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  • Throughout the battle of the space race between the United States and Soviet Union, both unions

  • experimented with remarkable and experimental technologies in the pursuit of the data and

  • wisdom required to conquer this new frontier. The task of gathering this data itself was

  • a tremendous challenge that required new aircraft, capable of reaching the edge of space and

  • pushing the boundaries of human understanding. One plane that stands out during the ascent

  • of the space race was the X-15. A plane designed to be the first to break into the hypersonic

  • regime and climb past thermán line, 100 kilometres above the earth’s surface, and

  • break into space. The plane would help NASA develop the materials needed to survive the

  • intense heat of re-entry, the structures need 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 required to allow humans to survive

  • the vacuum of space, like the first of its kind fully pressurised space suit. This was

  • the world’s first space plane. [1]

  • The plane laid the groundwork for both the Apollo Program, the Space Shuttle and the

  • SR-71.

  • To this day the plane holds the record for the fastest ever crewed flight with a top

  • speed of 6.7 mach. Leaving even the SR-71 in the dust as this rocket powered plane powered

  • through into the edge of space. This is the insane engineering of the X-15.

  • When the X-15 was first proposed in the 1950s, no other aircraft came even close to its proposed

  • capabilities in both max altitude and max speed.

  • The closest any previous plane came, was the X-2, which topped out at a max speed of Mach

  • 3.2. Less than half of the eventual record the X-15 would achieve.

  • The X-15 wasn’t just a step forward in capabilities, it was a tremendous leap that would require

  • the best minds in NASA, or the NACA as it was called then. The first step on this road

  • to the record 6.7 mach, was developing an engine capable of powering such an aircraft,

  • and for this, the designers had to turn to rocket propulsion.

  • Even the advanced hybrid engines of the yet to be developed SR-71 couldn’t push into

  • the hypersonic regime, and no air-breathing engine would be able to function at the altitudes

  • the X-15 was targeting.

  • The engineers knew the engine they required would need to produce around 240 kilonewtons

  • of thrust at sea level with an ability to vary thrust output, while fitting into the

  • narrow body of the plane. This powerful engine did not exist, and developing it would prove

  • to be one of the greatest challenges facing the X-15.

  • The first problem to solve was this variable thrust output, which was desired to give pilots

  • more control over the aircraft and allow for testing at various speeds. Blasting straight

  • into the hypersonic regime without extensive testing at lower speeds would have proved

  • disastrous as the difficulties of frictional heating were solved.

  • Older engines, like those of the Bell X-1, the first plane to break the sound barrier,

  • achieved variable power output by simply selectively igniting 4 separate combustion chambers. This

  • provided stepped power output, but not true throttle.

  • The X-15 needed finer control than this, and needed to achieve it without adding significant

  • weight and complexity to the engine. Added complexity would decrease the safety, putting

  • any pilot in danger, while any added weight would significantly reduce the maximum altitude

  • the plane could achieve.

  • The X-15 achieved this control by varying the speed of it’s turbopump, which is the

  • pump which forces the oxidiser and fuel from their respective storage tanks into the combustion

  • chamber.

  • Pumping fluid at the rate a rocket consumes it is actually a tremendously difficult challenge.

  • The X-15 carried 8,165 kilograms of fuel and oxidiser, which the plane burned through in

  • just 85 seconds. That’s 5897 kilograms per minute.

  • That task would require a powerful pump, and that pump would need a powerful energy source.

  • Now, it may seem like an obvious choice to simply use a portion of that rocket fuel to

  • power the pump, and indeed this is how modern rockets, like the Space-X Merlin engine power

  • their turbopumps. [3]

  • Turbopumps operate by spinning a turbine using hot fast flowing gas, but using the products

  • of rocket fuel combustion in a spinning turbine would quickly lead to severely melted and

  • broken turbines.

  • The combustion products of rocket fuel are simply too hot for this application.

