While the plumbing in a rocket engine is seen as very complex (which it is), its premise is relatively simple, accelerating gasses down through a nozzle to produce thrust. In general, there are two main types of engines, solid rocket motors and liquid fuel engines. Solid fuel engines are all made in the same general way other than size. There are four major design philosophies around liquid fuel engines, open cycle, closed cycle, full flow, and cold gas. Each engine type has its own advantages and disadvantages.

Firstly, solid rocket motors use a solid fuel usually made from a slow, but fiercely burning material mixed with an oxidizing agent. Next, the solid fuel is packed into a high strength aluminum tank with ablative thermal insulation, that is material designed to decompose when exposed to heat that creates an evaporative cooling effect. Then, a channel is molded in the center of the tank from its base to where the nozzle sits or attaches (based on manufacturer) where the ignition system is located. Usually, the ignition system is an electronically activated detonator that starts a controlled burn of highly flammable magnesium compounds which chain react with the main fuel.

Solid rocket engines provide many advantages. Primarily, they are much cheaper to produce and can provide an obscene amount of thrust for a decent amount of time. Additionally, they are simpler to manufacture compared to their liquid fueled brethren. However, there are some disadvantages to this type of engine, mostly revolving

around a lack of control. These engines once started cannot be shut down until the fuel is depleted, this can be a safety hazard for crewed vehicles. The most famous example is during the Space Shuttle Era, where if a catastrophic problem occurred; the vehicle would have to detach itself from the center tank at a specific altitude to ‘glide’ to the runway. Another disadvantage is the lack of significant thrust gimballing, meaning the ability to change the orientation of the engine bell to vector thrust in a specific direction. All in all, solid rocket motors are most useful as additional booster stages for the main launch stage. For example, the Atlas rockets can hold up to eight solid rocket motors to create more thrust on launch and have the potential to kick the spacecraft into a higher orbit.

While solid motors are the cheapest the most common engines are liquid fueled engines. They come in many flavors, Gas generator cycle (open cycle), closed cycle, full flow, and cold gas/monopropellant.  The first type of liquid fueled engine to be used on an orbital rocket was a gas generator cycle (open cycle) engine. All liquid fueled engines (other than cold gas thrusters) require a pre-burner to power the turbo pumps that allow for the fuel to enter the combustion chamber at a high enough velocity. A simple way to think of a pre-burner as a mini rocket engine that points toward a turbine, the fuel to oxidizer ratio in the pre-burner is not perfect and can (depending on the fuel) create a black sooty substance that clogs and sticks to everything, which becomes problematic when trying to reuse the pre-burner exhaust making it impossible, so it is dumped overboard. All open cycle engines use carbon derived fuels such as RP-1 or cryogenic kerosene.

These engines are relatively cheap for a liquid fueled engine and can produce large amounts of thrust. In fact the F-1 engines aboard the Saturn V moon rocket were open cycle. But there are some serious downsides to consider for this engine. They are very inefficient since some fuel is wasted during the pre-burner stage. Also, they are notoriously hard to start since the pre-burner requires fuel flow and oxidizer, but the fuel and oxidizer pumps also require the turbo pump to function.

Other than open cycle engines, closed cycle engines are different due to the way fuel and oxidizer enter the pre-burner and power the turbo pump. Essentially, the fuel and oxidizer can enter the pre-burner in two different ways, either fuel rich or oxidizer rich, both function in the same way by redirecting the fuel or oxidizer rich exhaust gases into the main combustion chamber, whether or not they are fuel/oxidizer rich

depends on the type of fuel used, for example Hydrogen and oxygen are typically used in fuel rich closed cycle systems t the low density of hydrogen causing less of a need for a larger oxidizer tank, thus reducing the overall weight of the fuel tank. A prime example of a closed cycle system is the legendary Aerojet Rocketdyne RS-25 or the space shuttle main engine.

