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 the Kármán 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