  • The Merlin engine gets around this by using a very fuel rich mixture for the turbopump

  • pre-burner, which leads to incomplete combustion and lower exhaust temperatures. That exhaust

  • has a large portion of useful fuel contained within it, but the sooty exhaust is not suitable

  • for addition to the main thrust chamber.

  • So, that fuel is simply dumped overboard. You can see that fuel rich sooty gas coming

  • out of this exhaust here on the Merlin engine.

  • The X-15 used an entirely separate fuel to power it’s turbopump. A monopropellant in

  • the form of hydrogen peroxide. A monopropellant, like hydrogen peroxide, decomposes in an exothermic

  • reaction when in the presence of a catalyst. In this case hydrogen peroxide was passed

  • through a silver screen catalyst which caused the hydrogen peroxide to decompose into oxygen

  • and superheated 737 degree steam. It was this superheated steam that drove the turbine,

  • and the speed of the turbine could be controlled by simply adjusting the amount of hydrogen

  • peroxide passing over the silver catalyst with the use of control valves.

  • The exhaust of this system was then simply dumped overboard through this exhaust port.

  • This was not the only use for hydrogen peroxide on the X-15.

  • A similar system powered the auxiliary power system, or APU, which powered the plane’s

  • electronics.

  • The pilot would also need some form of control when outside of earth’s atmosphere, where

  • the plane’s aerodynamic control surfaces would no longer work, so the plane was fitted

  • with thrusters on the wing tips and nose to provide control while in space, these thrusters

  • were also powered by hydrogen peroxide.

  • Using hydrogen peroxide to power the turbopump came with some challenges. This turbine operated

  • two separate impellors, one for the liquid oxygen storage tank, which operated at 13,000

  • RPM, and one for the anhydrous ammonia tank, which operated at 20,790 RPM. These different

  • pumping speeds ran on the same drive shaft, which necessitated gearing to achieve the

  • appropriate fuel mixtures, but also incorporated serious safety concerns over accidentally

  • fuel leakage from the respective hydrogen peroxide, liquid oxygen and anhydrous ammonia

  • lines, as a spinning shaft is more difficult to ensure adequate sealing. Double seals were

  • placed between each section in an effort to prevent mixing, while a system of pressurised

  • helium purged the system.

  • The choice of liquid oxygen and anhydrous ammonia was an interesting one. This engine

  • needed to be powerful, extremely powerful, and getting it to the required thrust levels

  • was going to need the right fuel and oxidizer combination. [Page 95 of Ignition]

  • When speaking of 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 over the entire burn period of the engine. We can graph this rather easily, by

  • plotting the thrust the engine is providing in each second of its flight, that may look

  • something like this.

  • The total impulse is found by finding the area under this graph, which gives us the

  • total energy the rocket released.

  • This is a useful metric in itself, but specific impulse is better, because not all propellants

  • are born equal. Two different fuel and oxidiser combinations could provide the same total

  • impulse, but we need to consider the weight of the fuel and oxidisers 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 total propellant weight

  • the rocket expelled. [4]

  • Going by this metric, a liquid hydrogen and liquid oxygen fuel mixture is by far the best.

  • Hydrogen has the lowest molecular weight of any known substance, each hydrogen atom consisting

  • of just 1 electron and 1 proton.

  • The H2 molecules used in liquid hydrogen fuel have a molecular weight of just 2.016, while

  • RP-1, the kerosene derived fuel used for the Space-X merlin engine, has a molecular weight

  • of around 175. [5]

  • However, molecular weight is not the only factor in determining specific impulse, we

  • also need to consider a multitude of other factors like fuel mixture ratios, combustion

  • temperatures, pressure ratios and specific heat ratios. [8] This is where a nice simple

  • specific impulse value gives us a clear understanding of how much thrust per unit weight a fuel

  • and oxidiser combination could potentially provide, without delving too much into the

  • complicated physics and chemistry.

