Transofrm heat into kinetic energy

[egom]

Is there a method to transform the heat from the rocket engines into kinetic energy (from what I have read like 80-90% of the fuel energy is transformed in heat).<br />If the heat would be transferred at fist in electriciy and from electicity inot kinetic energy the performance would increase dramatically.<br /><br />EgoM

 

[chriscdc]

If by heat you mean the radiated heat, then you could get energy out of it. Actually the main problem is heat getting absorbed by the rocket nozzle. Some rockets use ablative cooling where material takes heat away when it is shed from the bell. Other systems use active cooling to remove heat.<br /><br />There are pyro-electric materials that when they heat up they generate an electric potential that could be used. <br /><br />This 80-90% figure is probably mis-understood. The temperature of a gas molecule is proportional to its velocity. A gas particle can have a high temperature but not radiate any away. So a rocket can have an exhaust of X thousand degrees but this exhaust does not get cooler after it leaves the rocket (unless it interacts with something such as the atmosphere).

 

[egom]

I see. Well, instead of ejecting the result of the combustion at 1000 degrees and 10.000km/s you could eject it at 100 degrees at 15.000km/s. That would be a great achievent.<br /><br />In anycase, those pyroelectric materials sound cool. If they could improve the perfromance with 10% that would be great...<br /><br />EgoM

 

[tap_sa]

<font color="yellow">"Is there a method to transform the heat from the rocket engines into kinetic energy (from what I have read like 80-90% of the fuel energy is transformed in heat). "</font><br /><br />The convergent-divergent De Laval nozzle found in practically all chemical rocket engines is that method. In the combustion chamber all chemical energy in the propellants is released as heat. Well actually not all, complete burn would be nice but ~94-99% of the theoretical chemical energy gets released. Now we have very hot, slow moving, high pressure gas. 1-2% of the thermal energy in the gas is lost to engine walls. Some of that loss heats up the fuel in regenerative cooling and may drive the pumps in expander cycle engine so the energy is not entirely lost. Then comes the divergent part of the throat, gas laws dictate that the subsonic flow speeds up, cools down and pressure drops. At the throat gas speed reached Mach one and the divergent section begins. Now comes the funny part, gas laws say that the now supersonic flow continues to speed up while it expands in the nozzle. Pressure and temperature drop further. The lower you can allow the gas pressure to drop inside the bell the better thus rockets operate more efficiently in higher altitudes and best in the vacuum.<br /><br />Rocket is very efficient in turning the chemical energy into uniform gas flow, 50% or more turns into kinetic energy.<br /><br />

 

[mlorrey]

If it is so efficient, then why do other methods get higher Isp? I was under the impression that the rocket engine was the most inefficient means of propulsion, and that the De Laval nozzle only boosts its efficiency a few percent.

 

[mlorrey]

You make an excellent point, EgoM. However there is a trade off. Theoretically, the most effcient nozzle is one which has nearly infinite length, until the gas has expanded and cooled to the ambient solar wind level of concentration and thermal energy. The problem with this is that the longer you make the nozzle, the more mass this takes, thus dropping your T/W ratio. It would also be difficult to fit a near infinite nozzle into a rockets upper stage.<br /><br />Thus most rocket engines designed for vacuum operation really aren't, they are compromises between expansion vs engine mass and length.<br /><br />Engines designed for sea level operation are limited by their expansion to exhaust combustion gasses at the nozzle exit at or above sea level atmospheric pressure. Theoretically you could make these gases also ambient temperature at that pressure, but this would require a very long expansion nozzle that was very heavy. Like vacuum designed nozzles, they are shortened to improve thrust to weight ratios, and as a result the exhaust is still extremely hot. <br /><br />There is a means of converting kinetic energy of hot combustion gasses to electricity. One is called magnetohydrodynamic generation. The exhaust nozzle would be surrounded by electromagnets that impart a magnetic field on the exhaust gases, which are actually a plasma. The plasma acts as the rotor in a generator, imparting force upon the electric field of the stator, and thus generating current.<br /><br />The question then is: so what? Lets say you are able to slow the exhaust products through MHD to low speeds. You still have to do something with them. Carrying them on your spaceship only adds dead mass to your ship. If you don't have means to add energy to them again and run them back through the cycle, it is useless to you. Since you need to dump it from your spaceship, you might as well do it in a way that gives it the greatest velocity, not the lowest.<br /><br />Now, there are proposals to use a working f

