Rocketry is not a particularly well-studied field, at least among the vast majority of worlds that comprise the Interior proper. While general equations of motion and inertia are widespread and understood, the engineering knowledge required to build functional reaction drives and rocket engines are notably lacking, especially on worlds where the local magic density is high enough that such a field of study is relegated to the rarified air of pure research owing to its lack of useful practical applications.
This shortcoming is particularly evident among the thaumoscientific enclaves of the Exterior, whose access to high-powered interuniversal travel allowed them to neatly sidestep the need, or even desire, to physically travel significantly beyond the surface of whatever world they were standing on. The myriad worlds of the Interior represented far more variety, more interest, to explorers than the dark expanse of the cosmos above them, containing little more than dead wastelands, uniform and unchanging.
That isn’t to say that there are no civilisations within the Interior that have plied the heavens. Such worlds were of particular curiosity to the exploratory divisions of the Exterior, but in much the same way that one would consider a particularly enjoyable piece of art or music to be interesting - enjoyable to observe, but otherwise of no practical merit. They are outliers, statistical anomalies among an insurmountable trend of planetbound equine cultures.
It was for this reason that Twilight Sparkle was having to rapidly deduce the best purely-magical rocket design from first principles as the world she was standing on tumbled, out-of-control, through an eight-dimensional void.
Twilight stood at the tip of Mount Canter, having long abandoned the idea of moderating the Void magic flow that she had tapped, instead opting to open the floodgates as wide as they could possibly go, nudging the flow slightly to direct it along a particular avenue, but otherwise allowing it to do as it would. In front of her floated an intensely-glowing orb - a visual abstraction of a thaumically-assembled eight-dimensional inertial measurement unit, a plain scroll and quill, and a plain mechanical stopwatch, fetched via magic from one of Canterlot’s many clockmakers.
Around her, closing in at a speed well in excess of c, the universe disintegrated. Void fissures violently ejecting material and spacetime into pure nothingness and imparting commensurate reactive momentum upon the remaining, rapidly dwindling, portion of the universe, propelling it upwards and outwards through the fissure at the heart of the Ruins.
Slowly, almost agonisingly so, the universe stopped its eight-dimensional tumble. Jets of ejecting spacetime, directed off-axis by Twilight’s subtle nudging, nullified their angular momentum and allowed her a brief moment to tabulate the data that was streaming from her improvised IMU into her frontal cortex.
She knew where she was when Bastion detached from the Interior, give or take a few vert. She knew roughly how fast she was going, give or take a few multiples of c, and the direction of that motion in the eight-dimensional space her ‘vehicle’ occupied. Integrate over time, and that gave her the position she was at right now, and, more importantly, which direction was ‘down’, back towards the Interior.
Carefully, delicately, she directed the expanding void tendrils in the opposite direction, and the acceleration built up. Steadily, their upward momentum dwindled, replaced with an ever-increasing downward trend. By her guess, they would impact somewhere in the Exterior, if only due to the asymmetric thrust and uncontrolled tumble during the initial detachment. Ultimately, she had no way of knowing, since she completely lacked the ability to ‘see’ through the Void.
A twinge of fear shot through her as she considered the possibility that her calculations were off, and that she would miss her meta-universal target without ever realising it. It worsened seconds later when she realised that she was piloting an entire universe towards the Exterior at speeds that her mind couldn’t even comprehend, with little regards to how well her own vehicle, or the intended target, would handle the impact.
------
Moments after the first head became visible in the crosshairs of her rifle, Walleye’s world had turned from ‘tense anticipation’ to ‘neon white’. The first to round the corner was taken down cleanly and effortlessly by a shot to the base of their skull, the second and third followed suit as Rainboom opened up on the hallway with a chaotic, but nonetheless effective burst of polychromatic energy. Morbid though it was, it gave the following two ponies sufficient cover to avoid immediate death for long enough to loose a handful of shots at Team Fifteen’s hastily-erected cover and spoil their aim.
Another eight followed in their wake, and any pretense of being able to effectively return fire from their position immediately evaporated, forcing both Walleye and Rainboom fully behind the dividing wall, occasionally wincing as the fusillade of beams kicked chunks of the Exterior metal onto their heads.
“Certainly not the ideal way to go out,” Walleye muttered. “But it’ll do.”
“I don’t know about you, but I’d rather not die,” Lyra panted, breath slowly returning. “To be entirely honest.”
“What, you think they’ll accept surrender?” Walleye asked flippantly, pulling a grenade from her saddlebags, priming it, and casually lobbing it over the wall. “Somehow I don’t think they’ll go for that.”
A chain of hasty exclamations accompanied a marked reduction in the intensity of the gunfire raining down on the all-too-thin wall separating Team Fifteen from a quick death, followed seconds later by an explosion and screaming.
And as abruptly as it began, the gunfire stopped. Muffled, indistinct voices yelling through the smoke and haze, accompanied by the sound of increasingly-rapid hoofsteps.
After a few seconds, Rainboom poked her head over the wall, and scoffed.
“One grenade scares them off? Seriously? I would have expected more of a fight!”
“I know, right?” Walleye laughed. “How in Tartarus did they manage to take the Exterior when two of us fought them off with small arms and a bucking grenade?”
“You… didn’t scare them off,” Lyra stated flatly.
“Then what did, genius?”
“That,” Lyra replied, pointing directly upwards.
Walleye and Rainboom looked up, and almost immediately wished that they hadn’t. Descending out of the sky in a manner that could be most accurately described as brick-like, trailing debris and dust, was Mount Canter.
