The Insane Engineering of the X-15

Well, here we go along the battle of the space race between the United States and the Soviet Union. Both unions experimented with remarkable and experimental technologies in the search for the data and wisdom necessary to conquer this new frontier. The task of collecting this data itself was a tremendous challenge that required new aircraft capable of reaching the edge of space and pushing the limits of human understanding. One aircraft that stood out during the rise of the space race was the x-15, an aircraft designed To be the first to break into the hypersonic regime and pass the Carmen Line at 100 kilometers above the surface of the Earth and go into space, the plane would help NASA develop the materials necessary to survive the intense heat of re-entry, the necessary structures to ensure stability and control in the hypersonic flight regime and the development of control mechanisms for the vacuum of space while providing the impetus to develop several new technologies necessary to enable humans to survive the vacuum of space as the first of its kind fully pressurized spacesuit This was the world's first space plane The plane laid the foundation for both the Apollo program The space shuttle and the SR-71 To this day, the plane holds the record for the fastest manned flight ever performed with a top speed of mach 6.7, leaving even the sr-71 in the dust as this rocket-powered aircraft raced towards the edge of space this is the crazy

engineering

of the x-15 when the x-15 was first proposed For the first time in the 1950s no other aircraft came even close to its proposed capabilities in both maximum altitude and maximum speed.

The closest any previous aircraft came was the x2, which achieved a top market speed of 3.2, less than half of the final record that the x-15 would achieve, the x-15 was not a step forward in capabilities, it was a tremendous leap that would require the best minds at NASA or La naca, as it was called then, the first step in this The path to the 6.7 mach record was to develop an engine capable of propelling an aircraft of this type and to do this the designers had to resort to rocket propulsion, including advanced hybrid engines of the yet to be developed.

The sr-71 could not enter the hypersonic regime and no air-breathing engine could operate at the altitudes at which the x-15 was aimed. Engineers knew that the engine they needed would have to produce about 240 kilonewtons of thrust at sea level. With the ability to vary thrust power while fitting the narrow body of the aircraft, this powerful engine did not exist and developing it would prove to be one of the biggest challenges facing the X-15. The first problem to be solved was this variable thrust power which was desired to give pilots more control over the aircraft and allow testing at various speeds firing directly into the hypersonic regime without extensive testing at lower speeds would have proven disastrous as heating difficulties. due to friction had not yet been resolved.

Older engines such as those in the bell that this and needed to achieve without adding significant weight and complexity to the engine would decrease safety, endangering any pilot, while any additional weight would significantly reduce the maximum altitude the aircraft could reach. The x-15 achieved this control by varying the speed of its turbo pump, which is the pump that forces the oxidizer. and fuel from their respective storage tanks to the combustion chamber, pumping fluid at the rate that a rocket consumes, is actually a tremendously difficult challenge. The x-15 carried 8,165 kilograms of fuel and oxidizer that the plane burned in 85 seconds.

That is, 5897 kilograms per minute, that task would require a powerful bomb and that bomb would need a powerful power source. Now it may seem like an obvious choice to simply use a portion of that rocket fuel to power the bomb, and in fact, that's how modern rockets like Spacex's Merlin engine work. fuel their turbopumps Turbopumps work by spinning a turbine using hot, rapidly flowing gas, but using the products of rocket fuel combustion in a rapidly spinning turbine would lead to severely melted and broken turbines the products of rocket fuel combustion simply They are too hot for this application.

The Merlin engine avoids this by using a very rich fuel mixture for the turbo pump preburner, which causes incomplete combustion and lower exhaust temperatures. That exhaust contains a large portion of useful fuel, but the exhaust is not suitable for adding it. to the main thrust chamber so that the fuel is simply dumped overboard. You can see fuel-rich study gas coming out of the exhaust. Here in the Merlin engine, the x-15 used a completely separate fuel to drive its turbo pump. A monopropellant such as hydrogen peroxide is decomposed. in an exothermic reaction when in the presence of a catalyst, in this case hydrogen peroxide passed through a silver screen catalyst which caused the hydrogen peroxide to decompose into oxygen and superheated steam at 737 degrees, it was this superheated steam which drove the turbine and the speed of the turbine could be controlled simply by adjusting the amount of hydrogen peroxide passed over the silver catalyst with the use of control valves.

