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 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
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
wave and it’s angle becomes smaller as the mach number increases. Until, at hypersonic
speeds, the angle becomes so small that it almost matches the wedge angle. This looks
oddly a lot like what Newton predicted for subsonic airflow, and indeed his equation
becomes more and more accurate as the mach number increases.
And while this wedge shape begins to act predictably with this simple equation, normal thin curved
aerofoils designed with subsonic fluid dynamics in mind, begin to experience a dramatic loss
of lift, rendering them essentially useless at hypersonic speeds. The wedge tail continues
to perform and provide the stabilising pressure needed to keep the plane flying straight.
However, this does come with a tradeoff of high drag, as the blunt end creates a low
pressure zone behind it that drags the plane backwards, but this was of little concern
for a short range plane that needed to slow down quickly. In fact, the wedge tail was
fitted with extendable speed breaks to even further this breaking effect when coming back
from its high speed runs.
Flying at hypersonic speed did not just come with strange aerodynamics. The heat of hypersonic
speeds was one of the largest challenges that faced the X-15.
A very specialized metal was needed for this task. The SR-71 utilized titanium, and it
experienced a maximum temperature of about 300 degrees celsius on it’s pointed nose
and engine inlet spike during mach 3 flight. This temperature was vastly lower than what
the X-15 was expected to experience at Mach 6 and above. The effect of frictional heating
does not scale linearly. It would not be dealing with 600 degrees, but upwards of 1000 degrees.
Far beyond what the titanium skin of the SR-71 could handle. Having to deal with the extreme
external heat was difficult enough, but the designers also needed to contend with the
extreme cold emanating from the internal cryogenic liquid oxygen fuel tanks. In images of the
underside of the X-15 you can frequently see frost covering the belly of the plane where
the liquid oxygen tanks are located. There is only one metal on earth up for this task.
Inconel X.
Inconel X is a nickel, chromium, iron and niobium alloy that was capable of operating
at lower temperatures, while having extremely good heat resistance. Plotting tensile yield
stress against operating temperature for aluminium, titanium and stainless steel, looks something
like this. Now, if we plot Inconel X, we can see just how good it is at maintaining its
strength at extremely high temperatures.
However, Inconel is heavy. Designers estimated that an Inconel X airframe would weigh about
180% more than an equivalent airframe made from aluminium, and this was before the ablative
materials were applied, to allow for highest speed runs. [18]
The ability to maintain its strength at elevated temperatures was beneficial, but there were
plenty more problems to solve.
Non-uniform heat distribution made accommodating thermal expansion and stress extremely difficult,
and several redesigns of the plane’s structure was needed to fix problems that cropped up
along the way. During the plane’s first Mach 6 flight [21], one of the quartz windows,
quite worryingly, shattered mid flight when the inconel framing buckled due to thermal
expansion. Thankfully only the outer pane shattered and the pilot survived to tell the
tale. The framing metals were promptly switched to titanium, which experiences lower thermal
expansion, and the aft portion of the framing was removed entirely for a very interesting
reason.
The designers discovered during high speed tests that the plane was experiencing extreme
local heating in strange locations. One such hot spot was appearing behind the window,
and it was the result of shock waves creating turbulent flow. [20] These turbulent flows
created areas of elevated heat transfer into the skin of the aircraft that created dangerous
hot spots. To find and eliminate these hotspots the designers employed a special kind of heat
sensitive paint that would change color when exposed to certain temperatures.
After one high speed flight the plane returned with wedge shaped patterns emanating from
the leading edge expansion joints, which were small gaps in the leading edge to prevent
buckling when the inconel X expanded during flight. These gaps were creating this turbulent
flow and to fix it, engineers installed small strips of Inconel X over the expansion joints
in an attempt to minimize the turbulent zones. They did get smaller, but were not eliminated
completely.
For the eventual world record breaking flight, the inconel x alone would not ensure survival
of the plane. For this the plane would need an ablative material, a sacrificial material
designed to gradually burn and fall away from the aircraft, pulling the heat with it. One
of the principal missions of the X-15 was to develop these materials.
Multiple materials and application systems were tested throughout the X-15 program and
plenty of problems were found. Bonding the ablative materials to the surface of the metal
proved difficult. Some simply fell off when the underlying metal expanded underneath it
and it could not stretch with it. These problems were found at slower Mach 5 flights, but if
they appeared during the top speed attempt the plane likely would have been lost. [22]
Another problem arose when the ablative material, after burning away from the nose of the plane,
began attaching itself to the windows of the plane making it extremely difficult for the
pilot of see, which was a bit of a problem. The engineers looked at several solutions
to the problem. One involved deliberately exploding the outer pane of glass to remove
the ablative stained portion and leaving only the inner pane.
The engineers eventually landed on a less risky solution by installing a mechanical
eyelid to the left window that remained closed until the high speed portion of the flight
concluded, ensuring the pilot had at least one window to look out of during landing.
This was a relatively primitive solution and created some stability issues as once open
the eyelid acted like a canard, and caused the plane to slightly pitch up, roll right
and yaw right. An annoying but manageable problem.
A slightly more terrifying problem cropped up with the final ablative material. This
pink material, called MA-35S, was sprayed onto the surface of the plane in various thicknesses,
according to the local need. It worked well, but had one massive glaring draw back. When
mixed with liquid oxygen the material would become explosive and could be triggered by
a slight impact. [23] A bit worrying considering the plane’s oxidizer was liquid oxygen and
spillage was not rare, especially as the plane had to be continually topped up from it’s
B52 dock as it ascended to altitude. The spray on method could potentially introduce the
ablative material into the oxidizer lines too. To minimize this terrifying prospect
the plane was sprayed with a secondary white sealant coat to prevent the liquid oxygen
from mixing with the ablative, and came with the added benefit or disadvantage of hiding
the glorious pink colour.
After a decade of development, on the 188th flight of the X-15, the plane was finally
ready for it’s record breaking flight. On October 3rd, 1967, William Knight climbed
into the cockpit of the X-15 hanging from it’s perched underneath the wind of the
behemoth b-52, which carried the plane up to 45,000 feet. Here Knight dropped away and
ignited the rocket engine, and with the help of two external fuel tanks, they roared for
2 and a half minutes, pushing the plane to the yet to be broken record of 6.7 Mach flight.
[24]
In the attempt the plane was destroyed. The ablative coating hadn’t worked as well as
hoped and the plane landed with parts of it’s skin melted away. It would not fly again.
The remaining 2 planes in the program flew just another 11 times in total before the
program was shut down.
Through the 199 flights of the X-15, NASA gained some of the most valuable data it has
ever gathered. The X-15 not only broke speed records, but altitude records, when on July
17th 1962, Robert White, became the first man to fly a plane to space. The knowledge
NASA gathered through this problem advanced our understanding in 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, the pilot of the lunar lander and
first man on the moon, with invaluable experience in controlling a rocket powered spacecraft.
Armstrong was a fascinating man. Someone I knew very little about until I watched this
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Becoming a licensed pilot at the age of 16, entering an aerospace engineering program
at 17 through a military scholarship, before being drafted as an aviator in the Korean
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