FAQ

Rockets Are Complicated.

It took us a very long time to learn what we have, and we are constantly learning.

We’ve collected some questions we’ve received from varying levels of technical background, and hopefully this will answer some of your questions too! We’ve tried our best to make these answers concise yet accurate, but rockets are complicated.

If we’ve simplified too much or got too technical, or you have more questions that we haven’t addressed here, please feel free to contact us; we’re always happy to talk about rockets.

About Rockets

  • A rocket engine is a type of combustion engine, burning fuel with oxygen and using the resulting energy to achieve a purpose. Rockets carry not only fuel, but their own oxidizer as well. The high speed exhaust products are expelled through a nozzle, propelling the rocket forward per Newton’s second law.

  • Rocket engines can be liquid, solid, or hybrid depending on the type of propellants used. Solid rocket motors typically use relatively stable fuel and oxidizers mixed with a binder. Once ignited, the combustion occurs within a cavity through the center of the propellant grains, and the exhaust products exit through a nozzle at the end of the motor. Though simple to manufacture and use, they cannot be throttled, shut off, or reignited. The majority of hobby and student rocketry teams use premade commercial off-the-shelf (COTS) solid motors for propulsion.

    By contrast, liquid engines store their fuel and oxidizer in liquid form. In addition to the control not possible in solid motors, these engines can achieve much higher efficiency due to the propellants available, better mixing, and lack of inert binder. As a tradeoff, they are much more complex to develop and operate. The majority of orbital launch vehicles like the Saturn V, Space Shuttle, Falcon 9, and Electron are largely or completely liquid-fueled.

    Hybrid is another type of rocket engine which typically uses solid fuel grains with a liquid oxidizer. There are a variety of approaches to these systems, but in general they combine various advantages and disadvantages of both solid and liquid engines.

  • Over decades of rocketry, many thousands of combinations of fuels and oxidizers in various forms have been tested, some of them successfully. Bipropellants are a broad categorization of separately stored liquid fuel and oxidizers that are mixed and ignited upon injection into the combustion chamber. There are other classes and subclasses of liquid propellants like monopropellants and hypergolics, but bipropellant engines offer an excellent balance of performance, control, and safety, and are the standard for high-powered liquid rockets.

  • Like any engine, a liquid rocket requires a consistent and controllable flow of fuel and oxidizer to function. Because the chamber pressures in a rocket are extremely high, any backflow of combustion gasses into the propellant would yield undesirable consequences. Hence, a high-pressure, high-flow-rate, and high-reliability propellant feed system is required. The two most common methods of achieving this are “pressure-fed” and “pump-fed”. The former uses a highly pressurized tank of inert gas like nitrogen to force the propellants into the injector. The latter uses a turbopump instead, which is much lighter than a pressurization tank, but also much more complex.

    We currently use a three-tank pressure-fed system for its relative simplicity and reliability, but we are actively exploring self-pressurizing supercharged nitrous oxide for our flightweight system, as well as electric turbopumps for longer term projects.

  • Borealis was designed as a static test engine, this means it was never meant to be integrated into an airframe and flown. This can be seen in design elements such as the heavy cooling system, propellant tanks, and control computers. “Flightweight” refers to an engine, propellant, and control system designed with the weight and space constraints of a vehicle, in our case - a sounding rocket. Our first engine and propellant system designs were ambitiously flightweight, but we very quickly realized the scope of difficulty, time, and cost required for the development of liquid rockets. The Borealis was a refocusing of efforts to be more grounded, intended both technically and procedurally to pave the way for our following engines to fly.

    Additionally, a static engine allows us to add far more instrumentation and their supporting hardware than a flightweight system, drastically improving the primary goal of a combustion research engine.n

About Joining

  • Any of our members have become integral parts of our team without prior academic knowledge. Although material from STEM courses will give you a head start, much of what we’re doing is self-taught experientially-learned. With enough interest and dedication, anyone can contribute to the team on our many varied tasks. Additionally, we have non-technical subteams that are just as important and do not require any in-depth technical knowledge, they are also a great way to start and learn more about the technical subteams if you want to join them down the line.

    That said, onboarding is a lengthy process for us to catch new members up on the details of what we’re doing. As a result, our general recruitment isn’t open year-round, please see the Team page for current recruitment status.

