Hand-held space-jet

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tomandlu
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Hand-held space-jet

Postby tomandlu » Tue Mar 10, 2015 5:27 pm UTC

Hi,

For maneuvering in a zero-gravity vacuum, how practical would a hand-held unit be?

My thought is something rather like a small diver propulsion vehicle (http://en.wikipedia.org/wiki/Diver_propulsion_vehicle), with a simple handle/throttle that you basically hold out in front of you and aim where you want to go (with a fairly steep learning curve to handle inertia, etc.). Also, I guess the jets, which fire compressed gas (?) would need to be angled so as not to fire in the face of the pilot (not dangerous, just counter-productive).

Wouldn't that be easier to use than a back-pack, where, I assume, controlling your direction is a more complex business.
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Re: Hand-held space-jet

Postby Izawwlgood » Tue Mar 10, 2015 5:32 pm UTC

Like a fire extinguisher, a la Wall-E? Or a can of cheezewiz, a la Journey Man Project?
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Re: Hand-held space-jet

Postby tomandlu » Tue Mar 10, 2015 5:34 pm UTC

Izawwlgood wrote:Like a fire extinguisher, a la Wall-E? Or a can of cheezewiz, a la Journey Man Project?


Well, yes, but those are really bad designs... well, for space jets. For putting out fires or squirting cheeze, they're pretty good.

Edit: I'm imagining something like a short cone with a handle-throttle in the base, and four or five jets angled so that the wash misses the pilot. So, you hold it out in front of you and off you go...
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Re: Hand-held space-jet

Postby Draconaes » Tue Mar 10, 2015 6:45 pm UTC

tomandlu wrote:I'm imagining something like a short cone with a handle-throttle in the base, and four or five jets angled so that the wash misses the pilot. So, you hold it out in front of you and off you go...


Hm, that design seems really inefficient to me. A lot of the energy will be wasted cancelling out the opposite angled jet. To keep it in the "point in direction you want to fly" style, maybe some kind of pole, with the jet one one end extending behind you, and the other end you point in front of you. This will keep the jets from hitting the pilot without sacrificing efficiency.

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Re: Hand-held space-jet

Postby tomandlu » Tue Mar 10, 2015 10:09 pm UTC

Draconaes wrote:
tomandlu wrote:I'm imagining something like a short cone with a handle-throttle in the base, and four or five jets angled so that the wash misses the pilot. So, you hold it out in front of you and off you go...


Hm, that design seems really inefficient to me. A lot of the energy will be wasted cancelling out the opposite angled jet. To keep it in the "point in direction you want to fly" style, maybe some kind of pole, with the jet one one end extending behind you, and the other end you point in front of you. This will keep the jets from hitting the pilot without sacrificing efficiency.


But isn't that much harder to guide? I'm not thinking of opposing jets - more like: /\ (and two more at right angles - they only have to be angled enough to not wash over you). If the force is behind you, aren't you going to be a lot more unstable?
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Re: Hand-held space-jet

Postby scarecrovv » Wed Mar 11, 2015 4:36 am UTC

tomandlu wrote:But isn't that much harder to guide? I'm not thinking of opposing jets - more like: /\ (and two more at right angles - they only have to be angled enough to not wash over you). If the force is behind you, aren't you going to be a lot more unstable?


It's a common misconception that having your rocket above you is more stable than having it below you. This misconception is evident in some early rockets which put the fuel tank at the bottom at the rocket at the top. However, the stability math works out identically in both cases. See http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy.

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Re: Hand-held space-jet

Postby peregrine_crow » Wed Mar 11, 2015 9:48 am UTC

Draconaes wrote:
tomandlu wrote:I'm imagining something like a short cone with a handle-throttle in the base, and four or five jets angled so that the wash misses the pilot. So, you hold it out in front of you and off you go...


Hm, that design seems really inefficient to me. A lot of the energy will be wasted cancelling out the opposite angled jet. To keep it in the "point in direction you want to fly" style, maybe some kind of pole, with the jet one one end extending behind you, and the other end you point in front of you. This will keep the jets from hitting the pilot without sacrificing efficiency.


So, Harry Potter style broomsticks? :D
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Re: Hand-held space-jet

Postby tomandlu » Wed Mar 11, 2015 10:10 am UTC

scarecrovv wrote:
tomandlu wrote:But isn't that much harder to guide? I'm not thinking of opposing jets - more like: /\ (and two more at right angles - they only have to be angled enough to not wash over you). If the force is behind you, aren't you going to be a lot more unstable?


It's a common misconception that having your rocket above you is more stable than having it below you. This misconception is evident in some early rockets which put the fuel tank at the bottom at the rocket at the top. However, the stability math works out identically in both cases. See http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy.


Thanks for that. I think that assumes a rigid framework - does it make a difference that we have a non-rigid frame in this instance? (aka an astronaut). I guess I'm thinking of the difference between balancing on top of something (hard) and hanging from something (easy).
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Re: Hand-held space-jet

Postby Copper Bezel » Wed Mar 11, 2015 10:37 am UTC

Part of it is going to depend on how massive you expect the widget to be. It's not going to be very "point and click" if you have to seriously consider the inertia of the device itself relative to your body as you move it around.

But yeah, I think there's a part of that intuition that's applicable and a part that's not. At the very least, if the device isn't clamped to you, it's a lot easier to hold onto if it's in front and pulling than if it's in behind and pushing.

