Question about JETPACKS! (and physics)

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Question about JETPACKS! (and physics)

Postby The Great Hippo » Sat Oct 08, 2011 12:22 am UTC

There's a silly story I'm writing for silly reasons along with some silly people, and there's something I wanted to ask folks who are in a way better position to know than I.

It's a pulp serial hero story that takes place in the 1920s; one of the main characters is a girl with a jetpack--specifically, a highly experimental pulse jet. While most of the narrative is ridiculous (it involves an invasion from an alternate dimension Venus, an 'American Vampire' chapter of the Klu Klux Klan that opposes all those 'un-American' European vampires, an evil Doc Savage...), I wanted the jetpack itself to be semi-credible.

The girl has the ability to generate incredible amounts of electricity. The basic design I had for the jetpack right now is that it harvests her electricity and uses hydrolysis to generate hydrogen gas, which it then ignites; water is therefore the jetpack's primary fuel source. Would this be viable? Would there be any notable problems? Are there alternative designs available (particularly ones that use her ability to generate huge amounts of electricity)? Since this is very silly, very pulpy science fiction, everything doesn't have to be perfectly legitimate; advanced technology beyond 1920 is available.

I appreciate feedback from anyone with more knowledge on this subject than I (which authorizes 4 year olds to post!). Thanks for reading!

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Re: Question about JETPACKS! (and physics)

Postby jmorgan3 » Sat Oct 08, 2011 12:43 am UTC

You could have her use an arcjet rocket. Basically, you use electric heating to flash the water to steam, and direct the steam backwards to generate thrust.

Alternately, use a high-voltage arc to ionize the water, and use a strong electric field to accelerate the ions backwards.

Note that either of these methods will destroy anything close behind them when they are generating a reasonable amount of thrust.
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Re: Question about JETPACKS! (and physics)

Postby The Great Hippo » Sat Oct 08, 2011 1:01 am UTC

Would fuel usage be a problem, though?

Also, what would ionizing the water and using it as thrust look like? Would this basically be a giant waterspout?

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Re: Question about JETPACKS! (and physics)

Postby poxic » Sat Oct 08, 2011 1:14 am UTC

My biggest believability snag was always "how are their butts not being blasted out of existence?" I suppose you could make the jetpack actually a little platform with three or four downward-and-slightly-outward facing thrusters for stability. Or make the jetpack thruster point away at a 30 degree or summat angle, so the user has to lean backwards to go straight up.
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Re: Question about JETPACKS! (and physics)

Postby The Great Hippo » Sat Oct 08, 2011 1:34 am UTC

poxic wrote:My biggest believability snag was always "how are their butts not being blasted out of existence?" I suppose you could make the jetpack actually a little platform with three or four downward-and-slightly-outward facing thrusters for stability. Or make the jetpack thruster point away at a 30 degree or summat angle, so the user has to lean backwards to go straight up.
Oh, yeah, when I was reading about jetpacks, I noticed the talk concerning the necessity for stabilizing jets--a lot of pulp fiction described flying rockets as 'rocket belts' instead of 'rocketpacks', which made me imagine more of a platform (that you were strapped into). Although that's not quite as visually compelling as a girl with a simple jet strapped to her back, flying fist-first into the air.

The angled jets take care of that problem--so whenever she turns them on, she flies up and forward (unless she's got stabilizing jets on her hips). Another solution might be to have some form of alternative lift not based on explosive propulsion--some bizarre science or pseudo-science made possible by the absurdity of having a 'magical' electric generator?

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Re: Question about JETPACKS! (and physics)

Postby poxic » Sat Oct 08, 2011 1:47 am UTC

Anti-gravity! Because her superpowers allow her to temporarily move a large number of Higgs particles away from herself, making her (and her jetpack) much less massive than usual, so that gravity affects her (and it) much less!

And then she and her jetpack immediately slam into the nearest tree/wall/mountain to the west of them...
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Re: Question about JETPACKS! (and physics)

Postby Carnildo » Sat Oct 08, 2011 3:41 am UTC

The Great Hippo wrote:Would fuel usage be a problem, though?

Fuel usage is always a problem for realistic jetpacks.

