Why our "rockets" aren't really rockets — and what the science actually looks like
We call them "paper rockets" — and the name is fun, evocative, and wrong. A real rocket carries its propellant onboard and generates thrust throughout some portion of its flight. Our paper tubes do neither of those things.
The correct category for our projectiles is ballistic projectile — an object given an initial velocity and then left entirely to physics. The better analogies are:
This distinction matters practically. Model rocket safety codes, NAR/TRA certifications, and FAA regulations don't apply to ballistic projectile launchers the same way. But it also means the interesting physics shifts — away from propulsion and toward initial conditions and aerodynamics.
Here is one of the most counterintuitive things about our launcher system: adding weight to a paper rocket usually makes it go higher, up to a point. This is the exact opposite of how real rocketry works, and understanding why reveals a lot about the physics.
In real rocketry, the Tsiolkovsky rocket equation tells you that the mass ratio — the ratio of initial (fueled) mass to final (dry) mass — drives everything. Every kilogram of structure you eliminate means more delta-v for the same amount of propellant. Lighter is almost always better for a real rocket.
For a ballistic projectile, the equation changes completely. After leaving the launch tube, the rocket has a fixed initial velocity. What happens next is a battle between momentum and drag.
The critical insight: drag force depends on the rocket's shape and speed, not its mass. But deceleration equals force divided by mass. A heavier rocket with the same drag force decelerates more slowly — it "coasts" further on its initial momentum.
This doesn't mean heavier is always better. There's a sweet spot:
| Scenario | Real Rocket Effect | Our Projectile Effect | Verdict |
|---|---|---|---|
| Add 5g to nose | Reduces delta-v, worse performance | Improves BC, usually higher altitude | Opposite outcomes |
| Add 5g to body | Reduces delta-v | Improves BC; may shift CG forward (good) | Opposite outcomes |
| Lighter fins | Better mass ratio | Less total mass, worse BC; may shift CG rearward (bad) | Opposite outcomes |
| Reduce body wall thickness | Better mass ratio | Less mass — may hurt BC and structural integrity | Usually bad for us |
| Add payload compartment (empty) | Dead mass with no propellant benefit | Adds mass and moves CG forward | Opposite outcomes |
In rocketry, total impulse is the integral of thrust over the burn duration. It's measured in Newton-seconds (N·s) and defines how much "push" a motor delivers. The average thrust determines how fast the rocket accelerates during the burn.
Our launcher delivers impulse too — but in a fundamentally different way.
The National Association of Rocketry classifies motors by total impulse. Each letter class doubles the energy of the previous:
| Class | Total Impulse | Avg. Burn Time | Our Launcher Comparison |
|---|---|---|---|
| 1/4A | 0.313 N·s | ~0.3–0.5s | Below our range (low pressure/short tube) |
| 1/2A | 0.625 N·s | ~0.4–0.6s | ~30–40 psi, short tube |
| A | 1.25 N·s | ~0.5–0.8s | Typical 40–50 psi launch |
| B | 2.5 N·s | ~0.5–1.2s | 60–80 psi or longer tube |
| C | 5.0 N·s | ~0.7–1.5s | Above typical for our setup |
The extremely short impulse duration means our rockets reach their maximum velocity before they've moved more than a few inches. There is no sustained acceleration phase. The entire flight after tube exit is purely ballistic — identical to what model rocketry calls the "coast phase."
Aerodynamic stability is where paper rockets and real model rockets share the most physics. The fundamental requirement is identical: the Center of Gravity (CG) must be forward of the Center of Pressure (CP).
When a flying object is disturbed from its flight path — by a gust of wind, a slight launch angle error, or asymmetric drag — the aerodynamic restoring force acts at the CP. If CP is behind CG, this force rotates the nose back toward the velocity vector. If CP is ahead of CG, the force amplifies the disturbance. Stability margin is expressed in calibers (rocket diameters):
Both our rockets and model rockets use the Barrowman equations to predict CP location from fin geometry, body diameter, and nose cone shape. The math is identical.
Here is a meaningful difference: real model rockets generally do not spin intentionally. Their fins are mounted straight (aligned with the body axis). Our designs often use canted fins — fins angled a few degrees off-axis — which impart spin during launch, similar to rifle barrel rifling.
Spin stabilization provides robustness against small asymmetries. A slightly lopsided nose cone or uneven fin placement matters less when the rocket is rotating a few revolutions per second. The gyroscopic effect resists attitude changes.
Both types of rocket are susceptible to weathercocking — the tendency to turn into the wind. This happens because crosswind creates a net aerodynamic force at CP that rotates the nose windward. A well-stabilized rocket in a crosswind will arc into the wind rather than flying at the original launch angle. The physics is identical for both; neither can correct for it once airborne.
The Tsiolkovsky rocket equation is one of the most fundamental results in astronautics. Published in 1903, it describes how the exhaust of a rocket motor relates to the velocity change the vehicle achieves:
Notice what this equation requires: the mass must change. Propellant is consumed and expelled as exhaust. The rocket gets lighter as it burns. This mass ratio m₀/mf inside the logarithm is everything — it's why real rockets are 85–95% propellant by mass at launch.