The exhaust from this system was simply dumped overboard through this exhaust port. This was not the only use of hydrogen peroxide in the x. -15 A similar system powered the auxiliary power system or APU that powered the plane's electronics, the pilot would also need some form of control when outside the Earth's atmosphere, where the plane's aerodynamic control surfaces would no longer function, so The aircraft was equipped with thrusters on the wingtips and nose to provide control while in space. These thrusters were also powered by hydrogen peroxide. Using hydrogen peroxide to drive the turbo pump presented some challenges.

This turbine operated two separate impellers, one for the liquid oxygen storage tank operating at 13,000 rpm and one for the anhydrous ammonia tank operating at 20,790 rpm. These different pumping speeds ran on the same drive shaft, which required gearing to achieve the proper fuel mixtures but also incorporated serious safety concerns about accidental fuel leaks from the respective hydrogen peroxide, liquid oxygen, and anhydrous. The ammonia alliance as a rotating shaft is more difficult to ensure a proper seal. Double seals were placed between each section in an effort to prevent mixing while a pressurized helium system purged the system. The choice of liquid oxygen and anhydrous ammonia was interesting for this engine.

It had to be powerful, extremely powerful, and to achieve the required thrust levels, the right combination of fuel and oxidizer would be needed. When talking about rocket power capabilities, one of the first stops is specific impulse. Specific impulse describes how efficiently a fuel can convert its mass into thrust. To understand this, let's first look at total impulse which describes the thrust force generated during the entire firing period of the engine. We can graph this quite easily by plotting the thrust the engine provides each second of its flight, which can look like this This total thrust is found by finding the area under this graph which gives us the total energy released by the rocket.

This is a useful metric in itself, but specific impulse is better because not all propellants are born equal. Two different combinations of fuel and oxidizer could provide the same total thrust, but we must consider the weight of the fuel and the oxidizers themselves, after all, the initial weight of rockets is always dominated by the weight of their own fuel. To find the average specific impulse, we divide the total impulse by the total weight of the propellant. the rocket ejected by this metric a fuel mixture of liquid hydrogen and liquid oxygen is by far the best hydrogen has the lowest molecular weight of any known substance each hydrogen atom consists of only one electron and one proton the h2 molecules used in liquid hydrogen fuel has a molecular weight of only two, while rp1, the kerosene-derived fuel used for the spacex merlin engine, has a molecular weight of 175.

However, molecular weight is not the only factor for To determine specific impulse, we must also consider a multitude of other factors, such as fuel. mixture ratios combustion temperatures pressure ratios and specific heat ratios This is where a nice simple specific impulse value gives us a clearer understanding of how much thrust per unit weight a fuel and oxidizer combination could provide without delving too deeply into the complicated physics and chemistry and If we look at this value, hydrogen is best at about 381 seconds at sea level, while the kerosene and oxygen combination of the Merlin engine has a specific impulse of about 289 seconds.

However, once again, it is not as simple as choosing the highest specific impulse value because hydrogen has a very low density, which means we need a much larger volume tank. It is also a difficult fuel to handle, as it will evaporate if allowed to rise above its extremely cold boiling point of -250 degrees Celsius, requiring isolation of boiling valves and last-minute refueling to start. A small molecule can leak through the smallest holes, even the spaces between larger molecules of a seemingly solid metal, despite its potential, hydrogen was not up to this task, but would soon be used for the first time with the stage superior of the centaur after many years of development problems: there was a great deal of experimentation during this period to find a fuel and oxidizer mixture that would provide the specific impulse needed to take the aircraft to hypersonic speeds and it was not just a matter of loading the most powerful combinations of fuel and oxidizer.

