# How would a jetpack work? **URL:** \ **Category:** Factual Questions\ **Created:** [August 1, 2021, 1:02am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737 "2021-08-01T01:02:47Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 2, 2021, 2:44am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/21 "2021-08-02T02:44:11Z")
> [@mbh](#): > > The jet is above the horizontal centerline of the mass. Your weight keeps the jet pointing downward, and the thrust pointing upward. Doesn’t work that way, but it’s an understandable mistake. It’s so common that there’s a name for the error: the [pendulum rocket fallacy](https://en.wikipedia.org/wiki/Rocket#Pendulum_rocket_fallacy). A rocket does not become more stable simply because the engine is above the center of mass. In fact it’s irrelevant where the thrust exits. There are only a handful of things that affect the stability of a rocket: - Aerodynamics. A rocket can be stable if the center of pressure is behind the center of mass, and if the rocket is moving rapidly. - Rotation. You can make a rocket travel in a mostly straight line by spinning it, which causes most of the error to cancel out. - Gyroscopic and inertial action. External motions can be cancelled by internal devices, such as wheels spinning opposite to the desired correction. - Control over the direction of the jet relative to the center of mass. This can take the form of moving the jet (the usual approach) or moving the mass. Both of these seem to be used here, depending on your perspective. ---
**Author:** ![Limmin](https://avatars.discourse-cdn.com/v4/letter/l/f04885/32.png) [@Limmin](https://boards.straightdope.com/u/Limmin)\ **Post date:** [August 2, 2021, 3:03am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/22 "2021-08-02T03:03:14Z")
Nice info as always, Dr. S. At a low speed, balancing the rocket on top of the jet is possible if we have a control system to steer the jet a little bit as needed. I recall (can’t find a link to a video) an interview with Michael Collins, of Apollo 11 fame, describing the jerking / twitching he felt as the rocket was just getting moving. To keep the heavy Saturn V from toppling over, the engines would gimbal back, forth and side to side as needed, in quick bursts. Imagine holding a pencil, or better yet, a yardstick at the bottom. By moving your hand as needed, you can keep it balanced. Now…applying this to a jetpack, sort of like a Segway, we could get a nice stable flight, I’d imagine. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 2, 2021, 3:26am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/23 "2021-08-02T03:26:48Z")
Yep–most fully-fledged rockets achieve stability via gimbaling. I had not heard that interview with Michael Collins, but can certainly imagine that the gimbaling would be noticeable at low speeds. Also, while most modern rockets change the direction of the jet by pointing the entire nozzle in a different direction, there are other possibilities: - Putting vanes in the exhaust to redirect the flow (V-2 rocket) - If there are multiple engines, changing the throttle level of each engine (the Soviet Union’s N1 moon rocket) - Having additional “vernier” thrusters that alter the overall direction of thrust (the Soyuz launcher family) - Injecting fluid into the nozzle to weaken the thrust on one side (some Indian rockets) - Probably more things that I’m not thinking of right now… Balancing a stick on your finger gives some idea of the difficulty. Except that it’s actually a rubber cylinder filled with fluid in stacked chambers that are rapidly draining out. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 2, 2021, 11:22am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/24 "2021-08-02T11:22:20Z")
> [@Northern\_Piper](#): > > No wings; how would there be any stability? > [@Senegoid](#): > > Without airfoils or control surfaces like airplanes or helicopters, how is there any stability, and how are they steered? > [@Senegoid](#): > > How does it maintain stability? What keeps the pilot from tumbling randomly? Airplanes have vertical and horizontal stabilizers for a reason – what equivalent is there in a guy flying a jetpack? There are bajillions of consumer- and professional-grade quad-rotor and hex-rotor drones in operation that lack wings and control surfaces, and yet they are incredibly stable. They rely on computers and low-cost rate gyros and accelerometers to monitor orientation and position, and adjust the speeds of the rotors to achieve roll/pitch/yaw control. Positional control is achieved by managing roll/pitch to induce fore/aft/lateral acceleration as needed. Gravity’s jet suit (see video Richard\_Pearse lnked to) mentions a “control system,” but I think that’s just referring to the engine management system. I’m doubtful that they’re