  • And looking at this value, hydrogen is the best at around 381 seconds at sea level. While

  • the kerosene and oxygen combination of the merlin engine has a specific impulse of about

  • 289 seconds. [6]

  • However, it’s once again not as simple as picking the highest specific impulse value,

  • because hydrogen has a very low density, meaning we need a much larger volume tank.

  • It’s also a difficult fuel to handle, as it will boil off if allowed to rise above

  • it’s extremely cold boiling point of minus 250 degrees celsius, requiring insulation,

  • boil off valves and last minute fueling. To boot, this tiny molecule can seep out of the

  • tiniest of holes, even the gaps between larger molecules of seemingly solid metal. Despite

  • its potential, hydrogen was not ready for this task, but would soon be put to use for

  • the very first time with the Centaur upper stage after many years of development hiccups.

  • There was a great deal of experimentation during this period to find a fuel and oxidiser

  • mixture that would provide the specific impulse needed to get the plane to hypersonic speeds,

  • and it wasn’t just a matter of loading the most powerful fuel and oxidizer combinations

  • into the fuel tanks.

  • Increasing the specific impulse is directly linked to elevated combustion chamber temperatures,

  • since fuels with higher impulses generally release more energy when ignited. This is

  • one of the major hurdles engineers of this era had to contend with, as the materials

  • and designs needed to survive these extreme temperatures simply did not exist yet.

  • The traditional fuel of the time was a 75% alcohol 25% water mixture with a liquid oxygen

  • oxidiser, which has a specific impulse of about 269 seconds. Not high enough. The water

  • was added into this mix primarily to reduce the combustion chamber temperature, which

  • of course, reduced the impulse of the engine. [9]

  • To achieve that higher specific impulse, the engineers needed to figure out a way to allow

  • the engine to survive the elevated temperatures that would come with a higher impulse fuel,

  • and the only way to do this was by finding better materials or find a way of actively

  • cooling the engine. Ideally both.

  • One way they achieved this was through regenerative cooling. Regenerative cooling uses one of

  • the propellants, usually the fuel, as a cooling fluid. The fuel will be pumped through heat

  • exchange piping that wrap around parts exposed to dangerous heat, like the injector nozzle,

  • thrust chamber and nozzle, where it can draw heat away from the metals it comes in contact

  • with, before being injected into the thrust chamber.

  • This was not a new concept, the V2 rocket, which used that 75/25 alcohol mixture also

  • employed regenerative cooling, but the heat transfer rates were not terribly high.

  • To be an effective cooling fluid, the fuel needs to have a high specific heat capacity.

  • Meaning, it can absorb a lot of heat energy before it’s own temperature rises. Water

  • has a high specific heat capacity of about 4200 joules per kilogram kelvin. Meaning it

  • takes 4200 joules of heat energy to heat 1 kilogram of water by 1 kelvin. [12] We also

  • want the fluid to have a high latent heat of vaporization, which just means it takes

  • a lot of energy to vaporize the fluid. We don’t want the fluid turning into a gas

  • in the cooling tubes. Here water is strong again, boiling at 100 degrees celsius, and

  • that number will be even higher when it is pumped under pressure.

  • So now we are looking for a fuel that not only has high specific impulse, but with great

  • cooling properties too.

  • Kerosene was considered with a slightly improved specific impulse of 289 over the traditional

  • alcohol/water concoction, and was cheap and freely available at the time.

  • However, when passed through cooling tubes kerosene of this era had a nasty habit of

  • forming clumps of impurities, this process is called polymerisation or coking, and accelerated

  • when exposed to the heat of the cooling tubes, which could clog the thin tubes and cause

  • some major problems. The RP-1 grade kerosene fuel we used today was developed to combat

  • this problem by removing the impurities from the fuel.

