Understanding liquid fueled rocket engines

[jostpuur]

I'm troubled by the problem of pumping fuel to a combustion chamber. The big problem is that pressure cannot be directed in some particular direction. So when you see propellant leaving the chamber with a great pressure, it should be kept in mind that the propellant is attempting to leak, with the same pressure, back to the pipes through which the fuel is being injected. I have understood that most of the time the solution to this problem is that the liquid fueled rocket engines simply use extremely powerful pumps. I've found a following joke about this:

Turbopumps in rockets are important and problematic enough that launch vehicles using one have been caustically described as a 'turbopump with a rocket attached'- up to 55% of the total cost has been ascribed to this area.

http://en.wikipedia.org/wiki/Turbopump

My first question is that where do these pumps get their energy from, like in Saturn V and in Space Shuttle? If you don't think about it much, you might think that perhaps some battery is enough to run the pumps, because it's the combustion chamber from where the real energy to the propulsion comes from, but that wouldn't make sense to me, because the pumps need to fight against the incredible pressure created in the chamber.

Is part of the energy that is generated in the chamber, somehow directed back to the pumps? How could one accomplish that? Or do the pumps have completely own combustion engines, like cylinder engines burning their own fuel?

Another fact which I have found surprising is that the LEM did not use pumps at all. I have the https://www.amazon.com/dp/1844256839/?tag=pfamazon01-20 and it says that

The Bell engineer's approach was to try and eliminate anything which could go wrong with the engine. They started by doing away with the ignition system, instead using hypergolic propellants which would ignite spontaneously when exposed to the vacuum of space. Propellant pumps were discarded by forcing the hypergolic reactants into the engine bell using a pressurised helium system.

By introducing a high-pressure inert gas into the tanks, the two propellants would be forced out evenly and together without the need for unreliable pumps.

I am left puzzled by this, because according to my intuition the helium injected to the tanks would need to be in equally high pressure as the pressure created in the combustion, meaning that equal thrust might as well be achieved by simply shooting out the pressurized helium.

Earlier I had also considered a question that perhaps burning a fuel in the rocket engines is useless, because one might achieve the same propulsion by simply using the pumps to shoot some passive propellant, like water. I have not understood the thermodynamics of this, but I have been left under impression, that if there are two engines, which both create the same pressure at the mouth of the chamber, but other one shoots lighter particles, and other one heavier particles, then the one with lighter particles would create stronger thrust. Although I don't feel like understanding why that would be the case, I believe that that is the way it is, because otherwise, according to my reasoning, the rockets would work just as well by merely shooting passive propellant with pumps.

But I'm still left unable to come up with an answer to how one could avoid the need of the LEM's helium to be in an unreasonably high pressure when released to the fuel tank.

 

[jostpuur]

One possibility is that the LEM had valves which allowed propellant to only enter the chamber, but not leak back to the tanks. I guess this would mean that the burn was not continuous, but consisted of very rapid sequence of discrete explosions, the valves always closing when propellant explodes, and then opening again for new propellant to enter the chamber. I've never heard of such thing, but one cannot debunk such idea by watching videos only.

 

[russ_watters]

The pressure of the pump might be the same as the pressure of the gas in the combustion chamber, but the volume of the gas in the combustion chamber is much, much higher than the volume of the liquid being injected.

 

[jostpuur]

I see. That remark solves the biggest trouble.

 

[alphy]

Thank you for sharing your thoughts and questions about liquid fueled rocket engines. I can provide some clarification and answers to your inquiries.

Firstly, you are correct in your understanding that the pumps in liquid fueled rocket engines are extremely powerful and crucial to the propulsion process. These pumps are typically driven by a separate gas turbine engine, which is powered by the combustion of the propellants in the main engine. This means that some of the energy generated in the combustion chamber is indeed directed back to the pumps, but not directly. Instead, the energy is used to drive the gas turbine engine, which in turn powers the pumps.

Regarding the LEM's use of pressurized helium instead of pumps, this was a design choice made to reduce the complexity and potential failure points of the engine. By using hypergolic propellants, which ignite spontaneously when mixed together, and pressurized helium to force them out evenly, the LEM's engine was able to operate reliably without the need for pumps. However, this design would not be feasible for larger rockets with higher thrust requirements.

As for your consideration of using pumps to shoot passive propellant like water instead of burning fuel, it is important to keep in mind the principles of thermodynamics. Burning a fuel generates a large amount of energy, which is then used to accelerate the exhaust gases out of the nozzle at high speeds, creating thrust. In comparison, using pumps to shoot out passive propellant would not generate the same level of energy and thus would not result in the same level of thrust.

Additionally, the type of propellant used can also affect the thrust produced. In your example, the rocket using lighter particles would create stronger thrust because the exhaust gases would have a higher velocity, according to the equation for momentum conservation (mass x velocity). This is why rockets typically use lighter propellants, such as liquid hydrogen, to achieve higher thrust.

In summary, liquid fueled rocket engines utilize powerful pumps to inject propellants into the combustion chamber, and these pumps are powered by a separate gas turbine engine. The LEM's design choice to use pressurized helium instead of pumps was made to reduce complexity and potential failure points. And the type of propellant used can affect the thrust produced, with lighter propellants typically resulting in higher thrust due to their higher exhaust velocity. I hope this helps to clarify your questions and deepen your understanding of liquid fueled rocket engines.