guest blogger john hare
[Saturday afternoon post change. Argh. While replying to Marcus, I realized that this post could be considered Monday Morning Quarterbacking at its’ worst considering the loss just experienced by Armadillos’ team. I respect that team and consider the members I have met as friends. This post is bad timing that I put out just a few hours before reading about the burn through. This rollerthroat idea is not ready for use or even serious consideration by any active rocket company. It is meant as a thought exercise for something that might be useful in the future.
My condolences to the whole Armadillo team. My apologies to anyone that might be offended by my bad taste in posting this on the same day. Congratulations to Armadillo on winning level one.]
Two of the problems facing rocket developers are protecting the hardware from excess heat and persuading the propellants to enter the combustion chamber. The throat and its’ approaches are about the toughest areas to keep cool. Supplying power to a fuel pump is another major issue that is frequently dismissed as too difficult in favor of a pressure fed design. There might be a way to make both tasks easier.
The throat of a rocket experiences thousands of degree gasses at high pressure moving at sonic speed just small fractions of an inch away on the other side of a boundary layer. Extreme care must be taken in coolant flow through this area to keep everything from melting. Frequently a serious amount of fuel is used as film cooling to assist the cooling channels in protecting the throat and chamber walls.
I suggest mounting rollers at the throat normal to the direction of flow and let the fast gasses spin them so that no section of the roller experiences the maximum thermal load for more than a millisecond or so at a time during each revolution. A relatively small amount of fuel can flow through cooling channels in each roller to keep them cool, pump the fuel to a higher pressure so that it enters the chamber at substantially higher than tank pressure, and be a reverse flow film cooling blanket for the regular chamber walls.
Though there is a tremendous amount of power in the throat, it is not clear just how much of it could be applied to pumping fuel. 10% of the total fuel through the rollers would keep them cool through regenerative cooling in the impeller micro passages and heavy film cooling during the 50% of the time the roller is in the cooling cycle. 10% of the fuel pumped to much higher than tank pressure is not enough to be seriously beneficial. If there is enough power across the rollers though, running a shaft to a serious fuel pump makes sense. Depending on power actually available, the oxidizer might also have pump power available with a roller turbine. While making the surface of the roller rough would generate more power, it would also heat the roller more and restrict throat flow.
The relatively small amount of fuel pumped through the rollers will emerge on the full 360 degree perimeter of each roller. From the nozzle roller seal to the vertical chamber wall, the fluid will accumulate in the half volute and be directed in a reverse flow up the chamber walls providing some film cooling before the fuel boils out into the chamber to be burned. Film cooling from the upper injectors covers the balance of the walls before boiling out into the chamber . Since none of the film cooling fuel exits the throat against the walls, more fuel than normal can be used for the purpose to lower the work required by the regenerative cooling passages.
25% of the time the fuel from the rotor will be directed into the throat with no oxygen to burn or down the nozzle with no real benefit for the engine. CFD and experimental work would have to be done to minimize the amount of fuel through the actual roller if Isp losses are excessive. The rollers may not need internal fuel flow for cooling. It would be simpler and cheaper to use a heavier rich flow from the injector perimeter to flow down the walls as film cooling to the bottom of the roller before returning to the main chamber with the roller, cooling it before being reinjected into the chamber. With the cyclic heating and thermal mass of the roller compared to normal throat cooling methods, fuel required would be minimal.
The rollers could be used for minor TVC by shifting in from one side to bias the flow down the expansion nozzle. This would not be something to depend on without strong experimental proof of effectiveness. The more useful function of roller movement would be as variable throat area control as in a pintle nozzle. During deep throttling, the rollers both move to the center to create a smaller throat with the same expansion nozzle increasing the expansion ratio. An expansion ratio of ten at liftoff could become a ratio of one hundred at altitude when the engine is throttled to 10%. This would allow a high expansion ratio engine in a short package that is relatively light. More practical would be a ratio of three or four. See Jons’ ‘Thrust Augmented Nozzles” post for another more professional method of variable throat area.
The rollers naturally fit a modular installation method with a simple chamber build. A throatless chamber could be built with the roller throat assembly installed and removed as needed for modification, maintenance, or inspection. The heavy film cooling could reduce or eliminate regenerative cooling requirements in some conditions.