In fuel tanks, increased specific impulse is directly related to elevated temperatures in the combustion chamber, as fuels with higher impulses generally release more energy when ignited. This is one of the main obstacles that engineers of this era had to face in terms of the materials and designs necessary to survive. These extreme temperatures simply did not exist. The traditional fuel of the time was a mixture of 75 alcohol and 25 percent water with a liquid oxygen oxidizer that has a specific impulse of about 269 seconds, not high enough. Water was added to this mixture primarily to reduce combustion. chamber temperature which of course reduced the engine's boost to achieve that higher specific impulse, engineers needed to find a way to allow the engine to survive the elevated temperatures that would come with higher boost fuel and the only way to do this was by finding better materials or finding a way to actively cool the engine, ideally both ways to achieve this was through regenerative cooling regenerative cooling uses one of the propellants, usually fuel as the cooling fluid, the fuel will be pumped through of heat exchange pipes that surround parts exposed to dangerous heat such as the injector nozzle thrust chamber and the nozzle where it can extract heat from the metals it comes into contact with before being injected into the thrust chamber.

This was not a new concept, the v2 rocket that used that 75-25 alcohol. The mixture also employed regenerative cooling, but the heat transfer rates were not very high to be an effective cooling fluid. The fuel must have a high specific heat capacity, meaning that it can absorb a lot of thermal energy before its own temperature increases. Water has a high specific heat. capacity of approximately four thousand two hundred joules per kilogram kelvin, which means that four thousand two hundred joules of thermal energy are needed to heat one kilogram of water in one kelvin. We also want the fluid to have a high latent heat of vaporization, meaning it takes a lot of energy to vaporize the fluid, we don't want the fluid to turn into a gas in the cooling tubes.

Here the water is strong again and boils at 100 degrees Celsius and that number will be even higher when pumped under pressure, so now we are looking for a fuel. thatNot only does it have a high specific impulse but it also has great cooling properties, kerosene was considered to have a slightly improved specific impulse of 289 over the traditional water and alcohol concoction and was cheap and freely available at that time, however, when it happened through the cooling tubes, kerosene of this era had a nasty habit of forming clumps of impurities, this process is called polymerization or coking and is accelerated when exposed to the heat of the cooling tubes, which could clog the tubes thin and cause major problems.

The rp-1 grade kerosene fuel we use today was developed to combat this problem by removing impurities from hydrazine fuel, which has a specific impulse of about 303, but had a nasty habit of exploding when used in regenerative cooling. since its exothermic decomposition process can begin at a temperature as low as 97 degrees. which can lead to a violent explosion, eventually the engineers who were missing a few fingers at this point landed on anhydrous ammonia since its fuel, ammonia is a fantastic refrigerant fluid with an extremely high heat capacity and a high latent heat of vaporization , which makes it the ideal. rocket fuel for regenerative cooling with a higher specific impulse than its water and alcohol ancestors at 293 seconds, however ammonia has its own problems, it is toxic and would attack many metals such as copper.

The copper-containing pressure gauges of the difficulties and went over both time and budget while the airframe had to go through parallel development without the final engine, two xlr 11 engines that had previously powered the bell x1 were used instead, these provided enough power to take the aircraft to 3.3 and test some of the aircraft's flight performance characteristics, but did not meet the requirements for hypersonic flight in Meanwhile, data on the X-15's hypersonic flight characteristics were collected using advanced systems. hypersonic wind, but engineers had no idea whether this data would be accurate. This was still a very new field of research.

The design and requirements of a hypersonic aircraft that could possibly fly into space were so radically new and different that traditional aerodynamics textbooks had to be modified. Leaving it at the door this was going to require a completely new approach with all assumptions thrown out, for example during the development of the x-15 a debate developed at the Naca Ames research facility about nose design for hypersonic aircraft such as this. Julian Allen argued that any aircraft flying in this flight regime should be designed with a blunt body, something that completely contradicted the established thinking of the time which called for an extremely pointed nose in an effort to reduce drag.