The main advantage of this system is the amount of thrust that they can produce. Additionally, they are very efficient regardless of altitude while being some of the most reliable engines on the market. However, this performance comes at a cost, these engines are incredibly expensive and relatively complex and heavy.

The final category of engine derives from a closed cycle system and is called the Full flow system (staged combustion cycle). For a long time, it was once thought that this type of engine was impossible due to the materials needed to survive the extreme chamber pressures and temperatures.  Recently SpaceX has finally developed the materials for such an engine. Their new Raptor engine is the next generation of propulsion using an exotic methane/oxygen fuel. This engine works by essentially giving each pump for fuel and oxidizer its own turbopump that powers one component either fuel or oxidizer pumps. The fuel side is oxidizer rich and the oxidizer side is fuel rich to create an even burn ratio, the fuel and oxidizer rich turbopump exhaust is funneled directly into the combustion chamber. This design can produce more thrust than the RS-25 at a much smaller size while maintaining the high efficiency and even being cheaper than it. However, the Raptor has yet to be tested in space yet. It has been used in the latest Starship tests and has so far proven itself in terms of reliability, practicality, and feasibility as a first stage engine.

Ever since I was a small child, I knew that man was never meant to fly. However, I always wanted to challenge that fact. I was four years old when my love for aviation first took flight. It all started when my grandparents and I went to Calgary, Canada to visit my aunt. The only logical way was by flying.  As soon as we took off from Pittsburgh International Airport, my eyes were glued to the window in awe of the peaceful sky for the whole flight. The trip to Calgary changed me, that experience alone was enough inspiration to pursue my dream of one day being able to fly.

It was a Sunday Morning; I woke up and realized that my first introductory flight at RSA Flight Academy was today at the Morgantown municipal airport. Normally the drive from Waynesburg to Morgantown feels like an eternity, but today felt different. During the whole way there I was thinking about the way this experience will shape my future. I knew that knowing how to fly is just one aspect of aviation. The other aspect was all the technical work of the aircraft such as all the parts, what each part does, and how to check for defects. I didn’t have a single shred of doubt in my mind that these skills are going to be a great asset for my future career of being an aerospace engineer.

As we neared the airport, my mom asked me if I was excited, “Of course I am” I said, then she asked, “Are you nervous?” “No.” I replied. I was confident in my abilities and countless hours of personal simulator training but there was still a little doubt in the back of my mind as I was going to fly a real plane after all. Once we arrived at the airport, we walked into a hidden office by the airport’s restaurant That read RSA flight academy. We were met by a short man in his early twenties wearing a blue collared shirt. He shook my hand and said, “You must be Charles.” He then stated, “My name is Aron, and I will be your flight instructor.” “Today has great VFR conditions for flying, but there is a 10 knot direct crosswind.” The crosswind was a result of a hurricane that had struck land causing mild turbulence. “What plane will I be training in?” I asked. “The Piper Warrior II.” Aron said. He then added, “The plane is a bit older than the others here but is perfectly air worthy.” “So did you ever fly before.” he inquired. “Only in simulators, but not a real plane.” I answered.

I took note of how silent the inside of the airport was as we were walking to the door to the tarmac. “After you.” my instructor said as he pressed the button to open the automatic doors to the runway. As soon as he opened the door to the runway the silence was immediately drowned out by the sound of high pitched wails of Jet engines roaring to life- and the majestic shirk of the Jet engines of an Embraer E Jet as it lifts effortlessly into the sky. “I’ll never get used to that sound.” I said in awe. After about 5 mins of walking, we reached the hangar the plane was stored. “Wait here for a minute.” Aron said, as he was unlocking the side entrance to the hangar. About a minute later the hanger door noisily creaked upward. There it was, The Warrior II, a small blue and white gem of an aircraft with the callsign November eight two seven five seven. I helped push the plane on the taxiway for preflight, which was far easier than I thought. During the whole preflight checklist, I was studying the plane, checking all of the rivets, proper landing gear extension distance (these aircraft were known for having landing gear problems), all visible parts of the engine, all control surfaces such as the ailerons vertical stabilizer and stabilator. Since there is no fuel cross feed on this plane, the fuel selector needs to be set to the fullest tank and changed every hour to half an hour to maintain proper balance. After setting the fuel to the correct tank we were ready to start the engine. In that moment I felt a sudden wave of nervousness. I did not let my nerves get the better of me and followed through the engine startup checklist. “Mixture to full rich.” I then moved the red mixture handle all of the way forward. “Throttle to open ¼.” Then I just barely moved the throttle forward to a fourth inch.

