I’d like to continue the discussion about
rockets today and go into some details of
rocketry.
Before that I want to briefly mention that
there was work done in the mid 20th century
to develop airplanes that could go to space.
In the 1950s and 1960s, airplanes were more
developed as a means of transporting people
than were rockets so it wasn’t crazy to
think that more powerful airplanes would be
the vehicles that took humans to space. I’d
like to discuss one airplane in particular.
This is the X-15 airplane. The X is for experimental
and it still to this day holds the record
for the fastest crewed airplane flight. During
one flight in 1967, William Knight flew at
Mach 6.7, which is nearly 7 times as fast
as sound speed. This is about 7,200 kilometers
per hour or 4,500 miles per hour. Also, two
X-15 flights (Flights 90 and 91) got above
the Kármán line at 100 kilometers (or 62
miles) above sea level. Both flights were
piloted by Joseph Walker. This technically
made Joseph Walker an astronaut since he was
above the altitude that is generally considered
“space.” Note that Earth’s atmosphere
doesn’t abruptly end as you go up from the
surface of the Earth. It gradually becomes
less dense to the point that airplanes find
it difficult to generate lift since there
isn’t enough air flowing over their wings.
As such the Kármán line is approximately
the altitude at which an airplane would need
to go as fast as an orbiting spacecraft to
generate enough lift to fly.
This video shows the X-15 flying. It was taken
to an altitude of about 13.7 km (or 8.5 miles)
by a B-52 mother ship and then released while
already traveling at about 800 km/hr or (500
miles/hr). One famous test pilot of the X-15
airplane was Neil Armstrong who went on to
be the first human to walk on the Moon.
Last time we saw two endmembers of rockets:
solid-fueled rockets and liquid-fueled rockets.
The Apollo program largely used liquid-fueled
rockets, so I’ll be focusing on those today.
Liquid-fueled rockets can be further divided
into monopropellant and bipropellant rockets.
We have seen a few examples of bipropellant
rockets. As the name implies, these rockets
like the one shown in the diagram have two
propellants: an oxidizer and a fuel. A monopropellant
only has one propellant. For example, hydrazine
(N2H4) can be used as a monopropellant. We
saw last time that while solid-fueled rockets
are relatively simple in design, they are
generally not as powerful as liquid-fueled
rockets. Also, liquid-fueled rockets have
the ability to be stopped during a mission
and restarted when needed. You may recall
that the Saturn V’s third stage needed to
do this to get into Earth orbit and then leave
towards the Moon. We also talked about how
it’s difficult to make sure that propellants
are going into the combustion chamber at the
necessary rate when the tanks are moving with
the rocket. As propellants slosh around the
tanks, a mechanism is needed to push the propellants
through the pipes.
There are two main ways to force the propellants
into the combustion chamber. One kind are
pressurized systems and the other kind are
turbopump systems. Let’s first look at pressurized
systems.
Pressurized systems are also called pressure-fed
cycle engines. Shown on the right is a schematic
of this type of propulsion system. The fuel
tank and oxidizer tank combination that feeds
to a combustion chamber is likely familiar
to you by now. However, this system has an
additional tank with a pressured gas. This
gas is taken from the tank and heated by the
combustion chamber. It then is fed to both
the fuel and oxidizer tanks to force those
propellants down into the combustion chamber.
While this system is relatively simple, it
does add weight due to the extra tank of pressurized
gas. In rocketry, you want to reduce the mass
of the rocket as much as possible. Another
thing is that you need to have the pressured
gas at a very high pressure to ensure that
as time progresses there is enough pressure
towards the end to keep pushing the propellant
through the pipes. A famous example of a pressure-fed
cycle engine was the one used on the Apollo
spacecraft. They used a type of hydrazine
(N2H4) as fuel and nitrogen tetroxide (N2O4)
as oxidizer. Recall that these propellants
are hypergolic, meaning that they ignite upon
contact. By the way hydrazine is highly toxic,
so a lot of care needs to be taken since it
potentially can cause cancer.
Now let’s take a look at turbopump systems.
Turbo is short for turbine. A turbine is a
fan-like device that converts linear fluid
motion into rotational motion that can be
used for doing work. In the case of a car
turbo, it is used for adding more air into
the engine to get more power.
A famous turbopump system was the F-1 engines
of the Saturn V as shown on the left. There
are different types of turbopump systems and
the F-1 engines were of the gas-generator
cycle type. Shown on the right is a schematic
of a gas-generator cycle engine. As we have
seen already with liquid-fueled rockets, the
main idea is to get fuel and oxidizer to the
combustion chamber. Recall that for the F-1
engines the fuel was Rocket Propellant 1 (or
RP-1) and the oxidizer was liquid oxygen.
