The Thrill of a Rocket's Launch & Ascent - An Astronaut's Perspective

Science Buddies' founder, Kenneth Hess, was an astronaut on Blue Origin's NS-25 suborbital spaceflight. Follow along as he recounts the thrilling (and noisy) minutes of liftoff and ascent—and explore science projects students can do to experiment with the physics of rocket science.

Hatch closed, the crew capsule cabin was quiet—there was very little talking. Three minutes before liftoff, my heart rate was 60.

Soon the pace of activities picked up as the onboard computer of this fully automated space vehicle assumed control, cycling the fins and the rocket engine through their range of motion, shaking the vehicle. Plumbing hammered repeatedly as if a bass drum setting the tempo. It was time.

A rocket is a ferocious machine, roughly 80% fuel by weight, operating at the limits of what is mechanically possible. Nothing about one is subtle. Its job is weightlifter not ballerina, yet it's also sophisticated, superbly optimized, superhero not brute.

When the engine thundered to life, the ceiling of the capsule appeared to be on fire as it reflected the flames outside. Tumultuous noise came to us both through the rocket structure and from engine exhaust bouncing off the ground to our ears like a massive wave crashing over us.

Power You Can Hear

The noise from a rocket is of a very specific kind, created by countless particles in the engine and exhaust plume randomly and violently colliding into each other. Like the sound from an immense waterfall, but even more bass heavy, not cacophony or discordant in any way, it's predominantly what is called Brownian noise, and it innately conveys power to any human that hears it. A video does not do it justice—it must be heard in person.

To an astronaut on top, noise is not the rocket's only voice. Resonant sounds from other parts of the vehicle overlay the underlying noise in a complex and ever-changing pattern as the rocket responds to the forces of the engine and the resistance of the atmosphere. Sometimes I would feel like there was a musician woven into its essence, a multi-talented soloist changing instruments as needed to stand apart from the noise.

Rising above the roar of ignition, two deep musical notes resonated through the vehicle when the engine applied its force, sounding like the plucked strings of a symphonic double bass, as if it was singing, "Ready to lift."

Seven seconds later, the computer completed the vehicle diagnostics, committed itself to fly, and put the pedal to the metal. We immediately lifted off the pad in the loudest moments of the ascent. Perched atop this explosive package of electronics, cryogenic pumps, fire, and noise, I was stunned by the incongruity of the unfaltering, smooth ascent to space. All that raw, barely constrained power beneath me, yet here I was, gently but firmly pushed into my seat with Earth receding out of the corner of my eye, steadily lifted by an irresistible force, not hurled.

It was like riding in a glass elevator—just one that continues to accelerate, tops out at 2,240 miles per hour, and stops at a floor 107 kilometers above the ground.¹

Then at the 20 second mark the capsule began a planned rotation, 360 degrees every two minutes, enabling each member of the crew to scan the entire horizon.

Even laden with fuel, as it was at liftoff, the rocket's acceleration is immediate and continuous so long as the engine is running. At roughly 45 seconds, I could feel the engine throttle back ever so slightly, one tenth of G less acceleration according to my information panel, then a bit more. We were still pushed back in our seats, just not quite as much. The air pressure on the vehicle had reached a pinnacle known as max q, where the ever-vigilant computer reduced thrust so the force of the air's resistance to our progress didn't rise enough to do damage.

Transonic Flow

Before we could adjust to this momentary equilibrium, the atmosphere had thinned sufficiently for the computer to return our vehicle to full thrust, straight into the transonic regime—that liminal space between subsonic and supersonic flight where aerodynamics behave in mysterious ways. With the air moving at different velocities on each part of the vehicle depending on its shape, airflow is subsonic here and supersonic there, depending on where you look. Shock waves formed, detached, and reformed around our vessel in a complex pattern, invisible to us as we were punching through the speed of sound.

With the erratic shock waves of transonic flight came another peak in sound levels, a continuation of the dynamic acoustics that fell immediately after liftoff, the ground too far away to efficiently reflect noise back to our capsule, only to begin rising seconds later as our velocity increased.

Supersonic now at 73 seconds into the flight, we were outracing the sound of the engine in the air; it could only reach us through the rocket's structure. An interesting balance emerged: as the propellant tanks emptied, the structure became more efficient at transmitting vibration, while, simultaneously, the whisper of air resistance faded as we climbed higher into the thinning atmosphere. These opposing forces settled into an equilibrium of sorts, a sustained note in our ascent's composition. It was as if our vessel was finding its voice—no longer fighting against Earth's atmosphere, humming its way towards space with increasing clarity and purpose.

G-forces had been relatively benign at 1.6 G's, give or take, for most of the flight, but absent the atmosphere's restraint, and with the vehicle much lighter because half the rocket's fuel had been consumed, the G-forces now built relentlessly until the engine shut down, topping out at 2.9 G's and 2,236 miles per hour.

While I was seated next to a huge window, with my head in a restraint on the way up, I could only see out of the corner of my eye, and that corner was focused on Earth, not the darkening sky, bluish black at 80,000 feet and as black as it would get by the midway point to space when the engine shut down.

