Try it safely

1. A balloon rocket

Any age, with an adult · indoors · about 30 minutes · from NASA JPL

Thread a drinking straw onto a long piece of string or fishing line and tie the line between two chairs. Have an adult blow up a balloon and hold the neck closed with a clip, then tape the balloon under the straw. Let go of the clip and watch it zip along the line. Before each try, guess which way it will go and how far. Then repeat it to check.

• Let an adult blow up the balloons.
• Watch from the side, back far enough that no one gets in the balloon's way.

The physics: a rocket pushes gas out one way, and the gas pushes the rocket the other way. Try it with the forces drawn out in Newton's Third Law in the Science Lab.

2. A stomp rocket

Grades 4 to 9 · outdoors · 1 to 2 hours · from NASA JPL

A stomp rocket is a paper rocket that slides over a tube. Stomping on an empty plastic bottle at the other end of the tube sends a puff of air up the tube and launches the rocket. Use a store-bought stomp rocket launcher, or have an adult build the one in NASA JPL's Stomp Rockets guide, which also shows how to roll the paper rockets. Then change the fins or nose and see which flies highest.

• Launch in an open space, about 100 by 25 meters, with no trees, roofs or power lines overhead.
• Only the stomper stands at the launcher, and the tube points away from them. Everyone else stays clear of the tube and the landing area.
• Stomp across the middle of the bottle, not on its end.
• Aim into the wind. On a very windy day, launch beside a large building that blocks the wind, or wait.
• Pick up rockets only after they have landed.

The physics: the harder and faster you stomp, the more the air is squeezed, and the faster it pushes the rocket. See air pressure as bouncing atoms in Gas in a Box.

3. A water rocket

Older children and teens, with an adult · a large field · from NASA Glenn

A water rocket is a plastic drink bottle partly filled with water. A pump fills the space above the water with air under pressure. When the bottle is let go from its launcher, the air drives the water out of the neck, and the bottle shoots up. Use a launcher made for water rockets, and follow NASA's safety rules.

• Use only plastic drink bottles, best of all new ones. Retire each bottle after 10 to 15 launches, because bottles weaken and can burst.
• An adult supervises closely. Children can learn the rules but cannot be in charge of everyone's safety.
• Launch on a grassy field at least 30 meters wide, with the launcher anchored in the middle. On a windy day, set up near the upwind edge so the rocket drifts over the field.
• Rope off the launch area and keep watchers several meters back.
• Never point the rocket at people, animals or things.
• The person pumping wears eye protection. About 50 psi is plenty; never go above 90 psi.
• Count down with everyone behind the rope, and launch only when the landing area is clear.
• If a bottle does not lift off, it is still under pressure and can launch when touched. Do not stand over it.

The physics: the rocket gets its push by throwing mass out the back, just like a real rocket. How much speed it can get depends on how much of its mass it throws away. That is the rocket equation, and you can explore it in the course and in the Rocket Builder.

4. Model rockets with certified motors

For families, scouts and school clubs · a large open field · rules from the National Association of Rocketry

Model rocketry is the safe, modern form of what the Rocket Boys did. You build the rocket body from a kit, but the motor comes ready-made from a manufacturer, tested and certified. The National Association of Rocketry (nar.org) writes the safety code that the hobby follows. Read the full Model Rocket Safety Code before you launch. In brief, it says:

1. Materials: make the nose, body and fins only of light, non-metal parts.
2. Motors: use only certified, commercially made motors. Don't change them, and use them only as the maker says.
3. Ignition: launch electrically, with a launch switch that turns off when you let go and a safety key or interlock.
4. Misfires: if the rocket doesn't launch, remove the safety key or disconnect the battery, and wait 60 seconds before anyone goes near it.
5. Launch safety: count down, and make sure everyone is paying attention. Stand at least 15 feet back for small motors (D or smaller) and 30 feet for larger ones. Check a new design for stability before its first flight.
6. Launcher: launch from a rod or rail within 30 degrees of straight up, with a blast deflector. Keep the rod's tip above eye level or capped.
7. Size: a model rocket weighs no more than 1,500 grams (53 ounces) at liftoff, within the code's limits on motor power.
8. Flight safety: never aim at targets, into clouds or near airplanes, and never carry anything that can burn or explode.
9. Launch site: launch outdoors in a large open area, in winds under 20 miles per hour, away from dry grass or anything that could catch fire.
10. Recovery: use a parachute or streamer so the rocket comes down gently, with flame-resistant wadding.
11. Recovery safety: never try to get a rocket out of power lines, tall trees or other dangerous places.

A good way to start is with a club. The NAR's website lists clubs, called sections, that hold launches with experienced members.

5. Measure the height, like the Rocket Boys

Any rocket that goes up · a protractor, string and a weight · from NASA Glenn

The boys could not follow their rockets into the sky, so they measured an angle instead. You can do the same with any of the rockets above. Stand a measured distance from the launcher, say 30 meters. When the rocket reaches its highest point, sight it and read the angle up from level. A protractor with a weighted string, or NASA's simple altitude tracker, does the job.

In the right triangle, the tangent of the angle is the height divided by the baseline. So the height above your eye is the baseline times the tangent. Add your eye height, since you sighted from there.

Example: baseline 30 m, angle 40°. Since tan 40° ≈ 0.839, the height above your eye is 30 × 0.839 ≈ 25.2 m. Add 1.5 m for your eye, and the rocket reached about 27 meters. (The drawing is to scale: measure it and see.)

Why it's only an estimate: the method assumes the rocket flew straight up. If it drifted toward you, your number comes out too high; if it drifted away, too low. NASA's guide suggests two or three trackers standing in different directions, then taking the average. The course shows how two observers can do even better by triangulating.