A/N: Parts of this story will make a lot more sense if you've read the following stories: Cupcakes by Sgt. Sprinkles, Rocket to Insanity by Scherzo, and A Precious Rainbow, The Cupcake Killer and Apple of Her Eye by milesprower06. They're not critical to the understanding of the story, but if you plan on reading them, this story will spoil them.
"I still don't get how this is supposed to make me go faster, Twi." In her usual impatient tones, the blue-and-rainbow pony complained about the treatment she was being subjected to.
"Ugh, Rainbow Dash, I'm not going over how the suit works again. You're just going to have to trust me - I've spent months working on this!" Twilight Sparkle's claim was validated by her slightly frazzled mane and bagged eyes. Since witnessing the Sonic Rainboom first hand, she'd had a brainwave, and had been working nights and weekends on what she called the High End Velocity suit. HEV suit? Sounds like it's from a video game, Dash thought to herself every time the name came up. It wasn't exactly designed with high fashion in mind, and looked as much like a navy blue spacesuit as anything else. Possibly not quite so bulky, and not all-enclosing - Rainbow's wings were largely free-hanging apart from a set of rigid, hinged spines anchored to the main bone line of the wing for reinforcement - and rather than a helmet, there was just a pair of "dashing" (pardon Twilight's pun) goggles and something like a pilot's mask, hanging from a strap on the chest, and a chest panel that appeared to have some mechanism in it. "It's not too restricting is it?"
The pegasus responded with a prompt leap into the air, flying somersault, and a thud upon landing again. "Eh, needs to be abo-"
"So help me Celestia I will smack you if you finish that sentence." Glaring at her friend, the pair left the library and set off for the meadow.
In the meadow not far from Ponyville, Rainbow did a little more practice in the suit, starting to get a grip on how it altered her flying, not too concerned about the "how". Sure enough, there was definitely a little more speed there, and she was really feeling some of those G-forces on the cornering, but she still didn't quite get it. Twilight tinkered with a little box with some flashy lights and buttons.
"I don't get it Twi, I thought magic was your thing, not science," Dash inquired as she floated down next to her upside down, head resting on her forehooves. The unicorn chuckled in response.
"You'd be surprised how much the two have in common." Dash shrugged.
"So what do you want me to do again?"
"For the sixth time, I need you to go for a Sonic Rainboom, and then keep going. We're going to see just how fast you can really go!"
"Oh boy!" With nary another word, the cyan pegasus was already away, just a rainbow blur behind her, nearly forgetting to put on the mask and goggles. Sure enough, she took off and got up to a cruising speed, doing some rounds to warm up and accelerate to higher subsonic speeds. Already the run was going quicker than usual - the suit was definitely playing its part. Deciding to push herself a little further than usual, Rainbow started going higher in altitude, taking advantage of the mask's oxygen supply to ascend rapidly. After nearly a minute of almost perfectly vertical flying, taking herself to the edge of space, she banked around and turned back down, adding gravity to her acceleration. Not ten seconds after her course change, the Sonic Rainboom's signature Mach cone started forming, and in just five more, had already closed around her form, and she pierced the sound barrier, causing its signature explosion of radiant colours, high up enough in the air that, while barely visible, it could be seen all over Equestria. Not content with that, she continued her descent, pushing for more, really feeling like there was more in her. Banking up to level with sufficient centrifugal force to make her more than just a little bit dizzy, she continued on a level plane, about five hundred feet above the land surface, Ponyville coming up fast in her field of vision. With a gulp and renewed, grizzled determination, she pressed forward, flying faster than she had ever gone in her life. She passed by the meadow in a split second, a shockwave of wind following her (knocking Spike over as he took notes for Twilight).
As her speed increased, interesting side effects began to follow. To Rainbow, the centre of her field of vision began to become brighter, while around the edges it was starting to darken. To the citizens of Ponyville (but only those with quick enough eyes to even catch a glimpse of her), they would see a dazzling cone of bright white light with a rainbow streak behind it tear across the sky, followed by a brilliant, blinding flash, lighting up the sky like a supernova. But for the little blue pegasus as the centre of it all, it was a different story. The increasingly bright light in the centre of her vision outshone everything else engulfing her in pure white. Seconds ago, she was experiencing the intense forces of supersonic flight and the physical exertion needed to reach it, but now... it all went away. All sense of motion left her, all fatigue departed, even the feeling of the bulky suit disappeared.
Funny things happen when you break the light barrier.