# Tiny emergency propulsive device if stuck floating in a large volume in microgravity

The questions here in SXSE Can you swim in space? and in Physics SE How to escape the center of a room without gravity? [closed] both address aspects of how to move if stuck floating in the center of a volume in microgravity with nothing to grab. For the purposes of this question, let's say the nearest thing is now 100 meters away.

Because I worry about this all the time, I want to build a very small device I can keep on my person on a lanyard or ankle band that will help propel me safely to a wall. Although according to the answers it should be possible to escape naturally, I want to use a device.

How should my tiny device work? I want it to be as small and light-weight as possible. Should it be a laser, or a fan, or tiny tank of compressed gas, or a magnet, or an atmospheric ion thruster, or a rolled up $100 bill that I can bribe someone to help me? Basically, what would be the smallest, lightest technology device that could propel an average person 100 meters in say 10 minutes in a normal atmosphere? In the future I will sell them to space tourists, so it needs to be propulsive on its own. Space vacationers will expect getting around to be easier in low gravity; vigorously waving a fan won't be appealing to them. • If those recording took your clothes, why would they leave you with your device? ;) Just kidding. It's a good question. – called2voyage Sep 28 '16 at 13:54 • @called2voyage I didn't explain all the reasons it has to be small. I keep a spare hidden in my... hair. – uhoh Sep 28 '16 at 13:56 • Riiight, gotcha. ;) – called2voyage Sep 28 '16 at 13:57 • Right now it sounds like the$100 bill is your best bet. It's probably lighter than the next best option, the fan. – called2voyage Sep 28 '16 at 14:35
• freefall.purrsia.com/ff900/fv00900.htm – hobbs Sep 28 '16 at 15:29

I found a real world test of this. Dan Barry tried it when STS-96 was docked to the ISS. I've scanned his account from the book "Space Shuttle: the first 20 years."

tl;dr - he escaped by throwing his clothes.

Transcription:

DAN BARRY | Stranded in the middle of the room

STS-96

Entering the space station from the orbiter for the first time in 1999 was an exhilarating experience. Even with just two modules-Unity and Zarya-in place, the station's volume was 10 times that of Discovery's flight deck and mid-deck combined. I floated exactly to the center of Unity, where I could not reach the walls, and got stranded in the middle of the room. It's not easy to get stranded-I had to have my friends help me get perfectly still. Once I was stationary, my brain remained convinced that I could somehow maneuver or kick myself over to the wall. But when I reached out an arm, my body moved back and my center remained in the middle of the room. I instinctively tried moving fast, then slow, then bicycled my legs. None of it helped. I just had to wait for the air currents to drift me to the wall. Sneezing and spitting didn't do much good either. On the other hand, throwing clothing as fast as I could produced enough reaction to send me to the opposite wall.

AN OUTPOST IN THE SKY 117

• Aha - a first hand account. Somehow I imagined there was always stuff to grab on the ISS - I didn't realize this had any potential at all of happening there. It could even be a little dangerous if the air flow pattern had a null at the same point, or was not working properly. Hopefully a perfect storm like that could never actually happen! Great find, thanks! – uhoh Oct 4 '16 at 23:19
• Note the omission of which clothing was thrown !! – Organic Marble Oct 5 '16 at 14:24
• Excellent find. This hazard is much more critical/life threatening then previously evaluated. Esp, the need for CDF analysis at air condition down time, as region with lack of air flow occur at complex structures anyway. Text says breath/spit/sneeze as not to work. So theoretically, if you are naked, trapped in mid air with no air flow, no fluid/object left in bladder/intestine, and air conditioning goes down, you want/need clothing to throw away...or you are in trouble not only the legal, but even with the physics department. – GreenFox Oct 17 '16 at 16:41

What if you just carried a couple of uninflated balloons with you?

If you ever get stuck, just inflate the balloon, and then hold it near your center of mass, aim it away from you, and let the air out. Repeat until you make it to the wall.

The nice thing is that an uninflated balloon is light enough that you could even have a couple of spares.

• That's an ingeneously simple idea. Need propulsion? Make a rocket! Using your lungs etc. to do work and then use a nozzle to direct the expanding gas. Ellegant. – uhoh Sep 29 '16 at 3:45
• Is this faster than just moving by breathing? – JiK Sep 29 '16 at 14:58
• You don't need the balloon at all. Point your head towards your feet and breathe in. Tilt your head back and blow out hard and fast. Repeat. You'll develop some spin, but you'll get to the wall eventually. – Innovine Sep 29 '16 at 15:18
• @Innovine That's the natural way answered in the linked questions, and the OP specifically asked for a device. – user10509 Oct 5 '16 at 8:43

For comparison's sake, SAFER is what 3m/s-dV of compressed gas looks like. I doubt you'd be able to hide that.