 

[tap_sa]

<font color="yellow">"If it is so efficient, then why do other methods get higher Isp?"</font><br /><br />Because being efficient doesn't automatically guarantee high Isp. With chemical propulsion your Isp is limited by the heat of combustion in the propellants. In other forms of propulsion the kinetic energy put into propellant comes from outside source (electricity, nuclear), allowing much higher energy density ie exhaust speed ie Isp. But then the outside energy source usually puts serious limits to your thrust levels. Chemical engines release easily gigawatts of energy while electric propulsion tinkers at tens of kilowatts level.<br /><br /><font color="yellow">"De Laval nozzle only boosts its efficiency a few percent."</font><br /><br />Not having a covergent throat drops Isp only a few percent but pressure and overall thrust drop considerably. The divergent bell nearly doubles the the thrust and Isp.

 

[mlorrey]

Okay, then I suppose a good way to express EgoM's question would be: is it possible to have a nozzle that extracts more energy, possibly as electricity, from the exhaust's thermal energy (without slowing it down) which can then be used to accelerate the exhaust even more.<br /><br />I believe I gave a pretty good answer previously: you could have a very long nozzle that allows the exhaust to cool and expand more, so long as it was still at the same or higher pressure at the exit as the surrounding ambient environment. Tsiolkovsky dealt with this in his rocket by having a very long nozzle that was cooled by the propellant tanks which surrounded the engine and long nozzle. <br /><br />This is mass inefficient, though, so one would want to find a means of doing the same thing in a much more compact unit. The Tsiolkovsky rocket uses the engine as part of the fuel tank structure, though, so the real question would be is what the gain would be.<br /><br />Other than a long nozzle, you might use thermocouples to convert thermal energy to electricity. Currently, however, the thermal energy of the chemical rocket is used to power the turbo pumps in most engines (other than pressure fed), so there is energy reuse, thermo-mechanically speaking.<br /><br />There are also near IR versions of PV cells which could form a dish around a chemical rocket exhaust and derive power from the IR emissions of the exhaust gasses. Don't know what power and efficiency these would be.

 

[egom]

Well, <br />in the athmosphere the rocket shield heats because of the gas that hits it. That energy can be recovered <img src="/images/icons/wink.gif" /> if the mechanism to transform heat into kinetic energy you describe works. Basically this system would be very energy efficient (instead of a shield you have a heat conductive material that generates more energy). If the efficency of the transfer would be 100% then it would be as there would be no speed decrease because of the gas in the atmosphere.<br /><br />In the perfect system with 0 energy loss all that you need is to provide the energy to lift your ship to the desired position (that energy should be equal with the potential energy that the rocket would have when in the maximum position). I do not remember the formula right now but at conceptual level I hope that this is obvious to everyone.<br /><br />Another kind of engine is a combination of a chemical and an electrical one. What I am writing here is pure speculation (I am no professional in this area and I have only basic knowledge in phisics). The principle would be like that: we have the grids similar with the ones from the ion engine. We have the combustion chamber. It has an anode (a bar) on the middle and the chamber walls are the catode. We have a chemical reaction in the combustion chamber that has as a result 2 substances in gas form that are charged as a result of the chemical reaction (one is positively charged and the other is negatively charged).<br />In the middle section of the exit zone of the gas we have 2 grids that are negatively charged. Sice we have the anode in the middle then the negative gas goes to the middle and is further accelerated by the grid because of the difference of voltage. The same happens next to the walls of the combusiton chamber with the positive grids (basically we have a circle with negative grids in the middle and an outer circle with positive grids).<br />This is in fact a combination of the common chemical fuel with an

 

[tap_sa]

