Atmospheric Reentry in Mecha Anime
In Gundam and many other mecha anime the plot often takes place in two places, Earth and space. In the original Gundam, and the spiritual reboot of the series Gundam Seed, the action starts in space before heading to Earth, only to return to space again. A key issue in several Gundam series is atmospheric reentry, or the passing through the atmosphere as one returns to Earth from space. A key issue is often that the gundam the main character is piloting can’t survive, or isn’t tested for atmospheric reentry. This is usually hand-waved with some sort of technobabble, so today I will be taking a closer look at how realistic this technobabble is.
It might not seem like it while you are sitting down reading this post, but the atmosphere is actually quite dense and has multiple layers.
We live in the troposphere, which is from sea level to 7 miles (12km) up, and it becomes thinner or less dense the further you are from sea level. This is one of the reasons climbing very tall mountains like Mt. Everest is so dangerous, as the density of the air decreases to a point that it becomes difficult to breathe, but that is a lesson for another time. The atmosphere of the Earth continues out through the Exosphere which ends 6,200 miles away from Earth’s sea level, which seems very far. The question is then, where does outer space actually begin?
The Karman Line is the altitude set by the Federation Aeronautique Internationale (FIA) in an attempt to define where the atmosphere ends and outer space begins. Today the line is at 62 miles or 100km. By comparison the US air force sets the boundary of outer space at 50 miles. This might make it seem like the Karman line is an arbitrary boundary, but there is some scientific thought behind it.
Theodore Von Karman (1881-1963) (yes, the line was named after him), was a physicist/ aeronautics expert who calculated that the density of the air would be so low as to be unable to support aeronautical flight at 51.9 miles (83.6km). At this height he determined that any craft would have to travel faster than orbital velocity in order to remain in the air. I imagine that the reason the line is higher today than when it was originally calculated is due to improvements in technology over the aircraft of the 30’s, 40’s, and 50’s. So while technically you are still in the atmosphere, outer space begins when the Earth’s atmosphere is so thin as to make it a non-issue.
So, while the outermost reaches are not an issue for space flight, the lower denser regions are. An easy way to demonstrate why is to think back to the last time you were driving and had your hand out of the window. At low speeds it is fairly easy to keep your hand steady, but the faster you go, the hard it is to keep your hand in the same position. Another example is doing a belly flop off of the side of the pool doesn’t hurt nearly as much as doing one off the high diving board. Either way, the speed an object is moving at matters even when coming into contact with a material that we don’t consider to be very hard.
Another reason it matters is due to friction, which is the force that resists movement when 2 objects are moving past each other, which creates heat (like when you rub your hands together to warm them up when you are cold). The main considerations for determining friction are the material and the speed.
While it might not seem like much, friction due to air or air resistance is something to be concerned about at high speeds, and the space shuttle at atmospheric reentry is moving at 17,500mph. At those kinds of speeds the friction of the space shuttle and the air will create an enormous amount of heat. This is why all but the largest asteroids and meteors burn up when they enter the atmosphere. Thus, any space craft is going to need some serious protection when hitting the lower levels of Earth’s atmosphere.
What keeps the space shuttle and other space craft from burning to a crisp when they enter the atmosphere? The answer to that is a heat shield. A heat shield is a protective barrier covering the bottom of the space craft when it is passing through the atmosphere. This barrier prevents the heat from affecting the rest of the space craft. Today it is typically a ceramic, but in the early days of NASA it was made from a metallic ceramic composite.
Mercury Program Heat Shield
Space Shuttle Heat Shield
Time for the Anime
Mobile Suit Gundam
Now that all of the background material is out of the way, I can begin talking about atmospheric reentry in anime. We are going to start with the classic Mobile Suit Gundam. In episode 5 our protagonist Aumro Ray has to pilot the Gundam (RX-78) through atmospheric reentry. In the original episode Aumro deploys a heat proof film or cloak around the gundam to use as a heat shield to safely pass through the atmosphere, as seen here.
The idea of a heat resistant blanket or cloak is not completely out of this world as there are real world blankets that can survive extremely high temperatures. The blanket seen below can withstand temperatures up to 3,000 degrees Fahrenheit, which is the temperature of the space shuttle heat shield during reentry.
As you can see, this blanket is much more substantial that the transparent one seen around the Gundam during reentry. The second issue is that some of the heat resistant blankets do not have high resistance to friction, and would likely be destroyed by high air resistance during reentry. Thus, the Gundam’s cloak isn’t durable enough to survive the trip through the atmosphere and shield the Gundam at the same time.
Since I hadn’t seen the original Mobile Suit Gundam in a long time I decided to check out the wiki page, since the series has a large amount of ancillary material that might offer another explanation for the Gundam’s reentry system. As it turns out, I was right and the wiki page did have more information on how the Gundam survives reentry. Apparently, it has a system that pulls in air from the back of the suit and pushes it out the front to reduce the heat generated by friction as it reenters the atmosphere. I have a couple of issues with this system.
