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Hybrid Combustion-Nuclear Electric Rocket

  1. May 25, 2015 #1
    Ordinarily, combustion rockets and nuclear-electric rockets are considered as mutually exclusive categories. Would it be possible to have a launch vehicle engine which benefits from characteristics of both?

    For instance, chemical combustion generates a lot of kinetic energy from heat, and yet that energy is so chaotic that not all of it is harnessed as thrust. Meanwhile, ion and plasma engines are much more efficient, yet their thrust is low.

    Could it be possible to take the already hot exhaust gas produced from a combustion chamber, and either ionize it or break it down into plasma, in order to further accelerate it linearly using electric coils?

    The energy to ionize or "plasma-ize" exhaust gases would be supplied by a nuclear reactor which would supply energy for high-powered microwaves and magnetism to convert the hot combustion exhaust into ions/plasma. The ions/plasma would then be accelerated electromagnetically (Lorentz force?)

    http://en.wikipedia.org/wiki/High_Power_Electric_Propulsion


    I guess the overall goal here would be to have the high thrust of a chemical rocket, while boosting the Isp using nuclear power.
    (Yes, I've been previously reminded me that reactors require shielding, but let's assume this is an unmanned cargo rocket for the sake of argument)
    There are various designs for nuclear-enhanced chemical propulsion which have already been proposed, but most seem to involve using the nuclear reactor to increase the heat of the exhaust. I've not seen anything proposed which uses nuclear power for electromagnetic acceleration of the exhaust gas.

    What would be the pro's and cons of this idea?
     
  2. jcsd
  3. May 29, 2015 #2
    Ok, let’s break it down:

    In this case (actually in many cases) efficiency is misleading. Let’s take a look at this from mass/thrust ratio.
    Primary mass (discounting dry engine mass for simplicity) in chemical propulsion systems is fuel, in electric engines the primary mass is the sum of fuel mass and power generator mass (be it a nuclear generator or solar cells).
    As thrust is equal to exhaust mass flow × velocity of that mass flow, it is obvious that higher they are, the higher the thrust is.
    Thrust divided by total vehicle mass equals vehicles acceleration; and acceleration × acceleration duration equals to vehicles final velocity.
    Acceleration duration equals to total vehicles fuel mass divided by mass flow rate.
    And the last note: if vehicles acceleration is lower than a planets gravitational acceleration it cannot (obviously) take off from the ground without another aid (this is applicable only for vehicles which start from the ground instead from orbit).


    A typical chemical rocket can be designed to function with (almost) arbitrary mass flow rate, but the exhaust velocities are limited – currently somewhere between 2500 and 3400 m/s – and this limits the overall thrust. The ‘’chaotic’’ energy is manifested as heat and pressure, and both play a great role in increasing thrust. The kinetic energy of combustion is not used for thrust in rocket engines, but it is in some forms of pulsed engines and pulse detonation engines (check 1954 – 1966 project Orion).

    A typical electrical engine (like a Hall effect thruster, any ion thruster, or even an ion gun as a simplest form of this) gives kinetic energy to fuel by exerting certain forces on it – like the Lorentz force you mentioned. But the problem here is that these forces only act on certain constituent particles of the fuels atoms (on electrons). In order to get these particles free for use, we must first tear them away from their atoms, and we need to put in a lot of energy for that. The rest of the energy available is used for accelerating these particles to high exhaust velocities – which give use the high isp. Unfortunately these particles have a very low mass thus the generated thrust is also very low, furthermore electrons represent only a few percent of the total fuel mass thus most of this fuel is just dead weight.


    Due to high heat during combustion molecules which are initially formed, break down again (only a portion of them and not all), and during this process some free electrons are also generated. We could accelerate these electrons to high velocity but the isp wouldn’t increase by more than a fraction as isp would be calculated as ((gas mass flow × gas exhaust velocity + electron mass flow × exhaust velocity of electrons)/total exhaust mass). The electron mass would be miniscule in comparison to the rest of the exhaust mass, thus you would gain much more weight penalty than you would gain thrust.

    On the other hand if we use a generator to additionally heat the exhaust gas, we significantly improve isp, because the nozzle throat velocity increases.



    Someone please correct me if I’m wrong (or overly simplistic) at any part.
     
  4. May 29, 2015 #3

    rcgldr

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    I'm wondering how this works. After separation results in ions and free electrons, what happens to the oppositely charged particles that aren't accelerated out of the engine in the same manner that the targeted charged particles are accelerated out of the engine?
     
  5. May 30, 2015 #4
    In hall thrusters, as far as I understand, they are pulled out by the electron stream. And due to their much higher mass they produce the bulk of the thrust.
    I wonder if this would also be the case in a combined chemical/electrical engine, or would the interaction between ionized and non-ionized particles dampen the effect too much.

    Another problem with combined engines is that gas in the diverging part of the nozzle already has a high velocity, thus the amount of time during which particle acceleration can take effect is significantly reduced.


    p.s.: Wikipedia says that xenon hall thrusters actually have a fuel mass utilization of 90 to 99%.
     
  6. Jun 3, 2015 #5
    So I'm also just trying to understand that reason why electric propulsion is always limited to such low thrust levels. Is it because of what was said about only electrons being accelerated? And yet all the propellant is being expelled from the rocket, meaning that those accelerated electrons are at least dragging the positive ions with them.

    For some reason, VASIMR thrusters are supposed to produce more thrust compared to other electric propulsion. I read once that they've even been considered for shuttling a vehicle between the lunar surface and orbit. Why is it that VASIMR produces more thrust? Is it just because it tends to use a lighter gas like Helium or Hydrogen?
     
  7. Jan 21, 2016 #6
    Electric Rocket engines are not good to be used to launch the rockets, space crafts from the Earth Surface as they have a low thrust which is not powerful enough to do it. But they are the best for deep space missions........ because they provide the thrust for a much longer time than the chemical rocket engines do. Which means the continuous thrust for a longer time accelerates the space craft to a much higher speed that a chemical rocket engine can't give.......
    I wonder whether electric rockets can be used for Mars Missions.......
    Deeper the mission is, faster and quicker it can be completed with Electric Rocket engines...... Having a space ship assembled in the space(higher earth orbit) that is propelled by electric rocket engines, and using that for deep space missions will enhance the capabilities of the space missions..... (small capsules can be used to land on the planets and come back to the mother ship......)
    I think it is something the present technology is capable of doing....
    Well not in the scale that we find in the Scientific Fictions, but in a scale that we can afford with the existing technology...............!:smile:
     
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