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Re: Question about JETPACKS! (and physics)

Postby thoughtfully » Sat Oct 08, 2011 3:49 am UTC

More generally, reaction mass is a problem for any thrust source that isn't using the ambient air.
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Re: Question about JETPACKS! (and physics)

Postby The Great Hippo » Sat Oct 08, 2011 4:16 am UTC

And I guess there's no very sensible way to use air as your fuel source even if you have craploads of electricity?

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Re: Question about JETPACKS! (and physics)

Postby poxic » Sat Oct 08, 2011 4:26 am UTC

There's ramjets, but you pretty much have to be going Mach something already for that to work.
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Re: Question about JETPACKS! (and physics)

Postby jmorgan3 » Sat Oct 08, 2011 4:28 am UTC

The Great Hippo wrote:And I guess there's no very sensible way to use air as your fuel source even if you have craploads of electricity?

You don't need a fuel source, you need reaction mass. The most straightforward way to do this would be a propeller. More outlandish would be a turbojet or ramjet with the combustor replaced by an electrical heating element.
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Re: Question about JETPACKS! (and physics)

Postby ibgdude » Sat Oct 08, 2011 6:59 am UTC

You could also have a bismuth fuel supply, and a mini particle accelerator, then vaporize and ionize your bismuth, accelerate it to ridiculously high speeds, and eject it downwards. Based on Ion Thruster spaceships. Probably quite dangerous below, though.
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Re: Question about JETPACKS! (and physics)

Postby Hawknc » Sat Oct 08, 2011 7:31 am UTC

On hydrogen:

Let's say the combined mass of the girl and jetpack is 150kg, so the thrust required to hover is 1500N (ballpark figures here). Thrust is defined as: Image

All I have on hand is the specific impulse of liquid hydrogen when used in an LH2/LOx rocket (381 seconds, from here), so let's go with that. Solving for the mass flow we get a required H2 flow of about 0.4 kg/s. To generate that much hydrogen you would need about 3.5 litres per second of water. So the question is, how big is the tank on this poor girl's back?

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Re: Question about JETPACKS! (and physics)

Postby ibgdude » Sat Oct 08, 2011 8:08 am UTC

Clearly, you a higher specific impulse. If you use a Dual Stage Four Grid Electrostatic Ion Thruster, specific impulse 21000 s, or a jet engine, specific impulse 11000 s, you'll be fine. An electric jet engine, or scaled down model thereof, would be best, because no fuel would be necessary.
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Re: Question about JETPACKS! (and physics)

Postby The Great Hippo » Sat Oct 08, 2011 3:45 pm UTC

What about an electric turbofan? I don't know much about its design or the repercussions you'd face by strapping one to your back, but it sounds like something that might be mildly credible.

Also, yeah, Hawknc, the whole 'how much water do you need to fly this thing' detail was bothering me--though I have no science background, I've suspected that the amount of water you'd need for a hydrogen-powered engine would be kind of ridiculous. If there is an engine that can function efficiently enough on the hydrogen produced from hydrolysis, though, I thought it'd be cool for her to refuel by just dunking herself (and the whole jetpack) into a pool of water and opening the valves.

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Re: Question about JETPACKS! (and physics)

Postby Jplus » Sat Oct 08, 2011 4:14 pm UTC

A turbofan is essentially just a propellor that is boosted by combustion of a fuel. If you make it an 'electric turbofan' by leaving out the fuel, you'll end up simply with a propellor (with a peculiar shape).

I think propellors (in whatever shape) might actually be the most realistic solution in your case, because you can drive them with an electric motor and as you said, the girl has enough electric energy at her disposal. Remember the air vehicles from Avatar (the 2009 movie) that looked somewhat like a helicopter? They had these propellors shaped like a short tunnel, maybe something like that would suit your needs.
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Re: Question about JETPACKS! (and physics)

Postby scarecrovv » Sun Oct 09, 2011 4:11 am UTC

Jplus wrote:A turbofan is essentially just a propellor that is boosted by combustion of a fuel. If you make it an 'electric turbofan' by leaving out the fuel, you'll end up simply with a propellor (with a peculiar shape).