“What…,” Walleye began, only to be cut off as the 200 trillion-odd kilograms of rock cheerily obeyed the orders given to it by the laws of physics, and hit the ground. The ground, meanwhile, being made of Exterior metal, steadfastly refused to acknowledge something as trivial as a mountain being dropped on it, and dumped most of the energy and momentum back the way it came.
Mount Canter, obligingly, disintegrated into a kilometres-tall plume of pulverized rock and smoke. Canterlot, meanwhile, still encased in the force field bubble that Twilight had reactivated minutes earlier, shot out the side of the impact plume at near horizontal, bounced twice as it bled speed, and rolled to a halt about a half kilometer away from Team Fifteen.
Walleye could do nothing but gape as the shield gingerly deactivated and the city settled on solid ground.
“Well,” Rainboom observed. “That… just happened.”
With a crack only barely audible over the cacophony of settling debris, Twilight Sparkle materialised next to Team Fifteen, a conscious, shivering ball of pink fluff clinging to her back, hammer floating at her side. Tentatively, she stretched her legs and looked around, her expression stadily shifting from intense focus to manic glee, and the tiniest squeal of triumph escaped her.
“You…” Walleye began, glancing back and forth between the adrenaline-soaked Twilight and the mountain still settling in the distance, words entirely failing to come to her aid. “What.”
“I told you I had this” Twilight explained, grinning. “I saved everypony. I Won.”
“You… won?”
“I Won,” Twilight repeated, audible emphasis on the second word as she attempted to dislodge her shivering passenger, to no avail. “It’s over.”
“You… wait,” Walleye stammered, examining the pink object. “Is that Theta?”
“Yes.”
“...How?”
“I dropped a moon on her.”
“Hold up,” Lyra interrupted. “You dropped a moon on her?”
“Yes.”
“Bollocks.”
“Sorry?”
“You’re having me on. I’m calling bollocks to this. No way did you win that fight by dropping a moon on somepony.”
Twilight raised an eyebrow, before pointedly turning her head to gaze at the mountain behind them, and then back at Lyra, slowly.
“Okay, point,” Lyra conceded. “Let's assume you’re not completely full of it right now, how is she still alive? Moons aren’t the sort of thing that are known for leaving survivors.”
“Fun fact,” Twilight began. “Force Dome, and similar force-field defensive spells are, according to modern understanding of thaumic field theory, impenetrable, provided sufficient source energy and local thaumic flux. She likely cast it in the moments before impact to protect herself.”
“Hold up,” Rainboom interrupted. “Unicorn forcefields are impenetrable?”
“If cast properly, yes.”
“Why has nopony told us that?” She asked offhandedly to Walleye, clearly not expecting a response from the still-wordless grey pegasus.
“Okay, next question,” Lyra continued. “If you didn’t kill her, why isn’t she still trying to kill you? This is Theta we’re talking about here.”
“Honestly? I think I just broke her,” Twilight replied, poking the shivering mass on her back, only to elicit a quiet squeak of distress. “She’s been gibbering like a frightened foal since I grabbed her.”
“Twilight,” Walleye said, slowly and carefully assembling her words. “I say this will all due respect and deference, but honestly? After… this?”
She gestured at Theta and the mountain beyond.
“You scare me.”
“Thank… you?” Twilight said, confused.
Walleye nodded and looked back at Mount Canter. There was nothing left to say.
“So, what about you four?” Twilight asked. “How did it go storming the castle?”
“They were about to overwhelm us until you showed up,” Rainboom replied. “Nice timing, by the way.”
“Oh, just my luck,” Twilight moaned. “More saving to do.”
“Excuse you?” Rainboom retorted. “They obviously held back a reserve for defense. You might have alicorn-like powers, but we’re...”
“Look, I am running on pure adrenaline at this point,” Twilight shot back. “I don’t even have words good enough to properly elucidate what I just did in the last five minutes, let alone something you would understand!”
“Well buck you too then!”
“And it is still not over!” Twilight barked. “I still have to save the day again, because of course I do.”
“Okay, question,” Lyra interrupted. “Why are we suddenly overwhelmed? We saw thousands attacking Bastion, why are there also thousands here?”
“It makes sense that they’d not commit their entire force to that attack,” Rainboom offered.
“No, that’s not it,” Twilight said. “There is no way they wouldn’t fully commit to an attack against the one remaining threat against them and risk failure. They fell back, and you got caught in the retreat.”
“Why would they fall back when they had the advantage?”
“First thing I did before attacking Theta was to teleport anything that was a threat to the city out of Canterlot, before re-starting their defensive spells,” Twilight explained. “They likely gated back here shortly after.”
“So, now we’re, what? Behind enemy lines facing a scared, retreating army?”
“Yes, but we still have an advantage,” Twilight mused. “They will all be congregated around the gates, but whoever’s in charge of everything going on wasn’t in the attack. They sent Theta in their stead.”
“Makes sense,” Walleye chimed in, turning back to the group. “The ringleaders would likely be back in Operations. You said from that crystal vision thing that the initial attack focussed there.”
“How long does it take to get from the gates to Operations?”
“For me?” Rainboom offered. “Five minutes. If you’re on hoof, closer to fifteen. Probably twenty considering they’re retreating in a panic.”
“So we just need to get to Operations before they do, capture the ringleaders, and let the bigger army of Outsiders that I brought with me from Canterlot secure the Exterior.”
“Great plan, except that we ran away from Operations to get here,” Rainboom explained, exasperated. “We’d have to fight our way through them again to beat them to Operations”
“...Point,” Twilight replied, thinking for a moment, before she started rummaging around in the tattered remains of her saddlebags for something specific.
“What are you looking for?”
“A counterpoint,” Twilight replied, triumphantly pulling a cool blue crystal from the depths of her bags, and holding it close to her head.