SpaceTEC® Resource Blog for Aerospace Technicians


Can You Measure Up?

A Weights and Measures office in England

When I watch the International Space Station fly overhead, I am always amazed at what a piece of craftsmanship it truly is.  Is it because of its large size or how high up it is?  No, not really.  It’s the fact that each and every piece was built in over 1 dozen countries by thousands of people and brought together for the first time 250 miles up in low Earth orbit.  The chances of everything fitting together the first time would be astronomical (and it did fit together the first time!) if it wasn’t for one thing, the standardization of measurement.

With things such as the ISS, the various countries involved use the United States Customary System of measurement.  In fact, as stated on the ISS tour at KSC, the ISS is the last international space project involving the USA that will use this system of measurement.  Afterwards, all international space projects are to be done in the metric or SI system.

Currently the USA uses the United States Customary System as its standard of measurement.  The rest of the world uses the International System of Units (SI or commonly known as “metric.”)  In 1959, an international standard was agreed upon so that both standards could be easily translated back and forth.  The table below shows a good example of conversions for common measurement between the American and SI standards.

Unit Divisions SI Equivalent
Exact relationships shown in boldface
inch (in) 2.54 cm
foot (ft) 12 in 0.3048 m
yard (yd) 3 ft 0.9144 m
mile (mi) 1760 yd 1.609344 km

The standardization of measurement is a basic building block of a successful civilization.   You cannot have trade, buildings, or complicated machinery without an agreed upon standard of measurement.   From standards on length, volume, etc. societies cannot function without some standard of measurement.  The next time you go to fuel your car, look at the gas pump.  Somewhere on the pump will be a stamp from an official state office certifying that the pump meter is in accordance with standards of measurement when it calculates flow of gasoline being pumped into your car.  This ensures the fair trade in your purchase of gasoline.

Many ancient societies had standards of measurement.  Some small villages that dealt in trade would post their “standards of measurement” on a board in the village square, while larger governments and cities would actually set up standards of measurement by decree and have officials to enforce the standards.

The earliest known examples of standards of measurements came from the 4th and 3rd mellennia BC from the civilizations of Indus Valley (covering modern day parts of Pakistan, India, Iran, and Afghanistan, Egypt, and Mesopotamia (covering modern day parts of Iraq, Syria, Turkey, and Iran).  Probably the most common story of government setting standards of measurement is the story of King Henry I of England, who ruled England from 1100 to 1135.  The standard for the “foot” was supposed to have been made by measuring the King’s foot.  This practice had been going on before his rule, but it appears that new rulers would frequently want to leave their “mark” in some way in the culture, and this was one way of doing it, hence the “foot” and quite likely the term “ruler” for the stick showing a foot.

So what can happen if standards are ignored or the wrong standard is applied?   Much can happen such as unfair trade practices, building collapses, machinery that cannot have interchangeable parts, and one famous example of a multi-million dollar spacecraft being lost.

Aerospace technicians working on the Mars Climate Orbiter. Credit

The Mars Climate Orbiter was launched on December 11th, 1998 as part of a two spacecraft team (the other being the Polar Lander which was also lost) and was declared lost September 23rd, 1999.  It was discovered that the loss of the spacecraft (total program cost of $327.6 million) was due to the wrong measurement standard being used.  Lockheed Martin was responsible for the thrusters and had used United States Customary Units to calculate the thrust in pound force.  The main computer was expecting the calculations to be in newton’s based on SI standards resulting in the spacecraft underestimating it’s thruster effects by a factor of 4.5 (1 pound force is equal to 4.5 newton’s) .  The software error was never caught during ground testing and the entire spacecraft ended up dipping too low into Mar’s atmosphere during orbit insertion causing the spacecraft to burn up.

The importance of having a standard of measurement cannot be stressed enough.  Because of these standards, a meter or foot means the same throughout the world ensuring fair and accurate trade, collaboration on international projects, and someday a human return to the Moon and on to Mars.

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Friday, December 31st, 2010 History, Introduction to Aerospace, Tests and Measurements Comments Off on Can You Measure Up?