using the same sort of automatic stability system described above, since the arm-mounted engines can be individually reoriented by the pilot, which could result in a sort of cat fight between the pilot’s stabilizing efforts and the computer’s stabilizing efforts. Unless someone knows otherwise, I strongly suspect that stability and control is achieved entirely by the pilot’s body english and thrust management. The best evidence that this is at least possible is the early date at which jetpacks actually achieved flight, i.e. before such computerized stability systems even existed. The **[Bell Rocket Belt](https://en.wikipedia.org/wiki/Bell_Rocket_Belt#RB2000_Rocket_Belt)** lacked aerodynamic control surfaces, relied entirely on pilot inputs for stability/control. It was first demonstrated in 1961, and worked well enough to actually partake in the opening ceremonies at the 1984 summer Olympics. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 2, 2021, 12:12pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/25 "2021-08-02T12:12:34Z")
Not exactly a jetpack, but with a flight time of more than 40 minutes, this seemed like both a fun idea… > **[Williams X-Jet](https://en.wikipedia.org/wiki/Williams_X-Jet)** > > The Williams X-Jet, created by Williams International, was a small, single-person, light-weight, Vertical Take Off and Landing (VTOL) aircraft powered by a modified Williams F107 turbofan aircraft engine — designated WR-19-7 — after some minor modifications. The vehicle was nicknamed "The Flying Pulpit" for its shape. It was designed to carry one operator and to be controlled by leaning in the direction of desired travel and by modulating engine output power. It could move in any direction, acce... … and a pretty badass looking implementation: [![](https://img.youtube.com/vi/6wuQB6S4zHc/maxresdefault.jpg "The Flying Pulpit") ](https://www.youtube.com/watch?v=6wuQB6S4zHc) ---
**Author:** ![GreenWyvern](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/greenwyvern/32/2834_2.png) [@GreenWyvern](https://boards.straightdope.com/u/GreenWyvern)\ **Post date:** [August 2, 2021, 12:44pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/26 "2021-08-02T12:44:33Z")
> “The Flying Pulpit” More like a flying chariot! Actually, this looks to me like a better idea than any of the jetpack designs. It’s probably easier to balance. The rider is not suspended from a tight harness and can easily mount and dismount. It doesn’t require strong arms. It probably needs less training. Even with 1980s technology it could stay in the air for 45min and travel at 60mph. I wonder why this style of personal flyer never took off (as it were)? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 2, 2021, 1:13pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/27 "2021-08-02T13:13:53Z")
I think it’s just a case of helicopters already existing and being better all-round transport for the same sorts of use cases. Leisure and wow factor aside, jetpacks are pretty much a solution looking for a problem. ---
**Author:** ![GreenWyvern](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/greenwyvern/32/2834_2.png) [@GreenWyvern](https://boards.straightdope.com/u/GreenWyvern)\ **Post date:** [August 2, 2021, 2:10pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/28 "2021-08-02T14:10:33Z")
> [@Mangetout](#): > > I think it’s just a case of helicopters already existing and being better all-round transport for the same sorts of use cases. Sure, although those British marines seem to be finding some use for the jetpacks – and I can image that flying sleds/chariots/pulpits would be even more useful for special forces. They could dismount quickly, not be encumbered with a heavy pack, and easily carry a weapon and a bit of equipment along with them. For civilian and recreational uses a sled certainly makes more sense than a jetpack, unless you’re hung up on emulating Marvel characters. ---
**Author:** ![Babale](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/babale/32/15666_2.png) [@Babale](https://boards.straightdope.com/u/Babale)\ **Post date:** [August 2, 2021, 2:32pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/29 "2021-08-02T14:32:33Z")
> [@Dr.Strangelove](#): > > Doesn’t work that way, but it’s an understandable mistake. It’s so common that there’s a name for the error: the [pendulum rocket fallacy](https://en.wikipedia.org/wiki/Rocket#Pendulum_rocket_fallacy). A rocket does not become more stable simply because the engine is above the center of mass. In fact it’s irrelevant where the thrust exits. > > There are only a handful of things that affect the stability of a rocket: > > - Aerodynamics. A rocket can be stable if the center of pressure is behind the center of mass, and if the rocket is moving rapidly. > - Rotation. You can make a rocket travel in a mostly straight line by spinning it, which causes most of the error to cancel out. > - Gyroscopic and inertial action. External motions can be cancelled by internal devices, such as wheels spinning opposite to the desired correction. > - Control over the direction of the jet relative to the center of mass. This can take the form of moving the jet (the usual approach) or moving the mass. Both of these seem to be used here, depending on your perspective These are all valuable lessons you learn when playing Kerbal Space Program ---