“Master Switch on.” I heard a faintly audible hum of the battery come to life as I also turned on all of the lights for visibility. Next, I turned on the Auxiliary fuel pump which is notoriously loud to pressurize the fuel system. “Check left, clear.” “Check right, clear.” “Clear prop!” I then set the magneto switch behind the yoke to starter which caused the engine to roar to life. “Good job on the startup Charles.” “Now are you ready to taxi the plane.” “Uh, sure” I said nervously, by this point I was sweating bullets. “This is easier than driving my mom’s SUV!” I spoke. My instructor chuckled. After taxiing for what felt like forever, we finally made it to the runway. “N82757, you are cleared for takeoff on runway 18 for southbound departure.” Explained the air traffic controller. “N82757, cleared for takeoff southbound departure.” I replied. We started rolling down the runway and I thought to myself “No going back now.” Then the nervousness went away and suddenly the menacing roar of the engine turned into a gentle hum, but we were still picking up speed and powering through the air until we leveled off at 3500 ft and Aron let me have the controls, by this time I was too focused on flying the plane to take in the serenity of the environment. The crosswind and unstable atmosphere made a very rough flight as I felt as if we were being blown around like a leaf on a windy day. My instructor took over for the second half of the flight and then I got time to look at the hustle and bustle of everyday life condensed down into a dense ribbon of Asphalt. Being 3500 ft above the ground was astonishing and I could actually see a slight curve in the Earth.

The whole way back into Morgantown airspace was incomprehensibly peaceful with distant radio chatter in the background. After an hour of flying, we returned with a smooth landing on the runway. We returned to the hangar and took a shuttle to the main office where I officially signed up for the flight academy.

Ever since my first flight Five years ago, I have never given up on my dream. I am soon to be a fully instrument rated pilot and planning to buy N82757 when it goes up for sale very soon. I reflect back on my first flight with the nerves racing and trembling breath for every word spoken to air traffic control, I now know that in retrospect, there was really nothing to worry about, as both the Controller’s and instructor’s job it to keep the pilot safe whilst in the air (especially an inexperienced one).

          NASA’s Space Shuttle program was always thought to be a success; however, the amount of money poured into the program just to make it survive eventually led to its demise. The Shuttle itself had small design quirks that seemed to be preventable, but in hindsight, they were inevitable. The unreliable design proved to be effective in short term use, but eventually two out of the one hundred and thirty five missions ended in the loss of the orbiter and crew. The Shuttles were a social success but a financial and engineering nightmare. The Space Shuttles provided the equipment and tools to push mankind’s ever growing quest for knowledge and discovery, but at what cost? The cost of the program was the lives of 14 astronauts and over $209 billion dollars in funding (“NASA”).

          The Space Shuttle program was financially inefficient as the Heat resistant tiles along the bottom and sides of the shuttle would wear down from the extreme heat from reentry. Before each flight, the tiles would be replaced and come to a total cost of about 1.6 billion dollars per flight. The amount of down time before each flight was about 90 hours, mostly due to the replacement of the thermal tiles. The Shuttles would drain NASA’s budget so much that they are still under the burden of the shuttle program even though the last flight (STS-135) was in 2011. Since the Shuttles had a limited cargo capacity, it costed roughly 20 million dollars per ton into space, which is why foreign designs like the Russian R-7 (Soyuz) rockets were (and still are) economically superior to the Space Shuttle. The repeated diversion of Federal funds to the Shuttle Program caused many budget cuts during the Orbiter’s development and use, which is why the shuttle never reached its full potential (“NASA”).