I think propellors (in whatever shape) might actually be the most realistic solution in your case, because you can drive them with an electric motor and as you said, the girl has enough electric energy at her disposal. Remember the air vehicles from Avatar (the 2009 movie) that looked somewhat like a helicopter? They had these propellors shaped like a short tunnel, maybe something like that would suit your needs.

You are quite correct. Just take one of these, and replace the gasoline engine with an electric motor.

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Re: Question about JETPACKS! (and physics)

Postby poxic » Sun Oct 09, 2011 4:20 am UTC

I approve this idea.

/makes me twitch to leave out the "of"
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Re: Question about JETPACKS! (and physics)

Postby The Great Hippo » Sun Oct 09, 2011 4:44 am UTC

I'll probably go with that, then; it seems a little snazzier than I'd expect for 20s tech, but this story's got eldritch horrors from beyond the veil of space and time and aliens made of sound who eat music and shit noise, so I'm not going to worry too much about it.

One important detail--I imagined her using the pack to 'leap-frog' really high--that's much less likely with a propeller setup, though, right? Because you need time for the propellers to reach the necessary speed?

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Re: Question about JETPACKS! (and physics)

Postby poxic » Sun Oct 09, 2011 5:01 am UTC

The Great Hippo wrote:aliens made of sound who eat music and shit noise

Hey, I think that's the band I play in.

Prop planes existed in the '20s, didn't they? Your brilliant, electrically-charged, happenin' girl (or woman, we haven't heard her age yet) simply stole a couple of small propellers off an abandoned small airplane, plus a motor from a wrecked electric railcar, and got her best buddy the slightly-autistic-yet-mechanically-genius nerd from down the street to bodge together a sweet little dual-rotor personal helilevitator for her. It doesn't take too long for it to rev up enough for her, since she's small and light, and she does this quirky sort of hopping dance to get off the ground.

Or not. Your call.
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Re: Question about JETPACKS! (and physics)

Postby The Great Hippo » Sun Oct 09, 2011 6:22 am UTC

Well, it would make a pretty sweet band name.

And yeah, something like that probably would work--it occurred to me that it would probably make her 'feel' a lot lighter on lower settings, which would give her an excuse to hop really high.

Thanks for the feedback, everyone! This has been really helpful.

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Re: Question about JETPACKS! (and physics)

Postby dainbramage » Sun Oct 09, 2011 7:36 am UTC

Hawknc wrote:On hydrogen:

Let's say the combined mass of the girl and jetpack is 150kg, so the thrust required to hover is 1500N (ballpark figures here). Thrust is defined as: Image

All I have on hand is the specific impulse of liquid hydrogen when used in an LH2/LOx rocket (381 seconds, from here), so let's go with that. Solving for the mass flow we get a required H2 flow of about 0.4 kg/s. To generate that much hydrogen you would need about 3.5 litres per second of water. So the question is, how big is the tank on this poor girl's back?


You need a mass flow of 0.4 kg/s. In this case it's water as exhaust, not just H2. So our girl would be going through 0.4 L/s of water. Assuming she's electrolysing the water then re-combusting it of course.


At any rate, the easiest way would just be to give her an electrically powered fan.

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Re: Question about JETPACKS! (and physics)

Postby Hawknc » Sun Oct 09, 2011 8:55 am UTC

Possibly. Chemistry was never my strong suit.

dainbramage wrote:At any rate, the easiest way would just be to give her an electrically powered fan.

Agreed.

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Re: Question about JETPACKS! (and physics)

Postby Giallo » Mon Oct 10, 2011 8:43 pm UTC

I'm surprised they (there's always a "they" somewhere...) didn't catch her and used her as a limitless source of energy...
A Faraday cage should be enough to capture an electric man (or woman/girl).
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Re: Question about JETPACKS! (and physics)

Postby idobox » Mon Oct 10, 2011 11:04 pm UTC

If she has illimited electricity available, the arcjet is the best solution. You use a badass electric arc to heat air, instead of burning fuel.
A pulse arcjet would be easier to design, especially in the 20's, but a turbo arcjet would also be a viable option.