Hand fans are your best bet, because unlike all your other options they don't carry their own reaction mass (well, if you can call the battery in a laser pointer 'reaction mass'.) They won't move the air particularly fast, but they'll be moving relatively large quantities of it for a long time compared to compressed-gas/mini-ion options.

In fact, fans are such a good option many aircraft use scaled up versions for highly efficient propulsion.

In the spirit of the question, let's find the simplest possible option.

We need to cover 100 meters in 600 seconds (10 minutes). This means we need a velocity of

$$100m/600s \approx 0.16 m/s$$

What if you're a professional baseball player? The world-record pitching speed is around 40 m/s, so this seems like a good place to start. Let's just assume you have a ball on you at all times, too.

Let's say you're a little skinny, at around 60 Kg, and the ball is a little heavy, around 150 g,

$$150 g / 60 Kg = 0.0025$$

The ball is going to get 99.75% of the velocity, and you only .25%.

$$0.0025 * 40 m/s = 0.1 m/s$$ $$100 m / 0.1 m/s = 1000 s$$ $$1000 s \approx 16.6\ minutes$$

So we're going nowhere fast, but we'll get there in a quarter hour. If you want to get there in the 10 minute mark, throw something heavier, or try to break the world record for pitching speed. You could also diet before you're trapped, but that would require some amazing foresight.

• OK that's great! But I'm asking for a device that will safely propel me somehow. It needs to do the work. I take it out and it somehow propels me. You need to consider a few more things as well. 1) How to throw a baseball that fast - directly away from your center of mass. You have to push it out from your stomach, otherwise you'll tumble helplessly for hours in place 2) You might brake something important, that baseball will continue to bounce off the walls until it does, and if your volume is spherical, that might be you! You can't block it because - remember - you are tumbling wildly! – uhoh Sep 28 '16 at 16:57
• Also, even a baseball would be hard to hide in your "hair". Not impossible, just perhaps not comfortable for the OP. – called2voyage Sep 28 '16 at 17:18
• What about the drag ? – Antzi Sep 29 '16 at 1:33
• In this situation it'll be most efficient to move a large mass (or a large quantity of air) at a low speed rather than vice versa. – pericynthion Sep 29 '16 at 17:03

Fling out a parachute that pops open when it reaches the end of a tether. Pull it back rapidly.This will give you momentum (more than it took to fling the parachute). It could double as a cape, turban, loincloth or mumu.

• Like a sea anchor or a drogue, could be informally called a "space anchor" perhaps. I like this one very much - it doesn't have batteries that could run down. It could be incredibly light and small when rolled up using advanced materials, but then you'd need to find some ballast to attach to it so it will actually go some distance before stopping due to air resistance. There's probably even a formula to calculate how many times you'd need to do it to move 100 meters. – uhoh Oct 4 '16 at 23:29

Here are a couple of (bad) ideas:

• Make a macramé belt out of 100m of string and make sure the buckle is magnetic. Take off the belt, untie the knots, then lasso something.

• Use a couple of hand fans. They'd be reasonably easy to hide and should move more air than your lungs.

• Carry a small radio and call for help. This probably won't work if you're the victim of a prank, but you never know.

I wouldn't bother with a tank of compressed gas - too easy to run out of fuel.

Likewise, a giant electromagnet big enough to draw you to your target might be a bit impractical and hard to hide in your hair.