           The Space Shuttle Program was doomed to fail from an engineering standpoint. One reason for the Space Shuttle to be considered an engineering failure is the reliability of it’s design. Two out of the 135 shuttle missions ended in tragedy, so  the Space Shuttles had the highest mortality rate of any other manned space fairing vehicle. The first reliability factor was the frail nature of the shuttle’s heat shield, or the thermal tiles. If debris or pieces of ice impacted the shuttle on lift off the first thing that is damaged is the heat shield, which is the most important part in any reusable spacecraft is the ability to survive the extreme temperatures and pressures of reentry (“Accidents”).

          In the case of STS-27, there was a concern of major damage along the bottom section of the Orbiter’s heat shield to cause the Shuttle to disintegrate upon reentry, thankfully Space Shuttle Atlantis landed at Kennedy Space Center safely. Damaged heat shields were never a real considered risk until STS-107 when The Space Shuttle Columbia, NASA’s oldest orbiter disintegrated while reentering the Earth’s Atmosphere (“Space”).

          The Space Shuttle’s Solid Rocket Boosters (SRB) would become brittle in cold weather. Each rocket motor had an aluminum fuel tank that connected to the motor itself, which was clamped in place by steel brackets. The engine had an O-ring seal around the inside of the fuel tank which prevented moisture and the elements getting to the motor. The External boosters were designed to be reusable, thus needing the O-ring for easy removal of the motor itself. The O-rings in the engine had one major flaw, in cold weather the O-ring could crack and cause engine exhaust to leak out of the engine and melt through the external fuel tank. The loss of Space Shuttle Challenger is associated to the catastrophic burn through of the engine (“Case”).

         The Shuttle had more reliability issues revolving around its poor flight characteristics. In fact, the Space Shuttle had the worst glide to weight ratio than any other spacecraft at the time due to it’s blunt shape and the delta wing design. The Shuttle’s “glide” characteristics were so bad that the engineers gave the orbiters the nickname “The Flying Brick”. The Shuttle’s design had extreme limitations in terms of it’s Delta-V, meaning that it could only scrape into LEO (Low Earth Orbit). The Shuttle’s had a very strict weight limit as it was only 22,000 lbs, and that also includes fuel and monopropellant for the shuttle and payload.

          The RS-25 main engine of the Space Shuttle had reliability issues in the combustion chamber that could be fatal if not fixed. Each engine had over 600 fuel-oxidizer injector ports, which ensured the fuel to oxidizer ratio was correct and the exhaust gasses would not be turbulent as they raced through the throat of the engine nozzle and out through the bell. Over an extended period of time, the extreme temperatures would melt the injection ports, making them unstable and could cause a catastrophic burn through of the engine causing the motor assembly to detonate. NASA engineers had developed a solution to the problem by manufacturing gold plugs to fit into the injector ports and effectively deactivate the port, little did they know that now the Shuttles could be destroyed from a golden bullet. The reliability of the engine decreased over time due to its repeated use over the course of 30 years. STS-93 is the prime example of the previously mentioned “golden bullet” incident. When the clock hit T-0 all was going fine until the flight computer realized that the chamber pressure had dropped, so more oxidizer was being pumped into the combustion chamber, which increased the temperature dramatically. However, the problem was not diagnosed until Space Shuttle Columbia squeaked into low earth orbit. The gold plug that impacted the engine punctured five hydrogen fuel tubes on the inside of the engine bell. NASA engineers concluded that no more than eight tubes can be punctured or the loss of the orbiter is inevitable (“Space Shuttle”).

