our manned rocket. Also, the energetic nature of nitrous oxide makes this oxidizer potentially explosive, especially if operated in large vessels. The company behind the Virgin Galactic space plane suffered a fatal accident with nitrous oxide when this was used in a large scale application.
In hybrid combustion processes, the chemical properties of the oxidizer has a significant impact on combustion stability. Oxidizers that are “energetic” as nitrous oxide or hydrogen peroxide, simply performs much better in a hybrid, than non energetic oxidizers like liquid oxygen, or LOX as we call it.
By 2009 we started development of a monstrous 65 cm diameter hybrid motor, destined to become the power plant of the HEAT 1X research rocket. During the full scale static testing it developed high amplitude, low frequency combustion instability.
We chose to accept this phenomenon during the first flight. The severe vibration of the big engine did not damage the payload, but it was decided not to accept it for later manned flights. We did not manage to suppress this combustion instability using baffles, and after consultations with professional rocket engineers fighting the same type of problem, we decided to use a bifuel liquid propellant engine in our continued developments. The professionals we consulted stopped work on LOX based hybrids, and now uses another energetic oxidizer in the form of hydrogen peroxide.
Hybrids continue to be a potential contender for future CS power plants, and our experience with this type of propulsion have been decisive for us.