Three and Half Things That Made My Head Explode Starting Out in Aerospace ...

Surprising things I learned when I started working on Space Flight Software ... from Newton to Maxwell to Einstein and the Constellation Urion (Apollo 13)

by David Ferreira, Fourier Aerospace

I've worked on some exciting and high-profile projects in my life, but nothing as exciting as the three aerospace projects I've worked on in the latest phase of my career. Part of the excitement for me has been learning some new things that made my head explode. Despite being a lifelong, hardcore fan of space exploration, science, and learning, there are a few technologies commonly used in spacecraft that were new and stunning to me - one, because it involves 5th grade science.

1. Reaction Wheels

Remember in 3rd grade you saw a demonstration about gyroscopes (gyros) and pendulums? Maybe it involved a bicycle wheel -- and the instructor erroneously told you that bicycles stay upright because of gyroscopic forces? (That's not what keeps bicycles up ... but keep reading.)

How does this work? Here's the simplest example: Imagine you're in a craft floating freely in space with a heavy spinning wheel inside driven by a motor. (Let's ignore friction for now.) This wheel is spinning on the Z-axis (the axis pointing directly toward the center of the earth.) The onboard camera is pointing directly north and the wheel is spinning clockwise as seen from above (a higher altitude).

Now let's apply a mild braking force to the wheel. What will happen? The craft will borrow energy from the wheel and begin to rotate clockwise until an accelerating force is applied to the wheel. (Here I'm using the word accelerate referring to an increase in angular velocity - RPMs.)

That's simple, right? Now imagine we have 3 of these wheels, each rotating on a different axis -- the X, Y, and Z axes. As you might extrapolate from the example above, we can now control the attitude (orientation) of the spacecraft in any direction by carefully sending commands to the motors of each wheel in concert.

But wait, there's more! 

Now imagine we have 6 or 8 of these wheels all spinning at high angular velocities, powered by motors. And let's take two of these redundant wheels and apply a force that rotates both wheels on their x-axes - the east-west axis on the diameter of the wheel. And let's make sure we rotate each in the opposite direction.

Now our craft won't rotate - it won't change it's orientation - because we've applied rotational forces that counteract each other. What will happen? The craft will move laterally or even vertically while maintaining it's orientation. This is called a translational movement (not a rotational movement). In human terms, it will move up / down or shift to the side, while still looking straight ahead.

In this manner, a spacecraft can rotate or move in any direction without the use of propellants -- these wheels are powered by electricity collected by its solar panels and stored in batteries and the spinning wheels themselves.

As you might imagine, there ARE limitations and vulnerabilities to this. Some of these maneuvers require an increase or decrease in angular velocity. But every motor has it's limitation -- it's max and min RPMs. As we approach these limits, we exhaust our discretion in using this mechanism. If we slow the wheel down, we would cause the craft to rotate in an undesired direction -- even if the reduction is caused by friction.

Additionally, reaction wheels are notorious for failing. Imagine two opposing wheels spinning at 3200 RPM, stabilizing a satellite and suddenly one of the freezes up. Result: Your spacecraft suddenly starts sinning at hundreds of RPMs.

2. Magnetorquers (Torque Rods)

Ok, this really made my head explode because it's so simple that any grade-school child could come up with it and it is used to assist satellites in maintaining their desired orientation, moment by moment. And yet, many of us have never even heard of this. This, again, makes active use of a device that we typically think of as a passive navigation device -- the magnetic compass.

We all know that a compass is simply a magnetized steel needle allowed to spin freely around a vertical axis in a very low-friction enclosure. Because the needle is ferromagnetic, it wants to align itself with the magnetic field lines of the earth with the same end always pointing North. 

Perhaps you remember a grade school science demo where an electrical wire is coiled around a bolt and an electrical current is passed through the coil - the bolt becomes a temporary magnet (an electromagnet). Aside from attracting metal objects nearby, that bolt wants to align itself to the Earth's magnetic field with much greater force than when the coiled wire is not electrified.

Now imagine that bolt is fixed to the chassis of a satellite or other spacecraft. When a current is passed through the wire coil around it, a magnetic field is generated that will cause the bolt to want to orient itself with the Earth's magnetic field. And since the bolt is fixed to the craft, it will exert a rotational force on the free-floating craft that will push the craft toward alignment with the Earth's magnetic field.

Such a device is called a magnetorquer or, more informally, a torque rod and virtually every satellite in Earth orbit carries one or more of them.

Now imagine we have 3 of these bars welded firmly to the chassis, each aligned with a different local axis (pitch, roll, and yaw, or if you prefer, the X, Y, and Z axes of the craft).