A question to ask might be why the Shuttle's MMU was so bulky and whether a device designed on similar principles could actually be made appreciably smaller. If not, well, if it's the size of a chair, it might as well be one, and then you get all that built-in multiaxial control.
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Re: Hand-held space-jet

Postby tomandlu » Wed Mar 11, 2015 11:15 am UTC

Copper Bezel wrote:Part of it is going to depend on how massive you expect the widget to be. It's not going to be very "point and click" if you have to seriously consider the inertia of the device itself relative to your body as you move it around.

But yeah, I think there's a part of that intuition that's applicable and a part that's not. At the very least, if the device isn't clamped to you, it's a lot easier to hold onto if it's in front and pulling than if it's in behind and pushing.

A question to ask might be why the Shuttle's MMU was so bulky and whether a device designed on similar principles could actually be made appreciably smaller. If not, well, if it's the size of a chair, it might as well be one, and then you get all that built-in multiaxial control.


I was thinking of something the size of a basket ball or smaller, depending on how much propellant you want to carry, so the main inertial problem is the astronaut him/herself (e.g. don't leave it too late to turn around and slow down).

The thing about the MMUs is those things look hard to use (and put on) - I was wanting something that seemed a bit more "oh, yeah, I get it."
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Re: Hand-held space-jet

Postby sevenperforce » Wed Mar 11, 2015 4:43 pm UTC

The Harry Potter broomstick is a great idea. Really.

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Re: Hand-held space-jet

Postby tomandlu » Thu Mar 12, 2015 10:43 am UTC

sevenperforce wrote:The Harry Potter broomstick is a great idea. Really.


Yep - I might well go with it
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Re: Hand-held space-jet

Postby Whizbang » Thu Mar 12, 2015 1:37 pm UTC

Flying broomsticks in SPAAAAAACE!

I'd read it.

Also, consider a flying carpet.

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Re: Hand-held space-jet

Postby sevenperforce » Thu Mar 12, 2015 2:03 pm UTC

I had a wacky wacky dream last night -- possibly prompted by this thread -- in which I was watching a theme park employee do all manner of aerial acrobatics using a pair of hand-held rockets. They were each approximately two feet long and had a thumb trigger on the front end.

However, each rocket exhaust nozzle was out behind the rest of the rocket assembly on a hose, which makes no sense whatsoever in real life but somehow worked in the dream.

Then there was also the part where I created my own hand-held rocket by cooking a turkey in the right kind of oven, but even in the dream I knew that didn't make any sense.

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Re: Hand-held space-jet

Postby Chen » Fri Mar 13, 2015 2:12 pm UTC

With limited fuel for propulsion, you don't want to be wasting any directing thrust in directions except the opposite of where you want to go. I'm pretty sure this is why you have backpack mounted units. I'm not sure why a hand-held thing would be any easier to use. Unlike the underwater unit you're not providing continuous thrust in space. You're providing bursts. Seems like a hand-held unit would be far less precise since the exact position of your arm at the time of the burst would determine where you went. Since it's not a fixed position getting somewhere accurately seems far more difficulty than activating the thruster on your backpack which is always in the same place, relative to where you're facing.

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Re: Hand-held space-jet

Postby tomandlu » Sat Mar 14, 2015 8:15 pm UTC

Chen wrote:Unlike the underwater unit you're not providing continuous thrust in space. You're providing bursts. Seems like a hand-held unit would be far less precise since the exact position of your arm at the time of the burst would determine where you went.


I think you've hit my conceptual problem on the head. Thinking in terms of zero-gravity and vacuum is basically the opposite of what we're used to. Hand-held (or broom) makes a lot of sense with any dynamic that requires continual thrust, but is pretty much hopeless in a short-burst system. Still better than a fire-extinguisher but that's about all one can claim.
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Re: Hand-held space-jet

Postby Sandor » Fri May 22, 2015 12:13 pm UTC

Whatever you go for, you have to make sure that the thrust vector passes through the astronaut's center of mass. If not, every burst will impart torque, which will set the astronaut spinning. You might want to make the aiming mechanism separate from the propulsion mechanism.

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Re: Hand-held space-jet

Postby Qaanol » Thu May 28, 2015 10:21 pm UTC

peregrine_crow wrote:So, Harry Potter style broomsticks? :D

More like Harry Potter and the Methods of Rationality style broomsticks (or at least that one broomstick) I think.
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Re: Hand-held space-jet

Postby tomandlu » Fri May 29, 2015 5:47 am UTC

Qaanol wrote:
peregrine_crow wrote:So, Harry Potter style broomsticks? :D

More like Harry Potter and the Methods of Rationality style broomsticks (or at least that one broomstick) I think.


Ah - I've only skimmed it (although my son's a recent addict and is halfway through reading the whole thing). Can you point me to a chapter?
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Re: Hand-held space-jet

Postby peregrine_crow » Fri May 29, 2015 7:49 am UTC

Qaanol wrote:
peregrine_crow wrote:So, Harry Potter style broomsticks? :D
More like Harry Potter and the Methods of Rationality style broomsticks (or at least that one broomstick) I think.

Well, yes :D .
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Though hopefully with a little more control and a little less power

tomandlu wrote:Ah - I've only skimmed it (although my son's a recent addict and is halfway through reading the whole thing). Can you point me to a chapter?

It's chapter 58 and 59 (http://hpmor.com/chapter/58), but I would strongly recommend against reading those two chapters out of context if you have any interest at all in reading the whole thing, they contain serious spoilers for one of the most amazing arcs in the story.
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