Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Energy needed for achieving this mass flow rate for a rocket

  1. Apr 21, 2016 #1
    Hey,
    Suppose you have an enclosed vessel that is full of water. You heat it up then you open a valve, and that will create enough thrust for it to act as a rocket.

    How much energy is needed for a steam rocket to get heated enough to achieve a mass flow rate of 2430 Kg/s.
    Mass of water= 256 Tons.
    total mass of the rocket with the propellant is 282 Tons.
     
  2. jcsd
  3. Apr 21, 2016 #2

    ChrisVer

    User Avatar
    Gold Member

    This is by no means a HEP or Nuclear Physics question.
    that mass flow rate you are giving will erase your water in just 2minutes... but nevermind...
    How big is your valve?
     
  4. Apr 21, 2016 #3
    it has the exact area of this rocket engine. But I don't know how to calculate the exit area of the nozzle
    Rocketdyne RS-27A
    Propellants: LOX/RP-1 Kerosene
    O/F Ratio: 2.245
    Dry Weight: 2,528 lbs
    T/W Ratio (sl): 79.11
    T/W Ratio (vac): 93.75
    Length: 149 inches
    Diameter:
    67 inches
    Expansion Area Ratio (ε = Ae/At): 12
     
  5. Apr 21, 2016 #4

    rcgldr

    User Avatar
    Homework Helper

    Shouldn't the problem also state the speed of the expelled water / steam as well as the mass flow to determine the power, in which case the total energy would be the power times the time it takes to empty the water from the rocket?
     
  6. Apr 21, 2016 #5

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    The required temperatures would instantly evaporate your rocket.
     
  7. Apr 21, 2016 #6
    It's hard to determine the flow of an unsteady two-phase fluid
     
  8. Apr 21, 2016 #7

    russ_watters

    User Avatar

    Staff: Mentor

    Just pulling a number out of the air for order of magnitude, but if it starts hot and pressurized at 300 C, the enthalpy of vaporization is 1400 kJ/kg. So 2,430 kg/sec would need 3,400 MJ/sec. For comparison, that's about the typical heat output of a commercial nuclear reactor.

    As mfb says, you can't make the starting mixture hot enough to vaporize all of the water upon release without it being thousands of degrees.
     
  9. Apr 21, 2016 #8
    imagine we had a meta-material that can withstand such temp.
     
  10. Apr 21, 2016 #9

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    You cannot maintain a constant flow by heating it in advance, flow will decrease over time.

    Flow depends on the unspecified size of the narrowest point of the nozzle, and also on the shape.
     
  11. Apr 21, 2016 #10
    it will be heated with a 400 MW power plant then a high insulation layer will contain it
     
  12. Apr 21, 2016 #11

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    400 MW for 2430 Kg/s gives a maximal exhaust velocity of 570 m/s. That is really, really bad for a rocket. You won't go to space with such a tiny exhaust velocity.

    What do you actually want to do?
     
  13. Apr 22, 2016 #12
    I just wanna see how much power should I provide to a steam rocket as heat to make it have a same thrust and mass flow rate of NASA's Delta 2 rocket
     
  14. Apr 22, 2016 #13

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Why don't you look up how much power the combustion of the rocket fuel releases?

    Those powers are typically in the range of tens of gigawatts.
     
  15. Apr 22, 2016 #14
    But even if the power differs those two systems have completely different reactions. one is combustion and the other is straight expansion because of ;pressure deference,Maybe the combustion has less efficiency ? :/
    Also I'm kind mixed up with what exactly im looking for. Why do we care about the power input instead of energy. Can't I determine the input energy and then see how much time will it take a 300MW to reach that energy ?
     
  16. Apr 22, 2016 #15

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    The combustion is highly efficient (you heat the material with the chemical reaction - how could this be inefficient).
    To improve on that, you have to deliver even more power.
    To launch a rocket, you need some minimal mass flow - you have to counter gravity. Rockets typically start with thrust of about 1.5 to 2 times their weight, which means they waste 1/2 to 2/3 of their initial thrust already. Going down more makes it worse.
     
  17. Apr 22, 2016 #16
    What I mean is can we treat the steam rocket as a thermal battery. we store a huge amount of energy through heating and then launch. The power plant charges the rocket once not constantly. for example if a rocket a mass flow rate of 2,430 kg/sec would require 1000 Gjoules of input energy then we bring a 400MW power plant to charge it (heat it) for ⇒ Time= Energy/Power= 1000 Gjoules/400MW = 3333.333 seconds.

    What did I miss here?
     
  18. Apr 22, 2016 #17

    jbriggs444

    User Avatar
    Science Advisor

    The result is essentially identical to a huge flywheel. You spin it up so that the rim is at the selected exhaust velocity and then unwind a thin strip from one side. Both solutions fail for essentially the same reason. You have to be able to contain the immense pressure.
     
  19. Apr 22, 2016 #18

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    As discussed before, storing the energy as heat in the rocket does not work, the rocket would evaporate.
     
  20. Apr 22, 2016 #19
    if we hypothetically had a material that would withstand the high pressure/temperature .would it work?
     
  21. Apr 22, 2016 #20

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Sure. If you have indestructible materials that do not conduct heat at all, fill them with incredibly hot plasma at high pressure, let the plasma out, done.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Energy needed for achieving this mass flow rate for a rocket
  1. Mass flow rate (Replies: 8)

  2. Flow rate (Replies: 18)

Loading...