That was the first Sonic Rainboom. Now the real test starts...
Rainbow Dash was already known as the undisputed fastest flier in all of Equestria, but she had long since stopped comparing herself to other ponies. All that remained was beating her own records. She sped past the rainbow precipitation speed as she had hundreds of times before - it had been years since that mattered to her.
The resistance she felt as she accelerated, intensified.
Second Rainboom. A quick glance behind her confirmed the second rainbow was enveloping the first.
This would be a droll day if she didn't at least manage to blast through Boom 4.
She felt like she was flying into a brick wall, but kept on pushing herself. Soon, the brick wall was actively trying to break her neck, and it didn't last long before she had to grind to a halt. Luckily, deceleration was a lot faster than acceleration, and she stopped almost instantly.
With a hasty descent, she landed in the meadow beneath her, almost too tired to take another flap of her wings. Tomorrow, she would try again, flying back to Ponyville the same way. A look back whence she had come presented beautiful quadruple rainbows. To Rainbow Dash it was a melancholic sight.
It would be twenty-five percent cooler if I just managed to break Boom 5...
Rainbow Dash sat on the library floor, too tired to hover as she usually did.
"Come on, Twilight! You gotta know a way to reduce that drag!"
She stooped her head in defeat. If she wanted to go faster, it was evident she needed help from someone. Who better to turn to than the Element of Magic herself?
"I've reached my limit here, and it's really frustrating. There's nothing new left for me to do if I can't do it..."
Twilight gave her a concerned look.
"You know, Rainbow Dash, you shouldn't push yourself too hard. Based on what you've told me, the resistance scales cubically with your speed. Flying any faster could be dangerous, so you should at least wait until we manage to send probes that fast."
Bah! The egghead was way too cautious. It was no fun just waiting around so Twilight could put numbers on how much she was being slowed. What mattered was that she was being slowed by that drag, and she needed it to not slow her.
"Come ooooon, Twi! You're the Element of Magic and there's nothing you can do?"
Twilight let out a sigh, an indicator of the number of times she had heard that exact phrase.
"As I told you, it's not safe. You told me yourself you felt like you were being crushed, and I don't want that to happen to you, Dash. If you collapse injured during one of your trips, I don't think anypony will find you in time to help... That's why I don't want you to try going any faster."
Now it was Rainbow Dash's turn to sigh. Twilight could be as stubborn as Applejack sometimes.
"Then can't you just cast something on me that stops it from crushing me, at least?"
Perhaps if she could get rid of that part of the drag, she could manage to go faster through sheer willpower. Surprisingly, her suggestion wasn't rejected.
"You know... Maybe I can. I found an old tome on spatial alteration in the Canterlot archives, and there's one that might help. I'm not completely certain, though."
Her wings rose in painful excitement as she saw the dusty book float across the room. If it worked, she might finally make it!
"It wraps you in a field of magic that sets up a personal inertial frame. The force you experience should be at least partially absorbed or deflected by..."
Rainbow Dash wasn't paying attention, despite her hardest attempts. All she could think of was going faster than ever before.
"... but since this hasn't been tried out before, please don't..."
"Sounds good, so when do we start?" Rainbow interrupted.
The lecture stopped, if only for a second.
"Rainbow! You have to promise me! You can go as fast as you have before, but promise me you won't try going faster until the probes are finished checking that it's safe!"
Twilight's stare was intimidating. Rainbow Dash idly wondered how fast her magic could travel.
"Yeah, sure, I promise! You can count on me, I'm the Element of Loyalty after all!"
After receiving the wrap field or whatever Twilight had called it, Rainbow Dash set off to Cloudsdale. She'd take a night's rest before she-
The Sonic Rainboom actually surprised her for once, and she stopped. Surely she hadn't been going that fast? She was far too tired to attempt it, anyway. She looked around, but couldn't see anyone else practicing. Strange. She'd better go home before she started having delusions from being exhausted.
The next day was a calm, sunny day. Despite her exhaustion, Rainbow Dash had hardly slept. Her excitement kept waking her, so for once she had seen the sunrise.
I wonder if Princess Celestia ever oversleeps, she asked herself. For that matter, I wonder how fast she can fly. Does she push the sun or does she use magic?
As she ascended to the height she had gotten reserved after she was shown to be almost twice as fast as anypony else, she decided to focus on the task at hand: Breaking Boom 5. She felt a tinge of guilt at breaking her promise to Twilight.
Hey, I'm the element of Loyalty, not Honesty! I'm not betraying anypony! she thought to herself. Time to fly.
As she blasted off, she reached Boom 1 faster than ever before. Boom 2. She barely even felt resistance as she passed Boom 3. Still no resistance worth mentioning. Boom 4.
She had done it. She was faster than anypony ever before, including herself!
Awesome! Twilight's spell is even better than I thought!
The landscape rolled beneath her faster than she could remember. Meadows became sea, sea became forest and forest became mountains so quickly she could hardly follow the ground with her eyes. She kept accelerating. Boom 6. Boom 7. Boom 8. Boom 9! Now, looking anywhere but straight forward was dizzying. The resistance was far more noticeable now, but it felt like it was pressing on something other than her body. Probably that wrap of Twilight's.
She wasn't even tired, so she decided to give one try at breaking Boom 10, then she would stop for the day and tell Twilight that her spell worked. Forging ahead, the resistance more than doubled without another boom arriving. She felt the onset of the brick wall effect again.
When it feels like yesterday, I'll stop.
Giving one final push, she barely had time to hear the tenth boom before the brick wall gave way in a searing explosion of colors. Everything faded to black silence.
She was no longer moving.