Also, a quite cool, and easier alternative to a real jet pack, is to put small wings, and basically fly like a plane. Of course stationary flight becomes impossible, but you require much less thrust
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Re: Question about JETPACKS! (and physics)

Postby The Geoff » Tue Oct 11, 2011 12:22 am UTC

This requires some fairly sophisticated engineering, but should work in theory unless I've misunderstood relativity:

If you've got as much electricity as you could use, then make it a backpack mounted particle accelerator. Whiz your reaction mass, almost atom by atom, up to 99.lots-of-9s% the speed of light. You can use as little reaction mass as needed, down to a limit set by single-atom impulses on the human body. You'd need two counter-rotating accelerators to stop torque effects I'd guess.

(Yes, I realise this involves several LHCs, massively upgraded and shrunk to backpack size, using currently unknown materials, but hey)

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Re: Question about JETPACKS! (and physics)

Postby Technical Ben » Tue Oct 11, 2011 8:48 am UTC

The Great Hippo wrote:And I guess there's no very sensible way to use air as your fuel source even if you have craploads of electricity?

Burn ambient water vapour as hydrogen oxygen (after separating in your rocket chamber of cause). ;)
She cannot fly in a desert though. :(

Oh, idobox, nature wants to give you it's stationary flight mechanisms back.

I like the idea of using an electric turbo fan to gather reaction mass (air) to force out some sort of arc-jet or ion thruster.
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Re: Question about JETPACKS! (and physics)

Postby Tass » Tue Oct 11, 2011 9:18 am UTC

Technical Ben wrote:
The Great Hippo wrote:And I guess there's no very sensible way to use air as your fuel source even if you have craploads of electricity?

Burn ambient water vapour as hydrogen oxygen (after separating in your rocket chamber of cause). ;)
She cannot fly in a desert though. :(

Oh, idobox, nature wants to give you it's stationary flight mechanisms back.

I like the idea of using an electric turbo fan to gather reaction mass (air) to force out some sort of arc-jet or ion thruster.


If you separate it in the chamber to immediately combust it for heat, why not just use the electricity for heat directly.

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Re: Question about JETPACKS! (and physics)

Postby idobox » Tue Oct 11, 2011 3:19 pm UTC

Technical Ben wrote:Oh, idobox, nature wants to give you it's stationary flight mechanisms back.


I'll expand a bit on this one.
If you have wings to provide lift, you need less thrust, and thus can use a smaller engine. In this case, you won't have enough thrust anymore to fly without using the wings.
It will still be possible to fly against the wind,or in circles, but this is nothing like the stationnary flight you would achieve with a "standard" jetpack.

What I'm thinking of would look like that
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Re: Question about JETPACKS! (and physics)

Postby PM 2Ring » Tue Oct 11, 2011 3:45 pm UTC

Shouldn't this thread be in Fictional Science?

I like the particle accelerator idea, but even a somewhat simpler electrostatic ramjet system should be ok: she uses electrostatic attraction to suck air into her jetpack, ionizes it & then uses electrostatic repulsion to get maximum thrust from it.

Or she could use electrostatic effects more directly. This option doesn't require a jetpack, but it does require her to have precise control over where & how the electricity manifests. She'd give her body a negative charge (for example) and also make the air (or ground) beneath and behind her negative. Like charges repel, so she'd effectively be pushed from below & behind. Alternatively, she could make the air above and in front of her positive so that she'd be moved up & forward by the attraction of unlike charges.

Of course, she'd have to continually adjust the charges of the air around her as she flew, which would probably require a bit of concentration and limit her speed, but that shouldn't be too much of a problem. Besides, for narrative purposes it's nice to have a natural way to limit your characters' super-powers.

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Re: Question about JETPACKS! (and physics)

Postby The Great Hippo » Wed Oct 12, 2011 2:15 am UTC

It totally should, and I beg your pardon--I had no idea the fictional science subforum existed! I would be totally down with this thread being moved there.

The electrostatic ramjet strikes me as a very promising idea, largely because it lets me keep some of the color I wanted (one of the ongoing jokes is everyone calls her jetpack a 'rocket', and she gets angry because it's a jet); the explanation of how she could fly on her own would be something I could work with later, too.


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