Written by Peter Alexander, Education Director, BEEC
I had cleared my early afternoon schedule, waiting on the edge of my seat. It was 1:10pm PDT. I was watching the first American manned launch attempt in about 9 years. The Dragon capsule was poised on the Falcon 9 launch vehicle. The first time a private company was granted the status as a potential main player in America’s space program, featured two men in fully vested space suits ready on the launch pad.
And then flatly—typical of military monotone communication, even slightly cryptic—the mission specialist reported “The 40% margin of violation has been reached and you are good for launch abort sequence.” CNN representatives then jumped in to explain.
Before we hop into the nitty gritty, let’s have a word from our sponsors:
This disappointing message–although delivered on May 27, 2020–is actually being brought to us by three of the greatest physicists in human history — Johannes Kepler, Isaac Newton, and James Clerk Maxwell.
What a wonderful lesson they have to teach us. EVERYTHING that unfolded in those few seconds, when the decision to abort the launch was made public, can be traced back to crucial ideas students learn in AP Physics C, specifically.
Here is the play-by-play action analysis for those of you at home:
- A 40% margin of violation. This is the language the military uses to describe probabilities when determining if an occasion for a safe launch is compromised. Everything here is contingent upon the idea of “safe launch.”
- Safe launch probability. The concept of safety in normal flight is complicated. Even more issues need to be considered when “flight” includes leaving the earth’s atmosphere. Enter James Clerk Maxwell — we usually learn about him in the second semester of Physics C. He helped unify the relationship between electricity and magnetism. In the upper atmosphere there are electromagnetic fields which ebb and flow in strength due to variations in pressure and temperature. These fields can dissipate energy in the form of lightning. Physicists still are not sure of all the dynamics of upper atmospheric lightning, but we do know a bit about how to make predictions based on the weather.
- But the launch was from Florida! It turned out that pressure events in the atmosphere off the coast of the Carolinas was the culprit. Why should this matter? And even more basically, why do we launch spacecraft from Florida anyway? Enter Isaac Newton — we learn about him in the early part of AP Physics when we study mechanics. Newton’s theory of gravity gives us a precise account of the velocities required for the spacecraft to achieve its orbital objective. We also learn that the angular velocity of the earth is greatest near its equator. So, a launch from near the equator of the earth greatly reduces the demand on the rocket’s propulsion system to achieve these extreme velocities since, near the equator, the earth gives you the greatest “boost.”
- So what about the weather in the Carolinas? Again, thanks to Isaac Newton, his second law of motion describes exactly the PATH the rocket needs to take to achieve its orbital objective — in this case, we want to dock with the INTERNATIONAL SPACE STATION! That path can be thought of as an arc that starts in Florida and continues up along the eastern coast out into space. The spacecraft could, in principle, experience catastrophic failure leading to an “abort” in the atmosphere, throughout the duration of the rocket’s atmospheric flight. (A nice AP Physics C Mechanics Problem: Approximately where, relative to the Florida launch coordinates, will the rocket leave the upper atmosphere of the earth?) The weather off the coast of the Carolinas was well within that potential failure range. It is “uncertainties” in the stability of the electromagnetic fields which presented cause for concern. Essentially, in that area, they were afraid lightning could have hit the spacecraft.
- Why can’t we just wait until later this afternoon when the weather clears? I know … bummer! Why do we have to wait until Saturday at 3:22pm EDT exactly (more or less …there is a two-minute fudge factor). Enter Johannes Kepler — we learn about him too in AP Physics — Mechanics. He tells us, along with Newton, about orbital motion. The space station is in orbit around the earth. To dock with the space station, the launch vehicle must send the Dragon capsule into what is called a “transfer orbit” — that is, an orbit which will intersect the orbit of the space station. Since fuel costs are enormous constraints, we need to utilize all the help we can get from things like earth’s movement, position of launch, etc. So, when you do the math, it turns out that the next best time to try to “get along the right path” is this Saturday at 3:22pm EDT.
I hope you will be there watching with me!