The majestic world of power cycles in rocket engines part 1.
Welcome, ladies and gentlemen, to a new series of technical articles focused on teaching and interpreting the most common and, at the end, complex rocket engine power cycles in rocketry.
This first part will consist of the three most simple rocket engine cycles. I have decided to do so because they are easy to understand compared to the rest, as they serve as a first contact with this beauty of the propulsion systems. We will discuss everything from the simple and cheap cold gas thruster to the engines powered by an electric pump.
Take a seat, check the ground support equipment and let's get started.
Being simple doesn't mean being unimportant.
When we think of rocket engines, we tend to imagine engines like the F1, which powered the Saturn V, or the RS-25, which gave the wings to the Space Shuttle, but we need to go to the other side of the spectrum and take a look at the rocket engine cycle that keeps the spacecrafts on their correct attitude. The cold gas thrusters, also known as RCS.
For that task, this simple but reliable type of rocket engine owes its qualities to its basic design, which operates by the third law of Newton, taking advantage of the natural behavior of a pressurized gas and using that expansion to generate thrust, thrust that can be controlled by a valve between the propellant storage and the nozzle.
This valve also controls the flow control, the ON/OFF behavior, regulates the thrust and the response time, which gives more precise and quicker adjustment of the thrust.
The performance of these thrusters varies as the type of propellant used; some RCS have a low ISP of only 28 seconds using xenon, also known as Xe, up to 165 seconds using helium, also known as H2, but know that we have a reaction control system, so it's time to start raising the complexity.
Schematic of a cold gas thruster. (Credits to Tummala, A.R.; Dutta, A. on wikipedia)
I need a jog?
Now that we know the simplest manifestation of a rocket engine, I am going to step up to introducing the monopropellant pressure-fed rocket engine.
This cycle is the first of this series of articles that doesn't just throw the propellant; now it sends it to a combustion chamber where it burns, produces hot gases, and then is expelled through the nozzle, but this propellant needs a way to be sent, so they use the same principle as the cold gas thrusters, using an inert gas like helium. The pressure forces the propellant into the combustion chamber without the need for pumps, reducing the complexity and providing a good solution for the necessities of, for example, satellites that are used for attitude control on upper stages like the Falcon 9 second stage for trajectory adjustments.
A well known monopropellant engine include the Aerojet Rocketdyne's MR-106 that uses hydrazine as the main propellant.
Pic of the MR-106L. (Credits to SatCatalog)
Disclaimer:
(This discussion will not cover the simplest variant of a bipropellant rocket engine. While seemingly distinct, the operational principles of bipropellant and monopropellant pressure-fed systems share sufficient commonalities, as the bipropellant cycle uses 2 types of propellant known as a fuel and an oxidizer, and in my opinion, this warrants their treatment as functionally equivalent within the scope of this document.
Electric pumps have something to say.
We have a way to control our rockets, but we need a way to power our first stage. It's a pleasure to introduce you to the electric-pump-fed rocket engine cycle.
We want thrust, and we need to stop relying on the pressure of a gas to send our propellant to the combustion chamber, ¡It's time to say hello to the Pumps!
More thrust means that our engines are hungry, the pumps can deliver the propellant to the combustion chamber, and now is when the electric motors enter the scene, but they are hungry also. Using a battery to generate DC electricity, which is converted to AC thanks to an inverter that converts DC to AC that can be used by our motor.
Now the pumps can pressurize the propellants, known as fuel and oxidizer, and send them to the combustion chamber, where they mix and ignite, producing the thrust we need.
An electric cycle engine has potentially worse performance due to the added mass of batteries; that said, they can be jettisoned when their job is done like the second stage of the Electron does. One advantage may be lower development and manufacturing costs due to its mechanical simplicity and the lack of high temperature turbomachinery.
The main exponent of this cycle is the small but powerful Rutherford, powered by liquid oxygen (LOX) and Rocket Propellant-1 (RP1). On January 21, 2018, Rutherford made history by becoming the first electric pump-fed rocket engine to power a rocket also reaching orbit, combining high performance, precise control, and simplicity.
Electric-feed rocket cycle schematic. (Credits to wikipedia)
Rocket Lab CEO peter beck holding a Rutherford engine on June 10, 2015. Image credit: Phil Walter.
And we reach the first stop of this series of articles, on the next episode, we will discuss some of the most common power cycles of the recent age. I hope you enjoyed this article. My name is Raptor. Thank you so much for reading it… and I'll see you next time.
