PHY: G-Forces, Jets, and more!
Hey everyone, David here, sorry about my lack of posts, I have been busy with school and some other things but here I am (da daaaa). Following the success of DM-2, SpaceX launched the Falcon 9 two more times for its StarLink project, which is very exciting, and as I was watching the live transmission, Youtube recommended me an interesting video about the physics of fighter jets, and I, being interested in Aerospace technology and science, clicked it out of curiosity, only to be amazed at the number of things these pilots have to deal with during awesomely complicated maneuvers.
One thing, however, really caught my attention. G-Forces... are... awesome. Let's get right into it.
What exactly are G-Forces?
When we are here on Earth, we are constantly being pulled down by this little thing called Gravity. It's what keeps us "down to Earth" (badum ts). Because it is constantly acting force, it induces acceleration and, in the case of our planet, that is approximately g = 9.8 m/s¨2 (although this varies across different altitudes, such variation is very little, so across all surface we have roughly the same value for g). We calculate our weight with this value, i.e, how much force is our mass making us have to sustain, therefore, if g increases, our body weight also increases (note that our mass is constant, but our weight, given by mass*g, varies with g).
With that being said, G-Forces are "multiplications" of this standard 1g that we experience on Earth. If someone is experiencing G-Forces equivalent to 3g, for example, they are experiencing 3 times the value of g. Because weight = mass*g, they will also experience 3 times their body weight, so for example, if someone weighed 150 pounds on the surface of planet earth, this same person would weight 450 pounds while undergoing this acceleration! This goes to show how powerful this force is, however, 3g's is nothing compared to what Jet pilots feel during maneuvers. Let's first look into what exactly causes these G-Forces.
What exactly causes G-Forces?
So we already know what G-Forces are and how powerful they can be, but what exactly causes someone to experience G-Forces higher (or lower) than 1g?
The short answer is acceleration. G-Forces, although containing “Force” in its name, are really just a measure of acceleration. When a body accelerates, inertia comes into play, which makes the object want to move at a constant speed. When a human, for example, is under these circumstances, if they were seating on a chair they would be pushed into the chair (due to inertia) and the chair would also push into them (due to Newton’s Third Law), creating this feeling of a force acting on you, but it truly is just inertia. The faster you go, the harder you will get pushed into your seat as you will accumulate more inertia, therefore, the more “g’s” you will experience. There are many scenarios that cause G-Forces in many ways, so let’s take a look at some of them.
G-Forces in fighting jets and rockets
Fighting jets are really freaking fast. The famous F-16 Raptor Jet currently in use by the U.S.A.F., for example, can reach speeds up to Mach 2, which is twice the speed of sound! That is insanely fast, and something these jets can also do is maneuvers. Currently, it is said that the limiting factor of a Jet’s capabilities in combat isn't the systems itself, but instead, it is the pilot. These machines are capable of undergoing crazy amounts of aerodynamic stress on its hull, almost undestroyable by nature. We, however, are limited to many factors, one of the most crucial and notable being our G-Tolerance.
A typical human being can withstand 2-3 g’s without blacking out (more on that later). In order words, while under 3 times our body weight, we can still perform vital functions such as breathing and successfully pumping blood to our brains. During extreme maneuvers, however, pilots can experience up to 9 g’s !!! That is A LOT! Think about it: Get 8 copies of yourself, lay down and stack them all on your chest, then try to breathe. That is the feeling of 9 g’s. But how does that happen?
If a jet is moving straight, the only acceleration acting on your body is the one caused by the linear acceleration of the jet, which is big but due to its great acceleration, it doesn’t take much time for the Jet to reach a stable flight speed, when that acceleration starts to ease off. Nonetheless, when undergoing a maneuver, the jet enters a different type of motion that has another variable when considering the net acceleration: this motion is the Circular Motion, and the acceleration is the Centripetal (or Centrifugal, depending on your reference) acceleration.
The image on the right above shows the accelerations involved in a non-uniform (accelerating speed) circular motion. Similar to the picture, when a jet enters such motion, the centripetal acceleration pulls the whole vehicle to the center of the circumference that describes its trajectory, and the value of said acceleration is proportional to the second power of the velocity of the flying machine. Hence, when that fighter pilot pulls the stick down, the centripetal force acts on him, increasing by large amounts the net acceleration on his body, therefore increasing the G-Force the pilot is pulling.
Simple maneuvers can pull 3-4 g’s, very little compared to the 9 g’s a pilot can experience when going through some rougher moves. Even though this is hard on the pilot’s body, the capability of moving around efficiently with the jet is what will save one’s life in case of a dogfight, for example.