Our rocket's mass doesn't change at all. No propellant is consumed onboard. The compressed air stays in the launcher. The Tsiolkovsky equation has no meaningful form for our system because there is no mass ratio to compute.
Instead, our launch physics is an energy transfer problem. The expanding compressed air does work on the rocket as it travels down the launch tube. That work converts to kinetic energy:
The compressed air does a fixed amount of work (determined by pressure, volume, and tube length) regardless of rocket mass. A heavier rocket exits at lower velocity. A lighter rocket exits at higher velocity. But maximum altitude depends on how that velocity-momentum combination fights drag — which is why mass has a non-trivial optimum.
| Property | Real Rocket | Our Launcher |
|---|---|---|
| Governing equation | Tsiolkovsky: Δv = Isp × g × ln(m₀/mf) | Energy: v = √(2W/m) |
| Propellant location | Onboard, decreasing mass | Ground-based, stays behind |
| Mass during flight | Decreasing (burns propellant) | Constant |
| Key performance driver | Propellant mass fraction, Isp | Launch energy, ballistic coefficient |
| More mass effect on delta-v | Always worse | Lower velocity, but often higher altitude |
OpenRocket is a free, open-source model rocket simulator that uses numerical integration of aerodynamic forces, gravity, and motor thrust to simulate flight. It's built for model rockets with real motors — but it can simulate our rockets with a simple trick.
OpenRocket always needs a motor. The workaround is to define a custom motor that delivers approximately the same total impulse as our pneumatic launch, but over a very short time (0.01–0.05 seconds). The rocket reaches roughly the correct muzzle velocity at the end of this simulated "burn," and from that point forward the simulation is entirely aerodynamic coast — which is our entire real-world flight.
We validated the physics on this site against the NASA Rockets Educator Guide (EG-2020-11-46-MSFC), a public-domain educational resource from Marshall Space Flight Center. The guide covers Newton's laws applied to rocketry, paper rocket construction, foam rocket ballistics, altitude tracking, and water rocket design. Here's how our content compares.
| Topic | NASA Guide | Our Site | Status |
|---|---|---|---|
| Newton's Three Laws | Pages 21-25: F=ma, action/reaction, inertia | Physics Sec. 1-3, Math Sec. 4 | Aligned |
| Ballistic flight model | Foam Rocket (p.73): explicitly called ballistic, not a true rocket | Physics Sec. 1: "These Aren't Really Rockets" | Aligned |
| Nose weight for stability | Pop! Rockets (p.68): penny in nose; Water Rockets (p.92): clay in nose cone | Designer: nose weight config; Physics Sec. 2: mass paradox | Aligned |
| CG forward of CP | Water Rockets (p.94): CG forward, fins at rear, string swing test | Physics Sec. 4, Designer: Barrowman stability calc | Aligned |
| Range equation | Foam Rocket (p.77): Range = V₀²/g × sin(2A) | Math Sec. 4: Trajectory & Ballistics | Aligned |
| 45° optimal launch angle | Foam Rocket (p.75): max range at 45°; complementary angles give equal range | Math Sec. 4: trajectory analysis | Aligned |
| Altitude tracking | Pages 81-86: altitude = tan(A) × baseline, tangent table | Testing page: same formula, same method | Aligned |
| Weathercocking | Altitude Tracker (p.85): rocket turns into wind due to fin surface area | Physics Sec. 4: weathercocking discussion | Aligned |
| Drag as dominant flight force | Foam Rocket (p.74): gravity and drag determine trajectory | Physics Sec. 5, Math Sec. 3: drag equation | Aligned |
| Fin design affects stability | Pop! Rockets (p.69): different fin shapes shown; Water Rockets: beveled edges | Designer: swept/triangle/clipped delta fins | Aligned |
The NASA guide is aimed at K-12 educators and focuses on conceptual understanding. Our site extends into areas the guide doesn't cover:
The NASA guide includes several activities that directly parallel what our tools help with:
Reference: NASA Rockets Educator Guide, EG-2020-11-46-MSFC. Public domain (NASA/U.S. Government).
Here are some numbers to put the physics in perspective.
| Property | Estes A8-3 | Our Launcher (60 psi) | Notes |
|---|---|---|---|
| Total impulse | 2.5 N·s | ~2.0 N·s | Similar class |
| Burn time | ~0.5 s | ~0.015 s | 33x faster |
| Peak thrust | ~9.5 N | ~200 N | 21x higher peak |
| Peak G on rocket | ~10–15 G | 100–300 G | Far higher for us |
| Typical max altitude | ~45 m (150 ft) | ~60–70 m (200–230 ft) | We often go higher |
| Mass during flight | Decreasing | Constant | Key difference |
| Propellant mass fraction | ~30% | 0% | None onboard |
100–300 G sounds extreme. It is — but only for milliseconds. A human can survive about 5 G for a few seconds, or 45 G for 0.044 seconds (the tolerance curve is highly time-dependent). Our rockets experience forces their entire structure is designed to survive because the duration is so short. Paper rolls and tape joints that would fail under sustained load are perfectly adequate for an impulse that's over before the structure fully deflects.