# Parachutes

You're screaming through the sky, safely tucked up in the cockpit of a jet fighter, when there's a sudden loud bang and the engine judders to a halt. Well that's just great, isn't it? Here you are zooming along at maybe 2000 km/h (1200mph), several kilometers/miles above the ground and your plane has chosen this exact moment to break down! What do you do? Eject as soon as you possibly can, wait for the plane to fly clear, and then hit your parachute. With luck, you glide safely to the ground and live to fly another day. When it comes to saving lives, parachutes are among the simplest and most effective of inventions. How exactly do they work? Let's take a closer look!

Photo: A traditional round parachute. Although military paratroopers still use them, they are now largely obsolete for recreational diving. Photo by Chris Desmond courtesy of US Navy and Wikimedia Commons.

## How does a parachute work in theory?

Throw a ball up in the air and, sooner or later, it always falls back to the ground. That's because Earth pulls everything toward it with a force called gravity. You've probably learned in school that the strength of Earth's gravity is roughly the same all over the world (it does vary a little bit, but not that much) and that if you drop a heavy stone and a light feather from the top of a skyscraper, gravity pulls them toward the ground at exactly the same rate.

Photo: In theory, different things should fall at the same speed; in practice, it's a bit more complicated.

If there were no air, the feather and the stone would hit the ground at the same time. In practice, the stone reaches the ground much faster, not because it weighs more but because the feather fans out and catches in the air as it falls. Air resistance (also called drag) slows it down.

### What causes air resistance?

Just because the air's invisible, doesn't mean it's not there. Earth's atmosphere is packed full of gas molecules, so if you want to move through air—by walking, in a car, in a plane, or dangling from a parachute—you have to push them out of the way. We only really notice this when we're moving at speed.

Air resistance is a bit like the way water pushes against your body when you're in a swimming pool—except that air is invisible! If you jump off a diving board or do a belly flop, the awkward shape of your body will create a a lot of resistance and bring you rapidly to a halt when you crash into the water. But if you make a sharp pointed shape with your arms and dive in gracefully, your body will part the water cleanly and you'll continue to move quickly as you enter it. When you jump or belly flop, your body slows down quickly because the water can't get out of the way fast enough. When you dive, you part the water smoothly in front of you so your body can glide through it quickly. With parachutes, it's the slowing-down effect that we want.

If you fall from a plane without a parachute, your relatively compact body zooms through the air like a stone; open your parachute and you create more air resistance, drifting to the ground more slowly and safely—much more like a feather. Simply speaking, then, a parachute works by increasing your air resistance as you fall.

Photo: Testing a supersonic parachute for NASA's Mars Perseverance rover in a wind tunnel. Photo courtesy of NASA.

### Terminal velocity

When a force pulls on something, it makes that object move more quickly, causing it to gain speed. In other words, it causes the object to accelerate. Like any other force, gravity makes falling objects accelerate—but only up to a point.

If you jump out of a plane, your body ought to speed up by 10 meters per second (32ft per second) every single second you're falling. We call that an acceleration of 10 meters per second per second (or 10 meters per second squared, for short, and write it like this: 10m/s/s or 10m/s2). If you were high enough off the ground, then after about a minute and a half (let's say 100 seconds), you'd theoretically be falling at about 1000 meters per second (3600km/h or 2200 mph), which is about as fast as the fastest jet planes have ever flown!

Artwork: When you reach terminal velocity, the upward force of air resistance exactly balances the downward pull of gravity and you stop accelerating.

In practice, that simply doesn't happen. After about 12 seconds, you reach a speed where the force of air resistance (pushing you upward) increases so much that it balances the force of gravity (pulling you downward). At that point, there is no net acceleration and you keep on falling at a steady speed called your terminal velocity. Unfortunately, the terminal velocity for a falling person (with arms stretched out in the classic freefall position) is about 55 meters per second (200km/h or 125 mph), which is still plenty fast enough to kill you—especially if you're falling from a plane!

