Twilight Teaches Rainbow About Overpenetration
The resounding sound of an impact against the castle walls drew Twilight’s attention. She sighed and got up from her readings, heading over to the window. She stuck her head out and saw Rainbow Dash attempting to peel herself off the solid exterior wall.
“You know, this isn’t the library,” said Twilight.
“Ugh, tell me about it.” Rainbow shook the stars out of her vision and barged through the window. “You should put up some branches and have the walls converted to wood. Did I at least make a cool crash?”
“You didn’t crash, you splatted.”
“Oh come on, it wasn’t that bad,” Rainbow protested.
“When a pegasus hits a castle, that’s what happens,” Twilight replied. “It’s physics.”
“Yeah, but I'm no ordinary pegasus! I bet physics can’t explain me!”
Twilight grinned slyly. “Oh really? Want to prove that?”
Rainbow puffed up her chest. “Sure. Any challenge you can throw at me.”
“Well then, Rainbow. I challenge you to stay awake through the physics lesson that I’m about to give you.”
It was about that time that Rainbow realized that slamming into the castle had been a really bad idea. She was a slow learner.
Before she could protest, Twilight whisked her off to the castle study. It was crammed with books and whiteboards. Twilight found some markers and a clean space to write. “Okay, let’s begin. A body in motion tends to stay in motion unless acted upon by an outside force. What happens when a pegasus hits a wall?”
“Well if it’s me, I punch right through it!”
“Didn’t you just splat against my castle?”
“That wasn’t a splat, that was a crash!”
“You’re a meatbag hitting a stone wall. It was a splat. The wall exerted an outside force, and your body in motion came to a sudden halt.”
“Well…” A lightbulb finally clicked on in Rainbow’s mind. “All I’d have to do is go really fast. If I hit it hard enough, I’d punch right through.”
“Even if you did, then you’d die with a splat. A lot less dignified than going out with a bang, I’d say.”
“Well, I took out Applejack’s old barn with a bang.”
“That was a rickety wooden building that you’d already weakened. You couldn’t do the same to a sturdy stone wall.”
Maybe Twilight was right. There were a lot of things Rainbow could do, but she’d never tried punching a hole in a castle wall before. But it sounded awesome.
“Then teach me. I want to bang things.”
Twilight’s eyebrows went up and she concealed a grin. “Are you sure? You’re asking me to teach you?”
“If anyone knows physics, it’s you, Twilight! I’ve got get busy and learn how to punch holes in things!”
“If you really want to know about penetration, then I think I can help you.” Twilight picked up a marker. “Let’s start with force. Your squishy body applying a force to a sturdy castle wall, for example.”
“I’m not squishy! Do you even know how much I work out?” Rainbow flexed.
“You’re a lot softer than a castle. But that means as your body deforms while splatting on the stone, you’re transferring all your force to the wall. You aren’t bouncing off.”
“What’s the point?”
“My point is that if you want to apply maximum force, keep squishing and do less ricocheting.”
Twilight drew a simple diagram of a pony splatting into a wall. “What do you think happens when all that force hits the wall? Obviously, you didn’t punch through.”
Rainbow shrugged. “I don’t know. Maybe a picture falls off the other side or something.”
“Could be. A body slamming into a wall and causing damage to the inside of the wall is called spalling.” Twilight added a few cracks in the wall to her drawing. She turned back to Rainbow. “But what if you weren’t squishy?”
“I told you-”
Twilight rolled her eyes. “What if you were as hard as stone yourself?”
“Aside from being popular? Well, I’d probably fare a lot better against the wall.”
Twilight nodded. “That’s right. So if you were flying towards a wall and wanted to go through it, what shape would you make your body?”
“As small as possible, I guess. Just like going through air. Less drag.”
“Right again. If you concentrate all your force on a smaller area, you exert a higher pressure on the target than if you splatted. The higher pressure gives you a better chance of punching through.” Twilight drew a diagram of a smaller pegasus knocking a hole in a wall.
“Wait, you’re saying if I get smaller that will make me penetrate things deeper?” Rainbow frowned. “Are you sure?”
“There’s another way to get more penetration,” said Twilight. “If you’re flying towards something, how do you generate more force?”
“Go faster.”
“Right. Wow, Rainbow, the one time when that actually turns out to be the answer.”
“Hey, going faster is the answer a lot of times!”
“Except for the fact that you’ll still die in a sudden, embarrassing splat at the end.”
“Are you teaching me how to penetrate things or not?”
“Right, right, we were just talking hypothetically. So back to the lesson, once you’ve got a sufficient pressure to plastically deform the object you’re striking-”
“Wait, what?”
“To bend it so much it can’t bounce back to its original shape. Stone, for example, isn’t very flexible. It takes a lot of force to deform it, but once you do it’s broken.”