• Why would you worry about running out of fuel? Unless the compartment is utterly huge, all you need is a tiny push in the general direction of the nearest wall. Sure, there will be friction against the contained air, but nowhere near enough to pose a significant problem even if the compartment is tens or hundreds of meters across. – user Sep 28 '16 at 14:49
• @MichaelKjörling What you say is very true. With my luck however, I would carry the cylinder for months, then discover it's empty when I need it most. I might also aim it poorly and put myself into a spin instead of propelling myself across the room. – Dan Pichelman Sep 28 '16 at 15:18
• That's a really good point! I saw how hard Matt Damon and Sandra Bullock had to work in their movies, yikes! Tom Hanks made it look easier but he had extra thrusters. Ah - you mean REAL hand fans, not battery operated ones. That's a good point. In the future - when we have 200 meter spherical spaces filled with air in microgravity, there will be advanced, foldable yet super-stiff materials. Instead of the $100 bill, one can unroll (or unfold) that. I like that one a lot, but I'm looking for the best thing that provides whatever power it needs - either storing it or harvesting it. – uhoh Sep 28 '16 at 16:47 If you need to be able to cover 100 meters in 10 minutes, and you have a mass of 70 kg, then the minimum momentum you have to be able to give yourself is$70\cdot \frac{100}{600} \approx 12 \rm{~kg~m/s}$. Assuming you have the arm of a professional baseball pitcher, who would be capable of throwing a 145 gram ball at speeds up to 160 km/h (45 m/s), we see you would have to carry two baseballs with you (or objects of equivalent mass). This is why carrying a very small collapsible fan quickly becomes more attractive as an option - you don't need to carry all the mass if you can move the air around you. A simple handheld fan might give an air speed of 2 m/s over a disk with a diameter of 20 cm. Such a fan would move 75 g of air per second, for a net force of 0.15 N; applying that force to your 75 kg body, you would accelerate with a=0.002 m/s$^2$. This means that in 10 minutes, you can cover a distance of$\frac12 a t^2 = 380 m, which is plenty.

The difference is in the fact that your fan leverages the mass of the air around you. This is why rockets are so big - they have to carry not only their fuel, but in essence their propulsion is entirely the result of spitting mass out of the tail pipe. When you can move the medium around you, you can be much more effective.

Note that a can of "emergency air" will again not give you the result you desire. It's that pesky product of mass and velocity that matters...

As for the question of power: if we assume 25% efficiency of the motor, the electrical power has to be four times thekinetic energy of the air being moved per second. So the motor power has to be $$P = 4\cdot \frac12 m~v^2 = 2\cdot 0.075 * 4 = 0.6 \rm{~W}$$

Keeping this up for 10 minutes would require 0.1 Wh of energy.

A small portable fan with dual AA size batteries would have about 1600 mAh * 3.0 V = 4.8 Wh capacity. Which is 48 times more than you need. Note that as the area of the fan gets smaller, the power required for the same momentum goes up (you need higher velocity of less air, basically). This approach has some scope for further miniaturization - I will leave it up to you to figure out the details of that. Incidentally, this is why the human-powered "helicopter" that won the Sikorsky challenge had rotors that were 10's of meters in diameter...

• I like this kind of answer - quantitative analysis combined with insight. The fan could have a flywheel and a pull-string, like a toy gyroscope if the battery wears out. But if it's a battery, can you estimate the mA-hours? Would it be a tiny battery or a big one? If I unwrapped a thin photovoltaic sheet, could I power it with room lights? – uhoh Sep 29 '16 at 15:19
• @uhoh - I have updated my answer. A small battery is plenty. – Floris Sep 29 '16 at 15:27
• Wow! I think that's quite an important result. – uhoh Sep 29 '16 at 15:35

Your question requires a very low speed. 100 meters in 10 minutes is 0.166m/s.

By my calculations using $Ek=\frac 1 2 mv^2$, assuming you weigh 80kg then you need 1.10224J of energy. If you have something on your person that you can throw away from you at 5m/s, then rearranging the equation gives the following:

$m=\frac 2 {v^2} Ek$

$m=\frac 2 {5^2} 1.10224J = 0.088kg= 88g$

I believe my edited values and units are correct this time, so I think 88g is still low, although would be uncontrolled once moving.

Any extra equipment you might carry to help you escape could also be thrown. As UIDAlexD said, a fan could work, but also just throwing the fan away from you would be enough to get you moving.

• Your math works out a lot different than mine. I'll check both when I get home. – UIDAlexD Sep 28 '16 at 16:16
• You seem to plug a value in kilojoules where you need to have joules, so you're off by a factor of 1000. – Peteris Sep 28 '16 at 16:18
• I'm asking for a device that will safely propel me. @called2voyage figured out the no clothes part but I realize I didn't mention it explicitly (it's in the linked question). But I need something that I can hold that generates propulsion, not throw. – uhoh Sep 28 '16 at 16:35
• I'll give you a point for taking a mathematical approach! But I'm curious, should it be momentum that you conserve in a calculation like this - $m_1 v_1 \ = \ m_2 v_2$ ? – uhoh Sep 28 '16 at 17:18
• @uhoh Yeah, you're right. In a scenario like this it's momentum that's conserved. Rob seems to have based all these calculations on the mistaken assumption that the total kinetic energy of the projectile has to be the same as the total kinetic energy of the person who threw it, which is simply not true. The correct result for m in this case is actually 2.656 kg: wolframalpha.com/input/… – Ajedi32 Sep 28 '16 at 18:04

Breath in with mouth, and generate thrust from nostril, continue until you have escaped danger

Provide external stimulants pass through the nasal hairs to reach the nasal mucosa, allowing one to trigger controlled directional upper body muscle movements (for quickest results).

