Talk:Pressure-fed engine

Pressure-fed cycle? or Pressure-fed engine?

I'm wondering if we have a number of sources that refer to this as a pressure-fed cycle rather than a pressure-fed engine? In the literature, it seems to be used to define a type of engine, and the pressure-fed/pump-fed distinction does not seem to actually be about the thermodynamic cycle at all, but rather just a source for the pressure to drive the propellant into the combustion chamber.

The article itself mentions "cycle" only once, in the obligatory first line of the lede where the article title is supposed to be recapitulated. Yet it mentions pressure-fed engine or design several times.

If I'm right about this, then the name of the article may just be an inadvertent artifact of how the article was originally created in Wikipedia, and it might be time for a move to make the article name more in line with the terminology that is widely used in the rocket engine technical literature.

Anyone have good sources that show this commonly referred to as a cycle? Cheers. N2e (talk) 15:48, 22 November 2013 (UTC)

I'd have to check my sources, but offhand I don't think any rocket can have a "cycle". Thermodynamically, it's not a cycle; the working fluid is once-through, unlike a steam turbine where the steam from the turbine is condensed and returned to the boiler. The engine itself is also not cyclic in that its "moving parts" do not and cannot return to their starting positions, otherwise there could be no net impulse. Karn (talk) 17:20, 22 November 2013 (UTC)

I think you are exactly correct, Karn. I borrowed a textbook yesterday: Fundamentals of Classical Thermodynamics by Van Wylen and Sonntag, published by Wiley, 1973. I believe you are completely correct, it is not a thermodynamic cycle. Here's the relevant text with definitions from page 22-23:

"When a system in a given initial state goes through a number of different changes of state or processes and finally returns to its initial state, the system has undergone a cycle. Therefore, at the conclusion of a cycle all the properties have the same value they had at the beginning. Steam (water) that circulates through a steam power plant undergoes a cycle.

A distinction should be made between a thermodynamic cycle, which has just been described, and a mechanical cycle. A four-stroke cycle internal combustion engine goes through a mechanical cycle once every two revolutions. However, the working fluid does not go through a thermodynamic cycle in the engine, since air and fuel are burned and changed to products of combustion which are exhausted to the atmosphere. In this text the term cycle will refer to a thermodynamic 'cycle' unless otherwise designated." (emphasis added)

So, from that new information, I have a few observations:

1. I believe I was incorrect in phrasing the second part of my question in terms of a thermodynamic cycle, and in linking to that term in the Wikipedia.
2. The contrasting explanation of a mechanical cycle for a four-stroke cycle internal combustion engine is helpful. While the rocket-engine cycle/process is clearly not a mechanical cycle, it clearly shows that the term cycle in these technical subjects can have more than one sense.
3. "Cycle" can definitely be used as a noun for more than merely thermodynamic cycles. That said, while I've seen the term referring to types of rocket-engine cycles, I've not seen it used to refer to something as simple as whether the pressure to move propellant into a rocket engine combustion chamber is provided via a pump, or provided via stored compressed-gas energy in the working fluid of the propellant tank.
4. I've got to learn more about rocket engine cycles or rocket engine processes; it's not a subject I ever studied. I may try to find a book to borrow on it. But the main part of my question, I believe, remains valid. For clarity, I'll restate it below.

—N2e (talk) 12:35, 23 November 2013 (UTC)

Pressure-fed cycle? or Pressure-fed engine? (restated)

(here I am restating the question, without my mis-presumption about thermodynamic cycles)

I'm wondering if we have sources that refer to this as a pressure-fed cycle rather than a pressure-fed engine? In the limited rocket engine literature I've run into (admittedly, only a few AIAA papers), the term seems to be used to define a type of engine, and the pressure-fed/pump-fed distinction does not seem to actually be cycle of the rocket engine at all, but rather just a source of power for the fluid pressure to drive the propellant into the combustion chamber.

The article itself mentions "cycle" only twice , in the obligatory first line of the lede where the article title is supposed to be recapitulated, and in an introductory sentence to a list of pressure-fed engines at the end of the body of the article. Yet the article mentions pressure-fed engine (or pressure-fed engine, pressure-fed engines, or pressure-fed liquid fuel engines) five times.

If so, then the name of the article may just be an inadvertent artifact of how the article was originally created in Wikipedia, and it might be time for a move to make the article name more in line with the terminology that is widely used in the rocket engine technical literature.

So, still looking for any sources in the literature that show this is commonly referred to as a cycle. N2e (talk) 12:51, 23 November 2013 (UTC)

With no source in the literature found for the propellant pressure source being considered a unique rocket cycle, have moved that page to Pressure-fed engine (rocket), per discussion. N2e (talk) 16:59, 6 December 2013 (UTC)

Ullage

Forgive my limited understanding of the subject matter, but isn't one of the fundamental advantages of pressure-feeding the lack of ullage problems, at least when you keep the fuel in a bladder? Ullage is currently not mentioned in the article. For RCS specifically, it'd be very detrimental to have the requirement of ullage motors (since RCS is often to provide the functionality of ullage motors to other engines). Again, my understanding is very limited, and I am simply wondering if (a) this assessment is correct and (b) it should be included in the article. --Tobias (Talk) 04:16, 8 April 2016 (UTC)

From what I have seen in pictures and drawings of ullage motors and RCS engines, it seems they usually don't use a bladder. Instead they simply use a spherical or cylindrical tank with pressure enough to feed the motors. That is, as you spend fuel and oxidiser the pressure will lower and thus engine effect will decrease, until the pressure is too low to feed the engines. Thus, you can't use all of the fuel and oxidiser, some of it remains in the tanks after the engine "runs out of fuel". I assume this means the control software (or the human pilot in manual flight) must take into account that the RCS engines will get weaker and weaker as you use them. (Disclaimer: I am not an expert in these matters, just an interested geek.)

--David Göthberg (talk) 22:01, 20 December 2019 (UTC)

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