Is SpaceX’s Full Flow Stage Combustion Cycle(in raptor) more efficient?
The American based well known Space Company SpaceX’s raptor engine using Full Flow Stage Combustion Cycle as its propulsion system with liquid methane as its main fuel and liquid oxygen as its oxidizer in their Starship Research and development program . So first lets talk about the full flow stage combustion cycle and what is the specialty in their when comparing it with other combustion cycles (Viz., Expander Cycle , Combustion Tap-off cycle).
Full Flow Stage Combustion Cycle:
Let’s take a deep look into the gas generator cycle(Open Cycle System) first . This works by pumping the fuel and oxidizer into the combustion chamber using a turbopump. The turbopump has a few main parts, a mini rocket engine called the preburner, a turbine connected to a shaft and then a pump that push propellant into the combustion chamber. The turbopump assembly is called as the powerpack because it really is what powers the engine. In the open cycle system, the spent propellant from the preburner is simply dumped overboard and does not contribute any significant thrust. This makes it less efficient since the fuel and oxidizer used to spin the pumps is basically wasted.
Now the funny thing about a turbopump is that it makes it pretty difficult to start up since the preburner that powers the turbopump needs high pressure fuel and oxidizer to operate. So the preburners requires the turbopumps to spin before it can get up to full operational pressure itself, but the turbopumps need the preburner to fire in order to spin the turbopumps… but the preburner needs the turbopumps etc etc. This makes starting a gas generator tricky.
So lets talk about the turbopumps .In Turbopumps pressure always flows from high to low .So the turbopumps need to be a higher pressure than the chamber pressure. This means the inlets leading into the preburner is actually the highest pressure point in the entire rocket engine, everything downstream is lower pressure.
SpaceX’s Merlin 1D engine hot fire testing(above image) which runs on RP-1 or Rocket Propellant 1 and Liquid Oxygen. Observe the black smoke is coming out of the preburner exhaust .
Why would it be so sooty compared to the main combustion chamber which leaves almost no visible exhaust? Well that’s because rocket propellant can get super hot . Like thousands of degrees Celsius. So to make sure the temperature isn’t so hot it melts the turbine and the entire turbopump assembly, they need to make sure it’s cool enough to continually operate. Running at the perfect fuel and oxygen ratio is the most efficient and releases the most energy, but it also produces a huge amount of heat.
In order to keep the temperatures low, you can run the preburner at a less than optimal ratio, so either too much fuel known as fuel rich or too much oxidizer or oxygen rich. Running an RP-1 engine fuel rich means you’ll see some unburnt fuel appearing as dark clouds of soot. The highly pressurized unburnt carbon molecules bond and form polymers which is a process known as coking. This soot starts to stick to everything it touches and can block injectors or even do damage to the turbine itself!
So if you don’t want to waste the highly pressurized propellant and if you could just pipe that hot exhaust gas and put it into the combustion chamber. Then The Closed Cycle Welcomes you!
The closed cycle or staged combustion cycle increases engine efficiency by using what would normally be lost exhaust and connects it to the combustion chamber to help increase pressure and therefore increase efficiency. This Sounds really amazing and impossible. But let’s see how the Soviets solved this problem. The first operational closed cycle engine they made was the NK-15 designed for their N-1 moon rocket, they later upgraded it to the NK-33 and then many versions from there stemmed out including the RD-180 which is what is used on the Atlas V today.
But once more problem is their Since the NK-15 and NK-33 runs on RP-1 like the Merlin, you can’t run your preburners fuel rich because of the coking problem… so if you want to create a closed cycle engine with RP-1 the answer is running the preburner oxygen rich. Easy as that right? Well now you’re blasting SUPER HEATED highly pressurized gaseous oxygen which will turn anything into soup, right at a precision machined, crazy high tolerance turbine blade.
With a closed cycle engine, you don’t just use some fuel and some oxidizer and burn that in the preburner to spin the turbine, you actually shoot ALL of the rich propellant through the turbine. So with an oxygen rich cycle, all of the oxygen actually goes through the preburner and just the right amount of fuel goes to the preburner. You only need enough to give the turbine the right amount of energy to spin the pumps fast enough to get the right pressures for the preburner and the combustion chamber. But still like all engines, the chamber pressure cannot be higher than the pump pressure, so the pumps actually have a lot of weight on their metal little shoulders. Since RP-1 and LOX are relatively similar density and ratios, they can be run on a single shaft using a single preburner . But with Hydrogen you need to run even more Hydrogen per mass of oxygen, so between being less dense and needing a higher fuel ratio with Hydrogen, the pumps are drastically different between the Hydrogen and LOX. So the Engineers at Rocket Dyne Proposed an engine known as RS-25 which is used in Space Shuttle Program.Then the came to know that because of the large difference between pumps, they might as well just have two preburners, one for the hydrogen pump and one for the oxygen pump. But interestingly having two separate Shaft in one system makes an another trouble to the engine.
By Putting high pressure hot gaseous hydrogen on the same shaft and right next door to the liquid oxygen pump. If some of the hydrogen would leak out of the preburner, it would start a fire in the lox pump, which is catastrophically bad. Hydrogen is also very hard to contain, because it’s so not dense .So to make an elaborate seal to keep the hot hydrogen from sneaking out. The seal required for this is called a purge seal and it’s actually pressurized by helium so it’s the highest point of pressure, so if the seal leaks, it just leaks inert helium.
So atlast ready to talk about the Full Flow Staged Combustion cycle which basically just combines the two closed cycle methods we just talked about. With the full flow staged combustion cycle, you take two preburners, one that runs fuel rich and one that runs oxygen rich. The fuel rich preburner powers the fuel pump and the oxygen rich preburner powers the LOX. This means the full flow cycle needs to tackle the oxidizer rich problems, which again is solved by developing very strong metal alloys. SpaceX developed their own superalloy in house that they named SX500.Brilliant SpaceX!!
According to Elon, it’s capable of over 800 bar of hot oxygen-rich gas. That may have been one of the biggest hurdles in developing the Raptor engine.
This is literally Crazy invention by the SpaceX Engineers and of course Elon. The advantages of this system is that since both the fuel and oxidizer arrive in the combustion chamber as a hot gas, there’s better combustion and hotter temperatures can be achieved. There also is less need of that crazy sealing system , which makes for less refurbishment. That’s a good thing when you plan to reuse your engine over and over with little to no maintenance between flights . with a full flow cycle since all of the fuel and all of the oxidizer goes through the preburners , you can burn as much propellant as necessary to power the turbopump. But, your fuel to oxidizer ratio will be so crazy fuel rich and oxygen rich that the temperatures at the turbines will be much lower and this means longer life spans for the turbopump assembly. It also means more combustion happens in the combustion chamber and less in the preburner.
Also SpaceX Raptor Engine beats the Chamber Pressure of Russia’s RD-180 Engine
Raptor’s Chamber Pressure- 270 Bars
RD- 180’s Chamber Pressure- 252 Bars
So Finally in all the way Raptor Engine is considered to be the King of all rocket engine and also it is the most efficient one when comparing with others. The Full Flow Stage Combustion Cycle is the most efficient combustion and effective engine cycle in the rocket industry .