Julian Allen argued that this blunt body design would create an arc shock wave that would create a boundary layer of air around the vehicle and ensure that extreme frictional heat was kept away from the airframe and instead dissipated harmlessly. in the atmosphere. The X-15 incorporated these ideas into all aircraft. leading edges, including the nose and wings, and the idea would be applied to all reentry vehicles in the future, when the x-15 re-entered the Earth's atmosphere, it would take a very high angle of attack to reduce speed to 20 degrees. angle of attack, the upper vertical tail became completely useless as it was severely protected from airflow by the body of the aircraft, while the lower tail would experience a marked increase in effectiveness by plunging into the high pressure zone caused by the side compression of the wing, so this lower tail was essential to ensure yaw stability in these high angle of attack reentries, but this lower ventral tail was so large that it made landing on the aircraft's shorter skids impossible, so that the pilot had to ditch a section before landing. where he would deploy a parachute to land gently and hopefully without damage.

The X-15's vertical tail shape is one of the aircraft's most distinctive features. The primitive-looking wedge profile looks like something someone could have designed with 300 years of fluid knowledge. Interestingly, that's exactly what it was designed with in 1687. Newton described an equation in his groundbreaking book Principia that predicted the force a flat plate would experience in a moving fluid. He imagined the air as a stream of particles that would hit the plate and transfer everything. of their normal momentum to the surface and then traveled parallel to the plate, he also assumed that the particles did not interact with each other and that there was no random motion, this of course is incorrect, the complex fluid fields in this situation are much more complicated than what Newton predicted. but, interestingly, his equation fairly accurately approximates the forces on an airfoil in a hypersonic flow.

Let's look at the surface of the wedge tail as its Mach number increases at supersonic speeds. A shock wave will form at the tip of the wing. This is called an oblique shock wave. and its angle becomes smaller as the Mach number increases until at hypersonic speeds the angle becomes so small that it almost coincides with the wedge angle. Interestingly, this closely resembles what Newton predicted for subsonic airflow, and in fact his equation becomes increasingly more precise as the Mach number increases. The number increases and as the wedge shape begins to act predictably with this simple equation, normal thin curved airfoils designed with subsonic fluid dynamics in mind begin to experience a dramatic loss of lift, making them essentially Useless at hypersonic speeds, the wedge tail continues to function and provide stabilization. pressure needed to keep the aircraft flying straight, this carries a high drag trade-off as the blunt end creates a narrow zone of pressure behind it which drags the aircraft backwards, but this was of no small concern for a short-range aircraft. reach that needed to slow down.

In fact, the wedge-shaped tail was equipped with extendable speed brakes to further increase this braking effect when returning from its high-speed runs, flying at hypersonic speed not only had strange aerodynamics, the heat of hypersonic speeds It was one of the biggest challenges I faced. Took on the This temperature was much lower than the X-15 was expected to experience at Mach 6 and above. The effect of frictional heating does not scale linearly. SR-71 titanium could withstand. Dealing with the extreme external heat was difficult enough, but the designers also had to deal with the extreme cold emanating from the internal cryogenic liquid oxygen fuel tanks.

In the images of the underside of the x15 you can frequently see frost covering the belly of the plane where the plane was located. There are liquid oxygen tanks located. There is only one metal on earth for this task on channel x. Inconel for aluminum, titanium and stainless steel it looks like this now, if we plot in the x channel we can see how good it is at maintaining its strength at extremely high temperatures, however inconel is heavy, the designers estimated that a Inconel X airframe would weigh about 180 percent more. than an equivalent airframe made of aluminum and this was before ablative materials were applied to allow higher speed runs, the ability to maintain its strength at elevated temperatures was beneficial, but there were many more problems to solve.

The non-uniform heat distribution adapted to the temperature. Expansion and tensioning were extremely difficult and several redesigns of the airframe were necessary to fix problems that arose along the way during the plane's first Mach 6 flight. One of the quartz windows broke rather worryingly in mid-flight when the channel structure bent due to Fortunately, only the outer panel broke and the pilot survived to tell the tale. The structure metals were quickly changed to titanium, which experiences less thermal expansion, and the rear part of the structure was completely removed for a very interesting reason that the designers discovered during high speed testing that the aircraft was experiencing a extreme local heating in strange places, one such hot spot appeared behind the window and was the result of shock waves creating turbulent flow.