Oh, by the way, we can reverse the voltage in each wire coiled around these bars to reverse the poles of their temporary and variable magnetic fields.

Now, by varying the voltages to these electromagnets, we can exert some small rotational force on the craft without expending fuel (again, only solar power).

This navigational tool does have some limitations. It can only be used in the presence of a magnetic field (ie, Earth orbit ... or around Mercury, Saturn, Jupiter, Uranus, or Neptune). The forces created by these devices are very week. Their ability to rotate a spacecraft quickly is minimal. But they are very useful in maintaining or slowly rotating the craft.

For context, GPS satellites are "Earth-facing" - they must face Earth to be useful. A GPS satellite that looses its orientation is of limited use. 

3. GPS Time Dilation Adjustment

Actual everyday evidence proving Einstein's Theory of General Relativity

For hundreds of years, we relied on Newtonian theory to explain the motion of bodies through space and time. Isaac Newton's theories were built on the fundamental assumption that time is universal - it passes at the same rate everywhere in the Universe.

In the early 20th century, Einstein pointed out the flaw in this theoretical framework: Newton's assumption of a universal clock was wrong. Einstein asked us to consider a new ide that was very challenging to accept: that space and time are really two manifestations of the same concept, space-time. This leads to the idea that time passes at different rates in different places and at different velocities. This is called time dilation. As you can imagine, these ideas were very difficult for physicists to swallow. After all, their education and careers were based largely on Newton's theories.

Even today, much of the general public has great difficulty believing that time would speed up near a massive object (a planet or a black hole) or slow down in deep, empty space.

Einstein's main physical evidence of this were some photographs of a solar eclipse taken in Russia during WW I and his mathematical equations - neither of which were very accessible to lay people.

Nevertheless, in our time, we have much more solid evidence proving Einsteinian time dilation and it is constantly provided by hundreds of devices 24x7. These devices are GPS satellites.

(Einsteinian) Time dilation is a significant factor in GPS navigation. Without a time dilation adjustment, a GPS satellite is virtually useless.

If you're developing navigation software that consumes GPS data, or, if you're programming an actual GPS satellite you will find that every data packet from a GPS satellite must contain a TIME DILATION data point specific to that vehicle.

Because satellites operate in microgravity and at astronomical velocities, their clocks experience time at a different rate than ours here on Earth. This time dilation data point is used to adjust the time relative to the time on Earth. And the time is used to calculate your position as a GPS user.

This is concrete evidence that Einstein's Theory of General Relativity is not just a strong hypothesis, but a factual theory and ... extremely significant. Without this adjustment, GPS satellite position estimates would drift off by several kilometers per day.

3.5 The Constellation Urion

You may remember this amusing scene from Apollo 13 where Commander Jim Lovell (Tom Hanks) purges a urine holding tank and astronaut Fred Haise (Bill Paxton) cracks wise, calling the cloud of frozen urine crystals "the Constellation Urion" -- a reference to how the extravehicular urine immediately freezes in the cold vacuum of space forming a cloud of urine-ice crystals that twinkle like stars. Most of us found this line an amusing bit of low-brow "guy humor" revolving around what is likely a real-world phenomenon.

But when I started working on the Orion Spacecraft, I was surprised by how this topic figured into planning. I sat through several meetings where this came up. And I learned that there are strict rules around when a spacecraft can and cannot dump urine.

Here's why: In deep space (typically defined as beyond LEO - low Earth orbit), space craft up to and including our modern Orion craft, cannot rely on GPS navigation data. This is because the current constellation of GPS satellites all have their antennas pointed toward Earth. (And in the case of the Apollo missions, GPS satellites didn't exist.) The GPS satellites that may be pointed toward the spacecraft are blocked by the Earth and for those satellites not blocked by the Earth, the signal from their off-angle, directional antennas would not be strong enough for the spacecraft to use reliably.

Like the Apollo spacecraft, modern deep-space vehicles rely on celestial navigation -- they use the stars to determine their position and attitude in space. Unlike in the Apollo missions, Artemis era missions use video imaging and computers to accomplish what Apollo astronauts calculated manually.

A cloud of twinkling urine crystals twinkling around a spacecraft is highly likely to confuse these celestial navigation systems, including the human eye. If this happens, computers and astronauts could waste fuel or potentially wander tragically off course. Obviously, this could lead to catastrophic results including LoC ... sorry, "loss of crew".

This all makes good sense. What surprised me was the number of serious discussions I sat through where this topic came up. The expressions on the faces of us noobs slowly changed from amused to serious as we realized the gravity of this topic.