Photo: 1) Freefall in theory: In this training exercise, the skydiver is practicing freefall by floating over a huge horizontally mounted air fan. The force of the air pushing upward is exactly equal to the diver's weight pulling him downward so he floats in mid-air. Photo by Gary L. Johnson courtesy of US Navy and Wikimedia Commons.

Photo: 2) Freefall in practice: In reality, it's not the air that moves past you—you move through the air—but the physics is still the same: once you reach terminal velocity, the force of the air on your body pushing you upward exactly equals the force of gravity pulling you down. Photo by Ashley Myers courtesy of US Navy and Wikimedia Commons.

### How much does a parachute slow you down?

Feathers fall more slowly than stones because their terminal velocity is lower. So another way of understanding how a parachute works is to realize that it dramatically lowers your terminal velocity by increasing your air resistance as you fall. It does that by opening out behind you and creating a large surface area of material with a huge amount of drag. Parachutes are designed to reduce your terminal velocity by about 90 percent so you hit the ground at a relatively low speed of maybe 5–6 meters per second (roughly 20 km/h or 12 mph)—ideally, so you can land on your feet and walk away unharmed.

## What shape are parachutes?

Photo: Square-shaped "ram-air" parachutes are much more common than round parachutes because they're easier to steer and control. Photo by Shannon K. Cassidy courtesy of US Navy and Wikimedia Commons.

Traditionally, parachutes were round (dome-shaped) and, with their dangling suspension lines, looked a bit like jellyfish as they fell. They had vent holes that allowed air to escape, which helped to prevent them from rocking about as they came down, and their lines provided very basic steering.

Most modern parachutes are rectangular (a design known as ram-air). They have a number of cells that inflate as the air "rams" into them, so they form a fairly rigid, curved airfoil wing, which is much more steerable and controllable than a dome-shaped parachute.

Rounder chutes are still widely used by military paratroopers, because they work well for dropping lots of people together, in a fairly small area, at relatively low altitudes; paratroopers are simply trying to get to the ground quickly, not show off their skydiving technique. Recreational divers, on the other hand, consider round chutes obsolete: virtually all of them now use the ram-air design instead.

Photo: Paratroopers often still use round chutes because they're an effective way to get lots of people quickly and safely to the ground in a fairly small space. This parachute drop took place in Latvia in June 2018. Photo by Gina Danals courtesy of US Navy and DVIDS.

## What are the parts of a traditional, round parachute?

If you've ever seen a parachute spread out on the ground, you'll know it has lots of separate parts, and it can be a very tricky thing to pack back into its container so it opens correctly next time. What are all the bits and what do they do? Here are some of the more important ones, but there are quite a few more that I've missed out for clarity.

1. Pilot chute: A small parachute that opens the large, main parachute.
2. Bridle: Connects the pilot chute to the main chute.
3. Apex or top vent: Allows a slow escape of air from the top of the main chute. This prevents air from leaking out of the sides of the canopy, which tends to rock the parachute wildly as it falls.
4. Canopy: Main part of the parachute.
5. Skirt: Lower part of the canopy (think of a person's skirt hanging down).
6. Suspension lines: Spread the weight of the parachutist evenly across the canopy.
7. Links: Connect the suspension lines to the risers.
8. Risers: Connect the links to the harness
9. Control lines: I've drawn only one, but there can be several different ones for steering and braking.
10. Harness and container: The harness is the part you wear (itself made of numerous components); the container looks similar to a rucksack and holds the packed-up parachute and all its bits and pieces, ready for action!

## How does a parachute work in practice?

Skydivers make parachuting look easy, but it's all a bit more tricky in practice! What you're trying to achieve is to get a large piece of super-strong material opening out above and behind you in a perfectly uniform way when you've just jumped from a plane screaming along maybe ten times faster than a race car! How can you possibly pull something safely behind you under those conditions?