“But wouldn’t something softer be easier to penetrate?”
Twilight nodded and smiled slyly. “Are you saying the stone walls are too hard for you?”
“No! I’m just saying I could try to penetrate other things. I don’t always have to go around sticking myself into stone. Like, what if I tried jousting against other ponies? Or even swordfighting?”
“Sticking a lance in somepony, huh? Well, that sounds violent, but it does present a much different challenge.” Twilight began to rapidly scribble notes on the board. “A long, solid pole like a lance can go quite deep, the cone shape gradually stretching the hole apart. This is due to the stresses spreading radially and causing material failure around the penetration point.”
Rainbow made a face. “It isn’t fun when you say it like that.”
“Of course not. Would you want to be the one getting penetrated?”
Rainbow laughed. “Would you be doing it?”
“No, no, I stick to theoretical work. I like to just run experiments with myself.” Changing the subject, Twilight said, “So how much penetration do you think is enough?”
“As much as I can, of course. Why is that even a question?”
“Well, if you penetrate too much you could get stuck. You could also go through and hit something on the other side.”
“Like what?”
“Like the other wall of the castle. You’d go through the first and hit the second.”
“So on top of getting hard and going really fast, I need to go for double penetration.”
“The first impact would probably make that impossible. Not to mention it would probably be even more difficult due to being deformed by the first impact. A blunted object, or one breaking apart, would distribute its force over a wider area and therefore reduce the pressure it could apply. An object would have to be traveling at a completely ludicrous speed to go through a castle wall and still carry enough force to go through the second.”
“Twilight, you’re the most powerful magic pony in Equestria and I’m the fastest pony. I think realism went out of the conversation a long time ago.”
“Fair point. Okay, so once you’ve double-penetrated your target like a spit roast, then what? Assuming you had actually generated enough force to pull it off, then coming out the backside you’re still probably moving at a pretty good clip. I know we’ve been talking about destroying things here, but what if there’s something sensitive you don’t want to hit? What about an innocent bystander?”
Rainbow shrugged. “Then I’ll penetrate them too.”
“Rainbow! You can’t just go around sticking yourself in any pony that gets in your way!”
“Okay, jeeze. I’ll ask them first.” Rainbow paused. “But in the interest of the conversation, what would happen?”
“Due to elastic properties, a high velocity impact with a soft target would create a hole artificially larger than the object penetrating it, like a bullet wound.”
“So you’re saying if I stuck myself in somepony hard enough, the hole would get bigger and make it easier?”
Twilight gave her a look. “I’m trying to teach you the physics of bodies in motion here. I’m not sure you’re taking the right lesson. Why would you want the hole to be larger?”
“Because I’m ripping it open. I thought we’d established that I really like penetrating things.”
“How do you know? You’ve never actually penetrated my castle walls.”
Rainbow grinned. “But you just taught me the best way. It’s like you’re practically giving me permission.”
“Well, on the off chance that you actually do manage to go deep and come out the other side, look out for hapless bystanders. Surprise penetration from behind is the worst kind.”
“What do you know about that?”
“I’ve always studied the subject extensively. Princess Celestia taught me about that when I was back in Canterlot.”
“Isn’t Princess Celestia a little busy with running Equestria to think about penetration?”
“You might be surprised. She’s drilled quite a few things with that long horn of hers, a lot of them probably by accident.”
Rainbow snickered. “That must be embarrassing.”
“I can’t imagine. That’s probably why all the ceilings at Canterlot Castle are so high.”
“No, I’m talking about getting penetrated by an alicorn.”
Twilight glared at her. “Careful. I might decide theoretical work doesn't satisfy me anymore.”
As it turned out, Rainbow was more than capable of penetrating Twilight’s inner sanctum. This was made perfectly clear when she rammed into the study through the wall, knocking down bookcases and trashing the whole room in moments.
“Rainbow!” Twilight shouted. “Did you have to throw away all that I taught you?”
“I didn’t throw it away,” moaned Rainbow from under a pile of books. “I used it. Pretty cool, huh?”
“Well, now everything is torn open and gaping.” Twilight gestured to the massive hole in the wall. “The least you could do is help me fix it.”
Rainbow pulled herself out from under the books. “Are you going to give me another lengthy lecture or the quick tips version?”
“Just the tip.”
That cheered Rainbow considerably, although when she saw Twilight pulling out the heavy equipment, her face fell. “What’s that?”
“This is an air compressor unit that we can use to run various power tools.”
“It looks like a big unit.”
“It has to be, to blow so well.”
Twilight took out a nail gun and a pneumatic screwdriver, attaching them to hoses. “All right, let’s get something temporary up to cover the hole. Start putting up this plastic.”
Rainbow stretched it across the hole, but it didn’t seem to want to stay stuck. “This bag won’t fit.”
