What if you made a jetpack using rifles?

Quick Overview

Building a jetpack out of downward-firing rifles is theoretically possible, requiring over 300 rifles firing continuously for 150 rounds each to generate enough thrust (around 150 lbs) to lift a 10-pound object plus the rifle itself, but the fuel/ammunition mass and the immense recoil force (up to 30 Gs) make it impractical and lethal.

Key Points: A 10-pound rifle firing 10 rounds per second of 10-gram bullets at 700 m/s produces approximately 15 pounds of thrust, resulting in a thrust-to-weight ratio of 1.5, which is just enough to lift itself off the ground. To lift a 60kg (132 lb) rider, the calculation suggests needing over 300 rifles, each loaded with 150 rounds, firing continuously for 150 seconds to achieve a thrust-to-weight ratio greater than 1. The GSh-6-30 autocannon, which weighs half as much as the GAU-8 Avenger but has a higher rate of fire, produces a thrust-to-weight ratio of 30 when fired downward, offering extreme but unsustainable lift. Firing the GSh-6-30 generates an acceleration force of 30 Gs on the operator, which is lethal, and its massive recoil (about 5.5 tonnes) is enough to damage the aircraft it is mounted on. The main limiting factor for a rifle-jetpack is ammunition mass; carrying enough ammunition (over 150 rounds per 10-pound rifle) adds too much dead weight, eventually making the device too heavy to take off. Theoretically, if the ammunition weight could be ignored, a rifle jetpack could reach an altitude of about 200 meters before running out of ammo.

Context: This video explores the feasibility of building a personal jetpack using multiple downward-firing rifles, based on the principles of thrust derived from Newton's Third Law. It uses stick-figure animations and physics calculations, referencing concepts like the thrust-to-weight ratio seen in rocketry (like the Saturn V) to analyze the required number of rifles and the practical limitations imposed by ammunition weight and recoil.

Detailed Analysis

The video investigates whether a jetpack can be constructed using downward-firing rifles based on Newton's Third Law. The initial calculation uses a hypothetical 10-pound rifle firing 10 rounds per second, each weighing 10 grams at 700 m/s, which generates about 15 pounds of thrust, resulting in a thrust-to-weight ratio of 1.5, enough to hover the rifle itself. However, for a 60kg rider, this requires over 300 rifles firing continuously for 150 seconds each to achieve liftoff (thrust-to-weight ratio > 1). The analysis then shifts to real-world powerful weapons, noting the GAU-8 Avenger (5 tons recoil) and the even more powerful Gryazev-Shipunov GSh-6-30, which has a thrust-to-weight ratio of 30. While the GSh-6-30 setup could achieve a vertical acceleration of 0.5 Gs with a rider, the recoil causes 30 Gs of acceleration on the operator, and firing more than 30 rounds causes overheating. The ultimate limiting factor is the mass of the ammunition; carrying enough bullets (e.g., over 150 rounds per rifle) adds too much dead weight, making the entire assembly too heavy to ever leave the ground, despite the theoretical thrust capability.