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The Ephemerality and Reality of the Jetpack
First time accepted submitter Recaply writes "Here's a look back at the 1960's Bell Aerosystems Rocket Belt. 'Born out of sci-fi cinema, pulp literature and a general lust for launching ourselves into the wild blue yonder, the real-world Rocket Belt began to truly unfold once the military industrial complex opened up its wallet. In the late 1950s, the US Army's Transportation Research Command (TRECOM) was looking at ways to augment the mobility of foot soldiers and enable them to bypass minefields and other obstacles on the battleground by making long-range jumps. It put out a call to various aerospace companies looking for prototypes of a Small Rocket Lift Device (SRLD). Bell Aerospace, which had built the sound-barrier-breaking X-1 aircraft for the Army Air Forces, managed to get the contract and Wendell Moore, a propulsion engineer at Bell became the technical lead.'"
...as opposed to the rocket belt, which was merely eardrum-breaking...
Jetpacks make sense if you can get them to work. If you can get one to every person in your army, imagine the mobility you would have. Think of the kinds of flanking maneuvers you could perform. The same would be true for flying cars. Of course, the barriers are cost, controllability, range.....things like that.
Space elevators and asteroid mining can make sense too, but in those cases (assuming the space elevator can actually be built, which it can't with today's materials) it becomes a cost/benefit analysis. Is it cheaper to mine asteroids, or get the same materials here on earth? As soon as it's cheaper to get them from asteroids, we will get them from asteroids. Is it cheaper to get things into orbit via space elevator? We don't know yet, but if it is, then we will build a space elevator.
"First they came for the slanderers and i said nothing."
A big problem with jetpacks is that human ankles are weak landing gear. You can't do a parachute landing fall while wearing a jetpack; you have to do a standing landing. With all the mass of the gear on your back.
The other big problem is that rocket systems have a short flight time, and jet engine systems are too expensive. The jet engine powered backpack worked well, but cost too much. That used a small Williams jet engine. Williams International has tried and tried to make small jet engines cheaper. So have many others. Unfortunately, that's a very hard problem, which is why general aviation is still piston-powered. Below small-bizjet size, jet engines don't seem to get much cheaper as they get smaller. There was a big effort about a decade ago to develop "very light jets", but they ended up costing well over $1 million, most of that being engine cost.
So it can be done, and it has been done, but it just doesn't work very well.
Flanking maneuvers? "Hey everybody, shoot the loud flying things over there!"
A flanking maneuver doesn't need to be a surprise. It just needs to be fast enough to get into position before the defenders can rearrange themselves.
I don't know what place they would have on a modern battlefield.
Whether they have a place on the modern battlefield is equal to the question of whether infantry has a place on the modern battlefield. If they do, then having a mobile infantry is an advantage.
"First they came for the slanderers and i said nothing."
This is the reality of how to make a single man fly.
Williams WASP X-Jet
It worked, it flew, there was no military justification for it, it disappeared.
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but no one mentions the Martin http://www.youtube.com/watch?v...
Jetpacks make sense if you can get them to work.
As would many, many other things, like Warp Drive and the G Spot.
It must have been something you assimilated. . . .
I suspect they ran the numbers and decided that rather than making medicore-range quasi-flyers out of ground soldiers, the smart money was on just getting it over with and develop better helicopters, instead. Better speed; longer flights; bigger payloads - all much cheaper than adding limited flight capabilities to the individual.
For the search part, a UAV would probably be a cheaper way to put sensors in the air. For the rescue part, you need to hover and lift quite a bit of weight. Consider that many victims need some medical attention or at least assistance in getting aboard the rescue craft. So that means a crew of several people plus rescue gear (basket, stretcher, etc.). Well beyond the capabilities of a jet pack and even some small helicopters.
Have gnu, will travel.
We don't see jetpacks or flying cars for the very same physics reason. In order to hover against gravity you need to produce thrust > weight. Since thrust is proportional to (mass/second) X velocity, and power is proportional to (mass/second) X velocity^2, an efficient source of thrust you want to move a lot of material slowly (assuming you have unlimited reaction mass -> the atmosphere).
So, things that hover need to move lots of air, and have great big propellers. That is why helicopters work, and jet-reaction cars are too inefficient to be practical. It is why airplanes have big wings, not stubby lifting bodies. There may be a few spacial cases where you are willing to tolerate inefficiency, but they are rare.