Let’s start with the pre-burner. It’s
conveniently named since it ignites a little
quantity of fuel and oxidizer to make hot
exhaust. That hot gas is fed to the turbine,
which makes the turbine spin. The spinning
of the turbine operates the pumps of each
of the propellants helping the liquids flow
down to the engine or engines in the case
of the Saturn V’s first stage. You may notice
though that there is something odd with the
fuel line in this diagram. While the oxidizer
line feeds directly into the combustion chamber,
the fuel line goes past the combustion chamber
and wraps around the engine’s nozzle (or
the bell shaped part) a few times before being
directed into the combustion chamber. Why
do you think that is? One reason is that they
use the fuel to cool the extremely hot engine
nozzle (we will see typical temperatures in
a moment). So the fuel, which may be very
cold to start acts as a coolant much like
the case of radiator fluid in a car that helps
cool the car’s engine. Another reason for
wrapping the fuel line around the engine nozzle
is to heat up the fuel. Combustion is most
efficient when fluids are in a fine vapor
state.
You may wonder how exactly F-1 engines on
the Saturn V were started. Even though the
actual propellants were Rocket Propellant
1 and liquid oxygen, the starting of the engine
was done using a hypergolic propellant. Remember
that hypergolic propellants ignite on contact
so they are a convenient choice.
Not all liquid-fueled rocket engines are started
using hypergolic propellants. Here is a main
engine from the Space Shuttle, which used
liquid hydrogen as the fuel and liquid oxygen
as the oxidizer. This is a smaller engine
compared to the F-1 engine and the Space Shuttle
had three of these. These engines were started
using spark plugs much like car engines. So
an electric spark ignited the liquid hydrogen
and oxygen.
I took a picture of a nice graphic at the
Kennedy Space Center that shows various temperatures
for reference. Celsius is on the bottom and
Fahrenheit is on top. Take a look at where
the average human body temperature is on this
figure at about 100 degrees Fahrenheit or
37 degrees Celsius. Fireworks are about 2,500
degrees Fahrenheit or 1,371 degrees Celsius
and quartz sand turns to glass at 4,172 degrees
Fahrenheit or 2,300 degrees Celsius. Notice
that, “inside the engine,” meaning the
Space Shuttle main engine, was at 6,000 degrees
Fahrenheit or about 3,300 degrees Celsius.
This is well above the temperature at which
copper boils!
Now I would like you to either read or listen
to the communications leading up to the launch
of Apollo 11. Often documentaries of the Apollo
program speed up this moment but I would like
you to take in all the events that are going
on leading up to launch. You’ll mostly be
reading or hearing from NASA’s Public Affairs
Officer Jack King.
Public Affairs Officer: T minus 1 minute,
35 seconds on the Apollo mission, the flight
to land the first men on the Moon. All indications
coming in to the control center at this time
indicate we are Go. One minute, 25 seconds
and counting. Our status board indicates the
third stage completely pressurized. Eighty-second
mark has now been passed. We'll go on full
internal power at the 50-second mark in the
countdown. Guidance system goes on internal
at 17 seconds leading up to the ignition sequence
at 8.9 seconds. We're approaching the 60-second
mark on the Apollo 11 mission. T minus 60
seconds and counting. We've passed T minus
60. 55 seconds and counting. Neil Armstrong
just reported back: "It's been a real smooth
countdown". We've passed the 50-second mark.
Power transfer is complete - we're on internal
power with the launch vehicle at this time.
40 seconds away from the Apollo 11 lift-off.
All the second stage tanks now pressurized.
35 seconds and counting. We are still Go with
Apollo 11. 30 seconds and counting. Astronauts
report, "It feels good". T minus 25 seconds.
Twenty seconds and counting. T minus 15 seconds,
guidance is internal. Twelve, 11, 10, 9, ignition
sequence starts, 6, 5, 4, 3, 2, 1, zero, all
engine running. LIFT-OFF! We have a lift-off,
32 minutes past the hour. Lift-off on Apollo
11.
Armstrong: Roger. Clock.
Public Affairs Officer: Tower cleared.
Armstrong: Roger. We got a roll program.
McCandless: Roger. Roll.
Public Affairs Officer: Neil Armstrong reporting
their roll and pitch program which puts Apollo
11 on a proper heading.
One thing that you would have noticed is that
“ignition sequence start” is said at about
T-minus 9 seconds. It takes the five F-1 engines
a few seconds to get going at full throttle.
You can’t just turn on the Saturn V and
just go! Also, you may have noticed how they
have “liftoff” but then it take a few
seconds for the Saturn V to ‘clear the tower,’
meaning that the bottom of the rocket went
past the top of the tower. While the F-1 engines
were very powerful, the Saturn V was also
very heavy. This was why the Saturn V didn’t
just shoot off. It almost crawled vertically
into the sky at first. 
