Why do all rockets look the same?
All rockets use the same principle to generate thrust, pushing a lot of mass one way to generate a reaction force in the opposite direction. This reaction force is what we know as thrust. Most rockets you will have seen look relatively similar; they all use a big bell nozzle to guide the exhaust gas. The bell shaped nozzle is called a de Laval nozzle, first invented by Swedish inventor Gustaf de Laval in 1888. The de Laval nozzle provides two main functions. Firstly, the nozzle shape guides the exhaust gases in a controllable direction. Secondly, the shape and exit diameter of the nozzle allows the gas to expand to the atmospheric pressure, which allows the high pressure exhaust molecules to push all in the same direction and makes the thrust generating process efficient.
What are the most efficient rocket designs?
The de Laval nozzle works the best when the exhaust gas leaving the nozzle is matched with the ambient pressure. This means that all the exhaust molecules can push downwards and don’t waste thrust force also pushing sidewards. A rocket, however, will pass through a range of altitudes and therefore different ambient pressures. Rocket designers who use a de Laval nozzle have to choose an atmospheric pressure, and hence altitude, that the nozzle is tuned for. This means that at all other altitudes, the rocket is less efficient.
There are a few other nozzle shapes which can overcome the problem of a rocket operating at different atmospheric pressures. One of these is known as an aerospike. An aerospike nozzle is essentially an inverse de Laval nozzle and uses the outside atmospheric pressure to automatically change the diameter of your de Laval nozzle to the perfect size. Guided by the internal wall, the flow expands against the ambient air, allowing for an ideal expansion at all ambient conditions. This means that it can produce the optimum thrust at any altitude. In the rocket business, this is known as an ‘altitude compensating nozzle’. For the added benefit of the altitude compensating nozzle, however, comes a cost. The cost is that the aerospike is extremely difficult to build. One difficulty is that the spike structure (one half of the de Laval nozzle) must be supported inside the rocket exhaust. Another is that to keep the aerospike from melting, a very complex network of subsurface cooling channels must carry the heat away. These are some of the costs that cause rocket designers to stick with the simpler and traditional de Laval nozzle.
Additive Manufacturing (AM) sets us free
The recent progress of the 3D printing technology, known more generally as additive manufacturing, has enabled engineers to design exceptionally complicated parts that could not otherwise be made at all. In ProjectX, the opportunity for researchers working in aerodynamics and combustion to work with cutting edge additive manufacturers enabled some of the problems associated with the lucrative aerospike nozzle to be solved. Once the ProjectX team were unshackled from the usual constraints of traditional manufacturing, curves and shapes ideally suited to optimizing fluid flows and combustion gases could be brought to life. The product of this work is the ProjectX aerospike rocket engine.
The engine features a 3D curved combustion chamber and nozzle, which is optimised for propellant combustion, minimizing the stress on the chamber walls, and maximum thrust efficiency. The aerospike engine design proved a great choice for the additive manufacturing procedure because the engine can be significantly shorter than its de Laval counterpart. This means that a more powerful engine can be made in a smaller space within the confines of the additive manufacturing machines.
The ProjectX aerospike also has 3 chambers with fins to support the centrally running spike. While the fins support the spike, the chambers also allow for the concept of thrust vectoring. This means that instead of tilting the engine, which is done to control a rocket’s flight path, the engine can be firmly mounted to the rocket vehicle chassis and instead vary the direction of the thrust via the pressure in the combustion chambers. This offers weight and cost savings, which the team hope to investigate in the future.
Liquid hydrocarbons and complex geometry are going to feature in rockets of the future
The engine propellants are oxygen and natural gas, which is primarily composed of methane. Methane may feature as the rocket fuel of choice in the future due to its low molecular weight and reasonable energy density when stored as a liquid. Its safer handling characteristics will also mean that liquid methane is an attractive rocket propellant for spacecraft-to-spacecraft refueling.