1- The system would be extremely bulky and complicated as it would have to have air vents on a large portion of the Gundam.
2- Each vent is another point of failure and potential weak point, reducing the Gundam’s combat effectiveness.
3- There isn’t enough air in the upper layers of the atmosphere to properly function
FYI- That being said, I did see an interesting article (for the lack of a better term) on an active system to reduce air resistance in cars. It uses turbines to alter the airflow around a car to improve air resistance. I wasn’t able to find anything else on this system besides the one article and I have doubts as to whether or not it would work.http://www.citon.ro/Proiecte%20de%20cercetare%20-%20eng/9.%20Passive%20and%20active%20systems%20of%20aerodynamic%20resistance%20reduction%20for%20automobiles%20that%20generate%20electricity%20SERB%20ARR-01.pdf
For the run of the mill mobile suits and not the prototype Gundam, they show a more traditional heat shield, which anyone who has a passing fancy for real world spaceflight will be familiar with.
When it comes to the original Gundam (RX-78) that started the Gundam craze, its reentry systems are in no way realistic.
All of the other mobile suits which use of heat shields that resemble something that might be found in the real world are quite realistic in my opinion.
Gundam Seed is something of a retelling or reimagining of the original Mobile Suit Gundam that first aired in 2002. As such it also has an atmospheric reentry scene that handles reentry as serious business, with several ships exploding due to the heat of reentry. The protagonist’s ship the Archangel avoids this fate by deploying an ablative heat resistant gel on the bottom of the ship.
If I had to speculate, I would imagine that the writers could have been inspired by the stunt doubles who get lit on fire for action sequences in movies. In some cases, the actor is covered in a heat resistant gel.
The gel works by absorbing droplets of water, and water is surprisingly good at resisting temperature changes, (just think how long you have to wait for a pot of water to boil). All of the water has to be evaporated from the gel before it will burn. This is the same reason why a paper cup full of water doesn’t burn until the water is boiled off, because the water is absorbing all of the heat, preventing the paper from reaching a temperature where it will combust. These gels can resist temperatures up to 3000 degrees Fahrenheit for 6 to 36 hours depending on the condition. This is plenty hot, and long enough to survive atmospheric reentry, and the ship can release more if any is removed by air resistance.
So while it is a very novel idea, and one that is not used in space craft today, there is some actual science behind the ablative gel, making it plausible.
Gundam Seed also has Kira Yamato, the hero of the story, ride his Strike Gundam through the atmosphere, only this time it does not work out so well. The Strike Gundam lacks any system for atmospheric reentry and he ends up using his shield as a makeshift heat shield until he is able to land on the Archangel and ride out the rest of reentry.
I like how Gundam Seed made a point to show how the shield is partially melted by the heat, and that while the Gundam survived, Kira was injured by the trip as the cockpit overheated. While not much is known about the metal used in the construction of the Gundam or its shield, I like how the scene played out in what I think is a somewhat realistic manner. So, while I can’t call this one either way, I like the sense of realism it gave the scene.
Another staple of mecha anime that I couldn’t ignore is the Macross franchise, which many of us in the states saw as part of the Robotech series. During episode 27 of Macross or Robotech, Rick Hunter takes his Veritech fighter through atmospheric reentry. The show is cognizant of the heat that is generated during reentry, as Rick engages his heat shield, which we can see below.
The only problem is that the heat shield is on the top of his fighter and not the bottom where it is actually needed. Sure, it might cool the cockpit but the rest of his fighter is going to be burning up. So, while I give Macross points for talking about a heat shield, it fails in the application stage as it makes no sense from a scientific standpoint.
I am not going to be talking about how the SDF-1 survived reentry as it is a giant alien spaceship humanity converted for its own use, and who knows what it is made of.
Like the Gundam franchise, the Macross franchise has flourished over the years despite not being as well known in the West. It has had multiple sequels over the years, one of which is Macross Frontier, that came out in 2008, and was so successful it was reimagined as a series of two movies, the second of which, Macross Frontier: The Wings of Goodbye, we will be talking about today. Our heroes have to run a gauntlet of enemy ships, and then land on a planet in order to save the day. The problem is that at the speed they have to accelerate to, to avoid being destroyed as they run the gauntlet, means they will be going too fast to safely enter the planet’s atmosphere. The solution is genius from an artistic, narrative, and scientific perspective.
The solution as you can see is to use a piece of space debris from a previous battle as a heat shield during reentry. So in this one instance I have to give Macross Frontier credit for getting an aspect of space travel correct.
Atmospheric reentry is an often-overlooked part of anime involving space travel, but it can be done correctly from time to time. I hope you enjoyed the read and please leave any comments below.
FYI- I found this on fox news of all places