  • Hydrazine, which has a specific impulse of about 303, was also considered, but it had

  • the nasty habit of exploding when used in regenerative cooling. As its exothermic decomposition

  • process can start at a temperature as low as 97 degrees, which can lead to a violent

  • explosion. [10]

  • Eventually the engineers, who may have been short a few fingers at this point, landed

  • on anhydrous ammonia as their fuel.

  • Ammonia is a fantastic cooling fluid with an extremely high heat capacity [4.6 - 6.7

  • KJ/KG.K] and high latent heat of vaporization [1369 KJ/KG], making it the ideal rocket fuel

  • for regenerative cooling, with a higher specific impulse over it’s alcohol/water ancestors

  • at 293 seconds. [13]

  • However, Ammonia does come with its own issues. It’s toxic and would attack many metals,

  • like copper. The pressure gauges of the X-15, which contained copper were consistently failing

  • after 6 months of use, despite not being in direct contact with the fuel. This was annoying,

  • but deemed an acceptable trade off for the fuels benefits. [14]

  • This development process of the engine was fraught with difficulties and ran over both

  • time and budget, meanwhile the airframe had to undergo parallel development without the

  • final engine, instead using two XLR-11 engines, which had previously powered the Bell X-1.

  • These provided enough power to get the plane to 3.3 Mach and test some of the planes flight

  • performance characteristics, but fell well short of the requirements for hypersonic flight.

  • [16]

  • In the meantime, data on hypersonic flight characteristics of the X-15 were gathered

  • using advanced hypersonic wind tunnels, but the 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 left at the

  • door. This was going to require a completely fresh approach with all assumptions thrown

  • out.

  • During the development of the X-15 a debate was raging in the NACA Ames research facility

  • over the design of the nose for hypersonic aircraft like this.

  • Julian Allen argued that any aircraft flying in this flight regime should be designed with

  • a blunt body, something that completely contracted the established thought of the era, which

  • demanded for extremely pointed noses in an effort to reduce drag.

  • Julian Allen argued that this blunt body design would create a bow shockwave which would create

  • a boundary layer of air around the vehicle and ensured the extreme frictional heat was

  • kept away from the structure of the aircraft and instead dissipated harmlessly into the

  • atmosphere. The X-15 incorporated these ideas into all of the plane’s leading edges, including

  • the nose and wings. [15]

  • And the idea would be applied to all re-entry vehicles in future.

  • As the X-15 reentry earth’s atmosphere it would be taking a very high angle of attack

  • approach to bleed off speed. An angle of attack of 20 degrees rendered the upper vertical

  • tail completely useless, as it was severely shielded from airflow by the body of the aircraft,

  • whereas the lower tail would experience a marked increase in effectiveness as it dipped

  • into the high pressure zone cause by the compression side of the wing.

  • So this lower tail was essential for ensuring yaw stability at these high angle of attack

  • re-entries. But this lower ventral tail was so large that it made landing on the plane’s

  • shorter skids impossible, so the pilot had to jettison a section of it before landing.

  • Where it would deploy a parachute to land softly, and hopefully undamaged.

  • The shape of the X-15s vertical tail X-15 is one of its most distinctive features of

  • the plane. The primitive looking wedge profile looks like something someone may have designed

  • with 300 year old knowledge of fluid dynamics. Oddly, that’s exactly what is designed with.

  • In 1687, Newton described an equation, in his groundbreaking book Principia, that predicted

  • the force a flat plate in a moving fluid would experience. He imagined the air as a stream

  • of particles that would strike the plate and transfer all of their momentum normal to the

  • surface and then travel parallel to the plate. He also assumed the particles did not interact

  • with each other and there was no random motion. This, ofcourse, is wrong.

  • The complex fluid fields in this situation are much more complicated than Newton predicted,

  • but bizarrely, his equation rather accurately approximates the forces on an aerodynamic

  • surface in hypersonic flow.

  • Let’s look at the wedge tail surface as it increases it’s Mach number. At Supersonic

  • speeds a shock wave will form at the point of the wing. This is called an oblique shock