          In the end, NASA’s Space Shuttle Program was a social success among the masses, but in retrospect the Shuttles were a political, economic, and engineering failure. The Shuttles were meant to take mankind to Mars and beyond, but sadly hindered space exploration exponentially. The Shuttles cost NASA $209 billion dollars in funding and 14 brave astronauts. The people of the U.S. had a very positive attitude of the shuttles even though the program was holding humanity back. The Space Shuttles were a major step for reusable vehicles, but there were much cheaper options during its development. The Shuttles were supposed to accelerate mankind to the future; however, that dream never came true. Instead, the program turned into a financial and engineering nightmare for the engineers and NASA as a whole. The Shuttle program is a prime example of what happens when congress designs a spacecraft and not the engineers at NASA (“NASA”).

In the aerospace industry, there is a mythical type of propulsion that can create obscene amounts of thrust for the same amount of total fuel burned. These are called Aerospike engines. These differ from a traditional engine due to the lack of a singular engine bell, instead there is a central wedge or conical piece of metal, usually titanium or carbon infused tungsten. This unorthodox design is to solve the main source of inefficiency of De Levalle style engine bells.

As a spacecraft accelerates upward, the surrounding air pressure keeps the hypersonic exhaust from the engine laminar, allowing for the greatest amount of transfer of kinetic energy to the rocket. As altitude increases, the ambient air pressure decreases when this occurs, the exhaust gasses expand outward, thus losing efficiency. The outward expansion of exhaust is an aerospace engineer’s boogeyman. Aerospike engines solve this through engine bell geometry.  The shape of the engine bell and ambient air pressure are the two main determinants for thrust efficiency.

Keeping the exhaust gasses as laminar as possible for as long as possible can be done in two ways, altering the geometry of the engine bell or the velocity of the fuel and oxidizer mix in the engine’s injector plate and turbo pump assembly. For example, the space shuttle’s Aerojet Rocketdyne RS-25 main engines overcame this problem through sheer power, its exhaust velocity was well over Mach 4 which allows it to not experience the thrust loss through altitude. This comes at a cost for fuel efficiency, The shuttle’s main engines sucked through over 170 pounds of fuel every second. The aerospike engine avoids the fuel efficiency problem through a unique engine bell shape. The tapered wedge shape allows exhaust gasses to be focused down the sides of the slopes of the engine bell. This allows for the exhaust to stay laminar at extremely high altitudes without a significant loss in thrust and fuel efficiency as they are 20% to 30% more efficient.

This solution seems like the obvious from an engineering standpoint, but it has a significant number of drawbacks that make these engines extremely impractical. The most poignant issue is one of cooling. In propulsion waste heat can be done in two different ways, passively and actively. A primary example of passive cooling of a rocket engine is seen in ULA’s Delta IV heavy. The RS-68 main engines are lined with a graphite/silica mesh. This mesh is designed to disintegrate mid flight or ablate. The ablative material is designed to take the heat away with the shards that are discarded. This is the cheapest way of cooling an engine, but also unsustainable for repeated use. The second method is active cooling. This is done by running the cryogenic oxidizer through coolant pipes inside the engine bell itself. For example, the previously mentioned RS-25 uses the same system. The only problem is that this method is extremely expensive. The aerospike engines that are currently being tested have active and passive cooling measures in place, but it is still not enough to cool the mammoth engine.

Other than overheating issues, Aerospike engines are extremely complicated pieces of hardware, even compared to traditional engines. The exhaust cones themselves take hundreds of hours of careful machining and along with the added plumbing need to feed the 20 individual combustion chambers and additional calibration of each individual nozzle. This is since this design is relatively new and never done under a large production scale.

The added complexity also plays into the engine’s weight, these engines are extremely heavy weighing in at 1400kg per engine. The weight is where this engine loses its advantages, the fuel efficiency is negated due to the increased weight, and since they are so expensive most rockets would have better performance using a traditional style of engine.

Conclusively speaking, aerospike engines need to overcome the weight, complexity, and heating hurdles to be the next generation of propulsion systems. Aerojet has been making great strides in bringing the cost down and the simplicity higher, but there is still a long way until they are commonplace in the ever-evolving industry.