These turbulent flows create areas of high heat transfer to the aircraft skin that create hazards. Hot Spots To find and eliminate these hot spots, designers employed a special type of heat-sensitive paint that changed color when exposed to certain temperatures. After high-speed flight, the plane returned with wedge-shaped patterns emanating from leading edge expansion joints which were small gaps in the leading edge to prevent it from buckling when the inconel x expanded during flight. These gaps were creating this turbulent flow and to fix this, the engineers installed small strips of inconel x over the expansion joints in an attempt to minimize the turbulent zones they made.

They become smaller but were not completely removed for the eventual world record flight. Inconel X alone would not ensure the survival of the plane. For this, the plane would need an ablative material. heat with it, one of the main missions of the x-15 was to develop these materials. Multiple materials and application systems were tested throughout the x-15 program and many problems were found. Bonding ablative materials to the metal surface proved difficult, some simply fell off as the underlying metal expanded underneath and could not stretch with it. These problems were encountered on flights slower than Mach 5, but if they appeared during the full speed attempt, the plane would probably have been lost.

Another problem arose when the material ablated after burning. far from the nose of the plane it began to stick to the windows of the plane, making it extremely difficult for the pilot to see, which was a bit of a problem. Engineers looked for several solutions to the problem, one of which involved deliberately blowing up the plane's outer panel. glass to remove the ablatively stained portion, engineers ultimately opted for a less risky solution by installing a mechanical eyelid on the left window that remained closed until the high-speed portion of the flight concluded, ensuring that the pilot had at least one window to look during landing, this was a relatively primitive solution and created some stability problems, as once open the eyelid acted as a canard and caused the aircraft to pitch slightly upwards, roll rice and yaw rice, an annoying but manageable problem , a slightly scarier problem arose with the final ablative.

Material: This pink material called ma35s was sprayed on the surface of the plane in various thicknesses according to local needs. It worked well, but it had one major obvious drawback: when mixed with liquid oxygen, the material became explosive and could be activated by a slight impact. A little worrying considering the plane's oxidizer was liquid oxygen and spills were not uncommon, especially since the plane had to be continually refueled from its B-52 dock as it climbed to altitude, the spray method could also introduce the ablative material into the oxidant. To minimize this terrifying prospect, the aircraft was sprayed with a secondary coat of white sealant to prevent liquid oxygen from mixing with the ablative and had the added benefit or drawback of hiding the glorious pink color after a decade of in-flight development. 188. of the x-15, the plane was finally ready for its record flight on October 3, 1967, William Caballero climbed into the cockpit of the x-15 hanging from hishanger under the wing of the giant B-52 that took the plane up to 45,000 feet here they fell and ignited the rocket engine and with the help of two external fuel tanks they roared for two and a half minutes pushing the plane towards the simulated flight of 6.7 still for breaking, in the attempt the plane was destroyed by the ablative.

The coating had not worked as well as expected and the plane landed with parts of the skin melted away, it would not fly again. The two remaining aircraft in the program flew only 11 more times in total before the program was shut down. During NASA's 199 flights, NASA's x-15 obtained some of the most valuable data ever collected. broke speed records but also altitude records when on July 17, 1962, Robert White became the first man to fly an airplane into space. The knowledge that NASA collected through This program advanced our understanding of rocket engine design. , turbulent flow, localized heating, ablative materials, and hypersonic stability and control, all of which contributed to the design and development of the Mercury Gemini Apollo and space shuttle programs and provided Neil Armstrong with invaluable experience in rocket control.

Spacecraft Powered Armstrong was a fascinating man, someone I knew very little about until I watched this documentary out of curiosity. He aired an hour and 40 minute long documentary that captivated me throughout the entire process. I was inspired by the story of a young man who became fascinated with modeling airplanes at a young age and pursued that passion in every step of his life, becoming a licensed pilot at the age of 16, entering an aerospace

engineering

program at the age of 16. 17 through a military scholarship before being drafted as an aviator in the Korean War, a springboard toward his eventual career as an experienced test pilot and, of course, astronauts, the life of a man driven by a deep passion for aviation which led him to a life of greatness that will never be forgotten.

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