So the engines started a few seconds earlier
than launch. In this plot the time of launch
is marked as 0 in the horizontal axis. Both
vertical axes on the left and the right are
showing thrust in different units. The exact
numbers are not important. The main point
here is to realize how the engines were gradually
building up thrust before liftoff. The Saturn
V’s computer was programed to check that
all five engines were running at their indented
thrust levels before it launched. Also, notice
here that Engine 5, the center engine of the
five F-1 engines came on first. Then the other
four engines came on in opposing pairs. This
was a balance issue. Turning on engines on
opposite sides made sure that the whole Saturn
V didn’t fall over.
Now let’s take a look at accelerations experienced
by the astronauts during liftoff. The horizontal
axis shows time in seconds since liftoff and
the vertical axes show accelerations in either
standard units or in G-force. Using G-force
is nice because 1 means what you are experiencing
right now under the influence of Earth’s
gravity. Feeling 2 Gs would mean feeling like
you were twice as heavy than you are currently.
Apollo astronauts experienced a maximum of
4 Gs, meaning that they would have felt as
if they were 4 times as heavy than they were
on their seats before launch. The different
colored boxes in the figure divide the timeline
into the three stages of the Saturn V rocket.
The Number 1 indicates liftoff and the acceleration
builds up rather quickly due to the power
of the Saturn V’s first stage. Number 2
is the point when the center engine of the
first stage cutoff. Recall that’s the engine
that started first. The acceleration dropped
due to the cutoff and then started to build
back up until all four remaining engines of
the first stage cutoff. The G-force went to
0 as astronauts would have experienced a moment
of weightlessness, much like you experience
on a rollercoaster or an airplane flying through
turbulent air. Number 4 is the starting point
for the second stage burn. Again the acceleration
builds up to the point of center engine cutoff.
You’ll notice that there is an extra bump
in the acceleration for the second stage.
This is due to the changing of the amount
of fuel and oxidizer used. They changed the
fuel/oxidizer ratio to improve efficiency
of the burn to save propellant for the relatively
longer burn of the second stage. After the
remaining four engines cutoff, astronauts
would have again felt near weightlessness.
Number 8 is the start of the third stage,
which burned for a short time to get the astronauts
into orbit around the Earth. In a little over
10 minutes, the three Apollo astronauts were
in space!
Now I’d like to talk about pogo oscillations,
which rockets experienced during launch. It’s
a negative effect that needs to be worked
out. Otherwise, it can lead to the destruction
of the rocket. It is named after the pogo
stick, which is shown on the left.
I would like to show an example of the Tacoma
Narrows Bridge, which was a bridge in the
state of Washington in 1940. It is not a perfect
example to demonstrate pogo-like oscillations
since those have more to do with structures
shaking at their natural or resonant frequencies.
The breaking glass using sound example that
is often done in science or physics classes
is a better demonstration of what pogo-like
oscillations can do to solid objects. Yet,
I think the Tacoma Narrows Bridge is a dramatic
example of how seemingly solid structures
like bridges can be bent, twisted, and broken
by the motion of fluids like the wind. On
the day of the bridge’s collapse, it was
only four months old. The video shows the
bridge swinging erratically, while there is
one person left on the bridge. Leonard Coatsworth
was trying to get his daughter’s dog out
of the car. The dog, being scared, would not
leave the car. Leonard finally had to leave
the dog. He barely made it back to safety
himself.
Going back to rockets, pogo oscillations are
oscillations in the structure of rockets.
The structure of the rocket starts to vertically
stretch and compress like an accordion. It
is caused by combustion instabilities in the
rocket’s engines and may lead to the break
up of the rocket. This effect was studied
heavily during the Gemini program when Titan
II rockets were experiencing too much pogo
oscillations for use by Gemini astronauts.
Pogo oscillations were experienced by the
uncrewed Apollo 6 mission but the mission
wasn’t significantly affected. The crewed
Apollo 13 mission experienced pogo oscillations
when their second stage was burning. By the
time of Apollo 13, they had programed the
Saturn V’s computer to shutoff any erratic
engine, so the computer decided to shut off
the center engine. It didn’t affect the
mission in getting into orbit since they just
burned the remaining four engines of the second
stage for longer to get the necessary altitude
and speed that they needed. Of course Apollo
13 would go onto have many other problems,
but fortunately it wasn’t affected too much
by pogo oscillations.
Things like pogo oscillations are what makes
rocket science, rocket science. It’s not
easy because there are many things that could
go wrong and it’s difficult to predict what
those will be 100% of the time. We have of
course learnt a lot over the past several
decades but it’s a constant learning experience
and it’s utterly foolish to be nonchalant
when dealing with rockets.