Liquid Fueled Rocket Engine Basics

Bi-propellant rocket motor Credit NASA

As an aerospace technician you may find yourself around liquid fueled rocket engines daily or as in some cases of SpaceTec alumni, actually working on them.  Either way, it is a good idea to know some basics about these engines, especially the more common bi-propellant engines.


Liquid fueled rocket engines were first proposed by Konstantin Tsiolkovsky in his book The Exploration of Cosmic Space by Means of Reaction Devices published in 1903.  It remained a theory until the first successful flight by a rocket named “Nell”, lasting only 2 and ½ seconds and traveling 41 feet, done by Robert Goddard on March 16th, 1926.

Goddard and “Nell” Credit NASA

What is a bi-propellant liquid fueled rocket engine?  Well, as the name describes, it is a rocket engine that is fueled by a liquid fuel and a liquid oxidizer.   The entire set up is deceptively simple and consists of two tanks to hold the fuel and oxidizer, two turbo pumps, a combustion chamber, and a nozzle.

The two tanks can be small such as what was on Robert Goddard’s rocket or can be as large as the External fuel tank on the Space Shuttle system (which holds the tanks for both the fuel and oxidizer in one large shell).

The pumps are critical in pushing the fuel and oxidizer to their explosive meeting in the combustion chamber while maintaining adequate pressure to ensure proper flow rate and to prevent the combustion chamber from collapsing under its own weight due to atmospheric pressure.  As Dr. Jerry Grey wrote in his book Enterprise, “Much of the difficulty with liquid-propellant rockets arises in maintaining constant flow and rapid, efficient mixing of the propellants…the need for tanks heavy enough to withstand the pressure dictated the use of pumps instead…pumps the size of washing machines which had to have the power of a diesel locomotive…” (Grey, Ph.D., 1979)  One of the factoids listed at the Kennedy Space Center Visitor Center Launch Gantry says that one turbo pump from one Space Shuttle Engine (SSME) can actually drain an Olympic size swimming pool in just 25 seconds.  That’s a lot of fuel being pushed along the pipes leading into the combustion chamber, yet it takes all three Space Shuttle Main Engines nearly 8 minutes to drain the External Tank!

The combustion chamber is where all the action happens.  This is where the fuel and oxidizer meet and combust forming the explosive energy required to lift the rocket.  “Chamber walls less than a sixteenth of an inch thick that withstood pressures of one hundred atmospheres and temperatures of 6,000 F.” (Grey, Ph.D., 1979)  Basically an explosion occurs that is continuous as long as there is fuel or until the engine is shut down.  This is why it is said sometimes that a rocket launch is nothing but a vehicle or payload riding a controlled explosion.

After the fuel combusts the resulting energy has to go somewhere or the chamber will explode.  The resulting energy or flame exits out the familiar nozzle we see on all rockets.  Nozzles experience great heat from the flames and actually have to be made of a material that can withstand it and/or have a way of being cooled so they can maintain their shape and form during the launch.  The nozzles for the SSME’s actually have some of the liquid hydrogen routed through tiny radiator tubes lining the nozzle using the super cold liquid hydrogen to keep the nozzle cool and to warm the hydrogen before it reaches the combustion chamber making it easier to ignite.

As I said, deceptively simple but as many technicians and engineers that have worked on these engines for nearly 100 years now can attest, this “simple” system has resulted in many exploded rockets and engines (and some loss of life) during the trial and testing that still goes on today.

For further learning about this subject:

Grey, Ph.D., J. (1979). Enterprise. New York: William Morrow and Company, Inc.


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Tuesday, December 14th, 2010 History, Introduction to Aerospace Comments Off on Liquid Fueled Rocket Engine Basics

Why is there an American Flag on the Moon?

Apollo 11 displays the Flag of These United States of America.

How did six American Flags end up on the Moon?  “It’s because American astronauts put them there,” might be said, but it was a little more complicated than that.  A basic tenet of human nature is to take simple things and make them as complicated as possible.  Deciding which flag to put on the first manned mission to the Moon was no exception.