<font color="yellow">"in the athmosphere the rocket shield heats because of the gas that hits it. That energy can be recovered"</font><br /><br />I assume you are talking about atmospheric drag during ascent. Those losses are miniscule compared to gravitational losses and the energy required to accelerate the vehicle into orbital speed. Even if you somehow manage to recover the heat in the nose cone that's only part of the energy involved in the drag. Some of it turns into shockwave, other part moves and heats the air that never even touches the vehicle.<br /><br /><font color="yellow">"In the perfect system with 0 energy loss all that you need is to provide the energy to lift your ship to the desired position (that energy should be equal with the potential energy that the rocket would have when in the maximum position)."</font><br /><br />This perfect system forgets gravitation. It affects all the time demanding deltav from your vehicle's engines, unless you are at orbital speed. One must remember that the potential energy at LEO altitude is peanuts compared to the kinetic energy of the orbital speed. This is why even amateurs can send rockets briefly <i>up</i> to space but it's still an elite big boys club for those who actually can get things to <i>stay</i> there.<br /><br />About your electrochemical engine; the negative and positive products of the chemical reaction would immediately combine and form neutral substance, wouldn't they? I'm not even sure if a combustion of anykind could produce ions.

 

[w_ashley]

yes. <br /><br />PIEZO ELECTRIC is one way, but heat creates chemical change, chemical change creates chemical motion chemical motion is movement. You just need to direct it. Heat Creates Kinetic Energy by exciting matter. <br /><br /><br />Note you have heat going out it is directed to create 'thrust' this thrust is used for 'launch' and to counter act 'gravity' which has a slightly varying degree at different pointts due to mass being an accumulation of tangents of force. Launch is counteracting the tangent of force, efficient launch is working 'as closely' with the tangential force as posible. Thus orbiting etc.. <br /><br />EM Energy is different strengths of light (photons) these photonic forces are 'tangents of force' they are motion. <br />Energy eventually forms into nuetrons and protrons all this stuff being composed of energy in a tangent or 'spin' now. We can accumuate spins to create a desired motion or 'launch' heat itself is a representation of density of energy in relation to it's historic conductance. <br /><br />So what you do is create a 'dish' or director to focus the 'photons' Fireballs are excited gas etc... with many photons. Photons and EM are electricity (sorta). The issue is that if you send suff in one direction and collect it you create drag, so you need a way of making a drag 'orbit' so that you keep a constant motion, in complete efficiency though 0 you don't go anywhere. You need to discard energy to have motion you can collect it while you go though creating a 'conduit' that is if there were many rockets you could pass energy in a type of photon high way. <br /><br />If you understand physics and reality it is all quite novel.<br /><br />It goes to the box holding a box, your gonna have a box in the box, so unless you have something to hold the box you have an issue.<br /><br />What you do is make a box that is the photons, but you need something to direct them, such as a radio telecope.. When you get to the issue of putting a box inside th

 

[mlorrey]

W_ashley is partly correct: you use your heat to both excite your reaction mass, and to guide it in the same direction as much as possible at as high a speed as possible. You want the heat to go into the acceleration of the reaction mass.<br /><br />With chemical propulsion this is pretty straightforward. The only ways around the Isp limits of chemical combustion are to use pulse detonation (which requires high structural strenth and vibration dampening, i.e. increasing dead mass) or to have air breathing for as much of the propulsion cycle as is feasible with your materials technology.<br /><br />It appears that SHARP materials, HfB2 and ZrB2 may be the unobtainium we've been looking for. They allow mach 7 flight at sea level, and mach 11 at 100,000 ft. Using this material on leading edges, nose cones, and intakes and combustion chambers of airbreathing combined cycle engines seems to be the materials breakthrough we've been needing to make multi-mode airbreathing launch a reality. These materials perform so well because they are extremely fast radiators of heat that they absorb and have a super high melting point: almost 3600 C.<br /><br />That Boron is also a fantastic neutron absorber may make these SHARP materials useful in coolant to air heat exchangers for nuclear ramjets, radiating the thermal energy the coolant absorbs from the core while absorbing the neutrons emitted by the coolant. This may make this form of propulsion feasible.<br /><br />But it is in space propulsion we need some breakthrough too.<br /><br />The big obstacle, of course, is that you can't just generate power and exert it in some way that doesn't require that you keep throwing mass overboard. This is the area where breakthrough physics is required.<br /><br />We are all tied to the inertial frame by all the other mass of the universe acting gravitationally in opposition to acceleration in any direction. The needed breakthrough is some way of biasing that inertial drag, or else treating the gravity