So as not to savagely snap us against our five-point harness, the engine throttled down gradually over several seconds before main engine cutoff (MECO) at two minutes 22 seconds into the flight. As the engine turned off and its force was removed from the structure, once again the vehicle resonated like a symphonic double bass, yet differently because its cavernous fuel tanks were nearly empty. In a musical phrase inverted from the one at liftoff, 30 Hz lower and six decibels louder, in triumph now it sang, "Job done." No longer accelerating, we were immediately weightless, albeit still buckled in our seats. Halfway to our apogee above the Karman line signifying the beginning of space, we would coast the rest of the way in free fall.

The voice of the capcom crackled through the radio, "Prepare for separation," the process of detaching the capsule from the booster, which would return to Earth on its own, ready for another flight. The latches released, sounding like a massive, sharp hammer blow on wood, momentarily as loud as liftoff, immediately followed by the snap of the springs jolting us away from the rocket. For that fraction of a second, the G-force was higher than the ride up.

Then, as I floated weightless, it was silent.

Only Earth and the Sun were visible, no stars in the blackness of space because Earth was far too bright for them to be visible.

Even though Earth from space looked just like the pictures we've all seen for decades, it was almost out of body to see it with my own eyes. The surprise was the Sun. We live our lives seeing the Sun through the frame of reference of Earth. The Sun rises over the trees or the mountains with gorgeous red clouds; we watch it slip below the ocean as it sets at the beach. Even at noon it's high in the bright blue sky. Always we see it from Earth's frame of reference. In the blackness of space, the Sun is just "there," a star in all its naked brilliance, and it was clear that Earth and everything on it exists at the Sun's pleasure. I wanted to see Earth in its native environment—what I saw was the solar system in its native environment.

As the capsule rotated through a complete revolution, I spent the entire two and a third minutes that I was out of my seat at the window, transfixed by Earth and the Sun, occasionally glancing at my GoPro to make sure it was recording.

All too quickly the alarm sounded to strap back into our seats.

Student Project: Simulate a Rocket's Flight

The new A Simulator for Suborbital Spacecraft project guides students in asking and answering "how high can a rocket go?" In this project, students use an interactive code sandbox to simulate the trajectory of a sub-orbital spaceflight. Students can make changes to the code to explore different variables, like the mass of the rocket or how much fuel it carries, and rerun the simulation. How will their simulations compare to data from a real flight? The new A Simulator for Suborbital Spacecraft project guides students in asking and answering "how high can a rocket go?" In this project, students use an interactive code sandbox to simulate the trajectory of a sub-orbital spaceflight. This project has many layers, enabling a moderately curious student to learn something new about rockets in a few minutes, while containing the richness to engage serious students for many hours.

Students can make changes to the code to explore different variables, like the mass of the rocket, its fuel, its aerodynamic drag, and much more. They can even explore how the rocket would perform in the vacuum of space or the reduced gravity of a moon, and they can compare the results to actual suborbital rockets like the Blue Origin New Shepard Ken flew on, or the Mercury-Redstone that launched the United States first astronauts.

Other Student Projects

Students interested in the physics of a rocket's takeoff and ascent can conduct related experiments with projects like these:

• Bottle Rocket Blast Off!: Use a bottle rocket to explore how air pressure created before launching relates to the maximum height after launch.
• Solid Motor Rocket Propulsion: Launch a model rocket with different engines, predict its performance, and measure how high it actually goes.
• Model Rocket Aerodynamics: Stability: Explore the relationship between fin size and model rocket stability.
• Rocket Science: How High Can You Send a Payload?: Use a water bottle for a bottle rocket with compartments for fuel and payload explore the science of lifting mass vertically into the air.
• Study Bottle Rocket Performance with Electronic Sensors: Use sensors to measure and monitor a model rocket's flight and evaluate how changes in the design or launch procedure impact the rocket's performance.
• Make a Spacecraft Motion Simulator!: Build a cable-driven spacecraft motion simulator to simulate moving a model spacecraft around in three-dimensional space. (For an advanced version of this project, see Motorized Arduino Cable-Driven Spacecraft Motion Simulator.)
• Explore Satellites with Powerful Simulation Software: Use FreeFlyer® simulation software to explore how satellite sensors are configured and how orbits are planned.

Browse space science projects

A Firsthand Account of Suborbital Spaceflight—From Pre-flight Training to Landing

This post is part of a series of posts about Ken's experience as an astronaut on Blue Origin's NS-25 suborbital spaceflight. This series breaks space flight into individual segments, including pre- and post-flight, and connects Ken's first-hand account with space science projects for students.

Other posts in this series:

• A Firsthand Account of Suborbital Spaceflight - An Astronaut's Perspective
• Preparing to Go to Space! - An Astronaut's Perspective
• Heading Back to Earth—An Astronaut's Perspective

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¹ I learned later that not all rockets are this smooth. Blue Origin's New Shepard is sleekly aerodynamic with a single, smooth-running, liquid-fuel engine. Contrast this with the Space Shuttle, which was an ungainly affair with a huge surface area to catch every gust of wind, two widely spaced and rough-running solid boosters to supplement its liquid-fuel engines, and a configuration generating asymmetric propulsive and drag forces that the guidance system fought to control. The Space Shuttle was like riding a bulldozer to space.