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Air Expansion: Questions

  1. Oct 4, 2009 #1
    I suppose this is my first topic here at Physics Forums. You see, I'm working on an "Iron Man" suit, as you'd call it. I want some questions answered, because I suppose this explores a new idea yet not tried as of now.

    I'll make it short: air in, heated, expanded, and air out.

    First Question: How much thrust do you require to lift off a 100kg human off the ground?
    Secondly; can this thrust be produced from a 1000cc* intake of air heated to about 400[tex]\circ[/tex] C (refers to topic, "how much does air expand when heated"), blown through a nozzle 5 cm in diameter?
    And lastly; how do I produce this heat? A uranium, polonium, thorium, etc. reactor? A liquid, ruby, etc. laser? Or by any other means?

    *Clarification required
  2. jcsd
  3. Oct 4, 2009 #2
    The force required keep a human of 100kg comes from.


    So 100*9.81 = 981 N.

    Secondly thrust depernds on how much air you can shift, not how hot it is.

    The thust is a function of mass flow rate of air. So for starters you wouldnt want to heat the air as it reduces the density.
  4. Oct 4, 2009 #3
    Your proposed nuclear jet engine has really low thrust

    I suppose you mean 1000 cm^3 of air goes through it each second.

    1000 cc has a mass of 1.2 gram = 0.0012 kg
    you raise the absolute temperature by 50%. After adiabatic expansion the volume
    will be 1336 cc. [itex]PV^{\gamma} = n R T [/itex].

    1366 cc flowing out through an area of 19.6 cm^2 will have a speed of 70 cm/sec = 0.7 m/sec

    The thrust is equal to (exhaust speed) * (exhaust mass per second) =

    0.7 * 0.0012 = -.00084 N. Less than a millionth of what you need.
  5. Oct 4, 2009 #4
    Thrust is also a function of the speed of the air. Heating the air is one way to increase this.
  6. Oct 4, 2009 #5
    Mass flow rate includes speed...

    How does increasing the temperature increase the speed of the exhaust? (I have a good idea of what you are going to say, I just want to be sure)
  7. Oct 4, 2009 #6


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    As a practical aside, Univeruser, you appear to be overlooking the 200-500 kg that the suit itself will weigh. I designed a practical, buildable 'battle suit' about 25 years ago. It wasn't built, due to financial considerations, but the estimated weight was around 250 kg empty. The hydraulic system alone was over 100. There were no armaments, which are also very heavy, since it was designed for fire rescue operations. Flight was never an option.
    If you get your unit up and running, you would need something like a Wright-Williams Micro Turbo jet engine to get it airborne. Even then, you would encounter almost insurmountable stability and control problems. Maybe look into cruise missile technology, because that's pretty close to what you would be wearing.
    Good luck with it, but be prepared for disappointment.
  8. Oct 4, 2009 #7
    Well you let the air expand. The hotter it is, the faster it will expand. You should be able to
    use this in a kind of jet engine, even if the efficiency will be much worse, because the temperature
    will be far lower and no combustion products to increase the mass expelled.
  9. Oct 4, 2009 #8
    I wasnt expecting that....

    Hotter air will cause a (slightly) faster expansion, but it expands in all directions so gives no net gain. In the case of both a jet engine and rocket motor it wont help thrust in the slightest. Tempertaure isnt the driving force for the expulsion, you want the largest pressure gradient possible.

    I was expecting you to attribute temperature to the maximum throughput of the nozzle. As increased ambient temperature increases the speed of sound, which increases the choke speed. Higher choke speed = more flow. For an ideal gas of course.
  10. Oct 5, 2009 #9
    Listen, my friends. I ought to have made this clear, but I'm a 12-year old. So I do need starters, since knowing basic pooon.rinciples ain't enough. I'll post again with a clearer diagram in two weeks or less.
  11. Oct 7, 2009 #10
    Aw, that's all right. I'd prefer some tips on how to increase the pounds of thrust I'd require to get a load of, say, 700 kilograms, off the ground, or the average air intake per second that would suffice, or maybe the heating element (maybe a maser, or a 1500-mW laser).

    Coming to the metals of the suit, I propose a scandium-iridium alloy. This is so, since scandium is very light, and just as strong. 10% iridium added to the scandium, I think, makes it corrosion proof and stronger, but makes it heavier too (that's because iridium is the second densest element). What about that gold-titanium alloy? Any suggestions?
    Last edited: Oct 7, 2009
  12. Oct 7, 2009 #11
    Sorry for the pooon.rinciples. That's principles.
  13. Oct 7, 2009 #12
    oh, to be 12 again... :rolleyes:
  14. Oct 7, 2009 #13
    ...is nice, my friend...that's when you keep bubbling with ideas. I somewhat feel people are ignoring to post here because I'm 12...and older people generally don't like answering young people Anyway then, back on the track.