Parachutes are actually three chutes in one, packed into a single backpack called the container. There's a main parachute, a reserve parachute (in case the main one fails), and a tiny little chute at the bottom of the container, called the pilot chute, that helps the main chute to open. Once you're clear of the plane, you trigger the pilot chute (either by pulling on a ripcord or simply by throwing the pilot chute into the air). It rapidly opens up behind you, creating enough force to tug the main chute from the container. The main chute has to be carefully packed so the ropes that connect it to your harness (known as suspension lines) open correctly and straighten out behind you. The main chute is designed to open in a delayed way so your body isn't braked and jerked too suddenly and sharply. That's safer and more comfortable for you and it also reduces the risk of the parachute ripping or tearing.

The force on a parachute is considerable, so it has to be made from really strong materials. Originally, parachutes were made from canvas or silk, but inexpensive, lightweight, synthetic materials such as nylon and Kevlar® (a chemical relative of nylon) are now generally used instead.

Photo: Parachutes are made from strong lightweight nylon and have to be packed very carefully if they're to open correctly when they're deployed. Photo by Gary Ward courtesy of US Navy and Wikimedia Commons.

### Softer landings

Parachutes were invented about a century ago, but they continue to evolve, as inventors devise ever-better ways to improve their safety and handling. Here's a more advanced 'chute, designed for the US Army in 2001 (and patented in 2003). It contains the same basic features as other chutes: a canopy (10, blue), a skirt underneath (12), and suspension lines (14) in four groups called risers (16), attached to a bridle (22), which supports the harness (26) and parachutist (P). But it also has two improved safety features to reduce the risk of the parachutist landing too fast and too hard. At the top, the parachute has a bridle with an extra loop of rope on either side and an electrical cutting mechanism to release it (pink, top, labeled 28). In the middle, it has what's known as a pneumatic muscle (bright green, 24). There's an altitude measuring device (gray, top, 34, 36, 44), which projects radar beams to the ground to measure your height and speed and figure out when the safety mechanisms need to be deployed.

Artworks: Courtesy of US Patent and Trademark Office, from US Patent 6,575,408: Soft landing assembly for a parachute by Richard J. Benney and Glen J. Brown, assigned to the United States Of America (US Army), June 10, 2003.

How does it work? That's shown in the artwork on the right. If the wind blows you too fast horizontally, the appropriate electrical mechanism releases one of the extra side ropes, causing the parachute to tilt to the opposite side, so reducing your speed. When you near the ground, if you're going too fast, the pneumatic muscle shortens, pulling you much closer toward the canopy, and so reducing your speed.

## What a drag!

Using a parachute to bring a person safely to the ground from a plane is one thing. But what if you had to bring an entire plane to rest the same way? That was the challenge facing NASA every time the Space Shuttle (the reusable space plane, now-retired) came back to Earth.

During its launch phase, the Shuttle had a powerful main engine and rocket boosters to power it into space. But when it came back again, it was nothing but a glider, drifting through the air and counting on its stubby wings to carry it home.

Once it was safely back inside Earth's atmosphere, the Shuttle hit its 4.5km (2.8mile) long landing strip at about 350km/h (220mph)—rather faster than a typical jet airplane (which lands at speeds more like 240km/h or 150mph). When the wheels were on the ground, the crew applied the brakes to bring the craft safely to a halt, but they also used a horizontal parachute called a drag chute to help. It was about 12m (40ft) across and helped to cut the Shuttle's speed by about 75 percent before it was jettisoned.

Photo: The Space Shuttle Atlantis, coming in to land on May 26, 2010 after its last ever mission. Photo courtesy of NASA.

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## Find out more

### On this website

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### Useful organizations

• US Parachute Association: An organization representing people who enjoy skydiving.
• British Skydiving: A British nonprofit, founded in 1961, that promotes skydiving. (Formerly called the British Parachute Association.)

### Patents

For deeper technical detail, take a look at:

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