Honestly can not understand why the down vote, but here are the detail.

Airflow Dynamics of Human Jets: Sneezing and Breathing

The "Emergency Forced Nostril Thrust Generator", has an aim to provide controlled directional thrust using nothing more than human body movements.

As you can see from the image provided in this journal , the classical theory of utilizing breathing to accumulate velocity in order to escape is inefficient at best for two reasons.

1.The classic theory does not appear to hold in microgravity condition The traditional notion was, breath in slow from everywhere, breath out fast focused, and off you go. However looking at air/fluid movements in the ISS indicate otherwise. Here are the maths. Let Air density be p , surface area of mouth A, and velocity of airflow from mouth be u, the following holds when breathing out. $-pAu2$ of work is preformed over time. therefore, inverse thrust of $pAu2$ is generated to move forward. While the airflow from the lung to the mouth follows a single route, as soon as it reaches outside the mouth, the airflow scatters to every possible direction regardless of shape of nozzle. The net volume of in and out still remains the same unless you change the inlet and outlet vector to a different direction. If there was even a sight change in airflow direction, this will be due to the density of air itself. (as there is no such thing as natural flow of air under microgravity condition. Remember just WHY the ISS forced air conditioning to be turned on ? ). In a nutshell, when you breath out air in microgravity, the air just sits there, you gain no net thrust if you keep facing the same direction. You can not rely for external shape of airflow to propel your self.

The field of fluid dynamics appear to represent a closer model of airflow under microgravity condition than the classical earth model that we are used to...

2. Motion control problem When we attempt to use breathing as the self propellent method, the difficulty lies in changing the motion vector. You want to limit the body movement as much as possible here, yet generate a controlled thrust.

The nostril, is located to the far end of the body mass, yet faces downwards, towards the center of gravity, with the mouth opening located approximately 90 degrees angle, allowing the inlet and outlet to be positioned on a different plane at any given time, a very desirable property. And the thrust generated is always controlled by how much air one breathes in.

Why I asked to stick up a alien object up the nose The question we have is a very real and likely scenario, that can happen in emergency situations. Under the assumption, it would not be a good idea to just expect life support and other air flow systems to be working (you would not be stuck if they were functioning correctly the first place anyway)

"Provide external stimulants pass through the nasal hairs to reach the nasal mucosa, allowing one to trigger controlled directional upper body muscle movements" really is the quickest and most efficient way to instantaneously generate thrust to evacuate danger with minimal resource. (but I guess people just thought I was fooling around.)

Reasoning in Design Given the condition, you really only have Air as possible propellant. There is a known engineering problem between size/efficiency for providing air thrust by a mechanical turbine engine.

This leaves us in a well known problem of limited mass, limited power, a area that has been worked upon in Earth, under the field of Human powered Aircraft design.

The key in such constraints is to replace as much engineering components as feasible with the human body, using the human body as part of the design instead of being just a payload.

When designing for a theoretical thrust generator, you have two options (1) place it at the center of gravity, or (2) at the tip of the structure, directing the nozzle towards the center of gravity.

Using the nostril and facing downwards, I honestly believe that this is a very elegant solution for providing controlled thrust in a controlled manner. It allows one to replace the nozzle and air thrust generator with the nostril and lungs.

The final piece of puzzle is "controlled upper body muscle movements", and here I intentionally did not write sneeze as that was not what I meant. While it is true, that the MacGyver style experiment (pepper up the nose) proposed is unpleasant, it was to show just how much potential thrust the human body can generate when appropriate muscles are worked upon in the desired order.

It may appear silly, but I am always dead serious about my answers.

• The OP specifically asked for a device. – user10509 Oct 5 '16 at 8:46
• The linked journal does not in any way at all support your assertion that flow changes significantly in microgravity conditions. Typical explanations for microgravity airflow differences all involve loss of convection (which is powered by gravity and density differences), but the article takes pains to explain that the exhalations are not significantly affected by convection, and there should therefore in fact be no mechanism for microgravity to affect exhale patterns at all. – Nathan Tuggy Dec 7 '16 at 4:42