**Author:** ![gdave](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gdave/32/3683_2.png) [@gdave](https://boards.straightdope.com/u/gdave)\ **Post date:** [August 2, 2021, 4:01pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/30 "2021-08-02T16:01:56Z")
> [@GreenWyvern](#): > > Sure, although those British marines seem to be finding some use for the jetpacks – and I can image that flying sleds/chariots/pulpits would be even more useful for special forces. > > They could dismount quickly, not be encumbered with a heavy pack, and easily carry a weapon and a bit of equipment along with them. There was a fairly recent thread on the British marines and there jet pack. As @Mangetout says, for the most part, jet packs are a solution in search of a problem. Jet packs, or hover boards, or [flying platforms](https://en.wikipedia.org/wiki/Hiller_VZ-1_Pawnee), or similar devices, are loud, slow, clumsy, and hot. They make the “pilot” an easy and obvious target. They’re difficult to control and require extensive training. It’s effectively impossible to fire a weapon while also trying to control the device. And they have a very limited range and flight duration. The U.S. military has been experimenting with “jet packs” in various forms since 1955, and has yet to figure out a way to make them useful. In specific niche applications, like a boarding action, as portrayed in the British marine video, they _might_ be useful. But even then, under real world combat conditions, I’m personally doubtful the jet packs would really be very useful. Maybe they might have some utility for Air Force Pararescue and similar search and rescue functions, to quickly get a medic into an otherwise inaccessible location. Or maybe the current generation of jet packs have finally gotten to the point where they’re going to actually be useful, or someone will figure out an innovative use for them. But after 65+ years of failed projects, it’s maybe also possible that as cool as they are, jet packs just aren’t very practical. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 2, 2021, 4:26pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/31 "2021-08-02T16:26:53Z")
I keep thinking, I would not want to be taking (possibly incendiary) enemy fire while strapped to tanks of jet fuel. Or riddled with large-calibre AP bullets in any case. ---
**Author:** ![GreenWyvern](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/greenwyvern/32/2834_2.png) [@GreenWyvern](https://boards.straightdope.com/u/GreenWyvern)\ **Post date:** [August 2, 2021, 4:28pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/32 "2021-08-02T16:28:31Z")
> [@gdave](#): > > But after 65+ years of failed projects, it’s maybe also possible that as cool as they are, jet packs just aren’t very practical. I guess I’ll just have to wait for the flying cars! 😄 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 2, 2021, 5:42pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/33 "2021-08-02T17:42:51Z")
Oh, flying cars, we’ve had for over a century now. ---
**Author:** ![GreenWyvern](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/greenwyvern/32/2834_2.png) [@GreenWyvern](https://boards.straightdope.com/u/GreenWyvern)\ **Post date:** [August 2, 2021, 6:27pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/34 "2021-08-02T18:27:16Z")
I want one that you can fly in without having to drive to the airport first. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [August 3, 2021, 5:43am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/35 "2021-08-03T05:43:19Z")
> [@GreenWyvern](#): > > I want one that you can fly in without having to drive to the airport first.   You can have this – there are “airparks”, residential communities (often rather up-scale) built around a private little runway. Keep your airplane in your garage. When you want to go out for a little trip, just drive your airplane down the street to the runway, and there you are! [![](https://5ce351d9f3ccd9447e60e2eb.static-01.com/l/images/57a82587ae4835f6df11963c07d73131cd6f3958.jpg) ](https://5ce351d9f3ccd9447e60e2eb.static-01.com/l/images/57a82587ae4835f6df11963c07d73131cd6f3958.jpg) From this article: > **[Exit the runway and taxi all the way home! Cameron Park, CA - flywaze](https://flywaze.co/blog/want-to-turn-out-of-the-runway-and-taxi-home)** > >  I am sure every pilot and aviation enthusiast has probably dreamt of having a plane parked on their very own driveway. Many airpark communities are built to do exactly that ---