These accelerations also happen on rockets! Bob and Doug, for example, during the DM-2 launch, experienced a maximum of roughly 3 g’s. It is little compared to jets, but that is due to the less brutal motion of rockets compared to the latter. Space Shuttles, however, pulled a little bit more than the Falcon 9 did, reaching almost 4.5 g’s.
G-Force Tolerance
Although G-Forces are extreme and can hinder someone's consciousness really fast, pilots are trained for this. The exact number a regular person tolerates is about 2-4 g’s, however, that can fluctuate quite a bit, with people being able to pull up to 6 g’s naturally, and others passing out after hitting 2.5 g’s, for instance. Pilots, however, are trained to have a high tolerance, which actually makes up a huge part of their training. They spend hours and hours on centrifuges that simulate G-Forces by rotating really fast and creating centrifugal accelerations, acquiring tolerance by being put under continuously increasing G loads.
Besides G-Suits that we talked about, to help cope with G-Forces, pilots learn techniques, called Anti-G Straining Maneuvers (AGSM’s) that help them fly the plane more easily under the high weight of who knows how many times their own bodies’. One example is the “Hook” maneuver, which helps pilots breathe by squeezing their legs and breathing in a sort of hiccup, which essentially forces the diaphragm to take air rapidly instead of a slow fight against the g’s on your lungs.
G-LOC: the dangers of excessive G-Forces
Now we know a lot of things about G-Forces. But afterall, why are they so dangerous to fighter pilots?
When we are under G-Forces, not only we are being pulled down with a stronger force, but everything inside us also becomes really heavy, more noticeably the blood and the lungs. It is extremely difficult to breathe, hence the need for techniques and special suits, and what is also really hard to sustain is blood flow.
Because we are “heavier”, our heart has a hard time pumping blood as strongly as it would before, which may cause our brain to lack blood flow, and that is a huge problem because with no oxygen, the brain can’t work. If our brain is cut short of blood for too long due to G-Forces, a G-LOC (gravity-induced loss of consciousness) happens. In other words, you black out; sweet dreams, good night; see you on the other side; etc, etc.
The G-LOC itself is not dangerous: if it happens in a centrifuge, the centrifuge officer will just shut it down, bringing you back to the normal 1g, regaining a nominal blood flow, oxygenating the brain and making you regain consciousness in a matter of 5-10 seconds with no permanent damage.
Inside an aircraft, however, G-LOC’s are pretty much the end of you, not because of the G-LOC itself, but because of what happens next. Let’s go through a scenario: You are flying an F-16 Raptor maintaining a low altitude of 10,000 feet and decide to do a 360° horizontal loop, so you roll left and pull up on the stick way too hard, pulling 8g’s. At first you, are O.K, bring the stick a little bit down to the point in which you’re pulling 6g’s, but you try to maintain it. You start getting the first signs of a G-LOC as your breathing becomes more difficult and you start getting tunnel vision, and all of a sudden… black. This is a terrible scenario. The G-LOC itself won’t kill you, as due to blacking out your hand will let go of the stick, returning you to a stable position. However, for the next 5-10 seconds, your aircraft is completely uncontrolled, sinking 90° vertically down towards the ground… even if you are lucky enough to regain consciousness at, let’s say, 500 feet, at that point there is not much you can do besides ejecting.
Even doing that can be practically impossible as it takes a minute or two for your brain to completely understand what happened, where are you, etc., thoughts that you hope are 100% clear all the time, especially when, you know, flying a 20 million dollar plane.
Overall, not really fun, is it? Yeah, didn’t think so.
If you ever get a chance to go to Disneyworld, in Epcot there’s a ride called “Mission Space”, in which you sit down inside a mock-up space capsule cockpit where there are dozens of buttons and a screen in front of you that plays a little video of a Mars mission as if you were an astronaut in that mission. What is cool about this ride is that these “capsules” are all in a centrifuge that accelerates during certain times of the film so that it simulates G-Forces that you would experience in a mission to Mars! During launch, the peak of G-Forces, you pull around 2.5g’s, which might seem very little, but it is definitely an interesting feeling and gives you a good sensation of how strong this force can be. It also makes a lot of people vomit, but let’s not talk about that.
That’s all I’ve got for today! I’ll leave some links down below to some interesting videos about G-Forces, I hope you enjoy them!
Welp, it’s been a pleasure talking to you, and for now, that’s it, I’ll see you all later! Stay safe! Peace!
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