What is Jetpack and how does it work?

By Max Rogers

前阵子大家或许都看过一段疯传的视频：一个人背着沉重的“背包”，在“背包”动力的作用下飞了起来，并在空中盘旋许久。像超人一样脱离地心引力的影响不借助大型飞行器独立飞天的梦早已有之，我国早在明代就有一位万户将鞭炮绑在椅子上，试图通过此举升上天空。虽然最终结果很悲催，但是精神可嘉。成型的个人飞行器最早的设计来自于1919年俄罗斯发明家Aleksandr Fyodorvich Andreyev，但直到1960年第一款个人飞行器才以贝尔火箭背带（Bell Rocketbelt）的形式正式露面。不幸的是，个人飞行器的成本过于高昂，而飞行时间又太短，因此这项发明所引起的激动情绪在当时很快就消散了。2008年，来自瑞士的“火箭人”Yves Rossy首次使用其设计的一种动力翼套装飞越了瑞士的阿尔卑斯山。在技术发达的今天，我们终于实现了这个梦想：新西兰的马丁飞机公司（Martin Aircraft Company）付出了巨大研发努力，其研发产品马丁喷射背包是一种实验性的飞机，在2008年7月29日于实验飞机协会在美国威斯康辛州奥旭寇旭市举办的实验飞机协会航空冒险者聚会中公布，虽然产品名称为“喷射背包”，但它却不是使用喷射动力或火箭动力。这台飞机被联邦航空总署归类为实验性超轻量飞机。不同于以往的喷射背包，马丁喷射背包是第一个被认为是可实用化的产品。它历经27年的开发，使用汽油（高级汽油）引擎与两具导风管扇来提供升力。理论上可达到每小时60英里的速度与海拔8000英尺的高度，油箱加满油时约可飞行30分钟。售价大约十万美元一台。马丁飞机公司计划于2010年向10位顾客交付第一批喷射背包。

能够一飞冲天固然是好事，但是这项新产品也有一定的安全隐忧。当飞行人员离开地面后，如何保证其安全成为了一个最大的问题。在负载更多燃料的同时保持平稳性能则是喷射背包面临的另一个问题。好在仍有很多专家在继续钻研，努力为人类自由飞行寻求解决之道。或许在不久的将来，我们将有望真的实现人人飞行自由。

The jetpack, also known as rocket pack and rocket belt, is a device that uses gas jets (or sometimes liquid) to propel a person wearing it on the back through the air. Jetpacks have been around for decades, but those devices are more advanced today than ever before. The idea dates back to 1960s when the concept of flying machine emerged from sci-fi. As the technology developed, a dream of jetpacks has come true.

Where are the jetpacks used?

The jetpacks have mostly been used in extravehicular astronauts' activities so far. Despite a great development of technology over the past decades, many obstacles and issues remain the same. Those include the challenges of Earth's gravity and Earth's atmosphere, as well as a low energy density of the fuels we have at our disposal. By the way, the human body isn't being adapted to fly in a natural manner. All of this makes the field of use limited to the military, astronautics, and science.

Yet, we can expect to see jetpacks designed for commercial use in the coming years. One of them is the Martin Jetpack. This single-person aircraft is meant to be the most practical and safest OPHAVs in the world. It can be flown either manned or unmanned. There is also an option which involves the use of a mule train. Despite its name, Martin Jetpack doesn't use a jet pack for lift indeed, but some kind of ducted fans.

What types of jetpacks exist and how do they work?

One of the biggest challenges when it comes to the design is how to get the pilot safely off the ground. The challenge of creating a jet pack or rocket pack that is capable of carrying more fuel, while remaining stable in the air and responding promptly to any pilot's command, has been developed for decades by a few companies. Unfortunately, without a noteworthy progress.

The history of jetpacks has seen a lot of different models. They can all be categorized into the following 4 groups based on the type of drive:
- Liquid-fuelled jetpacks
- Hydrogen peroxide-powered jetpacks
- Turbojet packs
- Hydrojet packs

Liquid-fuelled jetpacks
Those flying devices were developed by Aleksandr Andreyev in 1919. Although a patent of this liquid-fuelled jetpack was issued later, it has never been constructed or tested. The device in question should has used oxygen and methane to work and had 3-foot long wings on both sides.

Hydrogen peroxide-powered jetpacks
These engines are powered by nearly pure hydrogen peroxide, running on the decomposition reaction of this liquid. When getting in contact with a catalyst, hydrogen peroxide gets decomposed into a blend of oxygen and superheated steam almost instantly, whereby its volume gets increased 5,000 times. This reaction is exothermic, meaning much heat is released. As a result of this process, a steam-gas mixture emerges at 1,360 °F. This scalding mixture serves as a reaction mass in jet rockets.

Turbojet packs
In this case, the traditional kerosene fuel runs a turbojet engine that is mated to the packs of rockets. The turbojet packs feature high efficiency, great propel height, and a flight duration of several minutes. On the other hand, they are very expensive and have complex construction.

One of the most known jetpacks of this kind is Yves Rossy's jet wingpack. This jetpack has rigid wings made of carbon fibers and is powered by 4 small jet engines that use kerosene. Yves Rossy has worn a special heat and fire resistant suit to protect his legs from burning. The engines are equipped with a heat shield that keeps the pilot to the safe distance by extending the jet nozzles around the jet pack's tail. However, this jet pack can't be launched from the ground. Instead, Rossy has been raised to altitude by a helicopter or plane before jumping out. He could then unfold his wings and fire his engines, which allowed him to fly horizontally for a couple of minutes before landing. He used a parachute to land safely.

Hydrojet packs
Hydrojet packs have a completely new approach to jetpack technology. In fact, those packs use water as a propulsion fluid because of the fact the water has a high density. Here, engine, fluid, and fuel supply is separated the from the flying apparatus. The water is conveyed to the jet nozzle pack through a flexible long hose. These devices use Jet Ski technology as the power plant that operates in a large body of water, such as a pool, lake, or ocean, to provide the required propulsion. The flow rate is controlled in two ways - by a remote actuator or a throttle operator. A few hydro jet packs are already in production.

Today, many companies offer jetpacks for sale to the public. Just to mention JetPack International (Jet PI) and Tecnologia Aeroespacial Mexicana (TAM). The first one is an American company that has modernized the rocket belt designs from the 50s with comprehensive materials and fuels. The flight time of these rocket belts is increased to slightly more than 30 seconds. T-73 is the most popular model of this company, which has a claimed flight time of 9 minutes. Its sale price is $200,000. TAM is a Mexican company which offers a variety of rocket-powered devices. One of the best selling TAM's products is TAM Rocket Belt that costs $125,000.

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