**Author:** ![GreenWyvern](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/greenwyvern/32/2834_2.png) [@GreenWyvern](https://boards.straightdope.com/u/GreenWyvern)\ **Post date:** [August 3, 2021, 6:02am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/36 "2021-08-03T06:02:32Z")
> [@Senegoid](#): > > When you want to go out for a little trip, just drive your airplane down the street to the runway, and there you are! I hope the local shopping mall also has an airstrip and parking spots for planes! 😀 ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [August 3, 2021, 6:09am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/37 "2021-08-03T06:09:44Z")
> [@GreenWyvern](#): > > I hope the local shopping mall also has an airstrip and parking spots for planes! 😀   Take a closer look at the pic just above. You may be able to see it enlarged if you click on it. Note that there is a STOP sign at the corner, mounted on a very low pole. Airplanes driving around on city streets have to obey all the regular traffic rules, STOP signs included. The signs are mounted low so the airplane wings can go right over them instead of bumping into them. (Explaining what this hijack has to do with jetpacks is left as an exercise to the reader. But bear in mind that we do not talk about hijacks at airports!) ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [August 3, 2021, 6:21am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/38 "2021-08-03T06:21:35Z")
> [@GreenWyvern](#): > > I hope the local shopping mall also has an airstrip and parking spots for planes! 😀 Airports for your shopping pleasure! Just fly your jetpack right in! > **[It's a Mall . . . It's an Airport; . . . It's Both: The Latest Trend in...](https://www.nytimes.com/1998/06/10/business/it-s-a-mall-it-s-an-airport-it-s-both-the-latest-trend-in-terminals.html)** > > Number of airports are turning their concourses into shopping malls; Pittsburgh International, major hub for US Airways, was one of first airport to emphasize shopping; it gives merchants access to around 60,000 passengers a day and has about 100... > **[10 Airports Where Shopping Is (Almost) as Good as Flying - The Points Guy](https://thepointsguy.com/guide/best-airport-shopping-north-america/)** > > Today, major airports contain between 50 and 100 stores — the same number as the average American shopping mall. > [@](#): > > [Airport shopping](https://thepointsguy.com/guide/how-to-shop-for-rare-and-exclusive-spirits-at-duty-free/) used to be simple: snacks, bottled water, a few magazines or a trashy novel (unless you were on an international flight, and had access to [duty-free shops](https://thepointsguy.com/2015/11/best-duty-free-shopping-scores/)). That was before air travel became commonplace, and prior to the explosion of travel retail as a category. Most major airports today contain between 50 and 100 stores, the same number as the average American shopping mall. ---
**Author:** ![GreenWyvern](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/greenwyvern/32/2834_2.png) [@GreenWyvern](https://boards.straightdope.com/u/GreenWyvern)\ **Post date:** [August 3, 2021, 6:25am UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/39 "2021-08-03T06:25:17Z")
> [@Senegoid](#): > > Airplanes driving around on city streets have to obey all the regular traffic rules, STOP signs included. The signs are mounted low so the airplane wings can go right over them instead of bumping into them. That sounds like great progress – driving around the city streets in a plane, and stopping at stop signs. But will these planes be full level 5 self-driving? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [August 3, 2021, 3:17pm UTC](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737/40 "2021-08-03T15:17:13Z")
> [@Senegoid](#): > > Airplanes driving around on city streets have to obey all the regular traffic rules When I was a kid and my mom was working at MYF, a pilot in (I think) a Cessna made an emergency landing on a street near the airport. He deiced to taxi back to the airport, and was issued a moving violation citation for operating an aircraft on a street. I’m guessing an aircraft is an ‘off-road vehicle’ much like a minibike or dirt bike that A) does not have the required equipment for operating on the street; and B) does not have a license plate. (My minibike had an off-road license sticker.) [Previous page](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737.md?page=1) [Next page](https://boards.straightdope.com/t/how-would-a-jetpack-work/947737.md?page=3)