 

[egom]

Kinetic energy is the same as the heat, but all the vectors of all the particles go in the same direction. I do not know how we can make this.<br />Now I am thinking that the atraction force between positive/negative particles and the atraction force between objects (gravity) uses the same principle (obvious). So all that we need is a way to opose the positive/negative atraction force to the gravity force. There must be a way to make this. I will think further to the concept to make such a spacecraft.<br /><br />And one comment to the guys that are bashing ISS: you can not study gravity on earth because you can not vary this factor (the gravity force). If they will start studiyng this domain this will be a giant leap for humankind and the progress can be very fast (like 1 decade). From this point only ISS values a lot more than the billions invsted in it.<br /><br />EgoM

 

[rocketman5000]

take for example everyones favorite high isp engine the vasimr. You can quantify the amount of heat kept in a molecule by how much its stagnation temperature multiplied by Cp (coefficent of pressure) to find the heat released it can be calculated by Cp*T at exit. The difference would be equal to 1/2mv^2. In a vacuum you can expand the fuel till it liquifies, in fact look at LOX and LH2 production to see this pratical application. therefore you can never get all of the energy out of the fuel. <br /><br />To figure out why non chemical fuels can get higher isp you have to look at the stagnation temperature of the gas being expelled out the nozzle. While LOX LH2 only gets to several thousands of degrees C the Vasimr LH2 will reach orders of magnitude higher. meaning if you expand to the same final temperature you have extracted much more energy per unit mass than a chemical rocket. A chemical reaction can only release a finite amount of energy and therefore can't equal the Isp of very efficient engines such as vasimr.<br /><br />btw vasimr even uses a Laval nozzle to expand the gass. This nozzle is constructed using magnetic field lines as the exhaust would melt any metal used.

 

[scottb50]

The problem being it would take a lot of LH2 just to get up to full power and quite a bit to keep the reactor from spewing out the back. The exhaust and operating temperatures would be dependent on materials just like they are with combustion engines. Thorium is liquid at 3300 C, but it would take a lot to contain it.<br /><br />In any case the weight would be prohibitive. <br /> <div class="Discussion_UserSignature"> </div>

 

[rocketman5000]

was that in response to my post? nothing that I talked about delt with a reactor...

 

[nexium]

Therocouples and special PN junctions convert waste heat to electricity. A more efficient converter may be available soon, so we can perhaps convert 1% of the rocket heat to electricity with little loss of thrust, and modest reduction in payload. Other than RTG = radioactve thermal generators, other space applications are still on the drawing boards, I think. Several kinds of electric propulsion will likely soon be available, but low efficiency limits applications. Neil

 

[mlorrey]

From what I've read, most electric propulsion systems are between 30-60% efficient, which is pretty good. The real limitation is amount of thrust. With electric systems, the higher your Isp, the lower your thrust, for a given power input.

 

[nexium]

Hi Rocketman: Vasmir is a reactor. Reactors are heavy with fisionable isotopes, but high energy per kilogram is possible in space where most of the usual radiation shielding and containmement mass can be omitted. All we need is radiation resistant waldos to make repairs and do maintenance. Fission reactors may allow unmaned probes etc to travel the solar system in 1/4 the time it takes chemical rockets. High temperature confinement breakthoughs may soon be available thanks to fusion research. Neil

 

[tomnackid]

Rocket exhaust is ionized and can therefore carry and electric charge. You can take a high speed stream of ionized gas and pass it through a magnetic field to create a Magnetohydrodynamic generator. MHD research was big in the 70s when it look like we might be going back to coal in a big way. You can pulverize coal into a powder and cleanly and efficiently burn it at ultra high temps in what is essentially a coal powered rocket or blowtorch. Pass the hot gas through a MHD generator and you can extract electricity. You can even use the "cooler" (relatively speaking) gas left over to heat water and turn a convectional turbine for added efficiency. Funding for MHD research pretty much dried up in the 80s, but who knows it might make a comeback.<br /><br />You could pass a chemical rocket exhaust through a MHD generator to extract electricity, but there is no such thing as a free lunch (or launch!). It will add weight and will reduce the speed of the exhaust.