The holy grail of chemical engines

 

Full Flow Staged Combustion Engines

 

Ok so rocket engines are weird, the action of mixing fuel and oxidiser is simple in theory but in practice its way more complicated. So starting from the beginning the first rocket used to be solid rocket boosters so fuel isn’t a big issue in them. The problem with solid rockets is that they are not efficient enough, enters liquid fueled rockets. Liquid fueled rockets are more efficient can carry more cargo up to higher speeds all good, the problem is how do we pump the fuel.

 

So rocket engines need something to pump fuel and oxidiser into the combustion chamber. At first  engines were pressure fed basically the pressure inside the fuel tanks will push the fuel in the engines. This was a very simple system got the job done but as the fuel is kept under pressure the skin of the rocket has to be very durable (and heavy) to hold it in which is bad. Enter turbopumps, turbopumps are well pumps which pump the fuel in the engine but it has to be powered somehow. This is were the complications begins, there are a million different ways to power a turbopump if we talk about all of them this blog would be a book. We are just going to talk about staged combustion engines.

 

Staged Combustion Cycle engines are probably the most common type of rocket engines. So the thing that happens in it is some of the fuel and oxidiser is burned in a smaller combustion chamber (staged combustion) and the hot gas is ran through a turbine which spines and powers the pump. Now the type of staged combustion also has its types. The problem with staged combustion is that we cant burn fuel and oxidiser in a stoichiometric ratio because it would be too hot and burn the turbine, so we need to burn it in a fuel rich or oxygen rich ratio. This is were the western and soviet space researches separate. The western aerospace went with the fuel rich system because its easier on the turbine and the soviets went with oxygen rich cycle which has some benefits we will come back to that.

 

So the fuel rich turbopumps were good because the combustion by product consists of CO2 and hot fuel both of which are non corrosive to the turbine. It was a good system but the fuel rich cycle had a problem whatever fuel and oxidiser that goes through the turbopump has to be dumped out of the engine as it doesn’t has enough pressure to be pumped in the engine. So a significant percent of the fuel is wasted out of the engine. It would be better if we could somehow route the wasted fuel back into the engine.

 

Enters oxygen rich cycle engine, it’s the same thing as the fuel rich cycle but the pre combustion by product are hot O2 and CO2. Technically speaking hot oxygen is more corrosive to the turbopump turbine but it does allows us to put the oxygen what would have been wasted to be put back into the engine (actually there is no oxygen that is wasted the turbine is supplied by the compressor or pump and the turbine exhaust is injected into the engine). This is possible because the compressed fuel is used to cool the bell nozzle and not compressed oxygen which would have been corrosive. This design allowed the soviets to make the best engines for a long time. The RD-170 and RD-180 remained the most used engine by the whole world for a long time.

 

Also a point to note is that the soviet engines have 2 or sometimes 4 nozzles for one engine, this is because it allows them to use conventional fuel injectors which are very similar to injectors in cars and are cheaper than the more advanced type of injectors used in western engines are very difficult to build. Also these engines are fixed they don’t have any option of gimbal, turning is achieved by off axis thrusters which can be fired when needed. Were unique design definitely reduces cost and complexity.

 

Inspite of the near perfect efficiency achieved by the oxygen cycle engines we can still do it better. In a staged combustion engine the fuel pump, oxygen pump and the turbine are on the same shaft thus spinning at the same speed, different fluids at different temperatures need to be compressed by compressors spinning at different speeds. Another problem with have one shaft is that the fuel and oxidiser and flow into each other which obviously leads to catastrophic disassembly.

 

Enters Full Flow Staged Combustion engines it combines the benefits of both the engines into one by having two different shafts one of them runs oxygen rich cycle and the other one runs fuel rich. The fuel from tank comes into the engine into the compressor then a small amount of compressed oxygen is added and burned then the hot gas runs the turbine and goes into the bell nozzle to cool it and then into the main combustion chamber. The oxygen side of the engine follows the same route of the oxygen cycle engine. This cycle gets through the problem of compressor efficiency by simply running both the compressors at different speeds and also the chance of fuel and oxygen mixing is non existent.

 As you probably would have noticed that the fuel and oxygen enters the compressor so the engine can’t run if the propellants are not already compressed this is achieved by a starter mono propellant usually Bromine(Br) compounds which spin the turbine before starting.

 The full flow staged combustion engine is probably the most efficient, mechanically reliably and powerful engine that we would ever make. It literally archives 98% thermal efficiency better than any engine. There are engines with higher power to weight ratio sometimes Even higher efficiency (by using better propellant). the soviet RD-180 still beats it in chamber pressure and power to weight ratio but it can’t be started or throttled as we wish. The only free flow staged combustion engine is development is the SpaceX raptor engine which is still on the test stand and yet to be put on a rocket.

 

We hope to see the raptor or similar engines being put on a reusable rocket and fly.