    Yup. And replying to Danger, he is right about the flight stabilization problems. But present technology can solve this problem, I believe, using computer stabilization and the other geek stuff.
    Last edited: Oct 7, 2009
  15. Oct 7, 2009 #14
    It's just, that when you're 12, usually, you don't have the knowledge yet to be able to work out the finer details of these ideas. I had lots of ideas when I was your age and I still do, but now I understand better why they're not as easy to make real as they are to imagine. This doesn't mean that you can't come up with ideas, of course you can and I encourage that with all my being. I became an engineer to find out how to make my ideas work and how to understand limitations. Some of them are possible, others are further out of reach. To explain to you how to build something like what you're thinking about would take many experts and would require teaching you everything from physics through calculus to jet engine design and aerodynamics. These things take years to learn. I, for one, don't have years to teach you these concepts and I suppose there are a lot of PFers who feel the same way. Once you get some drawings and a better grasp of your requirements (what you want you bodysuit to acheive (flight, weapons, armour etc?), I'm sure you'll get the feedback and ideas you need to be able to develop your design abilities. That being said, I think you've gotten quite a good response from some very knowledgeable and capable members, who have taken you seriously.

    Choosing metals/materials for the construction, for example, usually happens when you have a better idea of your requirements. Strength vs weight vs flexibility vs maintainability and workability and of course, cost.

    Make a list of the requirements of your design and some skecthes and we can go from there. your first requirement is "the ability of the suit to allow a 100kg person to fly", like you said above, that's a good start.
  16. Oct 8, 2009 #15
    You're right, Regardon. I learnt the hard way that the arc reactor woudn't exist. I did work out the finer details as you said, which I found under the Jet Pack article in the english Wikipedia. Here, under the section, "Winged jet and rocket packs", I found out that the flight time of Yves Rossy across the Channel was primarily supported by aircraft fuel. Since you can't have fuel to last on for a, say, hour, or so, even for longer lengths of time, the superheated air was my last resort as propellant. The air can be heated by a 1500-mW laser, which I think can be able to heat the air to the 1000 degrees Celsius or so in the relatively short span it gets through by the heating element.

    As for the laser, it shall be provided by a fission (who knows, in the near future) or fission reactor. RTGs wouldn't be reliable enough.

    Every theory meets with opposition. It might or not work, and so it was, for example, with the legendary astrophysicist R. Goddard. That is,the try shhould come first before the can't. Had the world been the first-time-trial-or-give-up type, we wouldn't have come this far.

    I chose an alloy of scandium, vanadium, and steel. The bad thing is, vanadium is heavy. So about 10% vanadium, 60% scandium, and 40% high-carbon steel.

    The sketches are kind of ready. The downside is that I'm not able to scan them. No weapons for the suit, as of now, I'm sure.

    There could be wings that would panel the armor on the arms, to provide additional lift.
    Last edited: Oct 8, 2009
  17. Oct 8, 2009 #16
    how did you work that out? did you assume the flowrate is 1366cm³/s at a pressure of 1bar?

    I think maybe this would be more pertinent, as it takes pressures into account:

    [tex]V_e = \sqrt{\;\frac{T\;R}{M}\cdot\frac{2\;k}{k-1}\cdot\bigg[ 1-(P_e/P)^{(k-1)/k}\bigg]}[/tex]

    Ve = Exhaust velocity at nozzle exit, m/s
    T = absolute temperature of inlet gas, K
    R = Universal gas law constant = 8314.5 J/(kmol·K)
    M = the gas molecular mass, kg/kmol (also known as the molecular weight)
    k = cp/cv = isentropic expansion factor
    cp = specific heat of the gas at constant pressure
    cv = specific heat of the gas at constant volume
    Pe = absolute pressure of exhaust gas at nozzle exit, Pa
    P = absolute pressure of inlet gas, Pa
  18. Oct 8, 2009 #17

    I know you are 12 mate but that made me chuckle.

    Sc and V are alloying elements, they would be added to steels or aluminium/titanium to alter the properties slightly. This means that of a steel alloy you'd probably add 0.1 - 4% weight of the alloying elelment.

    The 60% is what gave me a giggle for scandium, that is almost exclusively a micro alloying elelemnt, so 0.1 - 0.5% ish.
  19. Oct 8, 2009 #18
    Friends, please tell me by how much I'd have to increase the intake when it meets the laser for the short few milliseconds, to get a net gain or 10,000 N.

    It's more or less like the jet engine, where you gunk in aerofuel with the air, then set it on fire. The temperature rise caused by the fuel makes it move forward. Same it is with my engine, only that the heating element is a laser.
  20. Oct 8, 2009 #19
    It not the temperature rise that makes it move forward. It's the pressure and speed of the flow.
  21. Oct 9, 2009 #20
    Yes, but the aerofuel mixed in with the air raises the temperature of the air inside the mixing chamber and increases the volume, and of course, as you said, the compressor propels it out. In fact, the hot escaping, expanded gases are those which rotate the rotate the shaft connecting to the compressor fan. The fan spins to suck in air only at takeoff.

    I've prepared one such diagram to illustrate my system. You can see it below. For the time being, look at this simulation.

    http://upload.wikimedia.org/wikipedia/en/e/e5/Turbofan3_Labelled.gif [Broken]

    1. Nacelle
    2. Fan
    3. Low pressure compressor
    4. High pressure compressor
    5. Combustion chamber
    6. High pressure turbine
    7. Low pressure turbine
    8. Core nozzle
    9. Fan nozzle

    You can see that the hot air is marked by the flow of red arrows. You my also notice the many fans connected to the shaft that are rotated by the air (9. Fan nozzle) before it finally escapes out, hence drawing in more air. In simple terms, heated (and compressed) air (should be about 2000 degrees) power the engine.

    And here's my engine, attached.

    P.S. The compressors are missing when you look at the attachment. This does not approve of its non-existence.

    Attached Files:

    Last edited by a moderator: May 4, 2017
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