“Have you even tried nailing it yet?”
While Rainbow was busy with that, Twilight made a quick sketch of her plan for fixing the wall. If Rainbow was going to make things so hard, then she was going to improve the resistance.
She called Rainbow over to look at a diagram of two sturdy steel plates with a space in between. “Here’s what we’re going to do. This is called a Whipple Shield. It’s used in some spacecraft. The outer surface causes objects to begin to deform, and the inner void allows them to spread before slamming hard into the inside.”
“We’re going to put this on the wall? How are we going to attach it?”
“We’re going to screw it.”
“It would take some strong screwing to keep it attached to stone,” Rainbow noted.
“Long, hard, screws,” Twilight confirmed. “The metallurgy won’t allow for softer, more malleable metal.”
Applejack arrived as they were assembling the materials. She looked around at the mess and decided that she didn’t really want to hang out. She had a lot to do at the farm.
“Twilight, I came to drop off the bushel of apples you ordered,” she said.
“Thanks, AJ.”
“It’s against my better judgement to ask, but what happened here?”
“Rainbow took it as a challenge to penetrate my castle walls. She didn’t quite get the double penetration, though.”
“Twilight taught me how to get hard,” Rainbow said.
“Shoot,” said Applejack. “You didn’t know how?”
“She said I was squishy. Do you think I’m squishy, AJ?”
Applejack backed up as Rainbow flexed and tried to get her to come closer. “Now sugarcube, I’m all for feelin’ you up, but should you have known what the hardest part of your body was.” She grinned. “Your head.”
“I actually used my hoof to break into Twilight’s private space,” said Rainbow, gesturing to the books strewn around.
“Stuck your hoof right up in there, huh?” said Applejack. “So what took so long to figure this out?”
“Rainbow was splatting too much,” said Twilight. “What was it I said? ‘A meatbag hitting a stone wall.’”
“Instead, I just moved my body faster,” said Rainbow.
“Well of course,” Applejack remarked. “It ain’t the meat, it’s the motion.”
Applejack turned to go. “All right, I’ll let y’all get to work before this becomes a threesome. Stay cool in this weather, now. Big Mac was workin’ in the orchard and was all hot and bothered.”
Rainbow and Twilight went back to work. The screwing wasn’t going well.
“Your walls are just too hard!” Rainbow complained.
“I guess we’ll have to start drilling,” said Twilight. She connected an air drill to the compressor. “Here, try this. Make sure you use the proper lubrication. That drill bit could overheat if it’s not oiled up.”
Rainbow rubbed the lube all over and started drilling. She easily reamed the holes they needed and the two of them went back to screwing.
“‘It’s not the meat, it’s the motion,’” Rainbow muttered as they worked. “Did I hear Applejack right?”
“That’s what she said,” Twilight confirmed.
Rainbow shook her head.
In another few minutes, they finished drilling. With the holes in place, they could hang the first metal plate. “That looks good,” Twilight commented. “It’s a tight fit, but it works.”
“You know, I think this section of the wall might end up being stronger than it ever was before,” said Rainbow.
Twilight nodded. “I think you’re right. Harder, better, faster, stronger.”
“Faster?”
“It sure is going up quicker than resetting new stone blocks.”
That was true. They were already installing the outside metal plate. “Once we fill this gap in between, this place will be impregnable,” Twilight commented. “This could function as the breastworks for a more formidable defensive line. Earthworks, ironworks, parapets, towers, the whole shebang. Just one thing left to do. We have to fill that crack.”
“With what? Some kind of liquid? Would something sticky help hold it together?”
“In a way. We’re using foam.”
“Expanding foam?”
“Not your average foam. We’re building a hard, solid wall, remember? This stuff does triple duty of buffing up the armor value, as well as insulating from noise and temperature. I’ve got the canister right here.” Twilight showed Rainbow an industrial-sized container of liquid foam mixture. “Here’s the hose. Stick it in there.”
“Just stick it in?”
“Push it deep. You want it to spray everywhere, fully impregnating and integrating the slit between the two metal plates. Fill it up completely. There shouldn’t even be room for air.”
“I don’t know, it’s pretty tight. I’m pushing.”
“Harder!”
Rainbow finally got it in and started shooting her stuff, making sure it went everywhere and giving everything a good coating.
Twilight watched her work, giving her a reach around to help hold the hose. “Yeah, that’s it, impregnate the breastworks.”
“Should we throw some solid screws in there to help tighten it up?” Rainbow asked.
“Couldn’t hurt.”
The two of them continued working hard, sweating. But when everything was done, they stepped back to admire their work.
“It looks great,” said Rainbow. She tapped a hoof on the metal. “It’s so hard.”
“Yeah,” Twilight agreed. “It fits perfectly.”