Planes look like planes for a reason. Helicopters look like helicopters for a reason.
Whether they have a place on the modern battlefield is equal to the question of whether infantry has a place on the modern battlefield. If they do, then having a mobile infantry is an advantage.
Mobile infantry made me the man I am today.
Jesus was all right but his disciples were thick and ordinary. -John Lennon
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Whether they have a place on the modern battlefield is equal to the question of whether infantry has a place on the modern battlefield.
No, its not.
Infantry are mobile, versatile, able to perform precision strikes, able to adapt to many situations.
Rocket belts are heavy, unwieldy, unreliable, expensive, noisy, and incredibly niche. Im sure there are circumstances where it happens to be the right tool for the job, Im just not sure what that might be or why youd want to weigh all of your infantry down with it for that one circumstance.
GP didn't say they aren't possible, he said they aren't practical.
There are a few minor practical problems with jet packs, or more properly, rocket packs as these devices are rockets, not turbojets, and it matters. It matters because there are some pretty fundamental limits on how much fuel/reaction mass a soldier can carry, especially when they have to carry other stuff like body armor, weapons, helmets, ammunition, food, shelter. Let's imagine that a stripped down soldier carrying little but armor, ammo and a rifle has a mass of 100 kilograms, 90 kilograms of which are the actual soldier. The rocket pack described above had a mass (full) of 57 kilograms and required the pilot to wear heat-protective clothing -- let's call it 60 kilograms. So wearing it and suitable armor and carrying weapons would be a roughly 70 kilogram burden on a 90 kilogram soldier, sort of like wearing a liquid nitrogen and hydrogen-peroxide-filled teen-ager on your shoulders as you wade into battle.
Sadly, this model would not work at all for the current rocket pack designs -- they provide less than 1500 N of thrust, and our soldier now has a weight of 1600 N. He would burn half of his fuel (give or take) waiting for the fuel levels in the tank to drop to where he could take off at all. The troop of rocket-equipped soldiers would all have to be "feather merchants" -- mass 70 kg or less -- and be armed with plastic squirt guns to get off of the ground at all.
Even with modern improvements, nobody has been able to increase flight time beyond around 30 seconds. The practical range in 30 seconds is perhaps 200 to 300 meters, at a height of ten meters -- a height great enough that it is already dangerous to fatal if one falls from it wearing an explosive, superheated massive outfit on your back. One cannot expect to increase their range or flight time because rockets eject mass backwards at high speed in order to provide thrust forward. The backward speed of the reaction mass is determined and limited by thermodynamics and chemistry and the need not to cook the soldier to extra crispy in a 30 second flight. There isn't that much variation in what's available to use for thrust in this context -- one could probably improve on the 740 C exhaust temperature, but only at the expense of adding a lot more shielding (and weight) and much more protective clothing.
The more interesting possibility is to build an actual jet pack -- jets of course use air for thrust mass and use fuel just to heat and compress the air, so they potentially have a much greater range. Small jet engines are mostly hobbyist stuff at the moment, but can produce order of a kilonewton of force at a mass cost of maybe 20 kilograms for the engine itself. One would need two, still further efficiency improvements, serious hearing protection, better shielding in the clothing (jet exhaust is still hotter than the "rocket" exhaust of hydrogen peroxide catalysis to water and oxygen). There is even military technology associated with cruise missiles that could be adapted.
We could learn another lesson from cruise missiles as well. Wings help. Wingsuits, for example, increase the glide ratio of skydiver to six. Hang gliders can achieve 17 to 20. Equipping a small hang glider with a small jet engine (one engineered to run without overheating for indefinite periods of time, unlike many of the powered hang glider engines currently available that tend to be based on two stroke chainsaw motors) could conceivably result in a wearable harness with a comparatively small wingspan in which a fully equipped soldier would have a range of tens of kilometers in tens of minutes at heights ranging from 10s of meters to a thousand meters or so. After powering up and attaining height, the engines could be shut off and the gliders could passively and silently descend from a height of a kilometer to a target 10 kilometers away.
The wings would have to be designed to be "compressible" to a comparatively small pack and quickly and easily erected into a structurally stable functional form, and would probably ma
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