  • Each week, each subteam conducts 1-hour meetings during the weekdays; we also host 3-hour general meetings with all subteams every weekend. Additionally, team members are expected to work on their tasks for several hours outside of the weekly meeting times. Setting up meetings with smaller task groups within or across subteams is highly encouraged and facilitated on our communications platforms.

    As a student design team, we completely understand that our members get busy with exams, projects, and life in general. The most important thing is letting us know if you need some time away from the team so we can redistribute tasks and still meet our deadlines. We realize how difficult university can be, and no effort put into our team goes unappreciated.

  • While we try to accommodate virtual attendance for members who cannot physically attend any specific meeting, we highly encourage all members to join our in-person sessions. As we are currently in the manufacturing, assembly, and integration phase, being in our workspace is absolutely critical. The pandemic was one of the most difficult times for our team, and the momentum we’ve picked up since returning to campus has been incredible.

  • As students ourselves, we totally understand if life gets in the way sometimes. If you’ve taken a break from the team, but want to know what we’re doing now and what you can do to help, please reach out to the leads of the subteam(s) you were on or are interested in or use the contact form. We’d be happy to catch you up and have you back on the team.

About Borealis

  • The primary technical goal of the Borealis engine is to collect high quality combustion data. This data will be used to validate and improve our design methodology, and conduct research on the relatively little-explored combustion of ethanol-nitrous oxide rockets. The first hot-fire of the Borealis is a huge milestone for us, but only the beginning. The Borealis is reusable and modular, allowing us to prototype various injector and coolant system designs for future engines, as well as the effects of different propellant flow rates and ratios.

    Perhaps more importantly, through the development and construction of our first engine and supporting systems, our team has gained an incredible amount of experience. This invaluable knowledge of engine, coolant, propellant, telemetry, and control system design will allow MACH to take on even bigger projects, and give our members an incredible head start to their engineering careers.

  • Our goal has always been to fly a liquid rocket engine. A main goal of Borealis was to assist in the development of larger and lighter engines, but the expense of the reusable components prevents us from rapidly iterating and testing the design, a critical step in experimental rocketry. GAR-E represents a refocusing of our efforts into flight hardware, while Borealis is still being developed for our interests in combustion research. As they share the majority of propellant, control, and testing systems, the concurrent development of both engines does not pose a problem to the team.

  • Our first flightweight engine and propellant system is currently in the design phase. We are planning a carbon phenolic ablative engine, coupled with structural stacked ethanol and self-pressurized supercharged nitrous oxide tanks. The airframe integration will be a collaborative project with other established rocketry teams experienced in aerostructures, avionics, and ground support equipment. Another serious consideration for our team is a regeneratively cooled Kerolox engine, depending on budget and administrative approval.

  • Liquid rocket engines can achieve horrendous levels of complexity, so a successful flight test is only the beginning of our journey. Long-term projects of varying ambitions are under consideration, such as spaceshot (a rocket which reaches over 100 km of altitude), throttle control, in-flight reignition, propulsive landing, and many more. With rapid ongoing innovations in rocket technology and accessible manufacturing, MACH will never stop aiming higher.

Getting Technical

  • Our engines use ethanol as the fuel and nitrous oxide as the oxidizer; we’ve been calling this relatively uncommon combination “EthaNOS” for short. Our choice in propellants for Borealis was initially informed by our total inexperience at the time and budget limitations. Although ethanol and nitrous oxide have significant performance disadvantages, they are some of the safest propellants to handle (by the standards of rocketry). Our top priority is the safety of our members, which was the primary rationale behind our propellant choice. After the procedural and hands-on experience gained through a number of cold-flow and hot-fire tests with both inert and live propellants, we are confident in seeking approval for more conventional propellant combinations like Kerolox.

    During the research phase of our engine design, we realized that a relatively miniscule amount of research has been published on ethanol and nitrous oxide rocket engines. Although this resulted in less references to draw upon, it also represented an opportunity to do combustion research and engineer new flightweight system designs. After publishing a paper on our design methodology, we intend to follow it up with more novel research using experimental data collected during our hot-fires, exploring and testing injector designs, chamber cooling, propellant ratios, and much more. Our progress so far has led us to believe that there is real potential for low-cost liquid rocket propulsion using EthaNOS, such as the self-pressurization capabilities of nitrous oxide and excellent cooling properties of ethanol.