President John F. Kennedy first proposed using an American flag by saying, “…for the eyes of the world now look into space, to the Moon and to the planets beyond, and we have vowed that we shall not see it governed by a hostile flag of conquest, but by a banner of freedom and peace.”  NASA did not take much notice of that at the time for the Moon was still years away.  In fact, only spacecraft had American flags on them and it wasn’t until Astronaut Ed White’s first spacewalk that you even saw an American flag on a spacesuit along with one on his partner, James McDivitt’s suit.  Both of the men had bought the American flag patches themselves and had them placed on their suits.  Afterwards, NASA started having all spacesuits adorned with American Flags.

But putting a nation’s flag on a spacesuit is nothing compared to the historical significance of placing a flag on the Moon.   The political aspects internationally and domestically for such an event had to be considered.  Though it would be Americans landing on the Moon, they were representing all of humanity in this historic first visit to another world.

Christopher Columbus “Planting the Flag” and claiming the land for Spain

“Planting the flag” usually means making a claim to something, usually territory or land.  Throughout history men have “planted the flag” claiming ownership in the name of the king, queen, country, church, etc. marking the land as their own.  The United States had signed a United Nations Treaty in 1967 called the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies also commonly known as the Outer Space Treaty.  A section of that treaty forbids nations from claiming celestial bodies as their own through “claim of sovereignty, by means of occupation, or by any other means.”    Since “planting the flag” up to this time in history usually meant a “claim of sovereignty”, NASA had to explore if planting the American Flag would be perceived by nations of the world as a claim or would they understand it was only symbolic.

To solve this problem, NASA of course set up a committee to explore the issue.  In February of 1969, the Committee on Symbolic Activities for the First Lunar Landing was established.  “The committee was instructed to select symbolic activities that would not jeopardize crew safety or interfere with mission objectives; that would “signalize the first lunar landing as an historical forward step of all mankind (Sounds like something Neil Armstrong said a few months later doesn’t’ it?) that has been accomplished by the United States” and that would not give the impression that the United States was “taking possession of the Moon” in violation of the Outer Space Treaty.”

The committee looked at options such as planting the United Nations Flag, leaving a solar wind experiment that looked like an American flag, leaving little flags of all the nations of the world, or putting a plaque or marker on the surface of the Moon.   Arguments were made that since the first humans on the Moon were representing mankind, then some type of world flag such as the UN flag should be used.  Another argument in favor of the international type flag was the fact that even though most of the work and cost of Apollo was borne by the American people, NASA did have some international partners assisting in the program in a limited role including the Swiss with their development of the solar experiment, eight different countries assigned to examine any lunar rocks brought back, Brazil with its rocket sounding program, and the various nations that hosted tracking sites at their own expense.

In the end, the committee decided that only the American Flag should be planted on the Moon and also recommended the famous plaque left on the lunar lander that said, “Here men from planet Earth first set foot upon the Moon July 1969, A.D.  We came in peace for all mankind.” The plaque would not have any nation’s flag on it, but a picture of the east and west hemispheres.  Small flags of all 50 states and member nations of the United Nations were to be brought along, but returned to Earth with the crew and presented to each entity the flag represented.

The night before the launch of Apollo 11 a crew of technicians supervised by Jack Kinzler, the Chief of Technical Services Division at Marshall Space-flight Center, attached the American flag and plaque to the Lunar Module Eagle.  On July 20th 1969, Neil Armstrong and Buzz Aldrin deployed the American Flag on the lunar surface, a task that only took about 10 minutes but watched by the entire world.  There was no international outcry and only a few media outlets complained about the United Nations flag being left out.  A precedent was set and along with Congress’s blessing, all subsequent Moon landings had an American flag deployed at each site.

Anne M. Platoff of Hernandez Engineering Inc. has written an excellent paper on this subject, called “Where No Flag Has Gone Before: Political and Technical Aspects of Placing a Flag on the Moon,” that also includes the technical hurdles NASA had to overcome to deploy the flag.  Most of the information in this post is directly gleaned or quoted from her paper.  You can find the paper here.



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Saturday, October 9th, 2010 History, Introduction to Aerospace 1 Comment