The Oomph of Different Engines
Austin Morris, Director of Engineering
5 minute read
When designing a spacecraft, there are numerous systems and subsystems that you need to be aware of, and countless components that you need to consider, including how they all relate to each other and work together. When it comes to moving this spacecraft, the system that is generally considered to be in charge is the propulsion system. The purpose of this column is to address some of the different types of propulsion systems and what some of the trade-offs might be in selecting them.
Before we dive in fully, let me give an explanation and define “oomph”. Seeing as we just recently got our first real snow here in the Upper Peninsula, let’s make this a winter-themed explanation. Imagine that you’re standing on your slidey snow gear of choice, be it snowboard, skis, curling stones (although I don’t think you’re supposed to stand on those), or some other such device, and you determine that it is time to engage in some tomfoolery with your friends by throwing a snowball at them. Let’s also pretend that this snow or ice that you’re standing on is frictionless. In this scenario, you would pitch the snowball and would move by an amount equal and opposite to the force with which you threw the snowball (and the ghost of Sir Isaac Newton would be quite proud of you for obeying his laws). If you were to throw a larger snowball, or throw the same snowball faster, or even just throw more snowballs in the same amount of time, you would increase this equal and opposite force, following the formula F=ma or Force equals Mass times Acceleration. This is essentially the concept behind thrust, where a certain mass of propellant is accelerated away from a body, and the body is then accelerated in the opposite direction. The relationship between the masses and accelerations of the propellant and the body are what determines the resulting force, or oomph of that particular system. (Please note that I will henceforth only refer to “thrust” in this column as “oomph” and there is nothing you can do to stop me.)
One example of what a propulsion system might be is a chemical engine. Chemical engines are what most people think of when they think of spacecraft propulsion, because they are sometimes used as rocket engines as well (unsurprisingly, in aptly-named chemical rockets). There are numerous different types of chemical engines, ranging from solid-fuel engines and hybrid engines to monopropellant and bipropellant engines and beyond.
These types of engines use energy from chemical reactions to produce oomph, typically by expanding and expelling a hot gas and thereby propelling the craft. Chemical engines are notable for having a particularly high oomph but a relatively low specific impulse (Isp) which can be thought of as its efficiency. These engines can be used to make big changes to an orbit in a relatively short amount of time, sometimes with a quick engine burn of only a few seconds.
Electric propulsion on the other hand has a comparatively low oomph but very high Isp, which enables it to run efficiently for a very long time, but also requires it to burn for a while before making a substantial change to the orbit it is in. As with chemical engines, there are several different types of electric propulsion, including electromagnetic, electrothermal, electrostatic, and more. The ion engine that the KMI Laelaps spacecraft will use falls under the latter category, specifically under the umbrella of Hall-effect thrusters (I want to say “Hall-effect oomphers” but that one feels too weird so I will begrudgingly stick with “thrusters”).
Hall-effect thrusters, also known as HET, use a combination of electricity and, to a layman like me, magic to charge noble gas particles (commonly Xenon or Krypton) and fire them out at incredible velocities, resulting in a net force on the system. These particles, while fast, have an incredibly tiny mass that results in only a small net force, leading to the oomph being relatively low. Electric propulsion is generally used for craft that need to make consistent small adjustments over a long lifetime since electric propulsion requires only a small amount of propellant to make those adjustments when compared to chemical engines. Interestingly enough, there is a fellow Upper Peninsula aerospace company leading the charge on the development of commercially viable Hall-effect thrusters, Orbion Space Technology in Houghton. If you need further evidence that Michigan is leading the charge for sustainability in space, definitely check them out.
Also under the category of electric propulsion is pulsed plasma thrusters, also known as PPT, which operate on similar concepts to Hall-effect thrusters, but use a solid instead of a gas to provide ions to the system. Denser propellant storage is enabled by using a solid instead of a gas, which can help reduce the size and volume of a system using this type of propulsion. It also opens up new avenues for potential spacecraft refueling by repurposing existing solid objects in orbit, such as defunct satellites and rocket bodies, and turning them into propellant for this type of thruster. If you’re looking for further information on this innovation, Neumann Space of Australia has been working with CisLunar Industries of Colorado to develop this repurposing segment of the growing New Space economy.
Leaving space and coming back down to Earth for a moment, one type of engine that is the envy of any rocket scientist is the jet engine, specifically ramjets and scramjets. These engines are not used in space as they are air-breathing engines, but I can’t not mention them because of how cool and unfair they are. The coolness stems from the fact that these types of engines use their geometry alone, with no moving parts, to compress air, enabling it to mix with fuel in the combustion chamber and accelerate the vehicle. The unfairness stems from the fact that as the vehicle goes faster, the air goes faster and gets compressed more, which allows it to produce more oomph from the combustion chamber. Essentially, what you end up with is a system that produces more oomph every time it produces more oomph. You get more oomph per oomph.
While jet engines are air-breathing engines, there are still some potential applications for space, as this kind of system could be used to fly to orbit, deploying a payload before returning back down to the surface to be flown again. All of this and more comes with the future of science in space, and the paths to and through space are chartered with these various technologies. Each has their benefits and detriments, and as a collective offering, deliver acceptable tradeoffs whether on a delivery mission or one to the moon. It is likely a combination of these engines that will make deliveries to the moon with the Artemis program, but that’s a longer story for another column.
In the end, getting into space is a host of physical, financial, and engineering challenges, but operating in space has its own challenges. Activities in the cold, unforgiving, airless void of space aren’t often easy, but are rewarding in the scientific knowledge and “Wow!” factors they deliver. These different engines give the oomph of modern missions, and it’ll be exciting to see where they take us. Literally.
Recommended column to read next: Tyranny of the Rocket Equation