  • Rocket engines are notoriously known as contained and directed explosions. In the pursuit of not blowing up, rocket engineers have constantly pushed the limits of material sciences and thermodynamics. Even though air-breathing engines in cars or planes use fuels like kerosene and ethanol, the atmospheric air they burn with is comparatively thin and low in oxygen. When those same fuels are combusted with dedicated oxidizers injected at very high pressures, the combustion temperatures become hot enough to melt nearly any material. Not only is the melting point of the chamber, injector, and nozzle a concern, but materials become weaker at high temperatures - an undesirable trait for containing an ongoing explosion. Since the temperature and pressure inside the combustion chamber are the main determinants of performance, lowering those conditions to material limitations would be counterproductive.

    Ablative, film, and regenerative cooling are the main methods of preventing engine-rich exhaust. Regenerative cooling is by far the most effective of the three, and of course, the most complex. This process pumps fuel or oxidizer through the outside of the nozzle and combustion chamber, before injecting it back into the engine to be ignited. Not only does this method keep the hottest parts of the engine from melting, but it also preheats the propellant for better atomization during injection. While this sounds simple in theory, implementing it has a number of challenges which may take a large part of several years to explain and resolve. Regardless, the vast majority of orbital liquid engines use regenerative cooling.

  • A counterintuitive quirk of rocket engines is the scalability of cooling systems - larger engines are easier to cool. This is because combustion temperature is only dependent on the propellant combination, so a larger engine spreads a similar heat load across a much larger surface area. Water is one of the best coolants available, and we are confident it will provide a very large margin of safety for our first hot-fire test. As Borealis is a static instrumentation engine, the weight penalty of an external cooling system was not a factor for us. Our thin chamber walls will also allow us to collect the best combustion data possible compared to alternative methods like heatsink engines.

    From the beginning, our coolant system was also designed to withstand very high pressures and ethanol flow. After our initial hot-fire, we will use Borealis to conduct extensive regenerative cooling tests by using various ethanol-water mixtures and flow rates. The results of this testing will be the topic of another research paper as well as inform the viability of a flightweight regenerative ethanol-nitrous oxide liquid rocket.

  • Ablatives are materials with a special property to burn in a slow, controlled manner. Unlike most materials that completely deform and melt when exposed to extreme heat, only the top layer burns and forms a layer of charring. This layer, and the gasses from its combustion, serves to insulate the rest of the material underneath. Eventually, this layer breaks off and is forced out of the engine by the combustion pressures, exposing new material to undergo the same process. This method of cooling is extremely simple, and many materials can be used, from advanced carbon-composites to even wood. However, the erosion of the chamber and throat material poses problems for performance consistency, burn time, and reusability. Ablative cooling is used on many solid and liquid rocket engines such as the Apollo Lunar Ascent engine and the RS-68A.

  • For pressure-fed liquid engines, an inert pressurant like nitrogen or helium is typically used to push the propellants into the injector. This is the case with the vast majority of pressure-fed rockets, as stable liquid propellants like kerosene are incompressible, and cryogenic propellants like oxygen and hydrogen cannot vaporize fast enough to repressurize the tanks at the flow rates required for a rocket. Uniquely, nitrous oxide exists in both liquid and gas phases at ambient temperatures and common tank pressures. This means that as the oxidizer is fed into the engine, the remaining liquid nitrous oxide rapidly evaporates to repressurize the tank.

    “Supercharging” refers to the practice of increasing the tank pressure even further by adding inert nitrogen.Supercharging allows more of the nitrous oxide to remain liquid, which means more of it can be used before the back pressure drops too low to feed the engine.

    These two properties combined essentially allow a nitrous oxide tank to double as the oxidizer and pressurant tank,eliminating the weight and complexity of a third pressure vessel and its associated systems.

  • As excited as we are to begin hot-fire testing of the Borealis, it is even more important that we do so safely and collect useful data out of our testing. As a result, our testing schedule is heavily informed by industry standard rocket engine development. Beginning with a hydrostatic test to validate the structural design of our parts, we will move on to cold-flow testing with inert propellant stand-ins. After these initial tests, we will conduct a series of additional flow tests for fine-tuning our injector, propellant, and ignitor systems. In addition to these comprehensive tests, we will conduct various system-level experiments to validate transducer thermal isolation, cavitating venturi design, sensor data acquisition, and so much more. This is a packed testing schedule but we are confident that our team can pull it off to ensure a safe and nominal hot-fire test in the summer of 2023.